Hydrostatic Test - MAWP vs MAP
Hydrostatic Test - MAWP vs MAP
(OP)
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
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





RE: Hydrostatic Test - MAWP vs MAP
RE: Hydrostatic Test - MAWP vs MAP
R
RE: Hydrostatic Test - MAWP vs MAP
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
RE: Hydrostatic Test - MAWP vs MAP
RE: Hydrostatic Test - MAWP vs MAP
RE: Hydrostatic Test - MAWP vs MAP
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
RE: Hydrostatic Test - MAWP vs MAP
Mikeg7
RE: Hydrostatic Test - MAWP vs MAP
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
RE: Hydrostatic Test - MAWP vs MAP
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
RE: Hydrostatic Test - MAWP vs MAP
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
RE: Hydrostatic Test - MAWP vs MAP
"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"
RE: Hydrostatic Test - MAWP vs MAP
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
RE: Hydrostatic Test - MAWP vs MAP
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.
RE: Hydrostatic Test - MAWP vs MAP
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