Evaluating Safe Hydrotest Pressure for the Cold Tapping of Branch Pipe This article explains the problems and solutions for the problem of determining the safe hydro test pressure in case of cold cutting and the approaches to increase the limits of safe hydrotest pressure.
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T E C H N O L O G Y Cold Tapping
ot tapping of branches is a common occurrence in hydrocarbon industry. The hydro test pressure to test the integrity of the branch branc h weld for hot ho t tap has h as been bee n defined def ined under API 2201(Clause no 10.5). However cold taps are sometimes required when for a new installation, the line has been already hydro tested and some new branch requirements have to be made before commissioning of the line. For this, first the branch stub is welded. Local external hydrotesting of branch stub is done to ensure integrity of the branch weld without subjecting the whole of the already hydro tested header. After successful completion of the local hydro test, the header wall pertaining to the branch is tapped (cut) and a tested branch created without re-hydrotesting the complete header. This saves the construction effort as otherwise the already tested and complete header loop would become required to be included as part of hydrotesting resulting in wasteful re-hydrotesting effort. The problem with this cold tap is that the test pressure is governed by codes like B31.3 which makes the test pressure quite higher compared to what would have been if API 2201 was followed like that for HOT tapping. The solution for the problem of determining the safe hydro test pressure in case of cold cutting has been addressed in this technical paper. Also explained are the approaches to increase the limits of safe hydrotest pressure. In case of cold tapping the effect of internal
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pressure as in hot tap will not be there, thus further decreasing the ability of the run pipe to withstand external pressure i.e. resistance to buckling. So there is a need for a proper stress condition evaluation method to ascertain whether the hydrotest pressure employed for cold tapping is safe safe .
Problem Description. In case of hot tapping, per API 2201 (Safe Hot Tapping Practices in the Petroleum & Petrochemical Industries), with reference to the Article 10.5 (i.e. testing the weld and hot tapping machine): “The test pressure should be at least equal to the operating pressure of the line or vessel to be tapped. The test pressure should not exceed the present internal pressure by more than approximately 10 %, in order to avoid possible internal collapse of the pipe or vessel wall.” So if there is a header with operating pressure of say for example 4.0 Kg/cm2g Thus, Allowable test pressure for hot tap is 4.0 kg/ cm2 + 10 % of 4.0 Kg/ cm2 = 4.4 Kg/cm2. However, In case of cold tap, the test pressure are governed by codes like B31.3 and generally has Test pressure values on higher side say 6.0 Kg/cm 2 (Assuming no Stress correction in this specific case) than that by API 2201. During hydrotesting of the branch pipe the case of cold tapping situation is worse than hot tapping. The reason is since the header is experiencing no internal pressure during cold cutting the bulging due to hydro test pressure will be severe in compariso n to the hot tapping. tapping.
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For evaluating safe hydrotest pressure in case of will evaluate whether the Hydro test pressure as cold tapping, the finite element analysis approach given in the branch pipe as per the LDT i.e. 6 Kg/cm2 with use of ANSYS is being applied herein to check is acceptable or not for Cold Tapping. Snapshots are whether the hydrotest pressure as given in the branch being presented herein to show the results of the pipe as per the Line Designation Table (LDT) i.e. 6 analyses without reinforcement and also with the Kg/cm2 is acceptable or not for Cold Cutting Case. internal reinforcement.
Solution Approach Finite Element Analysis approach with ANSYS was adopted for evaluating the safe Hydro test pressure as otherwise it will be difficult to evaluate the localized yielding which occurs during the test. In this two cases for comparative study being taken into consideration. First is without providing any reinforcement inside the branch stub and second is with provision of reinforcement inside the branch stub. With results it will clear that with provision of reinforcement the blank deflection can be reduced to some extent and also corresponding stresses. Reinforcement can be provided by welding a pipe stub having smaller Outside diameter then the Branch stub on the Coupon portion or by another means. After testing during cutting of the header wall for creating the branch, the short pipe stub anyway gets removed.
Figure 1. Close up of Max Stress Affected Area (Cold Tap without Reinforcement)
Input Data. Max. Operating Pressure in Header pipe: 4 Kg/cm2 (In case of cold cutting Header pipe will have no internal pressure) Young’s modulus: 18525 Kg/mm 2 Poisson’s ratio: 0.3 Hydro test pressure of branch pipe as per the LDT: 6.0 Kg/cm2 Header Pipe to be tested is 30” nominal diameter, Material used A 671 CB 60 & 12.7mm thickness, corrosion allowance for header 1.5875 mm, thus effective schedule is 12.7-1.5875=11.1125 mm & Stub Branch is 20” Nominal diameter, Material Figure 2. Close up of Max Stress Affected Area (Cold Tap with Reinforcement) used A671CB60 & 9.525 mm thickness, Corrosion allowance for branch 1.5875 mm, Thus Having a closer look at the results of FEA analysis effective schedule is 9.525-1.5875=7.94 mm, For in- it can been easily depicted that generate Stress value ternal reinforcement 10 “pipe stub is used. in case of without reinforcement (8.864 Kg/ mm2) is Now with the use of Finite element tool ANSYS we higher than one generated during with reinforce-
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Figure 3. Close up of Displacements (Cold Cutting without reinforcement)
Figure 4. Close up of Displacements (Cold Cutting with reinforcement).It can be seen the buckling is having lower magnitude as comparison to Cold cutting without reinforcement 2
Figure 5. Plot of Membrane stress along with total stress across the Maximum stress affected zone (Cold cutting without reinforcement)
Figure 6. Plot of Membrane stress along with total stress across the Maximum stress affected zone (Cold cutting with reinforcement)
ment (8.262 Kg/mm ).So with provision of reinforcereinforcement(4.855 Kg/mm2) is higher than one ment the generated stress values drop. 2 Having a closer look at the results of FEA analy- generated during With reinforcement. (4.3 Kg/mm ). sis it can been easily depicted that generated So with provision of reinforcement the generated displacements due to hydro pressure in case of with- membrane stress values drops. FEA (ANSYS) Evaluation: From FEA analysis it out reinforcement(0.63 mm) is higher than one generated during With reinforcement.(0.546 mm). can be seen that both Von Misses as well as MemSo with provision of reinforcement the generated brane stresses are as follows: displacement values drops. Induced stresses Allowable From the results seen dur(At 0.06Kg/mm2 test pressure) ing generated membrane in Kg/mm2 stress across the Maximum Von Misses 8.864 (No reinforcement)/ 30 Kg/mm2(3*Sa) (As per stress affected zone it can be 8.262 (With Reinforcement) the BPVC Sec 8 Div 2 MA 4) easily depicted that generated Membrane 4.855 (No reinforcement) / 10 Kg/mm2(1*Sa) (As per membrane stresses due to hy4.3 (With reinforcement) the BPVC Sec 8 Div 2 MA 4) dro pressure in case of without
Cold Tapping
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In the detailed FEA analysis the key results deduced are:A) The generated stress for both in case of the Cold Cutting without reinforcement & Cold Cutting with reinforcement is compared with the allowable stresses and are found within the allowable. Thus the presumption of wall collapse as a result of higher test pressure can be safely and realistically evaluated with FEA. Also it has been established that with provision of internal reinforcement to Coupon during hydrotest the generated stresses & displacements can be further reduced. B) Also the Analysis for the requirement for any branch/header reinforcement can be done with more accuracy and ease taking into accounts the key inputs in a single model thus evaluating the overall effect of the different inputs.
Conclusion Two conclusions can be easily depicted with the available results. They are : 1. In case of Cold Taps As seen from the FEA results for Cold cutting with and without reinforcement the generated stresses are well within the allowable thus hydro test at 6.0 Kg/cm2 is safe from design point of view. The evaluation of safe hydro test pressure can be done easily by usage of FEA approach. 2. With usage of internal reinforcement the blank yielding as well as the generated stresses can be reduced. This approach can thus be used effectively in case if a generated stress exceeds the permissible limits. This technique can be used for assessment of safe hydro test pressure in line with code requirement for branches without involving the already tested header. Also in case of Problem, how the pressure enduring capability can be increased has been explained. HA
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This publication thanks Mr. M G Choudhury, Mr. Ameet Kulkarni and Mr. Vishal Ojha for providing this article. M G Choudhury is Sr Vice President & head of Piping Engg of RPTL Engg Ltd. He has around forty years of working experience in Piping Design Engg ,especially in pipe stress. He has also worked for Companies like EIL, TOYO, CHEMTEX and SABIC. Ameet Kulkarni is a Mechanical Engineering graduate from Visveshwariya Technical University (India) and has been associated with Piping Stress analysis in RPTL-EDMEC (Formerly BecRel engineering Pvt. Ltd.), Mumbai (India). Recently, he was involved with pipe stress analysis of various Reliance Plants. Vishal Ojha has been associated with Piping Stress analysis in RPTLED-MEC (Formerly BecRel Engineering Pvt Ltd), Mumbai (India). He is a Mechanical Engineering Graduate from Government Engineering College, Udaipur, Rajasthan. He has been working with RPTL-EDMEC for 4 Years and was also associated with pipe stress analysis of various plants.
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