Hydrostatic test pressure stress 7

[josedipofi]

Gent,
general question about test pressure according to ASME VIII div 1,

Hydrostatic pressure=1.3xPdxStest/Sdes (UG-99(b))
Static pressure can be neglect (very low)
Assume MAWP=Pd
Min. MAWP calculated < Hydrostatic pressure
Min. MAP calculated < Hydrostatic pressure also
In this case do I need to check the stress in the vessel at hydrostatic pressure(with 90% of the yield) or is implicit in the code that it will be always verify?

 

[TGS4]

...that will not stress the SS section enough to see any leak...

Just curious - why do you think that is the purpose of a hydrostatic test? Please refer to UG-99(g). The hydrostatic test is neither a proof test nor a leak test - although a leak check is required at a reduced pressure (UG-99(g)). The Code is silent on the rationale for performing a hydrostatic test. As an aside - that is not for lack of trying, BTW. There is not general agreement within the various Code Committees themselves about the purpose of a hydrostatic test.

jtseng123 - this situation is rather common and is one that is fully anticipated by the Code. Every other vessel that has followed these rules has turned out OK - why would you think that yours would be different?

vpl - thanks. There is no emoticon with a blue face

 

[jtseng123]

TGS4 and others: the term 'leak test" though not technically correct, but is commonly used across the entire industry, no need to picky on it. It will be still used for the next hundreds of years.
Can anyone explain why you have to time the stress ratio ? My understanding from mathematic point of view is, the stress ratio is to simulate the design condition, especially the design tempertaure that is impractical to heat it at shop to check if the vessel can sustain or not. In other words, it is to "fully stress" the vessel at ambient temperature. 1.3 factor, or 30% more, is just to stretch the vessel a little bit more, and if it won't fail, your vessel will be good at design condition plus additional 30% safety factor. Due to this 30% extra, code allow you to back down your test pressure by using the lowest stress ratio, or if the primanry stress exceed 95%, 90%, yield, or whatever other limits that have been talked about so much in this post, you can reduce the test pressure without causing hiccup.

simple math for hoop stress divided by allowable: P1*R/t/S1 = P2*R/t/S2 = 1(plate is fully stressed)
where P1=test pressure, S1=allowable stress at test temp
P2, design pressure, S2 = allowable stress at design temp
t , wall thickness, R: vessel radius.

So, P1=P2*S1/S2, now you see how the stress ratio appears there. Code just further adds 1.3 factor to it.
If stress ratio variation is within 30% for the entire vessel, it will be fine according to the math. In my case, the stress ratio variation is from 1 to 11. If I only test at 100 psig and in case there is defect in SS section, lack of fusion, internal crack, that is not detect, but can withstand 100 psig test pressure, does anyone think it won't burst when heating up to 1450 at 60 psig where the allowable stress is so low ?

 

[TGS4]

Burst - no. The failure mode at those temperatures is creep. The allowable stresses are based on lower-bound material properties - most materials perform much better.

Your exact situation has been discussed at the Code Committees tables. The writers of the Code are well aware of this possibility. They are OK with it. As I said - the purpose of the hydrostatic test is NOT necessarily to stress that material to 30% beyond the value that it will see at design. I get your math, though.

For your particular situation, remember that Division 1 does not place an upper limit on the hydrostatic test pressure. If you'd be happier with 500 psig, then there is nothing stopping you. And don't worry about exceeding yield in primary membrane - carbon steel has significant strain hardening. If you ran an elastic-plastic analysis of your hydrostatic test condition (at the pressure you want to test the SS part to) you'd probably find that it won't actually fail. If your simulation didn't show deflections exceeding about 1/2", I would likely say that that passes the "visible permanent distortion" test...

 

[jtseng123]

TGS4, thnaks. However, not so clear to me.
Let's disregard the 1.3 factor which to me is beef added by code, and also forget how good the modern materail is.(Note that base metal can be excellent, but welding is still controlled by human, so the hydrotest is to test more of the workmanship than base metal. though base metal does burst, but not so often. I use "burst" is from the failure appearance I have seen so many of them. No need to picky the word I use. Many of our overseas projects, the first piority is to get a good welder to minimize weld rejection rate),
just look at the simple hydrotest equation P1=P2*S1/S2. To me, it is to "simulate" the design condition from the math to make sure the vessel can "sustain" even there is defect in welding and base metal. (weld defects may exist because code allows spot or even no NDE). So as long as there is no leak, regardless how bad the weld and base metal are, the vessel is good to go for operation, becasue it pass the "simulation" test. (same as the financial meltdown that banks must pass simulated "stress test" to continue operating). That is my understanding of the purpose of hydrotest, straightforward with the technical basis by math. UG 99(g) saying inspection pressure shall not be less than the test pressure divided by 1.3, that exactly hits the math, and 1.3 is additional safety factor code added.

You have not clearly stated the purpose of hydrotest, would like to know your interpretation if other than mine.

In my case, it is not I am happier with 500 psig or not. I am the person to tell vendor what to do in this situation, and every decision I make, I must have technical basis. With 500 psig or higher that is way above the required for CS section, if something goes wrong, I will be responsible becasue "I tell vendor to do what is not desgined for". If tested at 100 psig, that means SS section is dramatically not pressure enough to "simulate" the design condition, then I have no techncial basis to state it will not fail when put to service.

Any knd of analysis, elastic-plastic, deformation, FEA, etc, you name it, can not substitute the real world hydrotest. That is my years experience in this field, and that is why code ask to do hydrotest after you have done your calcualtion and even with 100 % NDE on every weld and 100% UT on all base metal.

If my interpretation for the purpose of the hydrotest is correct, code committees shall take a second thought, maybe add limitation on the variation of stress ratios. And if variation is too large, just like my case, recommend what shall be done. I tends to ask vendor to test SS section separately at 1000 psig. Then weld CS and SS together and test at 100 psig entirely. The disimilar closing girth seam will be covered by refractory and set to 650F, so no need to worry.

 

[TGS4]

jtseng123 - I think that the hydrostatic test has many purposes - one of them is what you describe above. I completely agree that what you describe above is one of the purposes.

If my interpretation for the purpose of the hydrotest is correct, code committees shall take a second thought, maybe add limitation on the variation of stress ratios. And if variation is too large, just like my case, recommend what shall be done. I tends to ask vendor to test SS section separately at 1000 psig. Then weld CS and SS together and test at 100 psig entirely. The disimilar closing girth seam will be covered by refractory and set to 650F, so no need to worry.

I think that you may have a valid point about limiting the variation in St/S. Please submit an inquiry to the Code Committees, as outlined in "Submittal of Technical Inquiries To The Boiler and Pressure Vessel Committee - Mandatory" in the front matter of the Code.

 

[jtseng123]

My USA vendor intimidated me today if I insist hydrotesting C.S section and SS section separately, then weld together, it will have cost impact plus delay.
I told vendor, prove to me that testing the SS section at 100 psig and then put to service, if not causing failure, I will buy it.
I futher told vendor why not just put miminum welding deposite, and have all kinds of crack/voids in the base metal that nrmally does not receive NDE, even have crack/voids in the welds that NDE is not required, and I can guarantee 100 psig test pressure can be easily passed. And if he dares to put into service for 1450F/60 psig becasue he says it passes "100 psig" test pressure, I can guarantee him it's going to burst the first minute in service.

 

[doct9960]

jtseng,

If what you require is not in the contractual agreement, then you have to pay extra and expect a late delivery. That is not intimidation, that's business. Nothing is free.

If you did not specify any special hydrotest requirements in your PO, the vendor will use UG-99(b) for the hydrotest pressure.

What is the MAWP that will be stamped on the vessel nameplate? And what are the stress ratios for the CS section and SS section?

 

[jtseng123]

doct, the stress ratio is 1 for CS and 11 for SS. Actually, this is a long 40' duct, 56" diameter per B31.3. But for hydroetst, not much difference between B31.3 and Div.1. B31.3 has 1.5 factor in liu of 1.3 in Div. 1.

If you carefully study how the test pressure formula comes from and its purpose, you will see my point. I believe TGS4 has seen my point. So, basically, I disagree with you. You can not test a major piece of equipment at 100 psig, which supposedly to be tested at 1000 psig in order to see any meanfully defect. Is hydrotest vendor's responsibility ? yes it is. Shall vendor flag this special problem ? maybe they should when they bid the job and if they understand the formula. The truth is, hardly any vendor or anyone facing this kind of problem in their lifetime. Code is not all-powerful to cover all situation.

 

[doct9960]

jtseng,

You mislead a lot of us in this forum into thinking that your "vessel" is designed to ASME VIII-1, when in fact it is just a piping component. When TGS4 mentioned that "you may have a valid point about limiting the variation in St/S", he was referring to the ASME VIII-1.

There are a lot of differences in the hydrotest requirements of ASME VIII-1 and ASME B31.3. ASME B31.3 has a limit on the stress ratio; it should not exceed 6.5. There are also recommendations of reducing the hydrotest pressure if stresses exceed the material yield sstrength or 1.5x the component rating. ASME B31.3 is also specific on what hydrotest pressure to use if the piping system has more than one material or more than one design temperature. I suggest you get the latest copy of ASME B31.3 and carefully read paragraph 345.4.

So with a design pressure of 60 psig and assuming you got your stress ratios right (based on Table A-1 or based on component rating), your minimum hydrotest pressure would be 60 x 1.5 x 6.5 = 585 psig.

If you did not specify a 1000 psig hydrotest pressure in your P.O. and if you did not specify separate hydros for the CS and SS section, the vendor has every right to charge you extra and delay delivery.

 

[jtseng123]

Doct, thanks for pointing out 6.5 limit but I do not know what is the basis for that. The rest you talking about, yield and component limitation, etc, that I fully understand.

There are issues on this that I concern:
1. The integrity of the SS section that is not tested to its own presuure to simulate design condition. Even I discount the 1.5 muiliplier, the SS still must see 60x11= 660 psig test pressure for a meaningful hydrotest to guarantee it will be no problem in service.

2. Unneccessary testing the CS section at 585 psig for which it is not designed for, induces other risk. There are many manways and nozzles on the duct, so all reinforcement have to be rechecked, redesigned, refabricated. That for sure will cause cost and delay. I do not think it is wise to test at that high pressure for CS section.

So, comprimizing everything and then testing the system at low pressure becasue people keep referring "code" has not much meaning at all in my opinion becasue it defeats the purpose of hydrotest for SS section.

If you really have the project I have, are you willing to allow vendor to test at low pressure for SS section and you can endorse it " hey, SS section won't be a problem". And what will be your technical basis on that ? Becasue code says so ? Well, it's better to find out what's behind the 6.5 ratio limit and what kind of materal is involved. Is it based on the same dedesign temperature ? or similar material group ? Has code included my extreme case ?

 

[TGS4]

jtseng123 - you have a problem with the Code itself.

Please submit an inquiry to the Code Committees, as outlined in "Submittal of Technical Inquiries To The Boiler and Pressure Vessel Committee - Mandatory" in the front matter of the Code.

Since your case is really about B31.3, then follow the procedure in Appendix Z - Preparation of Technical Inquiries.

We can argue here all we want, but your beef is really with the Codes. Take it up with them. When you have submitted your inquiry (inquiries), please inform us here - many are Code Committee members, or work with Code Committee members. We may take an active role in shepherding your inquiry through.

The Code Committees have the background and history (and access to the history) of these requirements. Maybe there was a technical reason. Maybe there is test-data to back up what's there. Maybe it's completely arbitrary with no technical backup. Maybe it's a compromise position due to the consensus nature of the Committees. Maybe...

 

[jamesl]

It is common to run into a situation jtseng123 described when you use dissimilar materials on one vessel. For example, before the Code made revision in UG-99 several years ago, it was common to have bolting material for custom designed body flange limiting hydrotest pressure.

Hydrotest can be a test of structural integrity, leakage check and serves as mechanical stress relieve. I agree we should stress the vessel as more as possible, to 90%, 95% or even 100% yield so as more defects can be exposed as possible, if there is any. However we should realize it is almost impossible to stress test every weld seam to its maximum in a vessel during hydrotest. Think about those weld seams in a large nozzle neck or weld seams between flange and nozzle neck. The stresses there are not going to be very high even during a hydrotest.

Jtseng stated:

“If I only test at 100 psig and in case there is defect in SS section, lack of fusion, internal crack, that is not detect, but can withstand 100 psig test pressure, does anyone think it won't burst when heating up to 1450 at 60 psig where the allowable stress is so low ?”

If the stainless steel part of the vessel is stressed to 2600psi (I just use a random number here) at hydrotest without problem, is it unacceptably dangerous to stress it to 2000 psi @ 1450 F. I doubt it. Why would it burst? The safety factor built in the Code is still there in addition to the 30% higher stress during hydrotest. Besides, as pointed out by TGS4, structural failure at hydrotest temperature and at 1450 deg F would be a totally different mechanism any way.

If the carbon steel part of the vessel can be tested at 500 psi, why not just test the vessel @ 500 psi and be done with it?

 

[jtseng123]

Very good. Everyone has his opinion addressed. Here is the reality check: After disscussed with many heavy weight engineers, inside and outside the company, all in agreement that SS section shall be tested separately at its own high test pressure (585 psig), then welded to CS and do the second test at the CS low test pressure (112 psig). No one wants to test SS at pressure lower than 585 psig because there is no technical basis.
The other option is: test this SS/CS hybrid together at 585 psig to satify SS section, and beef up reinforcement on CS section for this high test pressure that it was not originally designed for.
I select the latter one and advise vendor to proceed. Be done with it.

I have the right to decide which code to be used. Fortunately, I select B31.3, not Div. 1. If I select Div. 1, I will be in much trouble to fight with the low stress ratio with 100 test psig on SS that wholy defeats the purpose of hydrotest.
This SS/CS hybrid is out of the code. And good engineering judgemnet must be called for.

If everyone still has interest, here is a simple question:
A pure 304H SS equipment at 1450F/60 psig, can be either B31.3 or Div. 1,
Per B31.3, test pressure is 585 psig because code says stress ratio to be 6.5 maximum.
Per Div. 1, test pressure is 860 psig because there is no stress ratio limit except yield.
Why so much difference in test pressure ?