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MIL-STD-1522A(USA28 MAY 1984SUPERSEDINGMIL-STD-1522 (USAF)1 JULY 1972

MILITARY STANDARD

STANDARD GENERAL REQUIREMENTSFOR SAFE DESIGN AND OPERATIONOF PRESSURIZED MISSILE ANDSPACE SYSTEMS

NO DELIVERABLE DATA REQUIRED BY THIS DOCUMENT

AREA SAI

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MIL-STD-1522A (USAF)28 MAY 1984

DEPARTMENT OF AIR FORCEWashington, D.C. 20301

Standard General Requirements for Safe Design and Operation ofPressurized Missile and Space Systems

MIL-STD-1522A (USAF)

1. This military standard is approved for use by the Departmentof the Air Force, and is available for use by all departmentsand agencies of the Department of Defense.

2. Beneficial comments (recommendations, additions, ordeletions) and any pertinent data which may be of use inimproving this document should be addressed to:

SD/ALMP.O. Box 92960Worldway Postal CenterLOS Angeles, California 90009

by using the self addressed Standardization Document ImprovementProposal (DD Form 1426) appearing at the end of this document orby letter.

ii



FOREWORD

This standard establishes the basic system safety criteriafor pressurized systems used on Missiles and SpaceSystem-Aerospace Vehicle Equipment (AVE) and its related GroundSupport Equipment (GSE). It is applicable to all AVE and GSEwhich contain pressurized systems, subsystems or components.All criteria listed herein are mandatory design criteria whenthis standard is placed on contract. Each criterion will bereviewed for specific applicability to the projected new design,and when systems are approved for modification. Specificapproval of the procuring activity is required prior to theexclusion, modification or revision of any criterion listed inthis standard during the generation of design specifications foritems of AVE and GSE which contain pressurized systems,subsystems or components thereof.

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3.383.393.403.413.423.433.443.453.463.473.483.513.523.533.543.55

4.

4.14.1.14.1.24.24.2.14.2.24.2.34.2.44.2.54.2.64.2.74.34.3.14.3.24.3.34.44.5

4.64.6.14.6.24.6.34.6.44.74.7.14. 7.24.7.34.7.44.7.5

CONTENTS (Cont)PAGE

Proof Factor . . . . . . . . . . . . . . .Proof Pressure . . . . . . . . . . . . . .Qualification Tests . . . . . . . . . .Residual Strength . . . . . . . . . . . .Safe-Life. . . . . . . . . . . . . . . .Service Life. . . . . . . . . . . . . . .Stabilizing pressure. . . . . . . . . . .Stiffness. . . . . . . . . . . . . . . .Stress Corrosion Cracking . . . . . . . .Stress Intensity Factor . . . . . . . . .Threshold Stress Intensity Factor . . . .Ultimate Load. . . . . . . . . . . . . .Ultimate Factor of Safety . . . . . . . .Ultimate Pressure . . . . . . . . . . . .Ultimate Pressure Factor. . . . . . . . .Verification/Re-Certification Tests . . .

GENERAL REQUIREMENTS . . . . . . . . . . . .

System Analysis Requirements. . . . . . .System Analysis. . . . . . . . . . . .System Analysis Data . . . . . . . . .

General Design Requirements . . . . . . .Loads, Pressures and Environments. . .Strength Requirements. . . . . . . . .Stiffness Requirements . . . . . . . .Thermal Requirements . . . . . . . . .Stress Analysis Requirements . . . . .Malfunction. . . . . . . . . . . . . .Miscellaneous Requirements . . . . . .

Materials Requirements. . . . . . . . . .Material Selection . . . . . . . . . .Material Evaluation. . . . . . . . . .Material Characterization. . . . . . .

Safe-Life Requirements. . . . . . . . . .Fabrication and Process Control

Requirements . . . . . . . . . . . . .Quality Assurance Requirements. . . . . .

Inspection Plan. . . . . . . . . . . .Inspection Techniques. . . . . . . . .Inspection Data. . . . . . . . . . . .Acceptance Test. . . . . . . . . . . .

Operations and Maintenance Requirements .Operating Procedures . . . . . . . . .Safe Operating Limits. . . . . . . . .Inspection and Maintenance . . . . . .Repair and Refurbishment . . . . . . .Storage Requirements . . . . . . . . .

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MIL-STD-1522A (USAF)

4.7.64.84.8.14.8.24.8.3

5.

5.15.1.1

5.1.1.15.1.1.25.1.1.35.1.1.45.1.1.55.1.2

5.1.2.15.1.2.25.1.2.35.1.2.45.1.2.55.1.3

5.1.3.15.1.3.25.1.3.35.1.3.45.1.3.55.1.4

5.1.4.15.1.4.25.1.4.35.25.2.1

5.2.1.15.2.1.25.2.1.35.2.1.45.2.1.55.2.2

CONTENTS (Cont)

28 MAY 1984

PAGE

Documentation. . . . . . . . . . . . .Special Requirements. . . . . . . . . . .

Re-Activated Pressurized Hardware. . .Multiple Proof Tests . . . . . . . . .Test Fluids. . . . . . . . . . . . . .

DETAILED REQUIREMENTS .

Non-composite Pressure Vessels. . . . . .Pressure Vessels with Non-Hazardous

Leak-Before-Burst Failure Mode(AVE) . . . . . . . . . . . . . .Factor of Safety Requirements . . .Safe-Life Analysis Requirements . .Qualification Test Requirements . .Acceptance Test Requirements. . . .Re-Certificate on Test Requirements.

Pressure Vessels with Brittle FractureFailure Mode (AVE). . . . . . . . .Factor of Safety Requirements . . .Safe-Life Demonstration RequirementsQualification Test Requirements . .Acceptance Test Requirements. . . .Re-Certification Test Requirements.

Pressure Vessels Designed EmployingStrength of Materials (AVE) . . . .Factor of Safety Requirements . . .Safe-Life Analysis Requirements . .Qualification Test Requirements . .Acceptance Test Requirements. . . .Re-Certification Test Requirements.

Pressure Vessels Designed Employingthe ASME Boiler Code (AVE & GSE). .Qualification Test Requirements . .Acceptance Test Requirements. . . .Re-Certification Requirements (GSE)

Composite Pressure Vessels. . . . . . . .Composite Pressure Vessels with Non-

Hazardous Leak-Before-Burst FailureMode (AVE). . . . . . . . . . . . .Factor of Safety Requirements . . .Safe-Life Analysis Requirements . .Qualification Test Requirements . .Acceptance Test Requirements. . . .Re-Certification Test Requirements.

Composite Pressure Vessels withBrittle Fracture or Hazardous Leak--Before-Burst Failure Mode (AVE). .

2424242525

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353535353535

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CONTENTS (Cont)PAGE

5.2.2.15.2.2.25.2.2.35.2.2.45.2.2.55.2.3

5.2.4

5.2.4.15.2.4.25.35.3.15.3.25.3.35.3.45.3.5

6.

6.16.1.16.1.1.15.1.1.26.1.1.36.1.1.46.1.1.56.1.1.66.1.1.76.1.1.86.1.1.96.1.1.10

6.1.1.116.1.1.126.1.1.136.1.1.146.1.1.156.1.26.1.2.16.1.2.26.1.2.36.1.2.46.1.2.56.1.2.56.1.2.7

Factor of Safety Requirements . . .Safe-Life Requirements. . . . . . .Qualification Test Requirements . .Acceptance Test Requirements. . . .Re-Certification Test Requirements.

Composite Pressure Vessels DesignedEmploying Strength of Materials(AVE) . . . . . . . . . . . . . .

Composite Pressure Vessels DesignedEmploying the ASME Boiler Code(AVE & GSE) . . . . . . . . . .Qualification Test Requirements .Acceptance Test Requirements. . .

Components. . . . . . . . . . . . . . .Factor of Safety Requirements. . . .Safe-Life Analysis Requirements. . .Qualification Test Requirements. . .Acceptance Test Requirements . . . .Re-Certification Test Requirements .

PRESSURIZED SYSTEM REQUIREMENTS. . . . . .

General Pressurized System RequirementsDesign Features. . . . . . . . . .

Assembly. . . . . . . . . . . .Routing. . . . . . . . . . . .Separation. . . . . . . . . . .Shielding. . . . . . . . . . .Grounding. . . . . . . . . . .Handling. . . . . . . . . . . .Special Tools . . . . . . . . .Test Points . . . . . . . . . .Common-Plug Test Connectors . .Individual Pressure and Return

Test Connectors. . . . .Threaded Parts. . . . . . .Friction Locking Devices. .Internally Threaded Bosses.Retainer or Snap Rings. . .Snubbers. . . . . . . . . .

Component Selection. . . . . .Connections . . . . . . . .Fluid Temperature . . . . .Actuator Pressure Rating. .Pressure Service Ratings. .Pump Selection. . . . . . .Fracture and Leakage. . . .Oxygen System Components. .

3738383838

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383838383839393939

41

4141414141414141424242

4242424242424242434343434343

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6.1.2.86.1.2.96.1.2.106.1.36.1.3.16.1.3.26.1.3.36.1.3.46.1.3.56.1.46.1.4.16.1.4.26.1.4.36.1.56.1.5.16.1.5.26.1.5.36.1.66.1.76.1.7.16.1.7.26.1.7.36.1.7.46.1.7.56.1.86.1.8.16.1.8.26.1.8.36.1.8.46.1.8.56.1.8. 5.16.1.8. 5.26.1.8.66.1.8.76.1.8.86.1.8.8.16.1.8.8.26.1.8.96.1.8.106.1.96.1.9.16.1.9.1.16.1.9.1.26.1.9.1.2.16.1.9.1.2.26.1.9.26.1.9.2.16.1.9.2.2

CONTENTS (Cont)PAGE

Pressure Regulators . . . . . . . . 43Flareless Tube Fittings . . . . . . 43Manual Values and Regulators. . . . 43

Design Pressures . . . . . . . . . . . 44Over Pressure . . . . . . . . . . . 44Back Pressure . . . . . . . . . . . 44Pressure Isolation. . . . . . . . . 44Gas/Fluid Separation. . . . . . . . 44Compressed Gas Bleeding . . . . . . 44

Design Loads . . . . . 44Acceleration and Shock Loads . . . 44Torque Loads. . . . . . . . . . . . 44Vibration Loads . . . . . . . . . . 44

Controls. . . . . . . . . . . . . . . 45Interlocks. . . . . . . . . . . . . 45Multiple Safety Critical Functions. 45Critical Flows and Pressures. . . . 45

Protection. . . . . . . . . . . . . . 45Electrical . . . . . . . . . . . . . . 45

Hazardous Atmospheres . . . . . . . 45Radio Frequency Energy. . . . . . . 45Grounding. . . . . . . . . . . . . 45Solenoids. . . . . . . . . . . . .Electric Motor Driven Pumps . . . . .

Pressure Relief. . . . . . . . . . . . 46Requirement . . . . . . . . . . . . 46Flow Capacity . . . . . . . . . . . 46Sizing. . . . . . . . . . . . . . . 46Unmanned Flight Vehicle Servicing . 46Automatic Relief. . . , . . . . . . 46

Low Safety Factor. . . . . . . . 46Confinement. . . . . . . . . . . 46

Venting. . . . . . . . . . . . . . 47Relief Value Isolation. . . . . . . 47Negative Pressure Protection. . . . 47

Testing. . . . . . . . . . . . . 47Storage and Transportation . . . 47

Reservoir Pressure Relief . . . . . 47Air Pressure Control. . . . . . . . 47

Contamination. . . . . . . . . . . . . 47Filtering. . . . . . . . . . . . . 48

Fluid Filters. . . . . . . . . . 48Air Filters. . . . . . . . . . . 48

Pressurized Reservoirs. . . . 48Unpressurized Reservoirs. . . 48

Bleed Ports. . . . . . . . . . . . 49Location. . . . . . . . . . . . 49Auxiliary Bleed Ports. . . . . . 49

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6.1.9.2.36.1.106.1.10.16.1.10.26.1.10.36.1.10.46.1.10.56.1.116.1.11.16.1.11.26.1.11.36.1.126.1.12.16.1.12.26.1.12.36.1.12.46.26.2.16.2.1.16.2.1.1.16.2.1.26.2.1.36.2.1.46.2.1.56.2.1.66.2.1.76.2.1.86.2.1.96.2.1.106.2.26.2.36.2.3.16.2.3.26.2.46.2.56.2.5.16.2.5.26.2.66.2.6.16.2.6.26.2.6.36.36.3.16.3.1.16.3.1.26.3.1.36.3.1.46.3.1.56.3.26.3.2.1

CONTENTS

Filler CapControl Devices.

(Cont)PAGE

Bleed. . . . . . .

Directional Control Values. . . .Overtravel. . . . . . . . . . . .Pressure and Volume Control StopsManually Operated Levers. . . .Limit Torque. . . . . . . . . .

Accumulators . . . . . . . . . . .Accumulator Design. . . . . . .Accumulator Gas Pressure Gages.Accumulator Identification.

Flex Hose. . . . . . . . . .Installation. . . . . . .Restraining Devices . . .Flex Hose Stress. . . . .Temporary Installations .

Hydraulic System Requirements .Hydraulic System Components.

Component Integrity . . .Component Selection .

Cycling. . . . . . . . .Actuators . . . . . . . .Shutoff Valves. . . . . .Variable Response . . . .Fire Resistant Fluids . .Accumulators. . . . . . .Adjustable Orifices . . .Lock Valves . . . . . . .Hydraulic Reservoir . . .

Pressure Limits. . . . . . .Cavitation . . . . . . . . .

Inlet Pressure. . . . . .Fluid Column. . . . . . .

Redundancy . . . . . . . . .Hydraulic Lockup . . . . . .

Emergency Disengage . . .Emergency By-Pass . . . .

Hydraulic System Pressure ReliefPump Pressure Relief. . . . .Thermal Pressure Relief . . .Location. . . . . . . . . . .

Pneumatic Systems Requirements. . .Pneumatic System Components. . .

Component Integrity . . . . .Configuration . . . . . . . .Compressors . . . . . . . . .Actuators . . . . . . . . . .Adjustable Orifice Restrictors.

Controls. . . . . . . . . . . . .Manual Takeover . . . . . . . .

4949494949505050505050505051515152525252525252525252525353535353535354

54545454545454555555555555

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Figure 1

2


FIGURESPAGE

Total Energy Contained in a PressureVessel . . . . . . . . . . . . . . .

Pressure Vessel Design VerificationApproach . . . . . . . . . . . . . .

Table I Stored Energy in a Pressure Vessel . .

II Qualification Test Requirements . . . .

III System Safety Factors . . . . . . . . .

IV Open Line Force Calculation Factor . .

APPENDIX . . . . . . . . . . . . . . . . . . . . . . .

28

PAGE

10

32

39

51

57

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SECTION 1SCOPE

1.1 PURPOSE - This standard establishes requirements for thedesign and test of all pressurized systems for missiles, spacevehicles and ground support equipment. These requirements, whenimplemented on a particular pressurized system, will assure ahigh level of confidence in achieving safe operation and missionsuccess.

1.2 APPLICATION - All the requirements listed herein aremandatory requirements when this standard is placed oncontract. Specific approval of the procuring activity isrequired prior to the exclusion, modification or revision of anyrequirement listed in this standard when design specificationsare generated for pressurized systems, subsystems andcomponents. When the system is intended for use on a launchtest range, specific approval for exclusion of any requirementmust be obtained by the procuring activity and the appropriatelaunch or test range approval authority.




2



SECTION 2REFERENCED DOCUMENTS

2.1 ISSUES OF DOCUMENTS - The following documents of theissue in effect on the date of invitation for bids or requestfor proposal, form a part of this standard to the extentspecified herein.

SPECIFICATIONSMILITARY

MIL-E-6051

MIL-S-8512

STANDARDSMILITARY

MIL-STD-1540

MIL-STD-1472

HANDBOOKSMILITARY

MIL-HDBK-5

MIL-HDBK-17

PUBLICATIONSFEDERAL

Title 49Code of FederalRegulation

Electromagnetic CompatibilityRequirements Systems

Support Equipment, Aeronautical,Special, General Specification for theDesign of

Test Requirements for Space Vehicles

Human Engineering Design Criteria forMilitary Systems, Equipment, andFacilities

Metallic Materials and Elements forAerospace Vehicle Structures

Plastic for Aerospace Vehicles Part 1Reinforced Plastics

Transportation DOT (Department of Trans-portation) CFR

3



OTHER

AFML-TR-68-115 Aerospace Structural Metals Handbook

MCIC-HB-01 Damage Tolerant Design Handbook, AirForce Materials Laboratory, Air ForceFlight Dynamics Laboratory

(Copies of specifications, standards, drawings, and publicationsrequired by contractors in connection with specific procurementfunctions should be obtained from the procuring activity or asdirected by the contracting officer.)

2.2 Other Publications. The following documents form a partof this standard to the extent specified herein. Unlessotherwise indicated, the issue in effect on date of invitationfor bids or request for proposal shall apply.

PUBLICATIONS

Codes

ASME Boiler and Pressure Vessel Codes, Section VIII,Division 1 and 2

ASME Boiler and Pressure Vessel Code: Section X.

(Applications for copies should be addressed to ASMEOrder Department P.O. Box 3199, Grand CentralStation, New York New York, 10163.)

The Standards of the Hydraulic Institute

(Applications for copies should be addressed toHydraulic Institute, 712 Lakewood Center N, 14600Detroit Ave., Cleveland, Ohio 44107.)

4



SECTION 3DEFINITIONS

The following definitions of significant terms are provided toinsure precision of meaning and consistency of usage. In theevent of a conflict, the definitions listed herein apply.

3.1 “A” BASIS ALLOWABLE - The “A” basis allowable of amaterial are the minimum mechanical strength values guaranteedby the material producers/suppliers such that at least 99% ofthe material they produce/supply will meet or exceed thespecified properties with a 95% confidence level.

3.2 ACCEPTANCE TESTS - Acceptance tests are the requiredformal tests conducted on hardware to ascertain that thematerials, manufacturing processes, and workmanship meetspecifications and that the hardware is acceptable for delivery.

3.3 APPLIED LOAD (STRESS) - The actual applied load (stress)on a structure is the load (stress) imposed on the structure inthe design environment.

3.4 “B” BASIS ALLOWABLES - The “B” basis allowable of amaterial are mechanical strength values specified by materialproducers/suppliers such that at least 90% of the materials theyproduce/supply will meet or exceed the specified properties witha 95% confidence level.

3.5 BRITTLE FRACTURE - Brittle fracture is a type ofcatastrophic failure in structural materials that usually occurswithout prior plastic deformation and at extremely high speed.The fracture is usually characterized by a flat fracture surfacewith little or no shear lips (slant fracture surface) and ataverage stress levels below those of general yielding.

3.6 BURST FACTOR - The burst factor is a multiplying factorapplied to the maximum expected operating pressure (MEOP) toobtain the design burst pressure. Burst factor is synonymouswith ultimate pressure factor.

3.7 COMPONENTS - Components for purposes of this standard,are all elements of a pressurized system.

3.8 COMPOSITE MATERIAL - Composites are combinations ofmaterials differing in composition or form on a macroscale. Theconstituents retain their identities in the composite. Normallythe constituents can be physically identified and there is aninterface between them.



3.9 CRITICAL CONDITION - The critical condition is the mostsevere environmental condition in terms of loads, pressures andtemperatures, or combination thereof imposed on structure,systems, subsystems, and components during service life.

3.10 CRITICAL STRESS INTENSITY - The stress intensity at whichunstable fracture occurs.

3.11 CRITICAL FLAW - The critical flaw in a structuralmaterial is a flaw of sufficient size and shape that unstablegrowth will occur under the specific operating load andenvironment.

3.12 DAMAGE TOLERANCE - The damage tolerance of a structure isits ability to resist failure due to the presence of flaws,cracks, or other damage for a specified period of unrepairedusage.

3.13 DESIGN BURST PRESSURE - The design burst pressure is atest pressure that pressurized components must withstand withoutrupture to demonstrate design adequacy in a qualification test.It is equal to the product of the maximum expected operatingpressure, burst factor, and a factor to account for thedifference in material-properties between test and designtemperatures.

3.14 DESTABILIZING PRESSURE - Any pressure that producescompressive stresses in pressurized structure.

3.15 DETRIMENTAL DEFORMATION - Detrimental deformationsinclude all structural deformations, deflections, ordisplacements that prevent any portion of the structure fromperforming its intended function, or that reduce the probabilityof successful completion of the mission.

3.16 DUCTILE FRACTURE - Ductile fracture is a type of failurein structural materials generally preceded by large amounts ofplastic deformation and in which the fracture surface isinclined to the direction of the applied stress.

3.17 FACTOR OF SAFETY - The factor of safety of a structure isthe ratio of the allowable load to the limit load.

3.18 FATIGUE - Fatigue is the progressive localized permanentstructural change that occurs in a material subjected torepeated or fluctuating loads at stresses having a maximum valueless than the ultimate tensile strength of the material.Fatigue may culminate in cracks or fracture after a sufficientnumber of fluctuations.



3.19 FITTINGS - Fittings are local elements of a pressurizedsystem utilized to connect lines, components and/or vesselswithin the system.

3.20 FLAW - A flaw is a local discontinuity in a structuralmaterial, such as a scratch, notch, crack or void.

3.21 FRACTURE CONTROL - Fracture control is a set of policiesand procedures involving the application of analysis and designmethodology, manufacturing technology and operating proceduresto prevent structural failure due to the initiation of and/orpropagation of flaws or crack-like deflects during fabrication,testing, and service life.

3.22 FRACTURE MECHANICS - Fracture mechanics is an engineeringconcept used to predict flaw-growth and fracture behavior ofmaterials and structures containing cracks or crack-like flaws.

3.23 FRACTURE TOUGHNESS (~IC) - Fracture toughness is amaterial characteristic which reflects flaw tolerance andresistance to fracture and is equal to the value of the stressintensity factor at flaw instability. Fracture toughness isdependant on the environment, geometry and loading rate.

3.24 HAZARD - An existing or potential condition that canresult in an accident.

3.25 HYDROGEN EMBRITTLEMENT - Hydrogen embrittlement is amechanical- environmental failure process that results from theinitial presence or absorption of excessive amounts of hydrogenin metals, usually in combination with residual or appliedtensile stresses.

3.26 INITIAL FLAW - An initial flaw is a flaw in a structuralmaterial before the application of load or environment.

3.27 LEAK-BEFORE-BURST (LBB) - A fracture mechanics designconcept in which it is shown that any initial flaw will growthrough the wall of a pressure vessel and cause leakage ratherthan burst (catastrophic failure).

3.28 LIMIT LOAD - The limit load is the maximum anticipatedload, or combination of loads, which a structure may be expectedto experience during the performance of specified missions inspecified environments. Since the actual loads that areexperienced in service are in part random in nature, statisticalmethods for predicting limit loads are employed whereverappropriate.



3.29 LINES - Lines are tubular elements of a Pressurizedsystem provided as a means for transferring fluids betweencomponents of the system. Included in this definition are flexhoses.

3.30 LOAD SPECTRUM - The load spectrum on a structure is arepresentation of the cumulative static and dynamic loadingsanticipated for the structure under all expected operatingenvironments.

3.31 MARGIN OF SAFETY - The margin of safety of a structure isthe increment by which the allowable load (or stress) exceedsthe applied load (or stress), for a specific design condition,expressed as a fraction of the applied load (or stress).

Margin of Safety = ALLOWABLE LOAD (OR STRESS) = 1APPLLIED LOAD (OR STRESS)

3.32 MAXIMUM ALLOWABLE WORKING PRESSURE (MAWP) - The maximumpressure at which a component can continuously operate based onallowable stress values and functional capabilities. MAWP issynonymous with MDOP (Maximum Design Operating Pressure) or“Rated Pressure”.

3.33 MAXIMUM OPERATING PRESSURE (MOP) (MEOP) - The maximumpressure at which the system or component actually operates in aparticular application. MOP is synonymous with MEOP (MaximumExpected Operating Pressure) or maximum working pressure. MOPincludes the effects of temperature, transient peaks, vehicleacceleration, and relief valve tolerance.

3.34 PRESSURE CYCLE - A pressure cycle is a pressure increasegreater than the threshold pressure (PTH) followed by apressure decrease greater than the pTH unless otherwisespecified.

3.35 PRESSURE VESSEL A pressure vessel is a component of apressurized system designed primarily as a container that storespressurized fluids and:

(1) Contains stored energy of 14,240 foot-pounds (19,310joules) or greater based on adiabatic expansion of aperfect gas, Figure 1; Table I; or

(2) Contains a gas or liquid which will create a mishap(accident) if released; or

(3) Will experience a design limit pressure greater than 100psi.

8




NOTE:energyfeet.

To obtain the pressure vessel equivalent, multiply theequivalent per cubic foot by the vessel volume in cubic

10



3.36 PRESSURIZED STRUCTURE - A pressurized structure is astructure designed both to carry internal pressure and vehiclestructural loads.

3.37 PRESSURIZED SYSTEM - A pressurized system, as addressedin this document, comprises the pressure vessels or pressurizedstructure, lines, fittings, valves, etc., that are exposed toand designed by the pressure within these components. It doesnot include electrical control devices, etc., required tooperate the system.

3.38 PROOF FACTOR - The proof factor is a multiplying factorapplied to the limit load or MEOP to obtain proof load or proofpressure, for use in acceptance testing.

3.39 PROOF PRESSURE - The proof pressure is the test pressurethat pressurized components shall sustain without detrimentaldeformation. The proof pressure is used to give evidence ofsatisfactory workmanship and material quality, and/or establishmaximum initial flaw sizes. It is equal to the product ofmaximum expected operating pressure, proof pressure designfactor, and a factor accounting for the difference in materialproperties between test and design temperature.

3.40 QUALIFICATION TESTS - Qualification tests are formalcontractual demonstrations that the design, manufacturing, andassembly have resulted in hardware conforming to specificationrequirements.

3.41 RESIDUAL STRESS - Residual stress is a stress whichremains in a detail part as a result of manufacturingprocessing, testing and operation.

3.42 SAFE-LIFE - Safe-life of a structure is the period duringwhich the structure is predicted not to fail in the expectedoperating environment. -

3.43 SERVICE LIFE - The service life of a component or spacevehicle is the total life expectancy of the item. The servicelife starts with the manufacture of the structure and continuesthrough all acceptance testing, handling, storage,transportation, launch operations, orbital operations,refurbishment, retesting, reentry or recovery from orbit, andreuse that may be required or specified for the item.

3.44 STABILIZING PRESSURE - Any pressure which producestensile stresses in a pressurized structure is a stabilizingpressure.

11



3.45 STIFFNESS - The stiffness of a structure is itsresistance to deflection under an applied load.

3.46 STRESS-CORROSION CRACKING - Stress-corrosion cracking isa mechanical-environmental induced failure process in whichsustained tensile stress and chemical attack combine to initiateand propagate a flaw in a metal part.

3.47 STRESS INTENSITY FACTOR (K1) - The stress intensityfactor is a parameter that describes the elastic stress field inthe vicinity of a crack tip.

3.48 THRESHOLD PRESSURE (pTH) - Threshold pressure is apressure change great enough to induce a stress which affectsthe flaw growth in a pressure vessel.

3.49 THERMAL STRESS - Thermal stress is a structural stressarising from temperature gradients and/or differential thermaldeformation in or between structural components, assemblies, orsystems.

3.50 THRESHOLD STRESS INTENSITY FACTOR (XTH) - The thresholdstress intensity factor is the maximum value of the stressintensity factor below which environmentally inducedflaw-growth, under sustained static tensile- stress, does notoccur for a given material in a specified environment.

3.51 ULTIMATE LOAD - The ultimate load is the product of thelimit load and the ultimate factor of safety. It is the maximumload which the structure must withstand without rupture orcollapse in the expected operating environments.

3.52 ULTIMATE FACTOR OF SAFETY - The ultimate factor of safetyof structure is the ratio of the ultimate load to the limit load.

3.53 ULTIMATE PRESSURE - The ultimate pressure is the productof the MEOP and the ultimate pressure factor. It is the maximumpressure which the structures must withstand without rupture inthe expected operating environment.

3.54 ULTIMATE PRESSURE FACTOR - The ultimate pressure factoris a multiplying factor applied to the MEOP to obtain ultimatepressure.

3.55 VERIFICATION/RE-CERTIFICATION TESTS -Verification/re-certification tests are tests conducted toverify/recertify the integrity of structures after some specificperiod of operation or storage or after exposure to some adverseconditions.

12



SECTION 4GENERAL REQUIREMENTS

This section presents general requirements for the analysis,design and verification of pressure vessels and pressurizedstructures. Included are requirements on system analysis,structural design, material selection, safe operating stresslevels, fracture control, quality assurance and other specialrequirements.

Pressure vessels and pressurized structures shall comply withthe requirements specified in Section 4. Pressure vessels andpressurized structures designed fabricated, inspected, andtested in accordance with the ASME Boiler and Pressure VesselCode, Section VIII Divisions 1 or 2, Section X; or (for GSEonly) Title 49 Code of Federal Regulation shall comply withsystem analysis requirements (Section 4.1) only.

4.1 SYSTEM ANALYSIS REQUIREMENTS

4.1.1 System Analysis. Perform a detailed systemfunctional analysis to determine that the operation,interaction, or sequencing of components shall not lead tounsafe conditions which could cause personnel injury or majordamage to the vehicle, its booster, or associated groundequipment. The analysis shall identify any single malfunctionor personnel error in operation of any component that willcreate conditions leading to an unacceptable risk to operatingpersonnel or equipment. The analysis shall also evaluate anysecondary or subsequent occurrence, failure, or componentmalfunction which, initiated by a primary failure, could resultin personnel injury. Such items identified by the analysisshall be designated safety critical and will require thefollowing considerations.

a. Specific Design Action

b. Special Safety Operating Requirements

c. Specific Hazard Identification and ProposedCorrective Action

d. Special Safety Supervision

4.1.2 Systems analysis data. Systems analysis data willshow that:

a. The system provides the capability of maintaining allpressure levels in a safe condition in the event ofinterruption of any process or control sequence atany time during test or countdown.



4.2

b. Redundant pressure relief devices have mutuallyindependent pressure escape routes.

c. In systems where pressure regulator failure mayinvolve critical hazard to the crew or missionsuccess, regulation is redundant and where passiveredundant systems are specified includes automaticswitchover.

d. When the hazardous effects of safety criticalfailures or malfunctions are prevented through theuse of redundant components or systems, it shall bemandatory that all such redundant components orsystems are operational prior to the initiation ofirreversible portions of safety critical operationsor events.

GENERAL DESIGN REQUIREMENTS

4.2.1 Loads, Pressures and Environments. The entireanticipated load-pressure-temperature history and associatedenvironments throughout the service life shall be determined inaccordance with specified mission requirements. As a minimum,the following factors and their statistical variations shall beconsidered :

a. The environmentally induced loads and pressures.

b. The environments acting simultaneously with theseloads and pressures with their proper relationships.

c* The frequency of application of these loads,pressures, environments and their levels and duration.

These data shall be used to define the design spectra whichshall be used for both design analysis and testing. The designspectra shall be revised as the structural design develops andthe loads analysis matures.

4.2.2 Strength Requirements. All pressure vessels andpressurized structures within the structural system shallpossess sufficient strength to withstand limit loads andinternal pressures in the expected operating environmentsthroughout their respective service lives without experiencingdetrimental deformation. They shall also withstand ultimateloads and internal pressures in the expected operatingenvironments without experiencing rupture or collapse.

14



Pressure vessels and pressurized structures shall be capable ofwithstanding ultimate external loads and ultimate externalpressures (destabilizing) without collapse or rupture wheninternally pressurized to the minimum anticipated operatingpressure.

All pressure vessels and pressurized structures shall sustainproof pressure without incurring gross yielding or detrimentaldeformation and shall sustain design burst pressure withoutrupture. When proof tests are conducted at temperatures otherthan design temperatures, the change in material properties atthe proof temperature shall be accounted for in determiningproof pressure.

Pressurized structures subject to instability modes of failureshall not collapse under ultimate loads nor degrade thefunctioning of any system due to elastic buckling deformationunder limit loads. Evaluation of buckling strength shallconsider the combined action of primary and secondary stressesand their effects on general instability, local or panelinstability, and crippling. Design loads for buckling shall beultimate loads, except that any load component that tends toalleviate buckling shall not be increased by the ultimate designfactor. Destabilizing pressures shall be increased by theultimate design factor, but internal stabilizing pressures shallnot be increased unless they reduce structural capability.

The margin of safety shall be positive and shall be determinedb analysis or test at design ultimate and design limit levels,when appropriate, at the temperatures expected for all criticalconditions.

4.2.3 Stiffness Requirements. Pressure vessels andpressurized structures shall possess adequate stiffness topreclude detrimental deformation at limit loads and pressures inthe expected operating environments throughout their respectiveservice lives. The stiffness properties of pressure vessels andpressurized structures shall be such as to prevent alldetrimental instabilities of coupled vibration modes, minimizedetrimental effects of the loads and dynamics response which areassociated with structural flexibility, and minimize adverseinteraction with other vehicle systems.

4.2.4 Thermal Requirements. Thermal effects, includingheating rates, temperatures, thermal stresses and deformations,and changes in the physical and mechanical properties of thematerials of construction shall be considered in the design ofall pressure vessels and pressurized structures. These effectsshall be based on temperature extremes which simulate thosepredicted for the operating environment plus a design margin asspecified in MIL-STD-1540.

15



4.2.5 Stress Analysis Requirements. A detailed andcomprehensive stress analysis of each pressure vessel andpressurized structure shall be conducted under the assumption orno crack-like flaws in the structure. The analysis shalldetermine stresses resulting from the combined effects ofinternal pressure, ground or flight loads, temperature andthermal gradients. Both primary membrane stresses and secondarybending stresses resulting from internal pressure shall becalculated to account for the effects of design discontinuities,design configuration and structural support attachments.

Loads shall be combined by using the appropriate limit orultimate safety factors on the individual loads and comparingthe results to material and/or geometric capabilities. Safetyfactors on internal pressures shall be as determined in Section5.0. Safety factors on external (support) loads shall be asassigned to primary structure supporting the pressurized system.

Classical solutions are acceptable if the design geometries andloading conditions are sufficiently simple and the results aresufficiently accurate to warrant their application. Finiteelement or finite difference structural analysis techniquesshall be used to calculate the stresses, strains anddisplacements for complex design geometries and loadingconditions. Local structural models shall be constructed, asnecessary> to augment the overall structural model in areas ofrapidly varying stresses.

Minimum material gage as specified in the design drawings shallbe used in calculating stresses. The allowable materialstrengths shall reflect the effects of temperature, thermalcycling and gradients, processing variables, and time associatedwith the design environments.

Minimum margins of safety associated with the parent materials,weldments and heat-affected zones shall be calculated andtabulated for all pressure vessels and pressurized structuresalong with their locations and stress levels. The margins ofsafety shall be positive against the strength and stiffnessrequirements of Section 4.2.2 and 4.2.3, respectively.

Records of the stress analysis shall be maintained. Theanalysis shall include the input parameters> data, assumptions)rationales, methods, references, and a summary of significantanalysis results, and safe-life analysis. The analysis shall berevised and updated to maintain currency for the life of theprogram.

16



4.2.6 Malfunction. Pressure vessels and pressurizedstructures are not required to withstand loads, pressures, orenvironments due to malfunctions that could create conditionsoutside the maximum expected mission requirements.

4.2.7 Miscellaneous Requirements. The structural designof all pressure vessels and pressurized structures shall employproven processes and procedures for manufacture and repair. Thedesign shall emphasize the need for access, inspection, service,replacement, repair and refurbishment. For all reusablepressure vessels and reusable pressurized structures, thestructural design shall permit these structures to be maintainedin and refurbished to a flightworthy condition. Repaired andrefurbished structures shall meet all stipulated conditions offlightworthiness.

4.3 MATERIALS REQUIREMENTS

4.3.1 Material Selection. Materials shall be selectedon the basis of proven environmental compatibility, materialstrengths, fracture properties, and service life consistent withthe overall program requirements. Material “A” allowable valuesshall be used for pressure vessels, and pressurized structureswhere failure of a single load path would result in loss ofstructural integrity. For redundant pressurized structures inwhich failure of a structural element would result in a saferedistribution of applied loads to other load-carrying members,material “B” allowable may be used. The fracture toughnessshall be as high as practicable within the context of structuralefficiency and damage tolerance.

For pressurized systems to be analyzed with linear elasticfracture mechanics, fracture properties shall be accounted forin material selection. These properties include fracturetoughness; threshold values of stress intensity under sustainedloading; subcritical flaw-growth characteristics under sustainedand cyclic loadings; the effects of fabrication and joiningprocesses; the effects of cleaning agents, dye penetrants,coatings and proof-test fluids; and the effects of temperature,load spectra, and other environmental conditions.

Materials which exhibit a low threshold stress intensity value,i~~, in the expected operating environments shall not be usedin pressure vessels and pressurized structures unless adequateprotection from the operating environments can be demonstratedby tests. If the material has a threshold stress intensityfactor, KTH, of less than 60% of the critical stress intensityfactor, KIC, under the conditions of its application, it shallbe mandatory to show, by a “worst case” fracture mechanicsanalysis, that the low allowable threshold stress intensityfactor will not precipitate premature structural failure.

17



4.3.2 Material Evaluation. The materials selected fordesign shall be evaluated with respect to material processing,fabrication methods, manufacturing operations, refurbishmentprocedures and processes, and other pertinent factors whichaffect the resulting strength and fracture properties of thematerial in the fabricated as well as the refurbishedconfigurations. (Reference Section 4.5)

The evaluation shall ascertain that the mechanical properties,strengths and fracture properties used in design and analyseswill be realized in the actual hardware and that theseproperties are compatible with the fluid contents and theexpected operating environments. Materials which aresusceptible to stress-corrosion cracking or hydrogenembrittlement shall be evaluated by performing sustainedthreshold-stress-intensity tests when applicable data are notavailable.

4.3.3 Material Characterization. The allowablemechanical properties, strength and fracture properties of allmaterials selected for pressure vessels and pressurizedstructures shall be characterized in sufficient detail to permitreliable and high confidence predictions of their structuralperformance in the expected operating environments unless theseproperties are available from reliable sources such asMIL-HDBK-5, MIL-HDBK-17, ASTM Standards, AFML/AFFDL DamageTolerant Design Handbook, MIL Specifications, AerospaceStructural Metals Handbook, and other sources approved by theprocuring agency. Where material properties are not available,they shall be determined by test methods approved by theprocuring agency. As a minimum, the characterization shallproduce the following strength and fracture properties for theparent metals, weldments and heat-affected zones as a functionof the fluid contents, loading spectra, and the expectedoperating

a.

b.

c.

d.

e.

environments , including proof-test environments:

Uniaxial tensile yield stress, Cys, and ultimatestress, ~u;

Fracture ’toughness KIC and KTH;

and, for pressurized systems to be analyzed withlinear elastic fracture mechanics:

Sustained-stress flaw-growth data, da/dt VS~K;

Cyclic-stress flaw-growth data, da/dn vsbK and loadratio, R; and,

Empirical constants associated with the chosenflaw-growth models.

18



Uniform test procedures shall be employed for determiningmaterial fracture properties as required. These proceduresshall conform to recognized standards, such as testspecifications of the American Society for Testing andMaterials, and the Society of Automotive Engineers. The testspecimens and procedures utilized shall provide valid test datafor the intended application.

Enough tests shall be conducted so that meaningful nominalvalues of fracture toughness and flaw-growth rate datacorresponding to each alloy system, temper, product form,thermal and chemical environments and loading spectra can beestablished to evaluate compliance with service liferequirements of Section 4.4. Test plans and results shall beapproved by the procuring agency.

4.4 SAFE-LIFE REQUIREMENTS

The safe-life shall be determined by analysis, test, or both andshall be at least four times the specified service life forthose pressure vessels and pressurized structures which are notaccessible for periodic inspection and repair.

For those pressure vessels and pressurized structures which arereadily accessible for periodic inspection and repair, thesafe-life, as determined by analysis and test, shall be at leastfour times the interval between scheduled re-certification.

All pressure vessels and pressurized structures which requireperiodic refurbishment to meet safe-life requirements shall bere-certified after each refurbishment by the same techniques andprocedures used in the initial certification, unless analternative re-certification plan has been approved by theprocuring agency.

4.5 FABRICATION AND PROCESS CONTROL REQUIREMENTS

Proven processes and procedures for fabrication and repair shallbe used to preclude damage or material degradation duringmaterial processing, manufacturing operations, andrefurbishment. In particular, special attention shall be givento ascertain that the melt process, thermal treatment, weldingprocess, forming, joining, machining, drilling, grinding, repairand rewelding operations, etc., P ‘ within the state-of-the-artand have been used on similar ha .re.

The fracture toughness, mechanical and physical properties ofthe parent materials, weldments and heat-affected zones shall bewithin established design limits after exposure to the intendedfabrication processes. The machining, forming, joining,

19



welding, dimensional stability during thermal treatments, andthrough-thickness hardening characteristics of the materialshall be compatible with the fabrication processes to beencountered.

Fracture control requirements and precautions shall be definedin applicable drawings and process specifications. Detailedfabrication instructions and controls shall be provided toinsure proper implementation of the fracture controlrequirements. Special precautions shall be exercised throughoutthe manufacturing operations to guard against processing damageor other structural degradation. In addition, procurementrequirements and controls shall be implemented to insure thatsuppliers and subcontractors employ fracture control proceduresand precautions consistent with the fabrication and inspectionprocesses intended for use during actual hardware fabrication.

4.6 QUALITY ASSURANCE REQUIREMENTS

A quality assurance program, based on a comprehensive study ofthe product and engineering requirements, such as drawings,material specifications, process specifications, workmanshipstandards design review records, and failure mode analysis,shall be established to assure that the necessarynon-destructive inspections and acceptance tests are effectivelyperformed to verify that the product meetsthe requirements ofthis document. The program shall insure that materials, parts subassemblies, assemblies, and all completed and refurbished

r~I are conform to applicable drawings and processspecfications; that no damage or degradation has occurredduring material processing, fabrication, inspection, acceptancetests, shipping, storage, operational use and refurbishment; andthat defects which could cause failure are detected or evaluatedand corrected. As a minimum, the following consideration shallbe included in structuring the quality assurance program.

4.6.1 Inspection Plan. An inspection master plan shallbe established prior to start of fabrication. The plan shallspecify appropriate inspection points and inspection techniquesfor use throughout the program, beginning with materialprocurement and continuing through fabrication, assembly,acceptance proof test, operation, and refurbishment, asappropriate. In establishing inspection points and inspectiontechniques, consideration shall be given to the materialcharacteristics , fabrication processes, design concepts,structural configuration and accessibility for inspection-Designs employing fracture mechanics techniques shall alsoinclude inspection for flaws. The flaw geometries shallencompass defects commonly encountered, including surface crack,corner crack, through-the-thickness crack at the edge of



fastener hole, surface crack at the edge of bolt hole, andsurface crack at the root of intersecting prismatic structuralelements . Acceptance and rejection standards shall beestablished for each phase of inspection, and for each type ofinspection technique.

4.6.2 Inspection Techniques. The selectednon-destructive inspection (NDI) techniques must have thecapability to determine the size, geometry, location andorientation of a flaw or defect; to obtain, where multiple flawsexist, the location of each with respect to the other and thedistance between them; and to differentiate among defect shapes,from tight cracks to spherical voids. Two or more NDI methodsshall be used for a part or assembly that cannot be adequatelyexamined by only one method.

The flaw detection capability of each selected NDI techniqueshall be based on past experience on similar hardware. Wherethis experience is not available or is not sufficientlyextensive to provide reliable results, the capability, underproduction or operational inspection conditions, shall bedetermined experimentally and demonstrated by tests approved bythe procuring agency on representative material product form,thickness, and design configuration. The flaw detectioncapability shall be expressed in terms of detectable cracklength, crack depth, or crack area. To minimize the possibilityof proof test failure in pressure vessels the selected NDItechnique(s) should be capable of detecting flaws smaller thancritical size with a 90% probability of detection at a 95%confidence level.

The most appropriate NDI technique(s) for detecting commonlyencountered flaw types shall be used for all pressure vesselsalong with their flaw detection capabilities.

The following criteria shall apply in defining thecharacteristics of part-through initial flaws in the event theselected NDI technique measures either the flaw length or theflaw depth, but not both. The depth of initial flaw shall beassumed to be one-half (1/2) the flaw length if the NDItechnique measures only the flaw-length; whereas, for an NDItechnique which measures only flaw-depth, the length of flawshall be assumed to be twenty (20) times the depth.

4.6.3 Inspection Data. Inspection data in the form offlaw histories shall be maintained throughout the life of thepressure vessel and pressurized structure. These data shall beperiodically reviewed and assessed to evaluate trends andanomalies associated with the inspection procedures, equipment

21



and personnel, material characteristics, fabrication processes,design concept and structural configuration. The result of thisassessment shall form the basis of any required correctiveaction.

4.6.4 Acceptance Test. Every pressure vessel, andpressurized structure shall be proof-pressure tested inaccordance with the requirements of Section 5.1, 5.2, and 5.3 asapplicable to verify that the hardware has sufficient structuralintegrity to sustain the subsequent service loads, pressure,temperatures and environments. The temperature of the vesselshall be consistent with the critical use temperature, or, as analternative, tests may be conducted at an alternate temperatureif the test pressures are suitably adjusted to account fortemperature effects on strength and fracture toughness.

Accept/reject criteria shall be formulated prior to acceptancetest. Every pressure vessel and pressurized structure shall notleak, rupture or experience gross yielding during acceptancetesting.

4.7 OPERATIONS AND MAINTENANCE REQUIREMENTS.

4.7.1 Operating Procedures. Operating procedures shallbe established for each pressure vessel. These procedures shallbe compatible with the safety requirements and personnel controlrequirements at the facility where the operations areconducted. Step-by-step directions shall be written withsufficient detail to allow a qualified technician or mechanic toaccomplish the operations. Schematics which identify thelocation and pressure limits of relief valves and burst discshall be provided, and procedures to insure compatibility of thepressurizing system with the structural capability of thepressurized hardware shall be established. Prior to initiatingor performing a procedure involving hazardous operations withpressure systems, practice runs shall be conducted onnon-pressurized systems until the operating procedures are wellrehearsed. Initial tests shall then be conducted at pressurelevels not to exceed 50% of the normal operating pressures untiloperating characteristics can be established and stabilized.Only qualified and trained personnel shall be assigned to workon or with high pressure systems. Warning signs with thehazard(s) identified shall be posted at the operations facilityprior to pressurization.

4.7.2 Safe Operating Limits. Safe o crating limitsshall be established for each pressure vessel and eachpressurized structure based on the appropriate analysis andtesting employed in its design and qualification per Section 5.0.

22



These safe operating limits shall be summarized in a formatwhich will provide rapid visibility of the important structuralcharacteristics and capability. The desired information shallinclude, but not be limited to, such data as fabricationmaterials, critical design conditions, MEOP, nominal operatingor working pressure, proof pressure, burst pressure,pressurization and depressurization sequence, operational cyclelimits , design and operating temperatures, operational s stemfluid, cleaning agent, NDI techniques employed, permissiblethermal and chemical environments, minimum margin of safety andpotential failure mode. For pressurized systems with potentialbrittle fracture failure mode, the critical flaw sizes andmaximum permissible flaw sizes shall also be included.Appropriate references to design drawings, detail analyses,inspection records, test reports, and other back-updocumentation shall be indicated.

4.7.3 Inspection and Maintenance. The results of theappropriate stress, and safe-life, analyses shall be used inconjunction with the appropriate results from the structuraldevelopment and qualification tests to develop a quantitativeapproach to inspection and repair. Allowable damage limitsshall be established for each pressure vessel and pressurizedstructure so that the required inspection interval and repairschedule can be established to maintain hardware to therequirements of this document NDI technique(s) and inspectionprocedures to reliably detect critical structural defects anddetermine flaw size under the condition of use shall bespecified. Detailed repair procedures shall be developed foruse in field and depot levels. Procedures shall be establishedfor recording, tracking, and analyzing operational data as it isaccumulated to identify critical areas requiring correctiveactions. Analyses shall include prediction of remaining lifeand reassessment of required inspection intervals.

4.7.4 Repair and Refurbishment. When inspections revealstructural damage or detects exceeding the permissible levels,the damaged hardware shall be repaired, refurbished, orreplaced, as appropriate. All repaired or refurbished hardwareshall be re-certified after each repair and refurbishment by theapplicable acceptance test procedure for new hardware to verifytheir structural integrity and to establish their suitabilityfor continued service.

4.7.5 Storage Requirements. When pressure vessels andpressurized structures are put into storage, they shall beprotected against exposure to adverse environments which couldcause corrosion or other forms of material degradation. Inaddition, they shall be protected against mechanical damagesresulting from scratches, dents, or accidental dropping of the

23



hardware. Induced stresses due to storage fixture constraintsshall be minimized by suitable storage fixture design. In theevent storage requirements are violated, re-certification shallbe required prior to acceptance for use.

4.7.6 Documentation. Inspection, maintenance, andoperation records shall be kept and maintained throughout thelife of each pressure vessel and each pressurized structure. Asa minimum, the records shall contain the following information:

a.

b.

c.

d.

e.

f.

g.

h.

Temperature, pressurization history, and pressurizingfluid for both tests and operations.

Number of pressurizations experienced as well asnumber allowed in safe-life analysis.

Results of any inspection conducted, including:inspector, inspection dates, inspection techniquesemployed, location and character of defects, defectorigin and cause.

Storage condition.

Maintenance and corrective actions performed frommanufacturing to operational use, includingrefurbishment.

Sketches and photographs to show areas of structuraldamage and extent of repairs.

Acceptance and re-certification test performed,including test conditions and results.

Analyses supporting the repair or modification whichmay influence future use capability.

4.8 SPECIAL REQUIREMENTS

4.8.1 Re-activated Pressurized Hardware. Pressurevessels and pressurized structures which are re-activated foruse after an extensive period in either an unknown, unprotected,or unregulated storage environments shall be re-certified toascertain their structural integrity and suitability forcontinued service before commitment to flight. Re-certificationtests for pressurized hardware shall be in accordance with theappropriate Re-Certification Test Requirement (5.1.1.5, 5.1.2.5).

24



4.8.2 Multiple Proof Tests. Multiple proof tests aregenerally not required or recommended except in specialcircumstances, as described below:

a.

b.

c.

d.

e.

f.

g.

h.

Re-activated pressurized hardware as described inSection 4.8.1.

Refurbished or repaired hardware (Section 4.7.4).

Hardware modification after the initial proof test.

Re-certification of hardware for additional serviceafter it has been in service for its intendedsafe-life.

Re-certification of hardware designed for safe-lifebetween regularly scheduled inspection.

Component testing prior to final assembly.

Proof test limitation resulting from inability of asingle proof test to envelope the criticaloperational pressure, temperature, and externalloading combinations.

Proof test limitation resulting from inability of theinitial proof test to verify the entire service lifecapability of the hardware.

4.8.3 Test Fluids. Proof-test fluids shall becompatible with the structural materials in the pressure vesselsand pressurized structures. Proof test fluids shall not pose ahazard to test personnel. If such compatibility data is notavailable, required testing shall be conducted to demonstratethat the proposed test fluid does not have a deleterious effecton the article to be tested.

25




26



SECTION 5DETAILED REQUIREMENTS

Four approaches for the design, analysis and verification ofpressure vessels are offered as illustrated in Figure 2.Selection of the approach to be used is dependent on the desiredefficiency of design coupled with the level of analysis andverification testing required. Final selection shall becoordinated and/or defined by the procuring agency and theappropriate launch or test range approval authority.

Approach A, Figure 2, illustrates the steps required forverification of a pressure vessel designed with a burst factorequal to 1.5 or greater. This approach is not acceptable forthe design and verification of ground support equipment. Basedon the results of the failure mode determination, one of twodistinct verification paths must be satisfied: 1)Leak-Before-Burst with leakage of the contents not creating acondition which could lead to a mishap (such as toxic gasventing or pressurization of a compartment not capable of thepressure increase), and 2) Brittle failure mode orLeak-Before-Burst in which, if allowed to leak, the leak causesa hazard. The verification requirements for path 1 aredelineated in Sections 5.1.1 and 5.2.1, and the verificationrequirements for path 2 in Sections 5.1.2 and 5.2.2.

Approach B, Figure 2, illustrates the steps required forverification of a pressure vessel designed with a burst factorequal to 2.0 or greater. This approach is not acceptable forUSAF/SD use, or design and verification of ground supportequipment. Verification requirements for this approach aredelineated in Sections 5.1.3 and 5.2.3.

Approach C, Figure 2, illustrates the steps required forverification of a pressure vessel designed employing the ASMEBoiler and Pressure Vessel Code. This approach is the onlyacceptable approach for ground support equipment. Additionalrequirements for airborne vehicle equipment and ground supportequipment are delineated in Sections 5.1.4 and 5.2.4.

5.1 NON-COMPOSITE PRESSURE VESSELS

This section is intended primarily for application to metallicpressure vessels but may be used for certain non-metallics withthe approval of the procuring agency (for fiber reinforcedcomposite pressure vessels, see Section 5.2).

27




5.1.1 Pressure Vessels with Non-HazardousLeak-Before-Burst(LBB) Failure Mode (Aerospace VehicleEquipment, AVE). The LBB failure mode shall be demonstratedanalytically or by test showing that an initial flaw of anysize, considering a flaw shape range of .05 z a/2c z .5, willpropagate through the thickness of the pressure vessel beforebecoming critical. (See the Appendix for an acceptableanalytical approach).

5.1.1.1 Factor of Safety Requirements. Pressure vesselswhich satisfy the LBB failure mode criterion may be designedconventionally, wherein the design factors of safety and testfactors are selected on the basis of successful past experienceor specified by codes and specifications. Unless otherwisespecified, the minimum Design Burst Factor shall be 1.5.

5.1.1.2 Safe-Life Analysis Requirements. In addition tothe stress analysis conducted in accordance with therequirements of-section 4.2.5, conventional fatigue-lifeanalysis shall be performed, as appropriate, on the unflawedstructure to ascertain that the pressure vessel, acted upon bythe spectra of maximum expected operating loads, pressures andenvironments will meet the safe-life requirements of Section4.4. Nominal values of fatigue-life characteristics of thestructural materials shall be taken from reliable sources suchas MIL-HDBK-5, ASTM Standards, MIL Specifications, the AerospaceStructural Metals Handbook, or other sources approved by theprocuring agency. Safe-life requirements are met when Minor’srule, expressed as

is satisfied. In this equation, ni = 4 times the number ofcycles applied at stress level i, Ni = the number of cycles tofailure at stress level i, and k = number of stress levelsconsidered in the analysis. The LBB failure mode criteriaaccepts the possibility of propagation of an existing flaw,through the thickness of the vessel wall, allowing leakage ofthe contained gas or liquid. In the event that this leakagewould present a hazard, fracture mechanics analysis shall berequired. (See Section 5.1.2.2)

5.1.1.3 Qualification Test Requirements. Qualificationtests shall be conducted on flight-quality hardware todemonstrate structural adequacy of the design. The testfixtures, support structures, and methods of environmentalapplication shall not induce erroneous test conditions. Thetypes of instrumentation and their locations in qualification

29



tests shall be based on the results of the stress analysis ofSection 4.2.5. The instrumentation shall provide sufficientdata to ensure proper application of the accept/reject criteria,which shall be established prior to test. The sequences,combinations, levels, and duration of loads, pressure andenvironments shall demonstrate that design requirements havebeen met.

Qualification testing shall include demonstration of theleak-before-burst failure mode by pre-flawed specimen testing,fatigue life by cycle testing and random vibration testing andultimate strength by burst testing. The following delineatesthe required tests:

a. Leak before burst testing

The test may be conducted on coupons which duplicatethe materials (parent material, weldment, and HAZ)and thicknesses of the pressure vessel or on apressure vessel representative of the flighthardware. Test specimens shall be pre-flawed andcycled through the design spectrum to demonstratestable flaw growth completely through the wallthickness. A sufficient number of tests is to beconducted to establish that all areas (thicknesses)and stress fields will exhibit a leak-before-burstmode of failure.

This test may be partially or completely omitted ifavailable materials data, directly applicable to thematerials and methods of construction, are availableto substantiate an analytical demonstration of theleak-before-burst failure mode.

b. Pressure Testing

Required qualification pressure testing levels areshown in Table II. Requirement for application ofexternal loads in combination with internal pressuresduring testing must be evaluated based on therelative magnitude and/or destabilizing effect ofstresses due to the external load. If limit combinedtensile stresses are enveloped by test pressurestresses, the application of external loads shall notbe required. If the application of external loads isrequired, the load shall be cycled to limit for fourtimes the predicted number of operating cycles of themost severe design condition (eg. destabilizing loadwith constant minimum internal pressure or maximum

30



additive load with constant maximum expectedoperating pressure). Qualification test procedureshall be approved by the procuring agency and theappropriate launch or test range approval authority.

c. Random Vibration Testing

Random Vibration qualification testing shall beperformed per requirements of MIL-STD-1540 unless itcan be shown the vibration requirement is envelopedby other qualification testing performed.

5.1.1.4 Acceptance Test Requirements. Acceptance testsshall be conducted on every pressure vessel before commitment toflight. Accept/reject criteria shall be formulated prior totests. The test fixtures and support structures shall bedesigned to permit application of all test loads withoutJeopardizing the flightworthiness of the test article. Thefollowing are required as a minimum.

a. Non-Destructive Inspection. A complete inspection bythe selected non - destructive inspection (NDI)technique(s) shall be performed prior to proofpressure test to establish the initial condition ofthe hardware. All pressurized hardware whichrequires periodic refurbishment to meet safe-liferequirements shall undergo a complete post-refurbishment inspection prior to additional proofpressure tests.

b. Proof Pressure Test. Every pressure vessel shall beproof-pressure tested to verify that the materials,manufacturing processes, and workmanship meet designspecifications and that the hardware is suitable forflight. The proof pressure shall be equal to:

‘proof = (1 + Burst Factor)/2 x MEOP or

1.5 x (MEOP) whichever is lower

5.1.1.5 Re-Certification Test Requirements. Allrefurbished pressure vessels shall be re-certified after eachrefurbishment by the acceptance test requirements for newhardware to verify their structural integrity and to establishtheir suitability for continued service before commitment toflight. Pressure vessels which have exceeded the approvedstorage environment (temperature, humidity, time etc.) shallalso be re-certified by the acceptance test requirements for newhardware.

31



TABLE II. Qualification Test Requirements

5.1.2 Pressure Vessels with Brittle Fracture orHazardous Leak-Before-Burst (LBB) Failure Mode (AVE)

5.1.2.1 Factor of Safety Requirements. Safe-life designmethodology based on fracture mechanics techniques shall be usedto establish the appropriate design factor of safety and theassociated proof factor for pressure vessels which exhibitbrittle fracture or hazardous leak-before-burst failure mode.The loading spectra, material strengths, fracture toughness andflaw-growth rates of the parent material and weldments, testprogram requirements, stress levels, and the compatibility ofthe structural materials with the thermal and chemicalenvironments expected in service shall be taken intoconsideration. Nominal values of fracture toughness andflaw-growth rate data corresponding to each alloy system, temperand product form shall be used along with a life factor of fouron specified service life in establishing the design factor ofsafety and the associated proof factor. Unless otherwisespecified the minimum burst factor shall be 1.5.

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5.1.2.2 Safe-life Demonstration Requirements. In


addition to the stress analysis conducted in accordance with therequirements of Section 4.2.5, safe-life analysis of eachpressure vessel covering the maximum expected operating loadsand environments, shall be performed under the assumption ofpre-existing initial flaws or cracks in the vessel. Inparticular, the analysis shall show that the pressure vesselwith flaws placed in the most unfavorable orientation withrespect to the applied stress and material properties, of sizesdefined by the acceptance proof test or NDI and acted upon bythe spectra of maximum expected operating loads andenvironments, will meet the safe-life requirements of Section4.4. Nominal values of fracture toughness and flaw-growth ratedata associated with each alloy system, temper, product form,thermal and chemical environments, and loading spectra shall beused along with a life factor of four on specified service lifein all safe-life analyses.

Pressure vessels which experience sustained stress shall alsoshow that the corresponding applied stress intensity (K1)during operation is less than the threshold stress intensity(KTH) in the appropriate environment

If the above cannot be shown: then analysis must show flawgrowth to failure or .9t, whichever is less, will not occurduring four times the time interval for which the pressurevessel is a hazard (eg. pressurized in the STS cargo bay).

The safe-life analysis shall be included in the stress analysisof Section 4.2.5. In particular, the fracture mechanics data,loading spectra and environments, flaw-growth model, initialflaw sizes, proof factors, strength and other input data,analysis assumptions, rationales, methods, references, summaryof significant results, shall be clearly presented.

Testing of structure under fracture control in lieu of safe-lifeanalysis is an acceptable alternative, provided that, inaddition to following a quality assurance program (Section 4.6)for each flight article, a qualification test program isimplemented on pre-flawed specimens representative of thestructure design. These flaws shall not be less than the flawsizes established by the acceptance proof test or the selectedNDI method(s). Safe-life requirements of Section 4.4 areconsidered demonstrated when the pre-flawed test specimenssuccessfully sustain the limit loads and pressures in theexpected operating environments for the specified test durationwithout rupture.

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5.1.2.3 Qualification Test Requirements. Qualificationtests shall be conducted on flight-quality hardware todemonstrate structural adequacy of the design. The testfixtures, support structures, and methods of environmentalapplication shall not include erroneous test conditions. Thetypes of instrumentation and their locations in qualificationtests shall be based on the results of the stress analysis ofSection 4.2.5. The instrumentation shall provide sufficientdata to ensure proper application of the accept/reject criteria,which shall be established prior to test. The sequences,combinations, levels, and duration of loads, pressure andenvironments shall demonstrate that design requirements havebeen met.

Qualification testing shall include life cycle testing, randomvibration testing, and burst testing. The following delineatesthe required tests:

a. Pressure Testing

Required qualification pressure testing levels areshown in Table II. Requirement for application ofexternal loads in combination with internal pressuresduring testing must be evaluated based on therelative magnitude and/or destabilizing effect ofstresses due to the external load. If limit combinedtensile stresses are enveloped by test pressurestresses, the application of external loads shall notbe required. If the application of external loads isrequired, the load shall be cycled to limit for fourtimes the predicted number of operating cycles of themost severe design condition (e.g., destabilizingload with constant minimum internal pressure, ormaximum additive load with constant maximum expectedoperating pressure). Qualification test procedureshall be approved by the procuring agency and theappropriate launch or test range approval authority.

b. Random Vibration

Random vibration qualification testing shall beperformed per requirements of MIL-STD-1540 unless itcan be shown the vibration requirement is envelopedby other qualification testing performed.

5.1.2.4 Acceptance Test Requirements. The acceptancetest requirements for pressure vessels which exhibit brittlefracture, or hazardous LBB, failure mode are identical to thosewith ductile fracture failure mode as defined in Section 5.1.1.4except that the test level shall be that defined by the fracture



mechanics analysis. Cryo-proof acceptance test procedures maybe required to adequately verify initial flaw size. Thepressure vessel shall not rupture or leak at the acceptance testpressure.

5.1.2.5 Re-certification Test Requirements. Allrefurbished pressure vessels shall be re-certified after eachrefurbishment by the acceptance test requirements for newhardware to verify their structural integrity and to establishtheir suitability for continued service before commitment toflight. Pressure vessels which have exceeded the approvedstorage environment (temperature, humidity, time etc.) shallalso be re-certified by the acceptance test requirements for newhardware.

5.1.3 Pressure Vessels Designed Employing Strength ofMaterials (AVE). Pressure vessels may be designed and verifiedutilizing the following criteria which does not employ fracturemechanics techniques. Pressure vessel failure modedemonstration is not required. The DoD user of this approachshould be aware that this approach is not acceptable for USAF/SDuse. The pressure vessel is to be designed, analyzed andverified as defined in Section 5.1.1 with the followingexceptions or changes to criteria.

5.1.3.1 Factor of Safety Requirements - The pressurevessel is to be designed to a minimum Proof Pressure = 1.5 xMEOP and a minimum Design Burst Pressure = 2.0 x MEOP.

5.1.3.2 Safe-Life Analysis Requirements - Requirementsper Section 5.1.1.2.

5.1.3.3 Qualification Test Requirements - Requirementsper Section 5.1.1.3 except that Leak Before Burst testing isdeleted.

5.1.3.4 Acceptance Test Requirements - Requirements perSection 5.1.1.4 except that proof test shall be conducted at 1.5x MEOP.

5.1.3.5, Re-Certification Test Requirements- Requirementsper Section 5.1.1.>.

5.1.4 Pressure Vessels Designed Employing the ASMEBoiler Code (AVE & GSE). Pressure vessels may be designed andmanufactured per the rules of the ASME Boiler and PressureVessel Code, Section VIII, Divisions 1 or 2.

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5.1.4.1 Qualification Test Requirements - (For airbornesystems required to satisfy Space Transportation System SafetyRequirements). Qualification testing shall consist of the cycletesting defined in Section 5.1.1.3b.

5.1.4.2 Acceptance Test Requirements - As specified inCode. AVE pressure vessels shall be proof pressure tested at105 x MEOP. This test may be accomplished when satisfying coderequirements.

5.1.4.3 Re-Certification Requirements (GSE) - When nodata is available concerning existing ground system pressurevessel or history of use, or if the origin of data is uncertain,perform a detailed investigation to determine the utility of thevessel. This investigation shall, as a minimum, include thefollowing:

(a) Paint removal and thorough internal and externalsurface cleaning including removal of corrosion.

(b) Thorough internal and external surface inspectionto determine extent of corrosion or any handlingdamage or cracks.

(c) Accurate measurement of minimum vessel wallthickness.

(d) Identification of construction materials.

(e) Determination of operating stress levels usingproposed system maximum expected operating pressure.

(f) Analysis to determine the operating safety factorbased on corrosion or other existing damage.

(g) Hydrostatic test to 150% of proposed maximumexpected operating pressure.

(h) The using activity shall maintain reports of theabove investigation as a part of the system records.

5.2 COMPOSITE PRESSURE VESSELS

Pressure vessels fabricated of composite materials must satisfythe non-composite requirements of Section 5.1 with the followingexceptions applicable to each design verification analysisapproach.

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Composite vessels with metallic liners may be designed employingeither of the two approaches. Composite vessels without a loadcarrying metallic liner may only be designed by the ASME code,(section 5.2.3 of this standard). The leak-before-burst orbrittle failure mode designation for a metal lined compositevessel shall be based on the characteristics of the liner.Fracture mechanics methodology is not applicable to thecomposite overwrap.

5.2.1 Composite Pressure Vessels with Non-HazardousLeak-Before-Burst (LBB) Failure Mode (AVE). Applicable fracturemechanics analysis and/or tests of metal lined compositepressure vessels shall verify the leak-before-burst failure modeof the metal liner.

5.2.1.1 Factors of Safety Requirements. Requirementsper Section 5.1.1.1.

5.2.1.2 Safe-Life Analysis Requirements. Requirementsper Section 5.1.1.2.

5.2.1.3 Qualification Test Requirements. Qualificationtesting shall consist of the leak-before-burst demonstration ofthe liner and cycle/burst testing of the composite vessel asdefined in Section 5.1.1.3. In particular the effects of theliner sizing operation on the fracture mechanics characteristicsof the liner should be accounted for in the LBB evaluation.

5.2.1.4 Acceptance Test Requirements. Acceptance testsshall be conducted as defined in Section 5.1.1.4. Thesubstitution of the metal liner sizing operation for acceptancetest is acceptable provided the requirements of Section 5.1.1.4are satisfied.

5.2.1.5 Re-Certification Test Requirements.Requirements per Section 5.1.1.5. (NOTE) Alternatere-certification procedure may be approved by the procuring andsafety agencies.

5.2.2 Composite Pressure Vessels with Brittle Fractureor Hazardous LBB Failure Mode (AVE). This section is applicableonly to composite pressure vessels with metal liners whichexhibit brittle fracture or hazardous leak-before-burst failuremode.

5.2.2.1 Factor of Safety Requirements. Unless otherwisespecified, the minimum burst factor shall be 1.5.

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5.2.2.2 Safe - Life Demonstration Requirements.Requirements of Section 5.1.2.2 shall apply to the metal liner.Conventional fatigue life analysis of the composite overwrapmust verify the liner is the critical safe-life component.Analysis shall show the safe-life of the overwrap to be a factorof 10 longer than the safe-life of the liner.

5.2.2.3 Qualification Test Requirements. Requirementsper Section 5.1.2.3.

5.2.2.4 Acceptance Test Requirements. Acceptance testrequirements shall be as defined in Section 5.1.2.4. Thesubstitution of metal liner sizing operation for an acceptancetest is acceptable provided the requirements of Section 5.1.2.4are satisfied. The metal liner shall not leak at the proof testpressure. An additional Cryo type proof test of the liner,prior to composite overwrap, may be required to adequatelyverify the largest allowable initial flaw size present in theliner.

5.2.2.5 Re-certification Test Requirements. There-certification test requirements for all refurb ifshed compositepressure vessels are as defined in Section 5.1.1.5.

5.2.3 Composite Pressure Vessels Designed EmployingStrength of Materials (AVE). Composite pressure vessels, bothload bearing metal Lined and all composite, may be designed andverified utilizing the criteria of Section 5.1.3 which does notemploy fracture mechanics techniques.

5.2.4 Composite Pressure Vessels Designed Employing theASME Boiler Code (AVE & GSE). Composite pressure vessels may edesigned and manufactured per the rules of the ASME Boiler andPressure Code, Section X.

5.2.4.1 Qualification Test Requirements. (For airbornesystems required to satisfy Space Transportation SystemRequirements). Qualification testing shall consist of the cycletesting defined in Section 5.1. 1.3b and random vibration testingdefined in Section 5.1.1.3c,

5.2.4.2 Acceptance Test Requirements. AS specified inCode. AVE pressure vessels shall1.5 x MEOP.

be proof pressure tested atThis test may be accomplished when satisfying code

requirements.

5.3 COMPONENTS

5.3.1 Factors of Safety Requirements - Componentsexcluding pressure vessels are to be designed to the minimumfactors given in Table III.

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TABLE III. Systems Safety Factors

5.3.2 Safe-Life Analysis Requirements. Not required.

5.3.3 Qualification Test Requirements. Not required onlines and fittings. Internal/ external pressure testing shall beconducted on all other components to demonstrate no failure atthe design burst pressure.

5.3.4 Acceptance Test Requirements. Acceptance testrequirements shall be satisfied by completion of leak and/orproof test requirements for the assembled pressurized system asdictated by the applicable range safety documentation, and orprocuring agency, requirements.

5.3.5 Re-Certification Test Requirements.Re-certification of lines, fittings and components shall be asdelineated in Section 5.3.5 as applied to the refurbishedsystems.

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SECTION 6PRESSURIZED SYSTEM REQUIREMENTS

Requirements which apply to all types of pressurized systems arein section 6.1. Additional detail requirements applicable tospecific system types are in the sections following.

6.1 GENERAL PRESSURIZED SYSTEM REQUIREMENTS

6.1.1 Design Features

6.1.1.1 Assembly. Design components so that, during theassembly of parts, sufficient clearance exists to permitassembly of the components without damage to O-rings or backuprings where they pass threaded parts or sharp corners.

6.1.1.2 Routing. Avoid straight tubing and piping runsbetween two rigid connection points. Where such straight runsare necessary, provisions shall be made for expansion joints,motion of the units, or similar compensation to insure that noexcessive strains will be applied to the tubing and fittings.Use line bends to ease stresses induced in tubing by alignmenttolerances and vibration.

6.1.1.3 Separation. Physically separate redundantpressure components and systems from main systems for maximumsafety advantage in case of damage or fire.

6.1.1.4 Shielding. Shield pressure systems from othersystems when required to minimize all hazards caused byproximity to combustible gases, heat sources, electricalequipment, etc. Any failure in any such adjacent system shallnot result in combustion or explosion of pressure fluids orcomponents. Shield or separate lines, drains, and vents fromother high-energy systems; for example, heat, high voltage,combustible gases, and chemicals. Drain and vent lines will notbe connected to any other lines in any way that could generate ahazardous mixture in the drain/vent line, or allow feedback ofhazardous substances to the components being drained or vented.Shield or isolate pressure fluid reservoirs from combustionapparatus or other heat sources.

6.1.1.5 Grounding. Electrically ground hydraulic systemcomponents and lines to metallic structures.

6.1.1.6 Handling. Provide fixtures for safe handlingand hoisting with coordinated attachment points in the systemstructure, for equipment that cannot be hand-carried. Handlingand hoisting loads shall be in accordance with MIL-S-8512.

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6.1.1.7 Special Tools. Design safety critical pressuresystems so that special tools shall not be required for removaland replacement of components unless it can be shown that theuse of special tools is unavoidable.

6.1.1.8 Test Points. Provide test points if required,such that disassembly for test is not required. The test pointsshall be easily accessible for attachment of ground testequipment.

6.1.1.9 Common-Plug Test Connectors. Common-plug testconnectors for pressure and return sections shall be designed torequire positive removal of the pressure connection prior tounsealing the return connections.

6.1.1.10 Individual Pressure and Return TestConnectors. Individual pressure and return test connectorsshall be designed to positively prevent inadvertentcross-connections.

6.1.1.11 Threaded Parts. All threaded parts in safetycritical components shall be securely locked to resistuncoupling forces by acceptable safe design methods. Safetywiring and self-locking nuts are examples of acceptable safedesign. Torque for threaded parts in safety critical componentsshall be specified.

6.1.1.12 Friction Locking Devices. Avoid friction-typelocking devices in safety critical applications. Star washersand jam nuts shall not be used as locking devices.

6.1.1.13 Internally Threaded Bosses. The design ofinternally threaded bosses shall preclude the possibility ofdamage to the component or the boss threads because of screwinguniversal fittings to excessive depths in the bosses.

6.1.1.14 Retainer or Snap Rings. Retainer or snap ringsshall not be used in pressure systems where failure of the ringwould allow connection failures or blow-outs caused by internalpressure.

6.1.1.15 Snubbers. Snubbers shall be used with allBourdon type pressure transmitters, pressure switches, andpressure gages, except air pressure gages.

6.1.2 Component Selection

6.1.2.1 Connections. Design or select components toassure that hazardous disconnections or reverse installationswithin the subsystem are not possible. Color codes, labels, anddirectional arrows, are not acceptable as the primary means forpreventing incorrect installation.

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6.1.2.2 Fluid Temperature. Estimate the maximum fluidtemperature early in design as part of data for selection ofsafety critical components, such as system fluid, pressurizinggas, oil coolers, gaskets, etc.

6.1.2.3 Actuator Pressure Rating. Specify componentsthat are capable of safe actuation under pressure equal to themaximum relief valve setting in the circuit in which they areinstalled.

6.1.2.4 Pressure Service Ratings. Pumps,valves/regulators, hoses, and all such prefabricated componentsof a pressure system shall have proven pressure service ratingsequal to or higher than the limit-load (maximum expectedoperating pressure) and rated service life of the system.

6.1.2.5 Pump Selection. Apply "The Standards of theHydraulic Institute" in evaluating safety in pump selection.

6.1.2.6 Fracture and Leakage. Where leakage or fractureis hazardous to personnel or critical equipment, design so thatfailure occurs at the outlet threads of valves before the inletthreads or body of the valve fails under pressure.

6.1.2.7 Oxygen System Components. Specify valves andother components for oxygen systems of 3000 psi or higher thatare slow opening and closing types to minimize the potential forignition of contaminants. Such systems shall also requireelectrical grounding to eliminate the possibility of thebuild-up of static electrical charges.

6.1.2.8 Pressure Regulators. Select pressure regulatorsto operate in the center 50 percent of their total pressurerange, or design to avoid creep and inaccuracies at either endof the full operating range.

6.1.2.9 Flareless Tube Fittings. In all cases flarelesstube fittings shall be properly preset prior to pressureapplication.

6.1.2.10 Manual Valves and Regulators. Design manuallyoperated valves and regulators so that over torqueing of thevalve stem or regulator adjustment cannot damage soft seats tothe extent that failure of the seat will result.

6.1.2.10.1 Valve designs which utilize uncontained seatsare unacceptable and shall not be specified.

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6.1.3 Design Pressures

6.1.3.1 Over Pressure. Specify warning devices toindicate hazardous over or under pressures to operatingpersonnel. These devices shall actuate at predeterminedpressure levels designed to allow time for corrective action.

6.1.3.2 Back Pressure. Safety critical actuations ofpneumatic systems shall not be adversely affected by any backpressure resulting from concurrent operations of any other partsof the system under any set of conditions.

6.1.3.3 Pressure Isolation. Components or lines thatcan be isolated and contain residual pressure shall be equippedwith gage reading and bleed valves for pressure safety check.Bleed valves shall be directed away from operating personnel.Fittings or caps for bleeding pressure are not acceptable.

6.1.3.4 Gas/Fluid Separation. Specify pressurizedreservoirs that are designed for gas/fluid separation withprovision to entrap gas that may be hazardous to the system orsafety critical actuation, and prevent its recirculation in thesystem. This shall include the posting of instructions adjacentto the filling point for proper bleeding when servicing.

6.1.3.5 Compressed Gas Bleeding. Bleed compressed gasemergency systems directly to the atmosphere away from thevicinity of personnel, rather than to reservoir. If the gas iscombustible, consideration should be given to methods forreducing the potential for accidental ignition or explosion.

6.1.4 Design Loads

6.1.4.1 Acceleration and Shock Loads. Specifyinstallation of all lines and components to withstand allexpected acceleration and shock loads. Shock isolation mountsmay be used if necessary to eliminate destructive vibration andinterference collisions.

6.1.4.2 Torque Loads. Specify the mounting ofcomponents, including valves, on structures having sufficientstrength to withstand torque and dynamic loads, and notsupported by the tubing. However, light-weight components thatdo not require adjustment after installation (for example, checkvalves), may be supported by the tubing, provided that a tubeclamp is installed on each such tube near the component.

6.1.4.3 Vibration Loads. Support tubing by cushionedsteel tube clamps or by multiple-block type clamps that aresuitably spaced to restrain destructive vibration.

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6.1.5 Controls

6.1.5.1 Interlocks. Specify interlocks where necessaryto prevent a hazardous sequence of operations and provide afail-safe capability at all times. For example, the "open"position of remotely controlled valves that can hazardouslypressurize lines leading to remotely controlled (or automatic)disconnect couplings shall be interlocked to preclude the "open"valve position coincident with the disconnected condition of thecouplings.

6.1.5.2 Multiple Safety Critical Functions. Pressuresystems that combine several safety critical functions shallhave sufficient controls for isolating failed functions, for thepurpose of safely operating the remaining functions.

6.1.5.3 Critical Flows and Pressures. All pressuresystems shall have pressure indicating devices to monitorcritical flows and pressures marked to show safe upper and lowerlimits of system pressure. The pressure indicators shall be solocated as to be readily visible to the operating crew.

6.1.6 Protection. Protect all systems for pressureabove 500 psi in all areas where damage can occur duringservicing or other operational hazards. Hazardous piping lineroutes that invite use as handholds or climbing bars, shall beavoided. Shield pressure lines and components of 500 psi orhigher that are adjacent to safety critical equipment to protectsuch equipment in the event of leakage or burst of the pressuresystem.

6.1.7 Electrical.

6.1.7.1 Hazardous Atmospheres. Electric components foruse in potentially ignitable atmospheres shall be demonstratedto be incapable of causing an explosion in the intendedapplication.

6.1.7.2 Radio Frequency Energy. Electrically energizedh draulic components shall not propagate radio-frequency energythat is hazardous to other subsystems in the total system, orinterfere in the operation of safety critical electronicequipment (Reference MIL-E-6051).

6.1.7.3 Grounding. Electrically ground pressure systemcomponents and lines to metallic structures.

6.1.7.4 Solenoids. All solenoids shall be capable ofsafely withstanding a test voltage of not less than 1500 V rmsat 60 cps for 1 minute between terminals and case at the maximumoperating temperature of the solenoid in the functional envelope.

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6.1.7.5 Electric Motor Driven Pumps. Electric motordriven pumps used in safety critical systems shall not be usedfor ground test purposes unless the motor is rated for reliablecontinuous and safe operation. Otherwise, the test parametersmay perturb reliability calculations.

6.1.8 Pressure Relief

6.1.8.1 Requirement. Specify pressure relief devices onall systems having a pressure source which can exceed themaximum allowable pressure of the system, or where themalfunction/failure of any component can cause the maximumallowable pressure to be exceeded. Relief devices are requireddownstream of all regulating valves and orifice restrictorsunless the downstream system is designed to accept full sourcepressure. On space systems, where operational or weightlimitations preclude the use of relief valves, and systems willoperate in an environment not hazardous to personnel, they canbe omitted if the ground or support system contains such devicesand they cannot be isolated from the airborne system during thepressurization cycle and the space vehicle cannot provide itsown protection.

6.1.8.2 Flow Capacity. Specify that all pressure reliefdevices shall provide relief at full flow capacity at 110% ofthe MEOP of the system, or lower.

6.1.8.3 Sizing. Specify the size of pressure reliefdevices to withstand maximum pressure and flow capacities of thepressure source, to prevent pressure from exceeding 110% of theMEOP of the system.

6.1.8.4 Unmanned Flight Vehicle Servicing. Where around system is specifically designed to service an unmanned

flight vehicle, pressure relief protection may be providedwithin the ground equipment, if no capability exists to isolatethe pressure relief protection from the flight vehicle duringthe pressurization cycle.

6.1.8.5 Automatic Relief.

6.1.8.5.1 Low Safety Factor. Where safety factors lessthan 2.0 are used in the design of pressure AVE vessels, providea means for the automatic relief, depressurization, and pressureverification of safety critical vessels for the event of launchabort.

6.1.8. 5.2 Confinement. Whenever any pressure volume canbe confined and/or isolated by system valving. provide anautomatic pressure relief device. Pop-valves, rupture discs,

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blow-out plugs, armoring, and construction to contain thegreatest possible overpressure that may develop are examples ofcorrective measures for system safety in cases not covered bythe above paragraphs.

6.1.8.6 Venting. Vent pressure relief devices for toxicor inert gases to safe areas or scrubbers, away from thevicinity of personnel.

6.1.8.7 Relief Valve Isolation. Shut-off valves formaintenance purposes on the inlet side of pressurized reliefvalves are permissible if a means for monitoring and bleedingtrapped pressure is provided and the requirements of ASME Codefor unfired pressure vessels, Appendix M, paragraph UA-354, andthe provisions for valve design in paragraph 6.1.2 are met. Itis mandatory that the valve be locked open when the system isrepressurized.

6.1.8.8 Negative Pressure Protection.

6.1.8.8.1 Testing. Hydrostatic testing systems forvessels which are not designed to sustain negative internalpressure shall be equipped with fail-safe devices for relief ofhazardous negative pressure during the period of fluid removal.Check valves and valve interlocks are examples of devices whichcan be used for this purpose.

6.1.8.8.2 Storage and Transportation. Thin walledvessels which can be collapsed by a negative pressure shall havenegative pressure relief and/or prevention devices for safetyduring storage and transportation.

6.1.8.9 Reservoir Pressure Relief. Design pressurizedreservoirs so that ull age volumes shall be connected to a reliefvalve that shall protect tune reservoir and power pump fromhazardous overpressure or back pressure of the system.

6.1.8.10 Air Pressure Control. The air pressure controlfor pressurized reservoirs shall be an externally nonadjustablepressure regulating device. If this unit also contains areservoir pressure relief valve, design the unit so that nofailure in the unit will permit overpressurization of thereservoir.

6.1.9 Contamination. The following contaminationrelated considerations shall be addressed in the design ofpressurized systems. Contamination includes solid, liquid andgaseous material.

a. Prevent contamination from entering or developingwithin the system.

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b. Design the system to include provisions to detectcontamination.

c. Design the system to include provisions for removalof contamination. Include provisions for initialpurge with fluid or gas which will not degrade futuresystem performance.

d. Design the system to be tolerant of contamination.

6.1.9.1 Filtering. All pressurizing fluids enteringsafety critical systems will be filtered through a 10 micronfilter, or finer, before entering the system.

6.1.9.1.1 Fluid Filters.

a.

b.

c.

d.

All pressure systems shall have fluid filters in thesystem, designed and located to reduce the flow ofcontaminant particles to a safe minimum.

All of the circulating fluid in the system shall befiltered downstream from the pressure pump, orimmediately upstream from safety critical actuators.Entrance of contamination at test points or ventsshall be minimized by downstream filters. The bypassfluid or case drain flow on variable displacementpumps shall be filtered.

lfWhen clogging of small orifices could cause ahazardous ma unction or failure of the system, theyshall be protected by a filter element designed toprevent clogging of the orifice. Note that thisincludes servo valves.

Do not use filters or screens in suction lines ofpower pumps or hand pumps of safety critical systems.

6.1.9.1.2 Air Filters.

6.1.9.1.2.1 Pressurized Reservoirs. Specify air filtersfor hydraulic reservoir air pressurization circuits and locateair filters to protect the pressure regulating equipment fromcontamination. Specify dry compressed air for hydraulicreservoir pressurization. Specify a moisture removal unit toprotect the pressure regulation lines and equipment.

6.1.9.1.2.2 Unpressurized Reservoirs. Unpressurizedhydraulic reservoirs shall have filters and dessicant units atthe breather opening to preclude introduction of moisture andcontaminants into the reservoir.



6. 1.9.2 Bleed Ports

6. 1.9.2.1 Location - Provide bleed ports where necessaryto remove accumulations of residue or contaminants. Providehigh point bleed ports where necessary for removal of trappedgases. The bleed valve shall be directed away from operatingpersonnel and possible ignition sources.

a. Equip components, cavities, or lines that can beisolated, with bleed valves which can be used torelease retained pressure, or will indicate thatcontinued pressure exists in the system.

b. Bleed valves used for reducing pressure on systemscontaining hazardous fluids shall be routed to a safedisposal area.

6.1.9.2.2 Auxiliary Bleed Ports. Provide auxiliarybleed ports where necessary to allow bleed off for safetypurposes. Locate bleeder valves so that they can be operatedwithout removal of other components, and shall permit theattachment of a hose to direct the bled off fluid into acontainer.

6.1.9.2.3 Filler Cap Bleed. Reservoir filler caps shallinclude design provisions which shall automatically bleed thereservoir on opening, so that possible ullage pressure can notimpart hazardous kinetic energy to either the filler caps, thefluid in the reservoir, or the system.

6.1.10 Control Devices

6.1.10.1 Directional Control Valves. Design safetycritical pressure systems incorporating two or more directionalcontrol valves to preclude the possibility of inadvertentlydirecting the flow or pressure from one valve into the flow orpressure path intended for another valve, with any combinationof valve settings possible in the total system.

6.1.10.2 Overtravel. Design control devices to preventovertravel or undertravel that may contribute to a hazardouscondition, or damage to the valve.

6.1.10.3 Pressure and Volume Control Stops. Allpressure and volume controls shall have stops, or equivalent, toprevent settings outside their nominal safe working ranges.

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6.1.10.4 Manually Operated Levers.

a.

b.

c.

Components that have integrated manually operatedlevers shall provide levers and stops capable ofwithstanding the limit torques specified byMIL-STD-1472.

Provide levers and stops on remote controls, capableof withstanding a limit torque of 1800 lb-in.

Because jamming is possible, do not use sheathedflexible-actuators for valve controls in safetycritical pressure systems; for example, push-pullwires, torque wires, etc., that are sheathed are notacceptable.

6.1.10.5 Limit Torque. Control components that haveintegral manually operated levers shall provide levers and stopscapable of withstanding the following limit torques.

Lever Radius (R) Design Torque

Less than 3 inches 50 x R lb-in.

3 to 6 inches 75 x R lb-in.

over 6 inches 150 x R lb-in.

6.1.11 Accumulators

6.1.11.1 Accumulator Design. Design accumulators inaccordance with the pressure vessel standards for ground systemsand locate for minimal probability of mechanical damage and forminimum escalation of material damage or personnel injury in theevent of a major failure such as tank rupture.

6.1.11.2 Accumulator Gas Pressure Gages. Accumulatorgas pressure gages shall not be used to indicate system pressurefor operational or maintenance purposes.

6.1.11.3 Accumulator Identification. Gas type andpressure level shall be posted on, or immediately adjacent tothe accumulator.

6.1.12 Flexhose

6.1.12.1 Installation. Use flexhose between any twoconnections where relative motion can be expected to fatiguemetal tube or pipe. Design flexhose installation to avoidabrasive contact with adjacent structure or moving parts. Rigidsupports shall not be used on flexhose.

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6.1.12.2 Restraining Devices. Design flexhoseinstallation which are six feet long or greater so thatrestraint is provided on both the hose and adjacent structure atno greater than six- foot intervals and at each end to preventwhiplash in the event of a burst. Restraining devices shall bedesigned and demonstrated to contain a force not less than 1.5 Xopenline pressure force. (See Table IV) The design safetyfactor shall be not less than 3. Sand or shot bags placed ontop of flexible hose is not an acceptable restraint. Do not usehose clamp type restraining devices.

6.1.12.3 Flexhose Stress. Design flexhose installationsthat shall not produce stress or strain of any nature in thehard lines or components. Include stresses induced because ofdimensional changes caused by pressure or temperaturevariations, or torque forces induced in the flexhose.

6.1.12.4 Temporary Installations. Temporaryinstallations using chains or cables anchored to substantialfixed points, lead ingots or other weights, are acceptableproviding they meet the requirements of paragraph 6.1.2.1.Protect flexhose from kinking or abrasive chafing from therestraining device or damage from adjacent structure or movingparts that may cause reduction in strength.

TABLE IV. Open Line Force Calculation Factor

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6.2 HYDRAULIC SYSTEM REQUIREMENTS

6.2.1 Hydraulic System Components

6.2.1.1 Component Integrity. When the system pressureprofile is indeterminate; perform safety tests at pressure nolower than 67 percent of the maximum allowable working ressurefor components rated up to 3000 psig and no lower than 80percent of the maximum allowable working pressure for componentsrated above 3000 psig.

6.2.1.1.1 Component Section. Select components whichare compatible with and rated for the viscosity of the hydraulicfluid to be used.

6.2.1.2 Cycling. Cycling capability for safety criticalcomponents shall be not less than 400% of the total number ofexpected cycles, including system tests, but not less than 2000cycles. For service above a temperature of 160°F, an additionalcycling capability equivalent to the above shall be required asa maximum.

6.2.1.3 Actuators. Safety critical hydraulic actuatorsshall have positive mechanical stops at the extremes of safemotion.

6.2.1.4 Shutoff Valves. Hydraulic fluid reservoirs andsupply tanks shall be equipped with shutoff valves, operablefrom a relatively safe location in the event of a hydraulicsystem emergency.

6.2.1.5 Variable Response. Do not use shuttle valves insafety critical hydraulic systems where the event of a forcebalance on both inlet ports may occur, thus causing the shuttlevalve to restrict flow from the outlet port.

6.2.1.6 Fire Resistant Fluids. Where system leakage canexpose hydraulic fluid to potential ignition sources or isadjacent to a potential fire zone and the possibility of flamepropagation exists, fire resistant or flame proof hydraulicfluid shall be used.

6.2.1.7 Accumulators. Hydraulic systems incorporatingaccumulators shall be Interlocked to either vent or isolateaccumulator fluid pressure when power is shutoff.

6.2.1.8 Adjustable Orifices. Do not use adjustableorifice restrictor valves in safety critical hydraulic systems.



6.2.1.9 Lock Valves.

a. When two or more hydraulic actuators are mechanicallytied together, only one lock valve shall be used tohydraulically lock all the actuators.

b. Do not use hydraulic lock valves for safety criticallockup periods likely to involve extreme temperaturechanges, unless fluid expansion and contractioneffects are safely accounted for.

6.2.1.10 Hydraulic Reservoir. Whenever possible, thehydraulic reservoir should be located at the highest point inthe system. If this is not possible in safety critical systems,procedures must be developed to detect air in actuators or othersafety critical components and to assure that the system isproperly bled prior to each use.

6.2.2 Pressure Limits. Hydraulic systems installationswill be limited to a maximum pressure of 15,000 psig.

NOTE

There is no intent to restrain development of systemscapable of higher pressures, however, the employment ofsuch systems must be preceded by complete development andqualification that includes appropriate safety tests.

6.2.3 Cavitation.

6.2.3.1 Inlet Pressure. Specify the inlet pressure ofhydraulic pumps in safety critical systems to prevent cavitationeffects in the pump passages or outlets.

6.2.3.2 Fluid Column. Safety critical hydraulic systemsshall have positive protection against breaking the fluid columnin the suction line during standby.

6.2.4 Redundancy. Hydraulic systems for primary flightcontrol of marine vehicles shall have redundant features for allmajor aspects of operation and control and be essentiallyindependent of systems non-critical to safety.

NOTE

Provision may be made for a safety critical system todraw power from a non-critical system, provided that nosingle failure can cause loss of both systems because ofthis connection.

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6.2.5 Hydraulic Lockup.

6.2.5.1 Emergency Disengage. Hydraulic systems thatprovide for manual takeover shall automatically disengage orallow by-pass of the main hydraulic system upon the act ofmanual takeover.

6.2.5.2 Emergency By-Pass.

a. Safety critical hydraulic systems or alternateby-pass systems provided for safety shall not berendered inoperative because of back pressure underany set of conditions.

b. Design the system so that a hydraulic lock resultingfrom an unplanned disconnection of a self-seatingcoupling or other component shall not cause damage tothe system or to adjacent property, or injury topersonnel.

5.2.6 Hydraulic System Pressure Relief

6.2.6.1 Pump Pressure Relief. Hydraulic systemsemploying power operated pumps shall include a pressureregulating device and an independent safety relief valve.

6.2.6.2 Thermal Pressure Relief. Thermal expansionrelief valves shall be installed as necessary to prevent systemdamage from thermal expansion of hydraulic fluid, as in theevent of gross overheating. Internal valve leakage not beconsidered an acceptable method of providing thermal relief.Thermal relief valve setting shall not exceed 150 psi above thevalue for system relief valve setting. Vents shall outlet onlyto areas of relative safety from fire hazard. Hydraulicblow-out fuses (soft plugs) shall not be used in systems havingtemperatures above 160°F.

6.2.6.3 Location. Pressure relief valves shall belocated in hydraulic systems wherever necessary to assure thatthe pressure in any part of a power system shall not exceed thesafe limit above the regulated pressure of the system.

6.3 PNEUMATIC SYSTEMS REQUIREMENTS

6.3.1 Pneumatic System Components.

6.3.1.1 Component Integrity. Pneumatic components(other than tanks) for safety critical systems shall exhibitsafe endurance against hazardous failure modes for not less than400% of the total number of expected cycles including systemtest. Pneumatic ground support emergency system componentsshall have safe endurance of a minimum of 5000 cycles.

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6.3.1.2 Configuration - The configuration of pneumaticcomponents shall permit bleeding of entrapped moisture,lubricants, particulate material, or other foreign matterhazardous to this system.

6.3.1.3 Compressors. Select compressors which aredesigned to sustain not less than 2.5 X delivery pressure, afterallowance for loss of strength of the materials equivalent tonot less than that caused by 1000 hours aging at 275°F.

6.3.1.4 Actuators. Safety critical pneumatic actuatorsshall have positive mechanical stops at the extremes of safemotion.

6.3.1.5 Adjustable Orifice Restrictors. Adjustableorifice restrictor valves shall not be used in safety criticalpneumatic systems.

6.3.2 Controls.

6.3.2.1 Manual Takeover. Provide for automaticdisengagement or by-pass for pneumatic systems that provide formanual takeover in the event of a hazardous situation. Providepositive indication of disengagement.

Custodians: Preparing Activity:Air Force - 19 Air Force - 19

(Project Number SAFT-F002)

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APPENDIX

FAILURE MODE DETERMINATION

The following is an acceptable approach for failure modedetermination. This approach is not manditory. An alternateapproach may be used if approved by the procuring agency andappropriate launch or test range approval authority.

The results of the stress analysis of Section 4.2.5 shall beused to determine potential failure modes of the pressure vesseland pressurized structure in terms of leakage (ductile fracturefailure mode) or complete fracture (brittle fracture failuremode). The structure is considered to exhibit a ductilefracture failure mode when

where KT~ is the plane strain fracture toughness of thematerial, ~ is the operating stress level, ~ is the prooffactor, o .,e is the yield stress of the material, and t is thematerial thickness at the location of aoP. Under thiscondition, further fracture mechanics analysis is not required.These pressurized systems shall be designed and tested inaccordance with the requirements of Section 5.1.1.

When

the pressurized system shall be designed and tested inaccordance with established linear elastic fracture mechanicsmethodology, as delineated in Section 5.1.2.

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