Increasing Importance of Rapid Crack Propagation (RCP)

for Gas Piping Applications- Industry Status

Dr. Gene Palermo – P Cubed ConsultingDr. Dane Chang – The Dow Chemical Co.

OutlineOutline

• Background• RCP Test Methods• RCP Requirements in Industry

Product Standards• RCP Test Data• Conclusions

BackgroundBackground

RCP is a fast fracture – 200 m/secDuPont – two RCP incidents in HDPE pipe – 300 ft and 800 ft distanceOnly a few incidents is gas applicationsTypical features are a sinusoidal crack path and “hackle” marks indicating the direction of travel

PE Pipe Dependant VariablesPE Pipe Dependant Variables

Wall thickness (pipe size)Internal pressureTemperatureMaterial properties/resistance to RCPPipe processing

OutlineOutline

• Background• RCP Test Methods• RCP Requirements in Industry

Product Standards• RCP Test Data• Conclusions

RCP Test MethodsRCP Test Methods

Full Scale (FS) Test – ISO 13478- long pipe samples- very expensive

Small Scale Steady State (S4) - ISO 13477- smaller pipe samples- test done in a laboratory- easier to get data and correlates to FS test

Critical Pressure (PCritical Pressure (PCC))

Constant temperature – 0◦CVary internal pressureDefined as the pressure above which the crack propagates and the pressure below which the crack arrests

Critical Pressure (PCritical Pressure (PCC))

FS/S4 Correlation EquationFS/S4 Correlation Equation

PC,FS = 3.6 PC,S4 + 2.6 bar

(ISO 4437 Equation)

Patrick LeversPatrick Levers’’ PaperPaper“Although the configuration of an ISO 13478 Full Scale RCP test does not exactly simulate that of installed pipe, the method is justifiably regarded as the ‘gold standard’. The S4 test method aims to simulate the internal pressure and wall hoop stress distribution around an axially propagating crack tip in the FS test. However, because of the difference in specimen sizes, these distributions diverge further away, and as a result the critical pressure results are different.”

FS FS –– S4 DifferencesS4 Differences

“The disparity originates mainly in the dynamics of the pressurizing fluid. In a full-scale configuration the contained nitrogen or air is relatively free to exhaust from an effectively infinite reservoir ahead of the crack tip to the flared-out fissure behind it. In the S4 test, this backflow is almost eliminated.”

The ISO Correlation EquationThe ISO Correlation Equation“The correlation factor was derived by assuming that the crack-arrest condition (i.e. the critical pressure) in each test corresponds to the same crack-tip pressure. A one-dimensional axial flow analysis for air or nitrogen then yields the equation enshrined in ISO 13477:

Pc(FS) = 3.6 Pc(S4) + 2.6 bar. (Eq. 1)”

Whence Commeth the NumbersWhence Commeth the Numbers

“This is based on the result that if a long pipe is ‘guillotined’ at a fixed plane along its axis, only a constant fraction, 1/3.6, of the initial pressure remains at the outflow. Critical pressures pc in Eq (1) are gauge pressures: the 2.6 bar offset arises because 3.6 bar absolute pressure is needed to drive the gas out against a 1 bar atmospheric pressure.”

FS FS –– S4 Correlation EquationS4 Correlation EquationIt is understood that the correlation equation in ISO 13477 is very conservativeIf a manufacturer does the FS test, then that is the value that is usedIf a manufacturer only does S4 testing, then he uses the conservative ISO 13477 correlation equationWe could determine 100 different correlation equations, one for each material, but the industry just uses the conservative ISO 13477 correlation equation

Critical Temperature (TCritical Temperature (TCC))

Constant pressure – 5 bar (75 psig)Vary temperatureDefined as the temperature above which the crack arrests and the temperature below which the crack propagates

Critical Temperature (TCritical Temperature (TCC))

OutlineOutline

• Background• RCP Test Methods• RCP Requirements in Industry

Product Standards• RCP Test Data• Conclusions

ISOISOISO has long recognized the importance of RCP, and has included an RCP requirement in its gas product standard for PE pipes –ISO 4437

Also, European gas companies have been installing large diameter pipes operating at high pressures, both of which increase susceptibility to RCP

ISO 4437 RequirementsISO 4437 Requirements

PC,FS > 1.5 MOP*PC,FS = 3.6 PC,S4 + 2.6 bar

PE pipe testing is only required when the wall thickness of the pipe is greater than the wall thickness of the pipe used in the RCP test to qualify the compound

* MOP = Maximum operating pressure

ISO/TC 138/SC 4/RCP AHGISO/TC 138/SC 4/RCP AHG

CSACSAProject to add requirement in CSA B137.4 (product standard) that RCP testing must be done has been completedSimilar wording to ISO 4437 – based on wall thicknessProject to incorporate RCP required values in CSA Z662 Clause 12 (Code) is completeSimilar critical pressure requirements as ISO 4437:- PC,FS > 1.5 X MOP- PC,FS = 3.6 PC,S4 + 2.6 bar

ASTMASTMASTM D 2513 had no RCP requirementsSeveral years ago, AGA PMC requested that ASTM D 2513 be revised to include the ISO 4437 RCP requirementsBalloting completedASTM F17 agreed to include a requirement that RCP testing must be done, but no values are included in ASTM D 2513

AGA PMCAGA PMC

PMC agreed with this approachPMC initiated a task group to develop a White Paper that would provide guidance on what the RCP test values should beDrafts of this White Paper have been, and are being, circulated

PMC White PaperPMC White PaperPC,FS > leak test pressure (1.5 X MOP)

PC,FS = 3.6 PC,S4 + 2.6 bar

Insure the critical temperature, TC, is below the minimum anticipated service temperature, TS

OutlineOutline

• Background• RCP Test Methods• RCP Requirements in Industry

Product Standards• RCP Test Data• Conclusions

Critical Pressure ValuesCritical Pressure Values

PE Material12” SDR 11 Pipe

S4 Critical Pressure at 32°F

Unimodal MDPE 1 bar (14.5 psig)Bimodal MDPE 10 bar (145 psig)

Unimodal HDPE 2 bar ( 29 psig)Bimodal HDPE 12 bar (180 psig)

Critical Pressure ValuesCritical Pressure Values

PE Material12” SDR 11 Pipe

Full Scale Critical Pressure

Unimodal MDPE 6.2 bar (90 psig)Bimodal MDPE 35 bar (500 psig)

Unimodal HDPE 9.8 bar (140 psig)Bimodal HDPE 46 bar (650 psig)

Critical Temperature ValuesCritical Temperature Values

PE Material12” SDR 11 Pipe

Critical Temp. at 5 bar (75 psig)

Unimodal MDPE 15°C (60°F)Bimodal MDPE -2°C (28°F)

Unimodal HDPE 9°C (48°F)Bimodal HDPE -17°C (1°F)

OutlineOutline

• Background• RCP Test Methods• RCP Requirements in Industry

Product Standards• RCP Test Data• Conclusions

ConclusionsConclusionsRCP is more important for larger pipe sizes and higher pressuresFS and S4 are the key test methods usedRCP requirements to do testing and required values are in ISO 4437RCP requirements are in CSA B137.4 and required values have been added to CSA Z662 Clause 12RCP requirements have been added toASTM D 2513, with suggested valuesin a proposed AGA White Paper

ConclusionsConclusionsIn general HDPE has slightly better RCP resistance than MDPEBimodal MDPE and bimodal HDPE have significantly better RCP resistance than unimodal PE – about 10 times higher PC and significantly lower TCBased on superior RCP performance, increasing recognition of RCP in product standards, and increasing demand in the gas industry for RCP resistance, the trend will be to the higher performing bimodalPE materials for gas applications

THE ENDTHE ENDPrepared by

Palermo Plastics Pipe (P3) Consulting