International journal of COMADEM Vol 20 No 1 (January 2017) 41-52 COMADEM International Journal of Condition Monitoring and Diagnostic Engineering Management Orbital TIG Welding and Evaluating Methods of Propulsion Feed Lines for Satellites M. Karthikeyan a *, V.N.A. Naikan b , and R. Narayan c a Liquid Propulsion System Centre (LPSC). Indian Space Research Organisation (ISRO). Bangalore –560008, India b Reliability Engineering Centre (REC). Indian Institute of Technology (IIT). Kharagpur–721302, India c LPSC, ISRO, Bangalore-560006, India ∗ Corresponding author. Tel.: +91-9448309700; fax: +80-25037109; e-mail: [email protected]1. Introduction Satellites need to be positioned in a particular orbit above earth for performing their intended functions and this demands self-correcting mechanisms in the form of thrusters to help the satellites for reaction control, attitude control, orbit rising and station keeping needs. Propellants in the storage tankages need to be transported to the thrusters for combustion through propellant feed lines that have more than hundreds of welded joints of tubes by Orbital TIG welding. The propulsion configuration consists of various 6 mm and 10 mm outer diameter plumb lines. During final integration of satellites, to join the end connection of the propellant feed line pipelines of thickness of 0.7mm requires Orbital TIG welding. The factors affecting the OTIG weld joints are the gap between work pieces (plumb lines with propulsion components). gap between electrode and work pieces, selection of electrode geometry, voltage input, precise control of weld current, bandwidth, electrode travel speed (RPM). flow rate of shielding gas used etc. The variation of any of these parameters will certainly affect quality of weld in propulsion system resulting in weld defects such as undercut, lack of penetration, lack of fusion, non-uniformity, cracks, excessive weld bead width, excessive weld-puddle overlap etc. Hence it becomes necessary to optimize the parameters. Figure 1. Feed lines (Propulsion schematic) ABSTRACT Propulsion system meant for Remote Sensing and Communication applications in satellites have built in them propulsion systems for reaction control, attitude control, orbit raising and station keeping purposes. These propulsion systems have propellant feed lines carrying propellants from tankages to thrusters. Orbital Tungsten Inert Gas (OTIG) welding is widely employed for the welding of the feed lines made up of 6mm and 10mm diameter stainless steel tubes of 0.7mm thickness. These welds need to be leak proof and strong and any defect in them will lead to propellant leakage resulting in the failure of the whole mission which means wastage of lot of efforts and Millions of dollars. Such failures seriously offset the programmatic goals and the nation’s need for remote sensing and transponders for communication. To ensure perfect weld joints, systematic process optimization and rigorous testing and qualification are essential. This paper outlines the criticalities of these weld joints and focuses on the various testing methods such as Visual inspection, Dye penetrant test, Hydrostatic tests, Helium mass spectrometer leak detection test, Pressure hold or Condition Monitoring test, X-ray Radiographic tests in addition to destructive tests such as tensile test and bend tests for this particular application of OTIG welding for satellite propulsion systems. Keywords: OTIG welding, Propellant feed lines, Visual inspection, Tensile test, MSLD, Leak test. 41
Transcript
International journal of COMADEM Vol 20 No 1 (January 2017) 41-52
International journal of COMADEM
COMADEMI n t e r n a t i o n a l J o u r n a l o f C o n d i t i o n M o n i t o r i n g a n d D i a g n o s t i c E n g i n e e r i n g M a n a g e m e n t
Click and type the title of your paper, only capitalize first word
The current energetic and economic context brings the need of a proper asset management of power generation facilities and components. This paper isintended to provide a wide view of the state of the art in asset management, with a special focus on fault detection and diagnosis. The different stages involved in asset management are defined and arranged as layers over the supervised process: Acquisition and pre-processing of data, fault detection, fault diagnosis and maintenance policies. Fault detection approaches are analysed in-depth, highlighting the main characteristics, the advantages and thedrawbacks of each approach. Fault diagnosis techniques and maintenance policies are also described and analysed. A detailed bibliography on these topicsis also provided.
Keywords: Asset Management; Power generation facilities/components; fault detection/diagnosis
Orbital TIG Welding and Evaluating Methods of Propulsion Feed Lines for Satellites
M. Karthikeyana *, V.N.A. Naikanb, and R. Narayanc a Liquid Propulsion System Centre (LPSC). Indian Space Research Organisation (ISRO). Bangalore –560008, India b Reliability Engineering Centre (REC). Indian Institute of Technology (IIT). Kharagpur–721302, India c LPSC, ISRO, Bangalore-560006, India ∗ Corresponding author. Tel.: +91-9448309700; fax: +80-25037109; e-mail: [email protected]
1. Introduction
Satellites need to be positioned in a particular orbit aboveearth for performing their intended functions and this demands self-correcting mechanisms in the form of thrusters to help the satellites for reaction control, attitude control, orbit rising and station keeping needs. Propellants in the storage tankages need to be transported to the thrusters for combustion through propellant feed lines that have more than hundreds of welded joints of tubes by Orbital TIG welding.
The propulsion configuration consists of various 6 mm and 10 mm outer diameter plumb lines. During final integration of satellites, to join the end connection of the propellant feed line pipelines of thickness of 0.7mm requires Orbital TIG welding.
The factors affecting the OTIG weld joints are the gap between work pieces (plumb lines with propulsion components). gap between electrode and work pieces, selection of electrode geometry, voltage input, precise control of weld current, bandwidth, electrode travel speed (RPM). flow rate of shielding gas used etc. The variation of any of these parameters will certainly affect quality of weld in propulsion system resulting in weld defects such as undercut, lack of penetration, lack of fusion, non-uniformity, cracks, excessive weld bead width, excessive weld-puddle overlap etc. Hence it becomes necessary to optimize the parameters.
Figure 1. Feed lines (Propulsion schematic)
A B S T R A C T
Propulsion system meant for Remote Sensing and Communication applications in satellites have built in them propulsion systems for reaction control, attitude control, orbit raising and station keeping purposes. These propulsion systems have propellant feed lines carrying propellants from tankages to thrusters. Orbital Tungsten Inert Gas (OTIG) welding is widely employed for the welding of the feed lines made up of 6mm and 10mm diameter stainless steel tubes of 0.7mm thickness. These welds need to be leak proof and strong and any defect in them will lead to propellant leakage resulting in the failure of the whole mission which means wastage of lot of efforts and Millions of dollars. Such failures seriously offset the programmatic goals and the nation’s need for remote sensing and transponders for communication. To ensure perfect weld joints, systematic process optimization and rigorous testing and qualification are essential. This paper outlines the criticalities of these weld joints and focuses on the various testing methods such as Visual inspection, Dye penetrant test, Hydrostatic tests, Helium mass spectrometer leak detection test, Pressure hold or Condition Monitoring test, X-ray Radiographic tests in addition to destructive tests such as tensile test and bend tests for this particular application of OTIG welding for satellite propulsion systems.
The propulsion system is configured as Gas module (gas bottle, pyro valves, filter, pressure regulators, check valve, latch valve, fill and drain valves etc.). Liquid module (propellant tanks, pyro valves, filters, fill and drain valves) and Thruster module as shown in the schematic (Fig.1). There are about 350 numbers of Orbital TIG weld joints to interconnect propulsion components using 6mm and 10 mm plumb lines.
As the feed lines carry the propellants to the propulsive device, any leak in the welds shall lead to catastrophic mission failure and the losses in terms of effort and crores of money will be substantial. Even a minor leakage will lead to disaster. In view of this, it is essential that all the joints in the propellant feed lines need to be totally leak proof and this demands the effective implementation of orbital TIG welding.
In order to ensure a defect free, strong weld in the propellant feed lines, Welding Procedure Specifications (WPS) and Procedure Qualification Record (PQR) for the OTIG welding process needed to be done meticulously. This demanded various testing methods such as visual inspection, Dye penetrant test, hydrostatic & Ultrasonic tests, helium mass spectrometer leak detection test, local helium spray vacuum test, X-ray Radiographic tests in addition to destructive tests such as tensile test and bend tests. This paper details initially the process of welding and then the various testing methods in depth.
2. Literature Survey
Mannion [1] studied orbital welding systems and captured the skills of the welder and enabled repetitive good welds without defects. Wolf [2] illustrates that Orbital welding has been around for twenty-five years, but its popularity in the last ten years in the semiconductor industry has highlighted its benefits. The time has come to apply orbital welding technology to a broad range of industries for critical system. Lin [3] cautioned about water hammer effect in propellant feed system due to rapid propellant/gas entry during pressurization and propellant feeding. Leca [4] analysed the water hammer effect in satellites due to sudden pyro valve opening by experimental and software computations. Bombardieri [5] brought out the importance of structural integrity of the propulsion system as sudden pyro surge due to priming in the beginning is a potential threat and hence weldments in the feed lines need to be strong and leak proof. Serafin [6] stressed the importance of high quality, integral weldments for space applications and outlined the merits of orbital welding towards this. Devereaux [7] covered the details of MMH/NTO bipropellant propulsion system involving propellant feed system that helps for transfer orbit maneuvers and station keeping operations. Orlowski [8] researched on these welding programs for orbital TIG welding i.e., pulsed current & increasing speed, constant current, pulsed current & decreasing speed and suggested that constant current provided better results based on mathematical tests. Selding [9] highlighted the importance of leak proof propellant storage and feed systems and outlined the failure of a communication satellite launched by Ariane 5 rocket due to leak in feed system. A small leak of propellant can lead to a failure of a big mission with huge cost and enormous efforts. Lafleur [10] enlisted spacecraft failures since 1957 and the reasons there of. Chang [11] clearly enumerated the huge losses due to launch failures and analyzing the failure between 1957 and 1999 indicated that out of 4378 launches, 390 failed and major causes pointed to propulsion system failure due to fuel leakage among other reasons, this point to the importance of defect free strong welding. Yousaf [12] Implemented the five phases of six sigma to improvement in welding processes. The optimum process sets suitably for
Shielded Metal Arc Welding. The welding process is explained using Flow chart and results are analysed using technique like fish bone diagram and Pareto analysis. Westin [13] stated that the need of stainless steel tubes is ever increasing and most of them are welded and with latest weld procedure and skilled welder, good quality welded tubes are a reality. Raied [14] depicts the testing of welded joints shows many welding defects and all these defects produced from different parameters. These discontinuities removed by adopting proper welding input. He also has given the remedy for the defects. Zapfe [15] enumerated various methods of leak detection in vacuum system including helium leak detections with focus on accelerated vacuum systems. Hartman [16] presented a survey of techniques used for ascertaining depth of penetration in welding. Akinlabi [17] highlighted the importance of visual inspection and X-ray radiography in NDT for evaluation of weldments. Wang [18] suggested computer based weld defect identification system utilizing image processing techniques. This helps identifying the weld defects. Bector [19] compared different NDT techniques to evaluate GTAW welded stainless steel cylinder. Vural [20] investigated application of ultrasonic testing to spot welded steel sheets to evaluate fatigue life of such welds. Tarng [21] constructed the relationship between TIG welding parameter and weld head geometry using optimization algorithm and fussy clustering technique and found that good weld quality can be achieved by optimal parameters. Lexington [22] employs the mass spectrometer leak detector which is given in the operation manual.
3. OTIG process in satellite propellant feed lines
3.1. OTIG welding setup
The Orbital TIG welding process offers a large range of benefits in welding of propulsion plumb lines. It is basically TIG welding process (tungsten inert gas welding) where arc is created between non consumable tungsten electrode and the spacecraft plumb line (tube). In Orbital TIG welding, electrode revolves around the stationary tube through a mechanical assembly. Hence it is called Orbital welding. Argon is widely used as shielding gas in the TIG process. It provides good arc striking characteristics and excellent arc stability even at low amperages due to low electron volt causing less potential energy. The quality of argon (shield gas) grade is 4.5 (purity level of 99.995 %) and is being widely used for welding of stainless steel tubes.
Figure 2. Marked are weldments in feed lines
The Orbital TIG welding is well suited to conduct in-situ welding on spacecraft structure as the plumb line orientation can be made in different planes. Post weld cleaning is also easy in this type of welding. Modern orbital welding equipment is
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designed for monitoring of the weld parameters; a complete weld protocol can be generated and stored or output can be taken as printed document. Automatic data transfer takes place without any interruptions to the weld procedure. This can ensure flawless leak proof propellant feed line joints for Satellites.
Figure 3. Orbital TIG Welding Experiment Setup
The aim of the welding is to have efficient and strong weld which can withstand propellant flow pressure, water hammer effect, slam effect, temperature, vibration and environments for satellite for its full life period.
Fig.2 shows locations of weldments being carried out in the part of the propulsion system for the understanding. To meet this requirement, the orbital weld should be free of defects. Fig.3 shows the Orbital TIG welding experiment set up consist of welding machine, shield gas control unit, alignment unit, Argon gas from cylinder for shielding the molten metal, Mass spectrometer leak detector (MSLD), welding head and satellite feed line tubes where welding need to perform on the satellite.
3.2. Material
Propulsion system employs both stainless steel tubes and titanium tubes. We consider here stainless Steel 304L tubes. The parent metal, its Chemical Composition and Mechanical properties are furnished in table.1.
Table 1. Chemical and Mechanical properties of SS 304L
Properties of SS 304 L Chemical
composition percent by weight
Mechanical properties
C 0.03 Tensile strength Ultimate 590 MPa
Cr 18-20 Yield Strength 250 MPa Ni 8-12 Comp. Strength 250 MPa Mn 2 Density 7890 kg/m3 P 0.045 Poisson ratio 0.29
S 0.03 Melting Point 1399 - 1454 °C
Si 0.75 Percent Elongation 35-50%
Al 0.1 Hardness 80(Rc) Fe Balance Young Modulus 200 GPa
3.3. Process parameters
The important parameters are Current, Voltage, RPM, Gap between electrode and welding feed line tubes, shield gas and purge gas, edge preparation and alignment of tubes to be welded etc. In automatic Orbital TIG welding, welding head rotates during the welding and pipes are motionless. This type of welding has high efficiency, quality and is a mobile welding process. In the fully automatic production line as well as aerospace applications, the maximal advantages and optimal application of this welding process are achieved. This welding process is performed by multi pass welding without filler material, due to smaller tube wall thickness.
3.4. Weld defects
The common defects encountered in the orbital TIG welding are: Porosity is a collective name describing cavities or pores caused by gas and non-metallic material entrapment in molten metal during solidification. There are many causes which include contamination, inadequate shielding, unstable arc, too short an arc gap and poor welding technique in general. Cracking discontinuities are some of the most dangerous, especially if they are subject to fatigue loading conditions. There are several different types of cracks and none are desired. They must be removed by grinding back (if superficial) or repaired by welding. Cracks can occur in the weld itself, or the base metal, or the heat affected zone (HAZ). Inclusions are generated by extraneous material such as slag, tungsten becoming part of the weld. These defects are often associated with undercut, incomplete penetration and lack of fusion in welds. Insufficient cleaning between multi-pass welds and incorrect current and electrode manipulation can leave slag and infused sections along the weld joint. This defect can only be repaired by grinding down or gouging out and re-welding. Incomplete fusion or lack of fusion that is difficult to detect and evaluate, is very dangerous. It occurs when the weld metal does not form a cohesive bond with the base metal or when the weld metal does not extend into the base metal to the required depth, resulting in insufficient throat thickness. Total thickness of the weld is less which reduces the strength of orbital TIG welding. Excess penetration occurs due to excess current and low rpm or low travel speed of the electrode which makes more penetration in the weldment causing reduction of internal diameter of the feed lines. Pin hole is formation of a hole in the weldment due to above said reason i.e excess current, low rpm. In addition, excess internal shield gas also can cause pin hole. Since the portion of the weld pool is open, it leads to leakage of propellant in the feed system. Mismatch or misalignment of tubes is due to improper butting or different thickness of the joining feed lines tubes. This also causes run out.
3.5. OTIG process in propulsion systems
3.5.1. Welding process flow chart The Orbital TIG welding process offers a large range of
benefits in welding of propulsion plumb lines. The Flow Chart for the initial specimen qualification for Orbital TIG welding is carried out as per the flow chart shown in Fig.4. Main parameters like Current, Revolution per minute (RPM). Gap between electrode and job, shield gas, edge preparation and cleanliness are deciding the sound welding of the specimen. The various test methods like visual inspection, dye penetrant, pressure hold test, MSLD leak check, X-ray and bend test are satisfactory, and quality clearance is given for carrying out the actual welding in the satellite. After the welding of feed lines in the spacecraft number of tests are reduced due to practical difficulties.
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Definition: Orbital TIG Welding
Objective: Efficient weld on s/c
Definition of weld defects
Setting/Adjusting Parameters
Lack of penetration
Excess penetration
Porosity
Inclusion
Crack
Run out
Current
Edge preparation
RPM
GAP:Tube Vs electrode
Shield Gas flow
Cleanliness Perform weld for specimen
Testing specimen
No
Visual inspection Pressure test
MSLD X-Ray
No
Ready for assembly
Figure 4. Flow chart for orbital TIG welding for satellite components
Visual inspection Dye-penetrant test (Pressure hold or
condition monitoring) MSLD X-Ray
Bend test
Start
Yes No
Yes
Performing weld on s/c
Testing
End
Rework
Cutting
Squaring
ng
Cleaning
ng
Yes
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3.6. Experimentation
Figure 5. Various current sequences
Initially bead-on-tube trials were done. Then, specimen level trials were carried out. Once the tentative ranges of the parameters were identified, experiments were planned for more influencing welding parameters at different levels. One such parameter i.e. welding current, ranging from 17.8 Amps to 18.9 Amps in steps of 0.1 Amp is detailed here. In each level, five specimens were tested and averaged. It is experimentally proved that the specimens with less current are showing lack of penetration and specimen with high current have resulted in excess penetration reducing flow path or forming pin hole and got damaged. Fig.5 shows that specimen welded with 10% less than optimum current (17.8 to 18.1Amps). optimum current range (18.2 to 18.5Amps) and 10% more than optimum current (18.6 to 18.9Amps). Fig. 6 shows that the specimen welded are in three ranges. That is, 17.8, 17.9, 18.0, 18.1, 18.2, 18.3, 18.4, 18.5 and 18.6, 18.7, 18.8, 18.9.
The results indicated that optimum current level is 18.35 Amp (middle value of 18.2 to 18.5). optimum RPM is 10 and the gap between electrode and tube is 0.8mm.
Figure 6. Current versus penetration level
A. Lack of penetration(Less Current) B. Perfect penetration (Optimum Current) C. Excess penetration (Excess Current)
4. Testing methods
Effective process parameters provide defect free weld with adequate strength. This calls for optimization of weld parameters and subjecting to extensive testing to evaluate the soundness of welds. Though there are many techniques available for testing these welds, some of the destructive test and non-destructive tests are used enabling evaluation of soundness of the weldments.
Visual Inspection is probably the most widely used among the non-destructive tests. It is simple and easy to do before and after any destructive and non-destructive tests.
All kinds of major surface defects can be found out by visual inspection, bend test are used for specimen before actual welding in the spacecraft. Pressure tests, leak tests and tensile tests evaluate the strength and integrity of the weld joints.
The following section explains each testing method in depth.
Types of Inspection The types of inspection followed for space application are
Nondestructive examination (NDE) and Destructive examinations consist of.
• Visual test (VT) • Dye penetrant test • Hydrostatic test • Pressure hold or Pressure decay tests • MSLD test • X-ray radiographic test (RT) • Tensile tests • Bend tests • Grey value test
Visual inspections or Visual test (VT) are performed to determine whether a weld meets expectations. Depending on the final use of the weldment, several types of inspections may be required, ranging from simple visual inspection to rigorous testing. Inspections and tests of a weld that do not destroy any portion of the completed weld are called nondestructive examination (NDE). Inspections and tests that destroy the completed weld, or samples of the completed weld, are called destructive tests.
4.1. Visual Test
A visual test (VT) or visual inspection is one of the most important methods of inspection and is widely used for acceptance of welds. VT is also used to identify weld defects. It is easy to apply, quick which says go or not go, and relatively very cheap. Visual testing equipment includes rulers, fillet weld gauges, squares, magnifying glasses, torch light, balls less than internal diameter of the tube and reference weld samples which are called specimen. It provides very important information about a weld’s general conformity to specifications. The weld features which are measured such as crown height, crown profile, weld size, weld length, dimensional variation, root side profile, root side penetration, surface color. In addition, a visual test may reveal discontinuities in the weld. A discontinuity is any disruption in the consistency of a weld. A flaw in the weld is a discontinuity that is unacceptable. A defect is a discontinuity that is serious enough to make the weld unacceptable. The common problems can be detected by visual tests are Under fill, Undercut,
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Overlap, Surface cracks, crater cracks, Surface porosity, joint mismatch, run out etc.
4.2 Dye Penetrant Test
A penetrant test (PT) is an easy and sensitive method of detecting and locating minute discontinuities that are open to the surface of the weld. Fig. 7 shows the different stages of dye penetrant test. The feed lines or tube which have joints by welding are cleaned by IPA (Isopropyl alcohol) well and dried before applying penetrant (stage.1 is before cleaning and stage.2 is after cleaning). A penetrating liquid (dye) is applied over the surface of the weld (stage.3). The dye then enters the discontinuity. After a short period of time, the excess penetrant is removed from the surface once again by IPA (stage.4). A developer is applied to the surface and allowed to dry (stage.5). The penetrant in the discontinuity or crack rises to the surface by capillary action, making the discontinuity or crack easy to see. After identifying the location of defect, tubes are cleaned by IPA (stage.6). A penetrant test is particularly useful on nonmagnetic materials like stainless steel and titanium alloys, where a magnetic particle test cannot be used. The penetrant is sprayed around all the weld areas. Penetrant works by capillary action and will identify any weld discontinuities or defects. Raied et al., in his research used liquid penetration methods which helped to detect fine defect and cracks opening to surface that cannot be detected by visual inspection.
The two types of penetrant tests are dye penetrant and fluorescent penetrant tests. A dye penetrant test requires the surface of the weld to be sprayed generously with penetrant and allowed to soak for a specified time.
Figure 7. Different stages of DP test
Excessive penetrant is then removed with an aerosol cleaner. All of the penetrant is then wiped from the weld area. After the penetrant is removed, the developer is applied. The developer is a powdery white substance that is lightly applied from an aerosol can. Any imperfections in the weld will hold the dye and bleed through the white developer, identifying the problem. A dye penetrant test can be done anywhere because it is portable, and it can be done in any position. The results can be detected in normal light, without the use of special equipment.
4.3. Hydrostatic test
When conducting a pressure test, one has to be alert to the possible failure of the unit. For hydrostatic test liquid media is employed (De-mineralized water or compatible oil). Before beginning the test, the detailed test procedures are made and reviewed by an appropriate committee to make sure that the test procedure ensures the safety of everyone in the testing area. Fig. 8 shows the hydraulic leak test set up where plumb lines (feed lines) having weldment is filled with water or by de-mineralized water or compatible oil and pressurized to one and half or two times of the MEOP (mean effective operating pressure) and held for three minutes as proof test and bring back to the MEOP. Further it is held for leak test. The pressure is monitored by a manual pressure gauge or pressure transducer as shown in Fig. 9.
The pressure decay gives the idea about the leak rate. This is done only lab level and not in the spacecraft level as feed lines are connected to the propulsion components which are intricate in shape and removal of water from them is not easy and leads to complication. Similarly ultrasonic test is also recommended only in lab level and not recommended for satellite level for the same reason.
Figure 8. Hydraulics leak test
Figure 9. Welded specimen vertically fixed
Pressure tests subject to a piping, or tubing to internal pressure by means of air or gaseous nitrogen or gaseous helium which are compatible for space application are called pneumatic test. The test program may require a number of cycles to be performed, simulating the use of the feed lines in actual service in satellite. The numbers of cycles are lesser than hydraulic test. During the test, the part or feed line tubes will expand. This expansion should not be restricted, or undue stresses will build within the part. Before beginning the test, the detailed test procedures are made and reviewed by appropriate committees to make sure the test procedure ensures the safety of everyone in the testing area. In pneumatic test, plumb lines (feed lines) having weldment are filled with gaseous nitrogen or helium and pressurized to one and half times of the MEOP maximum and held for three minutes as proof test and bring back to the MEOP. Further it is held for leak test. The pressure is monitored by a manual pressure gauge or pressure transducer. If the leak test is conducted for the considerable amount of time, it is called pressure hold test to quantify the leak rate. Feed system with leak, with standard leak and combination of both are shown in Fig. 10. The pressure decay gives the idea about the leak rate. The leak rate is calculated by following formula:
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Figure 10. Pressure profile during leak test
4.4. Helium mass spectrometer leak detector test:
Figure 11. GHe MSLD
A Helium Mass Spectrometer Leak Detector (MSLD) is an instrument commonly used to detect and locate small leaks. It was initially developed to find extremely small leaks in the gas diffusion process. Helium leaks out of the container, and the rate of the leak is detected by a mass spectrometer. Helium is used as a tracer because it penetrates small leaks rapidly due to helium molecular weight being very low.
Helium has also the property of being non-toxic, chemically inert and present in the atmosphere only in minute quantities (5 ppm). Typically a helium leak detector shown in Fig. 11 will be used to measure leaks in the range of 10−5 to 10−12 Pa·m3·s−1 or Scc/ sec. This method requires the welded feed lines of spacecraft to be tested to be connected to a helium leak detector. The outer surface of the feed lines to be tested will be located in some kind of a tent in which the helium concentration will be raised.
Figure 12. Leak Testing with Tracer Gas (Helium)
During the test the entire spacecraft is in a clean room which
is as per federal standard 209 B. If leakage is encountered, the small molecules of helium will migrate through the cracks into the part. The vacuum system will carry any tracer gas molecule into the analyzer cell of the magnetic sector mass spectrometer. A signal will inform the operator of the value of the leakage encountered is shown in Fig. 12.
4.5. Local helium spray - vacuum test:
This method is a small variation from the one above. It still requires the feed lines to be tested is connected to a helium leak detector. The outer surface of the feed lines which contains weldments to be tested is sprayed with a localized stream of helium tracer gas. If leakage is encountered, the small molecules of helium will migrate through the cracks into the part. The vacuum system will carry any tracer gas molecule into the analyzer cell of the magnetic sector mass spectrometer. A signal will inform the operator of the value of the leakage encountered. Thus correlation between maximum leakage signal and location of helium spray head will allow the operator to pinpoint the leaky area. Prior to connecting a feed system of the spacecraft, MSLD calibration is carried out using standard calibrator. Similarly after the test also, system is detached from feed lines and once again it is validated using standard calibrator.
4.6. Pressure hold or pressure decay test (Condition monitoring)
Satellite propulsion components like Propellant tanks, Helium gas bottles feed line tubes and all propulsion components are functional tested and qualified for the required application in component level after the manufacturing. However, important components like propellant tanks, Gas bottles and feed lines after welding are once again pressure tested after integrating in to the spacecraft is called pressure hold test in the system level. To ensure the gross leak if any due to handling during integration or
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transportation the pressure hold test is carried out for minimum of 96 hours in the clean room (where all propulsion components are integrated) as well as launch pad. Fig.13 and Fig. 14 show the pressure hold set up of propulsion system after integration in to the satellite covered with blast shield (Protective shield) around the satellite to protect the human being as well as Pressurization and depressurization unit (PDE unit) during pressurization of propellant tanks and gas bottles.
Figure 13. Pressure hold test set up - satellite side
Figure 14. Pressure hold test set up - PDE side
The pressure is increased in steps to mean effective operation pressure (MEOP) and further to proof pressure of 1.25 times of the MEOP. It is held only for 3 minutes and is brought down to the MEOP to carry out the leak test using MSLD. After ensuring that there is no leak, entire system is held for 96 hours (four days) for the gross leak check. At the end of the pressure hold test, once again system verified for the integrity and the system is depressurized to the safe mode positive pressure.
4.7. Destructive Tests/ Bend test:
Destructive tests are used for welder qualification and certification, as well as welding procedure qualification. In large production runs, destructive tests are often made by pulling apart sample units or specimen. It is often less expensive to scrap a part to make a destructive quality test than to test the parts using more expensive nondestructive tests. Bend tests are used to determine internal weld quality. In bend tests, a weldment is sliced into test strips, called coupons or specimen. The welded tube is cut longitudinally in the middle of the tube and bent as shown in the Fig. 15. This process stretches the weld to test the weld’s integrity. There is a machine to perform this bending and
the machine bends the prepared test coupon into a U form over a specified radius, which is dependent on the thickness and strength of the material. After bending, the outer surface and the inner surface of the U are checked for cracks and other indications as required by the weld inspection criteria.
Figure 15. Weld specimens after the bend test
The outer face of the bend may be examined by a visual, penetrant, to detect defects such as cracks, lack of fusion, and lack of penetration etc. Bend test specimens shall be prepared and tested in accordance with ISO 5173/5177 or ASME IX and the acceptance criteria shall be according to ASME IX.
4.8. Tensile Test:
Tensile tests are used to compare the weldment to the base metal mechanical values and specification requirements. The weldment is sliced into coupons or specimen, and then each end of the coupon is pulled in opposite directions until the coupon fails (breaks). A tensile test machine is shown in Fig. 16. Tensile tests are made to determine ultimate strength of the weld. This is the point at which the weld fails under tension. The yield strength of the weld is the point at which the weld yields or stretches under tension and will not return to its original dimensions. Elongation is the amount of stretch that occurs during the tensile test. It is measured by placing gauge marks on the sample specimen or coupon before testing and comparing the after-break distance with the original gauge marks. The various specimens and the welded tubes were tensile tested to find the weld strength. Fig. 17 shows the tensile test completed feed line specimen to evaluate the weld strength for the respective loads for an experiment.
Figure 16. Tensile Test by Universal testing Machine
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Figure 17. Specimen after Tensile test
4.9. X-Ray Radiographic test:
The X-Ray Radiography (RT) test is carried out as a complementary Non Destructive Test (NDT) for propellant feed line of the satellite propulsion system. It is a nondestructive method that reveals the presence and nature of discontinuities in the interior of welds. The visual inspection and X-ray radiography tests are conducted on specimen welds realized with various parameter combinations.
The visual inspection of the welds is the best technique to inspect the surface discontinuities. However it is inadequate to qualify the weld for flight since it cannot detect internal defects. It was found that although all the welds passed the visual test and appeared as defect-free welds, the X-ray radiographic testing technique successfully detected the internal defects present in the weld and it can be said that X-ray radiography is appropriate for internal discontinuities like porosity, Lack of Fusion, Lack of Penetration, cracks, voids, inclusions etc.
The spacecraft propulsion system feed lines of 6mm and 10mm diameter join various propulsion system components with plumb lines. For these weld inspection, Radiography using X-rays is used. Since welds are opaque to ordinary light, X-rays being a form of electromagnetic radiation that penetrates most materials are used to reveal internal flaws.
The X-ray radiography test, which is similar to photography, an X-ray machine located in a fixed location transmits X-rays through the propellant feed lines being tested. A film is kept on the other side of the feed lines and it gets exposed by the X-rays that pass through the test material. Any defects or inconsistencies in the weld will change the amount of X-rays due to attenuations and differential absorption of radiation. If a high density inclusion is present, fewer X-rays pass through those locations; they look white in the developed film or display. On the contrary, a pore in the weld will appear little darker.
The area surrounding the X-ray machine needs to be lead-shielded to prevent the radiation leakage, a portable X- ray machine is used for in-situ radiography of satellite plumb line welds application as the locations are not in a plane and located all around satellite.
In order to effectively capture the defects in the weldments, an innovative fixturing method was evolved for this particular application. The advantage in this method is to make an X-Ray fall perpendicular to the welded tubes kept in the fixture and also travel the same distance to give uniform density level for comparison. Fig. 18a shows that the conventional method of X-ray keeping the film in horizontal and Fig. 18b shows the X-Ray
film kept in a fixture made of ∅1000 mm arc where the intensity of light incident on the film is uniform for the all the weld pieces kept on the film.
Figure 19a. Standard fixture Figure 19b. Modified fixture
Figure 20. Specimens tubes on fixtures with serial numbers
Radiography density is a measure of the extent of film darkening. It is the ratio of logarithm of two measurements namely the intensity of the light incident on the film (Io) and the intensity of light transmitted through the film (It)
O.D =Log10 {(Io) / (It)}
Where,
O.D = Optical Density
Io = intensity of light incident on the film,
It = intensity of light transmitted through the film
Contrast within a film increases with increasing density and hence higher the density, it is better. To ensure higher density, it is necessary that the X-Rays travel uniformly the same distance. Film Density is measured with a densitometer which has a photo electric sensor.
The number of photons reaching the film determines the film density which is the function of intensity of radiation and the time exposure. The film characteristic curve depicts the relationship between the exposure and film density. In our case, if the standard fixture is used as shown in the Fig. 19a the
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radiography density obtained is not truly representing the actual density had the incidence been similar for all tubes.
Hence this inadequacy is rectified by employing the modified fixture as shown in the Fig. 19b to ensure that the film density observed for each tube is under identical condition. This improvement has helped to effectively evaluate the radiographic interpretations.
The optical density specification for the feed line is 2.6±10%. In the present method of testing 18 out of 27 specimen are only meeting the required specification which are within allowable tolerance error. The remaining 9 specimen measurements are beyond the specification as tabulated in the Table 2. The Fig. 21 shows 135 welded specimens after the X-ray.
Figure 21. Welded specimen’s X-ray.
Table 2. Optical density details for standard fixture and modified fixture
Components identification
Optical Density for standard (horizontal) fixture
% of variation in Optical Density
Optical Density for Modified (curved) fixture
% of variation in Optical Density Remarks
1 2.22 14.6 2.61 0.38
Optical density=log10(Io)/log10
(It)
Where Io = intensity of light incident on the film, It = intensity of light
Table 2. gives clear idea about the difference between Optical densities obtained for standard fixture and modified fixture. Specimen’s tubes on fixtures with identification numbers are shown in Fig. 20.
The improved or innovative method suggested (modified fixture) in this paper for measuring the optical density is considerably reducing the variation well within specification as shown in the Table 2 where all the 27 specimen are within specification. The maximum variation in this method is 1.15% only.
Fig. 22 shows that the X-Ray Gun consists of a small machine which is focused at the centre of the pipe to X-ray each weld. In Fig. 22 (Fig-A) shows a typical X-Ray mobile gun used for radiography in spacecraft level as it is flexible to move around a
spacecraft. As per the specification, gun to film distance of one meter length is required for X- Ray purpose. The specification is X-Ray 160 KV machine. Since numbers of specimen are more, the incident angle of rays is not same for all the tubes which are arranged side by side as per Fig. 22 (Fig. A). For example, for the centre tube alone, the incident beam is normal and of one meter length and varies when goes to the side. This incident angle increases the distance from 1m to 1.025 meters. In order to meet the incident angle as same for all the tubes, a fixture is specially designed as shown in Fig. 22 (Fig. B) for the arc of 1000mm diameter such that the incident angle is same for all the tube which is shown in Fig. 19b also. Hence the results obtained are more accurate. This technique eliminated the correction to be incorporated for oblique angle incidence, and hence variation due to oblique incidence is nullified.
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Figure 22. X-Ray radiography
Figure 23. Less penetration due to current less than optimum
Figure 24. Optimum penetration due to optimum current
The automatic film-processing unit is used to process the X-ray films. The X-ray radiographic exposure time on the typical spacecraft propulsion system is in the order of few seconds or mille seconds.
4.10. Grey value profile
Grey value profile: Fig. 23, 24 and 25 show that the grey value line profiles of 10% less than optimum current, optimum current and 10% excess of optimum current respectively. X- axis shows the distance along line of scanning & Y-axis shows the grey values which is proportional to thickness weld penetration scanned by X-ray. More thickness of penetration is less the grey value and vice versa. Fig. 23 show the penetration without weld crown. Fig. 24 show optimum penetration with crown. Fig.25 show more penetration and hole formation due to excess penetration (profile goes up and down). Hence grey value profile predicts the soundness of the welding.
Figure 25. Excess penetration due to current more than optimum
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5. Conclusion
The various tests have been discussed in the paper are highly useful tool for exploring a high efficient welding for propellant feed system of any spacecraft. The aim of the authors is to state the practical problem of the weld quality, to point to the possible diagnostic techniques and to notice possible theoretical and practical ways to explain the change of the weld conditions.
The challenge lies in properly adapting exact testing methods or technique in system level test limits of the test's usefulness. This study also indicates different failure mechanism and different condition monitoring testing techniques which can be adapted in spacecraft level.
Feed system welding is not required a high skilled welder as automatic welding is adapted, however, only the testing methods ensure the welding integrity thereby the system meets specific requirement of orbit life.
Visual inspection is very simple, fast method and cheap which provides basic information before proceeding further tests. Dye penetrant test the safest which is good for surface crack and porosity. Pressure test and MSLD are quantitative which clearly ensures life of the spacecraft. X-ray radiography is the complementary which gives shape and size of the defect. In this paper gray values is also additional complementary which increases the confidence level. Bend tests method ensures strength of the welding and aid for fixing the parameters there by uniform welding quality is guaranteed by controlling welding current, RPM etc.
The global need of stainless steel tubes increases steadily for aerospace applications. Due to improved and cost-efficient production technologies, more than 80% of the tubes are today welded. Quality assurance systems have been developed where all welding procedure specification and welders qualification, the material and weld properties are certified with established test methods.
Acknowledgment
Authors wish to thank, Shri. M.Kumaran, Shri S.Ravi, Shri V.N. Misale, Shri D.Saravana kumar and team, and Dr. Heeralal Gargama, Dr.D.P.Sudhakar, Smt. Vidhya Karthikeyan for their useful contributions in this work. Authors wish to express their gratitude to Shri G. Narayanan, Deputy Director, SRQA, Shri.S. Somanath Director, LPSC and ASC Committee for their constant encouragements and kind permission to submit this paper in any technical journal.
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