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ColdTapping

TECHNOLOGY

Evaluating Safe HydrotestPressure for theCold Tapping of Branch Pipe

H ot tapping of branches is a common occur-rence in hydrocarbon industry. The hydrotest pressure to test the integrity of thebranch weld for hot tap has been defined

under API 2201(Clause no 10.5).However cold taps are sometimes required

when for a new installation, the line has beenalready hydro tested and some new branch re-quirements have to be made before commission-ing of the line. For this, first the branch stub iswelded. Local external hydrotesting of branchstub is done to ensure integrity of the branchweld without subjecting the whole of the al-ready hydro tested header. After successful com-pletion of the local hydro test, the header wallpertaining to the branch is tapped (cut) and atested branch created without re-hydrotestingthe complete header. This saves the constructioneffort as otherwise the already tested and com-plete header loop would become required to beincluded as part of hydrotesting resulting inwasteful re-hydrotesting effort.

The problem with this cold tap is that the testpressure is governed by codes like B31.3 whichmakes the test pressure quite higher compared towhat would have been if API 2201 was followedlike that for HOT tapping.

The solution for the problem of determiningthe safe hydro test pressure in case of cold cut-ting has been addressed in this technical paper.Also explained are the approaches to increasethe limits of safe hydrotest pressure.

In case of cold tapping the effect of internal

This article explains the problems and solutions for the problem of determiningthe safe hydro test pressure in case of cold cutting and the approaches to increasethe limits of safe hydrotest pressure.

pressure as in hot tap will not be there, thus furtherdecreasing the ability of the run pipe to withstandexternal pressure i.e. resistance to buckling.

So there is a need for a proper stress conditionevaluation method to ascertain whether the hydrotestpressure employed for cold tapping is safe .

Problem Description.In case of hot tapping, per API 2201 (Safe Hot

Tapping Practices in the Petroleum & PetrochemicalIndustries), 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 theoperating pressure of the line or vessel to be tapped.The test pressure should not exceed the presentinternal pressure by more than approximately 10 %,in order to avoid possible internal collapse of the pipeor vessel wall.”

So if there is a header with operating pressure ofsay for example 4.0 Kg/cm2g

Thus, Allowable test pressure for hot tap is4.0 kg/ cm2 + 10 % of 4.0 Kg/ cm2 = 4.4 Kg/cm2.

However, In case of cold tap, the test pressureare governed by codes like B31.3 and generally hasTest pressure values on higher side say 6.0 Kg/cm2

(Assuming no Stress correction in this specific case)than that by API 2201.

During hydrotesting of the branch pipe the caseof cold tapping situation is worse than hot tapping.The reason is since the header is experiencing nointernal pressure during cold cutting the bulgingdue to hydro test pressure will be severe in com-parison to the hot tapping.

ColdTapping

HYDROCARBON ASIA, JUL-SEP 2011 53

For evaluating safe hydrotest pressure in case ofcold tapping, the finite element analysis approachwith use of ANSYS is being applied herein to checkwhether the hydrotest pressure as given in the branchpipe as per the Line Designation Table (LDT) i.e. 6Kg/cm2 is acceptable or not for Cold Cutting Case.

Solution ApproachFinite Element Analysis approach with

ANSYS was adopted for evaluating thesafe Hydro test pressure as otherwise itwill be difficult to evaluate the localizedyielding which occurs during the test. Inthis two cases for comparative study be-ing taken into consideration. First is with-out providing any reinforcement insidethe branch stub and second is with provi-sion of reinforcement inside the branchstub. With results it will clear that withprovision of reinforcement the blank de-flection can be reduced to some extentand also corresponding stresses. Rein-forcement can be provided by welding apipe stub having smaller Outside diam-eter then the Branch stub on the Couponportion or by another means. After test-ing during cutting of the header wall forcreating the branch, the short pipe stubanyway gets removed.

Input Data.Max. Operating Pressure in Header pipe:

4 Kg/cm2 (In case of cold cutting Headerpipe will have no internal pressure)

Young’s modulus: 18525 Kg/mm2

Poisson’s ratio: 0.3Hydro test pressure of branch pipe as

per the LDT: 6.0 Kg/cm2

Header Pipe to be tested is 30” nominaldiameter, Material used A 671 CB 60 &12.7mm thickness, corrosion allowancefor header 1.5875 mm, thus effective sched-ule is 12.7-1.5875=11.1125 mm & StubBranch is 20” Nominal diameter, Materialused A671CB60 & 9.525 mm thickness,Corrosion allowance for branch 1.5875 mm, Thuseffective schedule is 9.525-1.5875=7.94 mm, For in-ternal reinforcement 10 “pipe stub is used.

Now with the use of Finite element tool ANSYS we

will evaluate whether the Hydro test pressure asgiven in the branch pipe as per the LDT i.e. 6 Kg/cm2

is acceptable or not for Cold Tapping. Snapshots arebeing presented herein to show the results of theanalyses without reinforcement and also with theinternal reinforcement.

Having a closer look at the results of FEA analysisit can been easily depicted that generate Stress valuein case of without reinforcement (8.864 Kg/ mm2) ishigher than one generated during with reinforce-

Figure 2. Close up of Max Stress Affected Area (Cold Tap with Reinforcement)

Figure 1. Close up of Max Stress Affected Area (Cold Tap without Reinforcement)

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ColdTapping

ment (8.262 Kg/mm2).So with provision of reinforce-ment the generated stress values drop.

Having a closer look at the results of FEA analy-sis it can been easily depicted that generateddisplacements due to hydro pressure in case of with-out reinforcement(0.63 mm) is higher than onegenerated during With reinforcement.(0.546 mm).So with provision of reinforcement the generateddisplacement values drops.

From the results seen dur-ing generated membranestress across the Maximumstress affected zone it can beeasily depicted that generatedmembrane stresses due to hy-dro pressure in case of without

reinforcement(4.855 Kg/mm2) is higher than onegenerated during With reinforcement. (4.3 Kg/mm2).So with provision of reinforcement the generatedmembrane stress values drops.

FEA (ANSYS) Evaluation: From FEA analysis itcan be seen that both Von Misses as well as Mem-brane stresses are as follows:

Figure 4. Close up of Displacements (Cold Cutting with reinforcement).It canbe seen the buckling is having lower magnitude as comparison to Coldcutting without reinforcement

Figure 3. Close up of Displacements (Cold Cutting without reinforcement)

Figure 6. Plot of Membrane stress along with total stress across theMaximum stress affected zone (Cold cutting with reinforcement)

Figure 5. Plot of Membrane stress along with total stress across theMaximum stress affected zone (Cold cutting without reinforcement)

Induced stresses Allowable(At 0.06Kg/mm2 test pressure)

in Kg/mm2

Von Misses 8.864 (No reinforcement)/ 30 Kg/mm2(3*Sa) (As per8.262 (With Reinforcement) the BPVC Sec 8 Div 2 MA 4)

Membrane 4.855 (No reinforcement) / 10 Kg/mm2(1*Sa) (As per4.3 (With reinforcement) the BPVC Sec 8 Div 2 MA 4)

HYDROCARBON ASIA, JUL-SEP 2011 55

Enquiry Number 07/09-02HA

In the detailed FEA analysis the key results de-duced are:-A) The generated stress for both in case of the

Cold Cutting without reinforcement & ColdCutting with reinforcement is compared withthe allowable stresses and are found withinthe allowable.Thus the presumption of wall collapse as aresult of higher test pressure can be safelyand realistically evaluated with FEA. Also it hasbeen established that with provision of internalreinforcement to Coupon during hydrotest thegenerated stresses & displacements can befurther reduced.

B) Also the Analysis for the requirement forany branch/header reinforcement can bedone with more accuracy and ease takinginto accounts the key inputs in a single modelthus evaluating the overall effect of the dif-ferent inputs.

This publication thanks Mr. M G Choudhury,Mr. Ameet Kulkarni and Mr. Vishal Ojha forproviding this article.

M G Choudhury is Sr Vice Presi-dent & head of Piping Engg ofRPTL Engg Ltd. He has aroundforty years of working experiencein Piping Design Engg ,especially

in pipe stress. He has also worked for Compa-nies like EIL, TOYO, CHEMTEX and SABIC.

Ameet Kulkarni is a Mechani-cal Engineering graduate fromVisveshwariya Technical University(India) and has been associated withPiping Stress analysis in RPTL-ED-

MEC (Formerly BecRel engineering Pvt. Ltd.),Mumbai (India). Recently, he was involved withpipe stress analysis of various Reliance Plants.

Vishal Ojha has been associatedwith Piping Stress analysis in RPTL-ED-MEC (Formerly BecRel Engineer-ing Pvt Ltd), Mumbai (India). He is aMechanical Engineering Graduate

from Government Engineering College, Udaipur,Rajasthan. He has been working with RPTL-ED-MEC for 4 Years and was also associated withpipe stress analysis of various plants.

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 reinforcementthe generated stresses are well within the allow-able thus hydro test at 6.0 Kg/cm2 is safe fromdesign point of view. The evaluation of safe hydrotest pressure can be done easily by usage of FEAapproach.

2. With usage of internal reinforcement the blankyielding as well as the generated stresses can bereduced. This approach can thus be used effec-tively in case if a generated stress exceeds thepermissible limits.

This technique can be used for assessment of safehydro test pressure in line with code requirement forbranches without involving the already tested header.Also in case of Problem, how the pressure enduringcapability can be increased has been explained.