Test pressure depends on flange loads?

[Bartolomew]

Hi everybody,

I am sure this is an easy question for you about ASME VIII-1:
Test Pressure depends on MAWP per UG-99(b).
MAWP depends on the MAWP of every essential part of the vessel per UG-98(a).
If external loads are applied on flanges, the rating of flanges decreases, so do the MAWP of the vessel and consequently also decreases the Test Pressure.
My question: Does the Test Pressure depend on the external loads on flanges?

 

[MJCronin]

Excellent question !!!!

However ... If we can assume most PV testing is done in the shop

Can you not also argue that PV flanges WILL NOT undergo any external loadings from OPERATING and connected piping systems during pressure testing ? ....

and therefore the test pressure DOES NOT depend on any external loading conditions from non-operating and probably, not connected piping systems

MJCronin
Sr. Process Engineer

 

[david339933]

MAWP doesn't decrease....the MAWP is the MAWP. The MAWP may be less than it would be if the flanges didn't have external loads, but you have to consider the loadings in establishing the MAWP.
Then as you said, "Test Pressure depends on MAWP."

 

[Bartolomew]

So... I understand now that the MAWP that we use in UG-99(b) is marked on the nameplate and basically it corresponds to the conditions of pressure test (no external loads on flanges from piping and room temperature).
Then in operating conditions, with external loads applied on the flanges there is a de-rating of the flanges and the MAWP of vessel could decrease below the MAWP marked on the nameplate. Am I right?

 

[david339933]

No, not right. You have to take into account the UG-22 loadings in designing the vessel. If the loadings mean your MAWP is less than if the loadings were absent, then the "lower MAWP" is what is stamped on the nameplate.

 

[Bartolomew]

Let's see if I get you:
1.- I calculate the shell, heads, nozzles and flanges for internal/external pressure and some thickness (plates), schedules (pipes) and ratings (flanges) come out.
2.- Then I also apply the external forces and moments in the flanges to see if they need pad reinforcement o increase the rating. Here there is a problem because usually I do not know the F & M, in fact the costumer ask me to tell him the Allowable F & M in each nozzle. Then, with PV Elite I put some hypothetical F & M in each nozzle/flange and apply WRC107. The more I increase the F & M the lower value I get for the MAWP of flanges (de-rating). I increase more and more the F & M until the MAWP of the flange is really low (but I keep it always higher than design pressure).
3.- Finaly the MAWP that comes out after applying these hypothetical F & M (the allowable F & M that my cliente wants) is the one to use in pressure test UG-99(b). This is strange for me because the real F & M will come later from piping analysis and they could be smaller than the allowables.
Does make any sense all this?

 

[david339933]

I suggest you read U-2(a) as it has been revised for the 2021 Edition. Have you used the required User's Design Requirements Form? The days of pleading ignorance for loadings is over.

 

[LittleInch]

The way I'm reading this is that unless you really know the F&M on the nozzles, you adjust them to get the max you can but not so high that they impact the MAWP??

Then it doesn't matter if the F&M in reality are zero - it won't affect the MAWP and that's your key number.

So unless your forces and moments on the flanges are the thing mainly impacting the MAWP, then it matters not what they are or could be.

Or did read that all wrong?

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

 

[r6155]

Also, seismic and wind loads are not present during the pressure test.
All calculations are performed in the corroded conditions. In UG-99 (a) the thickness of the corrosion allowance is not considered, but it is present during the pressure test.
I prefer UG-99 (c) where the actual thickness is present.

@ Bartolomew
Please, try to understand the purpose of pressure test.

Regards

 

[Bartolomew]

1.- So to perform the hydro test I calculate the test pressure using the MAWP [UG-99(b)]. This MAWP corresponds to the minimum of all the MAWP of all the parts of the tank (shell, heads, nozzles, flanges, etc.), without taking into account external forces on the nozzles and flanges due to the connections with the pipes, wind, seismic loads, etc., since during the hydrostatic test there are none. This MAWP is written on the nameplate.

2.-What I was doing (it seems wrong) in PV Elite was introducing the forces and moments in the tank connections, which causes a derating of the flanges and consequently a decrease in the MAWP of the flanges and since in my actual job flanges are the part that control the MAWP of the vessel , then the hydro test pressure is smaller than in case 1 above.

Is that right?

 

[david339933]

So basically you think it's correct to stamp a MAWP on the nameplate that is higher than allowed in operation because you hydrotested the vessel in a shop.
See any problems with this?

 

[Bartolomew]

Thanks everyone, even though some answers seem to be contradictory (as usual) this thread has been clarifying for me.

 

[Snickster]

It has been a while since I did a pressure vessel calculation but my recollection is that the pressure vessel MAWP (and required test pressure) is basically based on hoop stress considering corrosion allowance just like piping. Any additional local stresses on the vessel shell at the nozzle connection due to piping loads are analyzed per the requirements of ASME Code allowables Div. 2 (which provides allowable primary and secondary stress load combination allowables in the shell at the nozzle - vessel interface) using calculation methods such as WRC 107/297/368 or finite element analysis. The stress in the nozzle itself is just like piping stress insofar as hoop and longitudinal stress although there is additional stress in the nozzle at the shell intersection. This is also discussed in the codes. If stress in shell or nozzle exceeds allowables then reinforcement is added to shell or nozzle thickness is increased. Loads at nozzle are calculated using piping stress analysis programs such as Caesar II. When specifying pressure vessel requirements to fabricator you would need to supply them with the external piping loads Fx,Fy,Fz,Mx,My,Mz in addition to the required design pressure. The required design pressure is usually say 10 to 20 percent over the maximum expected operating pressure. The manufacturer/fabricator will calculate the actual MAWP of the vessel based on the weakest component and also determine the test pressure based on calculated MAWP. Typically a piping stress analysis would be done considering specified nozzle neck wall thickness and standard minimum shell reinforcement pad. If allowable stresses are exceeded then piping is arranged for additional flexibility until nozzle loads are within allowables so that no special added reinforcement or nozzle wall thickness increases are required by fabricator above the standard required by ASME Code.

 

[r6155]

@ Bartolomew
Let your computer work with UG-99(c) and you can see the difference.
I use my own computer programs.
Regards.

 

[GD2]

Bartolomew,
My question is why will you consider the flange external loads during the vessel hydrotest? These loads were already considered in the vessel design. You don't have to take into account the external loads during hydrotest.
Again hydrotest loads are different than the operating loads. So why are we even talking the operating load condition during hydrotest.
Yes, if you are hydrotesting a tall vessel on site and its a windy day, you will need to consider the wind load, otherwise plan the test on a normal day but make sure that the vessel supports are designed for on-situ hydrotest.

GDD
Canada