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?

 

[brkmech1234]

Please restate your question which seems to be not clear

 

[josedipofi]

Sorry if it was not clear, i try it with an example.
1)
Pd=50 bar
1.3xPdxStest/Sdes=65 bar=Ptest
MAWP of the vessel=53 bar
MAP=55bar
Is the vessel safe at P=Ptest? If yes shall I prove that and how?
2)
Now the test pressure is according to PED
Ptest=1.43Pd=71.5 bar
Same question as above

 

[brkmech1234]

In no case the stress due to hydro test shall not execeed the yield stress. Good practice is to keep it below 0.9 times yield stress. This check is mandatory as per the code. However the below is worth noting

Generally test pressure is 1.3 times times the design pressure. But some times it is as high as 1.5 times of design pressure/MAWP. Most of the cases it is Ok because, the allowable stress of material according ASME VIII Div1 is min(Tensile strength/3.5, yied stress/1.5). Thus when you back calculate the stress at hydrotest pressure as high as 1.5 times MAWP, it will not excced the yield stress. More over, yield stress at design temperature will be lesser than that at room temperature where the hydro test is carried out. Thus, the stress due to hydro will be less than 0.9 times yield. Howver, in few cases it may exceed yiled where the static head is high like columns.

 

[josedipofi]

brkmech1234 thank for your reply
only one remark, where is state in the code that the check (hydrotests with 90% yield) is mandatory?

 

[brkmech1234]

It is mentioned somewhere in UG 99 . Please check

 

[josedipofi]

that is the point, nowhere is mentioned that it is mandatory!

 

[brkmech1234]

josedipofi

Do not jump to conclusion. Please refer to last sentance of UG 99(b)

"All loadings that may exist during this test shall be given
consideration."

What do you read out of it. I read it like one should check the integrity of the vessel during the test. It is simple fact that any material stressed beyond yield point has essentially to fail.

 

[TGS4]

Please keep in mind that these limits on the state of stress during hydrostatic test are ONLY applicable to the general membrane stress - that is it does NOT include any local stresses.

It is permitted, during operation for some localized stresses to exceed yield - even by a factor of two. Therefore, it is logical that some localized stresses will exceed yield during the hydrostatic test. This is OK, though.

Please do not use generalizations like

In no case the stress due to hydro test shall not (sic) exceed the yield stress

Although that may be true for the case of general membrane stress, it is not true for all other situations.

Furthermore, your statement of

It is simple fact that any material stressed beyond yield point has essentially to fail.

No, it is not. If it has exceeded the ultimate strength in uniaxial loading, then it is considered to have failed. For many materials, significant strength improvement is obtained from exceeding the yield strength: strain hardening. In fact, there are some who argue that the strain hardening and the subsequent compressive residual stress in local areas is a primary function of hydrostatic tests; it's much more than a leak test.

 

[brkmech1234]

TGS4,

Your comments seems to be bit clumsy. Let me have some time to understand and comment on them.

I have a question. You are aware of the context which I put the above comments. Do you agree with Josedipofi's statement that check for hydrotest is not mandatory?

 

[EngAddict]

"where is state in the code that the check (hydrotests with 90% yield) is mandatory?"

UG-22 j)

 

[TGS4]

To answer the OP's question - are there any additional checks to be made when calculating the hydrotest pressure - according to UG-99, the answer is YES, but not of the type that you indicate.

One of the most important parts of calculating the hydrotest pressure is not the multiplication of the design pressure by 1.3 (for ASME VIII-1), but that this product is also multiplied by the lowest stress ratio (LSR) for the materials of which the vessel is constructed. The stress ratio for each material is the allowable stress value at the test temperature divided by the allowable stress at design temperature. Bolting is NOT included EXCEPT when 1.3xLSRxS_bolt(@ test temperature) > 0.9xSy.

Note article UG-99(d)

The requirements of (b) above represent the minimum standard hydrostatic test pressure required by this Division. The requirements of (c) above represent a special test based on calculations. Any intermediate value of pressure may be used. This Division does not specify an upper limit for hydrostatic test pressure. However, if the hydrostatic test pressure is allowed to exceed, either intentionally or accidentally, the value determined as prescribed in (c) above to the degree that the vessel is subjected to visible permanent distortion, the Inspector shall reserve the right to reject the vessel.

So, in Division 1 there is no longer a maximum based on general membrane being less than 0.9xSy. The limiting criteria is "visible permanent distortion". That definition is left to the discretion of the inspector.

EngAddict - you are incorrect.

The loadings to be considered in designing a vessel shall include those from:

(j) test pressure and coincident static head acting during the test (see UG-99).

and UG-99 does not have a limit as discussed above.

 

[jtseng123]

Talking about hydrotest, now come to my real case just happening:
I have a 40 feet long vessel, 56" diameter, one half is CS at 60 psig/650F refractory lined, the other half is SS at 60 psig/ 1450F.
Per code, test pressure for CS section =100 psig, but can be tested up tp 500 psig so the primary membrane stress is 90% of the yield.
Test pressure for SS section = 1,000 psig.

How am I going to run the hydrotest ? Test separately at their own test pressures and then weld together ? or weld together and then test at the highest pressure it can take (500 psig) ? or test entire vessel at 100 psig ?

 

[TGS4]

I should have qualified my above post that my quotations are from the 2011 Edition of VIII-1. However, looking back to previous Editions/Addenda, back in 2007, the wording was identical except that the prohibition on 1.3xLSRxS_bolt(@ test temperature) > 0.9xSy was not there (that specific item was added in 2010). Going back to the 1998 Edition (when the ratio was changed from 1.5 to 1.3, there was still no limit of 0.9xSy. The earliest edition that I have on-hand (1980) does not have the limit.

(On an historical note, even the 1962 Edition of Section VIII, had no mention of 0.9xSy)

My guess is that the use of 0.9xSy has crept in from the old (pre-2007) versions of ASME Section VIII, Division 2. There, article AD-151.1 stated

The hydrostatic test pressure of a completed vessel shall not exceed that value which results in the following stress intensity limits:
(a) a calculated primary membrane stress intensity P_m of 90% of the tabulated yield strength S_y at test temperature as given in the applicable table of Subpart 1 of Section II, Part D...

Note that this limit is different in the current Edition of Division 2 - see article 8.2.1 and 4.1.6.2(a)(1).

But, to conclude, there is no such limit for vessels constructed to ASME Section VIII, Division 1.

 

[EngAddict]

@TGS4, my comments were regarding requirement to consider the design for the hydrotest condition, which it does. I was not suggesting it gave this allowable limit, only that it must be considered, and where the code does not specifically cover details of the design it does not mean it can be excluded. In such absence you can refer to other codes (or standard industry practice) for this guidance provided the method is as safe as those provided by the rules of division 1. Yielding around structural discontinuities is permitted and some distortion is acceptable since this is a serviceability limit state not a strength limit state. My concern would be regarding exceeding yield in general primary membrane away from discontinuities.

@jtseng123, it is based on the 'lowest' stress ratio of all materials of construction. Must be tested as a complete vessel, after welding.

 

[prex]

EngAddict, UG-22(j) is simply reminding the designer that in some cases (specifically tall vessels where the process fluid is not liquid) the hydrotest with the water head added may represent a limiting design condition.
I agree with TGS4 that there is no mandatory per code stress limit on the hydrotest stress, and also even that there is no obligation to submit calculations for the hydrotest condition except where that condition may be limiting, as per UG-22(j). The only further example that comes to my mind of a hydrotest limiting condition is when a tall vessel is tested in horizontal position, the test pressure is calculated including the liquid (operating) head, but the upper portions of the vessel, that do not see the operating liquid head, might go overstressed.
It is however evident that going beyond yield (with a reasonable margin) on a general membrane stress would be a source of troubles, so any designer would check this in case of doubt, but I can't see how this could happen apart from the special situations mentioned above.
jtseng123, your test pressure seems to be 100 psig, but you are free to propose to your client to go up to 500 psig for the sake of safety. As pointed out by TGS4, there is no mandatory upper limit on test pressure per VIII 1.

prex
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[EngAddict]

prex, I don't think your comments contradict mine. Technically UG-22 j) applies to all vessels but will only affect some limited cases, for most vessels the hydrotest pressure will always be acceptable. I agree with your comments, I would not request calculations unless my checks found that the test pressure is limited by design. The designer must consider all loading conditions that affect the vessel, whether they then rule them out as a limiting factor or not is beside the point.

 

[TGS4]

EngAddict - I have no disagreement with you regarding that all loads during the hydrostatic test must be considered. The example that you provided is exactly the situation where this paragraph is important. However - the question that you answered was in regards to this supposed (turns out to be fictitious) 90% of yield limit. As I stated before, there is no specified upper limit on hydrostatic test pressure in Division 1.

jtseng123 - As EngAddict pointed out, the St/S ratio is based on the LOWEST ratio of the materials of construction. Assuming SA-516-70 for your CS (S=20ksi@70°F and S=18.8ksi@650°F) and SA-312 TP304H for your SS (S=20ksi@70°F and S=1.8ksi@1450°F), your LSR would be 1.064. So, your hydrostatic test pressure would be 60psi x 1.3 x 1.064 = 83 psi.


All - I am going to keep repeating this until I blue int he face - there is NO "90% of yield" limit in ASME Section VIII, Division 1. Stop using it. Stop referring to it. Stop.

Furthermore, in ASME Section VIII, Division 2, there is such a limit, as I discussed above (Article 4.1.6.2(a)(1)). That limit is now 95% of room temperature yield. It was 90% but is now 95%. So, please stop using 90% of yield. It doesn't exist in any current Code and hasn't for the past 5 years.

Please erase from your memory any reference to 90% of yield for hydrotest.

 

[vpl]

TGS4 I gave you a star -- but I should point out that your little soapbox man is actually quite red in the face rather than blue. You make a very good point that the Code has been updated and people need to be using the latest version.

Patricia Lougheed

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[jtseng123]

since my vessel has two significant design temperatures, if only test at 100 psig, or 500 psig based on CS section, that will not stress the SS section enough to see any leak so to simulate the design condition at 1450F. Does code ever address how to hydrotest for two significant design conditions ?