Hydrostatic Test - MAWP vs MAP

[RPRad]

Hi

Hopefully there is a straight forward answer to this, I recently had a client indicate that with vessels which have extra thickness (ie I assume due to larger corrosion allowances (3/16' - 1/4") or I suppose cases where the nominal plate thickness used is "notably" thicker than the minimum allowable thickness) that the vessel should be tested to 1.3 x the MAP and not the MAWP...cant say I have heard this before or been requested by an AI to do that.

I have heard of the test pressure being increased due to the LSR, static head or even by Owner request...but still it was all based on MAWP. Presumably it has something to do with not reaching the same stress level during the hydro if the vessel had a thinner wall - does that even make sense?...is there any benefit or even a requirement to anything based off the MAP?

R

 

[gr2vessels]

The new edition 2013 has not changed the UG-99, the 2010 edition is still calling for 1.3 x MAWP x LSR.

 

[RPRad]

Yes, but I did notice that in Para UG-99(c)(which admittedly I dont think I have ever really read in vengeance) it does indicate that you can test to the least MAP if you wish... given agreement between Owner, Fabricator and AI...but I dont really see what the benefit would be or what you would be trying to prove..min requirement is still 1.3 x MAWP x LSR as you stated.....I guess you could argue that in vessels that have more thickness than required you are not meeting the same stress level if it was thinner, but again too what end

R

 

[gr2vessels]

There is no mention of MAP in UG-99(c).
The MAWP for each element should be calculated and that will give you a range of values, the highest value being for the over designed component. The smallest value is the determining MAWP and could be as low as the design pressure (not lower). The selected MAWP is a figure which defines the entire equipment and is recorded on the nameplate for posterity. You can also use it to calculate the hydrostatic test pressure.
Cheers,
gr2vessels

 

[RPRad]

Interesting looking a the definition they mention in 3-2, as directed there from UG-99(c) it reads for calculated test pressure: "The basis for calculated test pressure is the highest permissible design pressure as determined by the design formula for each element of the vessel using nominal thickness with corrosion allowance included...is that not the maximum allowable pressure (MAP)?...the MAWP being the highest pressure for each element based on nominal thickness "exclusive of corrosion allowance"?...also defined late in 3-2

 

[gr2vessels]

It could be a coincidence which cannot be generalized. The intent is to test the equipment to the highest pressure allowed for the weakest link in that vessel. Yes, that will under test some of the components, but the uncontrolled over pressure during test can damage the weakest component and be the basis for rejection (refer UG-99). As you can see, that opens a wide testing scope, where you could isolate various components and test them separately for their appropriate hydro test pressure. I don't think that's feasible and productive. You won't change the overall rating of the vessel, down to the lowest MAWP. So, what's your point of evaluating MAP in the hydrotest context?

 

[vesselfab]

Yes, many "user" require testing at the MAP condition. We do it regularly.

We re-design the vessel with a "new MAWP" which is the MAP of the lowest pressure of head, shell, or flange, making sure of the stresses incurred at hydro-pressure.
This often requires a slightly larger re-pad, or attachment weld.
It is really not enough money to worry about.

Every body is happy when done correctly

 

[MikeG7]

I use Compress for my vessel designs. When I select UG-99(c) as the basis for hydrotest the pressure for testing is 1.3 * MAP. That is the MAP for the lowest component. It is written in the design report like this too. I says in UG-99(c) that the factor 1.3 is multiplied to the "basis for calculated test pressure". I guess if you use MAP as the basis, then it could be a requirement. It makes sense to me that it should be tested in the shop based on the new, un-corroded condition. This would obviously be different for field testing. I'm as confused as RPRad.
Mikeg7

 

[RPRad]

Hi

I am with you Gr2vessels,not sure what the overall benefit is, unless you are trying to increase the MAWP of the vessel after the fact (particularly low pressure vessels there could be a notable amount of excess material)...which I guess is perhaps what vesselfab is getting at.

So my question to vesselfab is: What specifically is the purpose of testing at the MAP?...In your case is it simply just to raise the MAWP to max possible for the as built vessel geometry or are there other goals involved?...Why is the User asking you to do this?

In my particular case this doesnt have anything to do with raising the MAWP, the User provides the MAWP, the vessel is designed to the MAWP, built with nominal material sizes that are readily available and then instead of testing at 1.3 x MAWP x LSR they have put in place a company policy that says they will test to 1.3 x MAP.......they seem to think that in such cases that the 1.3 x MAWP hydro is not and an adequate enough test....thats what has me baffled (technically you could make that argument but again why bother)

R

 

[RPRad]

Hi Mike

Dont you find when you use that feature in Compress its annoying, if I dont shut it off it seems to generate about 3x the amount of paper than a similar calc done under APV

R

 

[MikeG7]

It is asked for by some of my clients. I mean, the clients want it shown and recorded what the basis is for the hydrotest. It is also often asked to check that when the hydrotest is done, that the stress in the vessel does not exceed 90% of the vessel stress. Compress does a good job of showing the calculations, which is a good thing IMO.

I for one do see there is a logic / benefit to testing to 1.3*MAP, reason being as stated before that it is a check of the new condition of the vessel. What's the point of testing a new vessel treating it as a fully corroded vessel. That is not the best demonstration that the vessel is safely built and designed. If the corrosion allowance is large, then the hydrotest pressure in the shop is significantly reduced if the MAWP is the basis.

As an aside regarding your previous post to Gr2vessels, I can't agree that the USER specifies the MAWP or that you design to a MAWP. The MAWP (and the MAP) come out of the design checks.

Mikeg7

 

[MikeG7]

Sorry, a correction:

"It is also often asked to check that when the hydrotest is done, that the stress in the vessel does not exceed 90% of the YIELD stress"

 

[RPRad]

I agree that anytime you test above 1.3 x MAWP that you need to check against yield strength and the MAP calcs are good for that.

But I guess I would argue that designing off the MAWP is more conservative than designing off the MAP (and when I say MAWP the number used to start with is based on the max design pressure the Owner considers possible under upset conditions and some margin for PRV lifting). Generally for cost sake you are keeping the nominal in the neighborhood of the min required plus any corrosion allowance.

I can see if you have an extra large corrosion thickness or larger nominal thickness (which I guess you could see with low pressure vessels) than required where you could make that argument for the higher pressure test. But for the majority of vessels I dont think you are proving anything substantial other than a "feel good" thing. It seems to me that if this was a big issue with ASME they would have made it mandatory instead of making it optional or perhaps even left the test pressure at 1.5 x MAWP

Having said all that I guess I dont really have an answer with respect to the reverse of my original question which is if you are permitted to test at the higher pressure why not do it

 

[jtseng123]

RPRad,
In a typical project, the test pressure will be either using MAWP or MAP and specified upfront in the spec. It has nothing to do with corrosion or extra plate thickness used. If nothing specified, using the minimum test pressure will fulfill the code. Keep in mind the hydrotest formula is to simulate MAWP at design temp.

Our company standard uses MAWP to calculate test pressure. My current client's spec asking MAP to fully stress at new and cold condition so they can sleep much better. Does it have something to do with future re-rating that may require higher test pressure ? Who knows !

So, it is just preference. I am unwilling to proceed higher pressure unless it is mandatory because of the risk.
Ask your client which vessels or at what condition they what to test at higher pressure by MAP and issue a change order.

 

[RPRad]

jtseng123

I agree on all points you make, I guess the extra thickness thing is the argument for doing a higher hydro and sleeping better. I can safely say that in our local area I or anybody I have talked to (AIs included) have never been asked to design (in my 25 years) off anything other than the MAWP provided nor hydrotest using anything other than the MAWP....I have been asked to occasionally to have the hydro performed at 1.5 x MAWP...in which case I have to check against yield before proceeding...but never anything under UG-99(c). Also I cant say I cant recall an instance where a vessel that was hydro'd using the MAWP has had some problem down the road that would have been exposed by a higher hydro...perhaps its an oil and gas thing although I have done alot of those and have never been requested to use the MAP for anything related to a hydro...maybe its a new trend..but the gist i get from our clients and the AI is that they are happy with 1.3 x MAWP ... and I have to conclude that so is ASME or they would have been more specific with regard to when to use UG-99(c)

However now knowing that it is permitted if a client requested it, such as is the current case, I wouldnt object to it...but I also wouldnt go out of my way to recommend it either to other clients

R