Girth Flange Gasket Leaks due to Vessel Lifting or Handling 4

[VFK]

An existing BEU type heat exchanger, 1200 mm ID (~47") and weighing 10,000 kg (~20K pounds), needs to be removed from an offshore oil rig and brought to a workshop on land so that the tube bundle, weighing 5700 kg and 2000mm long, can be replaced. The reason this scope cannot be performed on the oil rig is the bundle puller, supplied to the client at project handover for performing mechanical handling of the tube bundle offshore, is in disrepair.
* The shellside is CS 150# and service is low pressure steam and the tubeside is DSS 600# and hydrocarbon condensate service.
* The tubesheet has kammprofile gaskets (1250 mm OD, 15mm wide, SS316 4mm core with 0.5mm graphite both sides) on the shellside and tubeside and sit in recesses between the tubesheet and girth flanges.
* The bonnet studs are B7's, 28 off, 2 1/4" x 1000mm long plus 8 off collar studs of the same diameter. They will be tension-tightened and the target bolt assembly stress will be limited to 70% of bolt stress yield but the final figure will be assessed to PCC1 with Sfmax derived per the WRC-538 method. The OEM under-cooked the original bolt target stresses and the client dervied new loads with the help of a flange analysis expert.
* The plan is to hydrotest the new tube bundle with the bonnet installed with the production gaskets and production bolting. The workshop is proposing to not remove the bonnet post-hydro (there are nozzles to drain/dry/inspect the bonnet). We would then perform a leak test on nitrogen to prove the tubesheet gaskets are not leaking prior to releasing the heater from the workshop. We are not anticipating over-stressing of the bolting, hence why we intend to use the production bolts.
* The heater's lifting points are a pair of rated lifting trunnions on both the shell and on the bonnet and the centre of gravity is at the tubesheet. So it is inevitable for the bonnet to see lifting loads.
* The heater might be lifted at least five times after workshop release and will be shipped to the facility. Best case, we could get it down to two lifts by transporting it in a container.
Before I speak to an engineering consultancy, I would like to refine my understanding:

1. Is it best practice to replace girth flange gaskets post-hydro? I'm aware typically gaskets experience not-insignificant relaxation during the hydrotest. Other than corrosion/contamination issues, are there any other reasons to replace the gaskets? We intend to request the N2 leak test @ 90% of design pressure post-hydro as a control to prove the gasket is tight. I'm anticipating many will be reluctant to disassemble and reassemble the bonnet due to the large qty, size and tensioning requirements for the bolts.

2. If the workshop leak test is a success, is it plausible the gaskets may leak after the heater is installed due to loads experienced during handling/transportation? Is it credible the root cause for the leak could be the girth flange gasket(s) being "disturbed" or seeing localised crushing due to the longitudinal bending moment imposed on the bonnet during lifting (200 kNm [5 tonne x 2 x 2000 mm]) and/or due to transport acceleration loads?

3. Hypothetical scenario: after the heater has been installed the tubeside gasket leaks on the final N2 test in commissioning; troubleshooting the leak is unsuccessful and the decision is made to replace the tubeside gasket; the shellside gasket did not leak. In this case it would be preferable to remove the bonnet but keep the tube bundle in-situ using the 8 off collar bolts, because there is no reliable means of pulling the bundle offshore. Is it plausible that removing 28 of the 36 bolts could cause the shellside gasket to be "disturbed" and may introduce a new risk of the shellside gasket leaking after the bonnet is reassembled?

Sorry for the verbose post, but the devil is in the detail...

 

[GD2]

First tell us, how did you pull out the bundle from the shell? Did you use a bundle puller? And you intend to re-insert the tube bundle with the channel head/Bonnet?

GDD
Canada

 

[r6155]

Welcome to the forum.

a) Never use lifting lugs. Use slings and spreader bar on shell side.
b) Use a hydraulic tensioner to control the elongation of the studs, not torque.
c) Avoid N2 for leak test, use air at 1 to 3 bar (see ASME V)
d) Try to avoid replacing gaskets.
e) I assume there is a small hole in the pass partition for the drain and vent in the hydrostatic pressure test.

Any photo?
Regards

 

[LittleInch]

Can't you just repair or replace the bundle puller and leave the thing in place?

But lots of words which still needs a drawing or three.

What is the MAWP of the shell? It might have class 150 flanges but that means nothing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SnTMan]

VFK, yes, welcome to the forum. And thanks for the detailed description of the problem.

First off, likely nobody can say for sure

However:

1) Service gaskets are rarely replaced after hydro. If N2 (or air) leak test is satisfactory I'd leave 'em. Pneumatic test at high pressures is very hazardous. Test at low pressure if possible. If not, exercise all precautions.

2) Lifting across bolted joints should be avoided, however, you got what you got. Kammprofile gaskets are quite rugged, I would not expect them to be damaged if a meaningful amount of bolt load is maintained. After your shop leak test, you might re-run the bolting before handling, and again afterwards.

3) The whole and only purpose of collar studs is that the channel may be removed with out damage to, or any need to replace the shellside gasket. This practice is widepread and has a long history of success. 25% bolting normally maintains sufficient contact pressure on the gasket to avoid any damage to it. Under your scenario I doubt you'd need be concerned about the shell gasket.

And again, likely nobody can say for sure

Best of luck,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[SnTMan]

I suppose it would be possible to weld a series of lugs, say four, to each girth flange and following successful pressure and leak test to weld shipping bars across the lugs. Said bars / lugs to be removed once all handling is completed. Kind of like what is used for bellows expansion joints.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[GD2]

Sorry, my comment in my first response went wrong.

I am assuming that you are removing the entire HX with the shell. This is ideal as it will ease off extra work at site and all work being completed in the shop under control conditions.

Your concern is on lifting and the subsequent leakage. Very genuine. But don't you see tested HX being lifted all time that goes out for site installation? As r6155 pointed out, use slings and spreader to minimize the bending moment.

There is no reason to replace the hydrotested gaskets as you will be using the production bolting and gaskets following PCC-1 procedure. Also, make sure that you don't exceed the Code allowable hydrostatic stress of any part of the HX in the pressure boundary during the hydrotest.

Any equipment will go through pressure and thermal variations during its operating life and these conditions are always part and considered in the design.

if you are still uncomfortable, do consult a lifting expert. I understand your concern being offshore and what effort it takes if something goes wrong.





GDD
Canada

 

[VFK]

Thank you all for the prompt and insightful feedback. Oh its not my first time here, just first time with this email address.

@GDD - this is an upcoming scope and the bonnet and bundle have never been removed.

@r6155 - a) the heater has lifting trunnions which means slings will be used. b) OP states bolts will be tension tightened - hydraulic tensioners would be used. c) It is standard practice at O&G facilities to perform N2 leak tests on pressure envelope systems when (re)commissioning up to max operating pressure or 90% design pressure. This is consistent with ASME B31.3. I'm aware of the H&S risks associated with using compressed gas for testing of pressure equipment and N2's asphyxiant risks, but these are mitigated by the afore-mentioned hydrotest which is at a much higher pressure, staged pressure increases, restricted access to non-essential personnel, and the test locations are outdoors, not congested and well ventilated. Asides from my thoughts, the ops team perform risk assessments prior to each activity and put appropriate controls in place. e) yes, there is a drain/vent hole in the partition plate.

@littleinch - refurbishing the bundle puller and sending it offshore to perform the bundle extraction is one option, but we're expected to provide balanced business cases for all options supported by risk assessments. Further developing/evaluating the feedback from this forum and from our consultant will contribute to it.

@SnTMan - Thank you Mike for addressing each of the points and the measured feedback. Haha I work in shades of "likelihood" and "credible/plausible" because I realise theres no guaranteed outcomes in this industry.
1) noted with thanks. I've considered the risks of testing with compressed gas.
2) Yes, I had considered going around one last time with the tensioning heads post-hydro to restore the original target bolt stress to account for the inevitable gasket relaxation from the hydro.
Noted that it is not good practice to lift across gasketed joints - that was the key thing I wanted to sanity check.
I really like your idea of adding welded-braces to tie the girth flanges together. On the flip-side, it might even introduce an incentive for the client and my colleagues to strongly consider getting the bundle puller back into service to avoid performing hot-work on the facility!
3) Great, this was my understanding too and I was puzzled by the anecdotal feedback I received. I'll ask the client for more evidence how they think this practice contributed to leaks in the past.
Thanks again all.

 

[VFK]

Hmm, just a thought on the welded lugs-bracing idea - I imagine they will need to be beefy to bear the loads so a considerable amount of weld would be required. Then we would need controls to limit the effect of the welds thermal expansion/contraction on the gasketed joint, right?

 

[SnTMan]

I doubt I would be too concerned. You'd be surprised at the capacity or a few inches of 1/4" fillet welds...

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

ASME B31.3 ???, this HX is a pressure vessel.
I don't agree with adding anything to the flanges. The 36 studs with a diameter of 63.5 mm are enough. Slings are installed around the shell diameter, not on the trunnion. Horizontal pressure vessels do not use lifting lugs (only in special cases).

Regards

 

[VFK]

@r6155 - ...a pressure equipment that is bolted to a piping system consisting of multiple flanged joints, valves, instruments, etc. Once everything is assembled the system, including the vessel/HX is leak tested and inerted prior to introducing hydrocarbons.
Thank you for the insights. Could you please elaborate on these couple of points which are salient to this scope.
-what is it about the bolting preventing localised over-stressing the gasket(s) when the bonnet sees the over-turning moment from the lift? I anticipated the stiffness of the bonnet, shell and both girth flanges would have a greater contributing effect and the bolting wouldnt come into it.
-on the do/dont on lifting lugs. I've worked in design, 3rd party design verification and construction and thats the first I've heard of it. I didnt come across this topic when i searched this site.
thank you

 

[LittleInch]

Do you have a drawing or sketch of this vessel or photo.

I'm struggling a bit to see how "the centre of gravity is at the tubesheet."??

But it is not possible at this point to say whether there would be an impact on the gasket sealing from the effect of transportation.

There's a risk for certain with anything this big getting some form of shock loading but the container idea sounds better than many.

Also what sort of test pressures are we talking about for shell and tube here?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[VFK]

@littleinch - the shellside is 150#, 16mm thk, has a few 14" nozzles, while the bonnet is 600#, 33mm thk and has two 28" nozzles. Hydro pressure shellside 25 barg, tubeside 116 barg. Pic attached.

I greatly appreciate the support to date and the requests for more detailed information, however, I'm not expecting to receive consulting advice on this site or a compliant/non-compliant assessment and would caution any other forum users doing so. As per my OP, I'm seeking feedback ("coaching" if you will) whether the risks I've mentioned are credible/plausible and to gather some insights that I will consider when I draft a scope to perform appropriate engineering checks. In other words, I'm looking for "yes, thats worth investigating further, a couple of focus areas are... and here is why...". Apologies if I didn't make this clear initially.

This forum has an incredible wealth of info recorded that is useful for refining my understanding or as a check-point to find other reference sources.
thank you

 

[r6155]

Why didn't you send this drawing before? I asked you on 13 Aug 23
You made me waste my time.

 

[SnTMan]

r6155, you are the only one causing a waste of your time. No offense.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[LittleInch]

Holy moses - that's why a drawing tells you as much as all your words....

I can understand your concern, but for flanges that big and pressures that high, I'm finding it difficult to believe that unpressurised the movement of the flange under the moment loads are anywhere close to what the load is under test pressure. I guess you could see what that moment means in terms of bolt force compared to test pressure force?

To reduce risk there would seem to be a couple of things you could do.

After the hydrotest and draining just lift the vessel up and down gently and then do an air test at 90% MAWP to see if lifting causes any issues.

Build a lifting frame and add a third strop connection point to go under the flange. Need to use some micrometers to make sure you don't flex it the other way and take more than say 25% of the load or use load cells on the lifting beam to connect the chains and monitor them as you attach the extra lifting straps. Maybe.

Sudden shock loading or transportation / lifting it back onto the platform could disturb the gasket enough to cause a very small leak, but looking at the size of that beast I would like to think the bolts are man enough.

HX gaskets are a bit notorious for being sensitive as they are generally custom made with a smaller PCD than standard pipe flanges so prepare for a leak but hope for a tight vessel once it's back on the stand.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Christine74]

Assuming that you're using 8-UN threads, (36) 2 1/4" diameter SA-193-B7 studs tensioned to 70% of yield would provide 13,900,000 lbs of clamping force. I don't think there's much chance that lifting/rigging the heat exchanger would cause a gasket leak.

(BTW thanks for providing a detailed, well articulated question with all of the required background info)


-Christine

 

[VFK]

@littleinch - thank you for your post. Yes, I'm working thru the asme-App2 and WRC538 calcs and was intending to compare the bolt loads and flange moments for the lifting case to the hydrotest case and design condition case as a sensitivity check, but tbh I think those calc methods have their limitations for this scenario. I was originally worried about the lifting moment making the bonnet shell/flange rotate/deflect and cause the flange to compress the gasket at the 12 oclock position beyond its max allowable stress, and a leak path potentially opening up after the HX is put down. I think running the hydro case calcs will only give me bit of a sighter, because I foresee that load will cause a different deflection in the flange - it will try to pry open uniformly about the bolts resulting in either insufficient load on the gasket, or over-rotation of the flange relative to the gasket. In other words the mode causing the leak path is slightly different between the sets of load cases. I think FEA would be the more reliable approach in this case. But I can't help but think, if this was a ‘thing’ the bpvc would have developed a suitable method to assess it haha! At this time, I'm telling myself its prudent caution to do the checks.

Good suggestion to 'trial' lift then leak test.

Per the OEM design there is a temporary transport saddle supporting the bonnet - the top of it is visible in the image I posted. The base of the lifting frame / container will be modified to secure that 3rd saddle and we'll be sure to review the packing procedure instructions to install the support adequately.

I've come around to needing a contingency plan offshore to pull the bundle in the event theres a leak... plus I've witnessed some awful lifts with equipment contact/damage that eventuated for a variety of reasons. Its Sods Law...

 

[BJI]

Yes, I'm working thru the asme-App2 and WRC538 calcs and was intending to compare the bolt loads and flange moments for the lifting case to the hydrotest case and design condition case as a sensitivity check, but tbh I think those calc methods have their limitations for this scenario

Flange strength under design or operating conditions will not provide much benefit for this assessment. Safe to say the lifting loads are negligible for this joint. The quickest check would be to convert your lifting (or transportation) moment to an equivalent pressure, and then compare to your residual bolt loads in terms of gasket stress. Post back with the relative utilization.