MAWP vs. Design Pressure 5

[jproj]

Recently while trying to finalize a nameplate issue, I had an interesting discussion with one of our fab shops. They kept showing the vessel design pressure (50 psig) as the MAWP on their code nameplate drawings. The code calculations showed an actual vessel MAWP to be much higher (~140 psig). After several discussions, our fabricator said their A.I. refused to stamp anything other than the design pressure on the nameplate "MAWP" section. Vessel was designed to ASME Section VIII Div. 1 (current ed.).

While this isn't the first time I've heard of this practice, I'm curious about the reasoning behind it. As I understand it, the MAWP is a product of actual materials / thickness used for fabrication and really has very little (if anything at all) to do with the design pressure (unless of course components are designed to have a MAWP equal to the design pressure).

Any insight?

Best Regards,
Jproj

 

[jte]

djack-

Your thoughts are on target. That's why I felt I was "being generous" when I commented that it might add 15 minutes. It really shouldn't add more than 2 minutes to determine the MAWP of the major components based on DP and then change the DP to that MAWP for the rest of the design effort.

jt

 

[ggarciae]

here are two factors not considered:
- Ratio pressure/temperature is much often related in PV. Most of pressure vessels have aquous media. You may design a vessel to 50 psig (3 bar) and 142 ºC. But water temperature to 140 psig (10 bar) is 183 ºC. So stresses are not the same. Securities are not the same and so on.
- If manufacturer says design pressure is 50 psig, why must ans AI asume that equipment can work to 140 psig? By his own responsability?


Regards from Barcelona
G. García

 

[jte]

G. Garcia-

First, most of the vessels I've seen do not have an aqueous process fluid. Some do, most don't. Just a different industry than you are familiar with. But that is really irrelevant.

What is important is that the MAWP is not, as you pointed out, a stand-alone number. It must always be related to a temperature. From ASME VIII-1 Appendix 3: MAWP (is) the maximum gage pressure… at the designated coincident temperature for that pressure… So if calculations are made for the MAWP at the DT then the AI is, in fact, assuming nothing. It is incumbent upon the operator to stay within their equipment's design envelope. The AI is not expected to evaluate the process into which the vessel will be placed (then, decades later, sold and put into another process). That's what process engineers are for.

jt

 

[TomBarsh]

jt, A very good point. In some cases the MAWP of a vessel component may be limited by the requirement to meet the specified design MDMT. The details of this calculation can get very complicated in the case of a nozzle with reinforcing pad.

 

[jte]

Tom-

Good point about MDMT. The design envelope has a min (zero gage or some external) and max pressure with corresponging min and max temperatures.

jt

 

[VeryPicky]

Just a different perspective on the issue, and apologies if I missed this in previous postings in the stream. If the MAWP stamped on the nameplate is 50 psi versus actual 140 psi, that might be a legal and beaurocratic nightmare in the future if you want to rerate the vessel.
Although I might understand the rationale behind this approach by AI, I don't concur.
Any thoughts?

Putting Human Factor Back in Engineering

 

[jte]

Picky-

If the nameplate is stamped to 50 psi while calc's show 140 psi it is a fairly straightforeward issue to rerate the vessel. No nightmares involved. Its done all the time.

Nonetheless it is something which the fabricator should be doing as a part of providing a quality service to their client. Otherwise when the time comes to rerate folks like me have to spend several days collecting documentation and running the numbers to put the rerate package together for the AI/Jurisdiction to approve.

jt

 

[ggarciae]

jte: May be you are right and most of equipment is not aqueous fluid, Of course, it was an example. I can tell you a lot of PV where design pressures are function of steam pressure of fluids, but as you state thats another work.
I agree too that and AI is not assuming nothing when a PV is calculated and designed with any caractheristics whatever they are.But, you have quoted MAWP with DT both in the same sentence.So, what's the diference between MAWP and desing? or What is valid for temperature isn't for pressure? Really I think MAWP and Design pressure are the same. If a mfr. really wants a higher pressure he can say that and justify it with calcs as he prefers. AI only certify what manufacturer says.
As VeryPicky states, legal and burocratic issues as well as security issues may be involved. Another example, PV riscs are bigger with higher factors pressure*volume. As power boilers and rooms for them are in some countries classified and they have got requirements for distances, concrete wall thicknesses, etc. some power boilers have a design pressure bigger than the set pressure for PSVs (in spain we call it MAWP).
I know an ASME vessel can be stamped with different ratios pressure/temperature. Authorities here in spain don't consider that point. I apologise for any opinion out of focus and for my english too.
So, there are different points of view (process eng., PV manufacturer's, AI for construction, facilities requirements, legal issues, etc.)
But for me, design pressure or MAWP in construction stage is the one a PV can withstand in that condicions as stated by mfr. I think that, as a user, you cannot change any of main characteristics without new checks. And fluids are within main conditions too.
If PV mfr. don't clarify what pressure is the vessel manufactured for I support AI's opinion.


Regards from Barcelona
G. García

 

[jproj]

G. García,

I'm not sure how familiar you are with vessel design calculations, but The design pressure (minimum and maximum) must be initially used along with the design temperature (minimum and maximum) to determine the minimum thicknesses required. As we all know, calculated values are not typically available. It's probably going to be very difficult, if not impossible, to find 0.326875" thick plate). The actual plate used will be the next largest available thickness (for the case given above, plate used would typically be 0.375" thick). The thicker plate is able to withstand more pressure at the design temperatures than the actual design pressure. Therefore, it was given another name = "Maximum Allowable Working Pressure" (MAWP). Obviously, as others have pointed out, there is quite a bit more involved than my simple example, but the point is still the same.... MAWP is not the same thing as the design pressure.

Regards,
Jproj

 

[rhg]

Please, see ASME VIII Div.1 UG-99(c)"A hydrostatic test based on calculated pressure may be used by agreement between the user and the manufacturer....."
Also see UG-99(d)".......The requirement of (c) above represent a special test based on calculations. ...."

The term "agreement" and "special test" is equal to money.
Use MAWP when agreement is reached.

Regards
rhg

 

[GenB]

In my opinion and understanding Design MAWP is the
allawable stamping per drawing means that you have an ASME Desingn; the design can be to the maximum allowable by the Code; then from there you can decide your desired "MAWP" to be stamped on the vessel.
ussually yo design to the MAWP to be stamped + whatever +marging you want to add as corrossion or simplier.
You can also design and stamp (on drawing at certain pressure then when you fab.the vessel you decide)to lower the Stamping MAWP it is allowed and possible,
but it will take a few steps to comply as Dwgs have to be deviated with the inspectors (AI) permission and ussually it is not a problem. I do not see one. I have some inspection background studies so from that point of view there isn'y any ASME Code or NBIC indicating the contrary.genb

 

[twistobar]

Another reason not to calculate MAWP for a new vessel, unless a customer required so, is a lucrative post-construction rerating business...

... a man's got to do what a man's got to do...

 

[unclesyd]

Look on page 15 of this document for a graphical representation of the MAWP, MOP, Test pressure.

http://www.llnl.gov/es_and_h/hsm/doc_18.02/doc18-02.pdf

 

[rjoaks]

The actual stamping could be the weak point of a vessal. Dies for stamping are usualy low stress type. The Stamped metal could control the MAWP unless the stamping is on an attached plate welded to the vessal.

rjoaks

 

[thermmech]

I read the thread with interest, and I do agree with Tom B. and others re MAWP. However, I have a comment re the "Design Pressure".

As of 18 months ago in my juristidction the local authority started insisting that the Design Pressure = MAWP + static head (if applicable).

Therefore, they required us to rename on our drawings the original "Design Pressure" to "Process Design Pressure".

In the cases where there is no appreciable static head (i.e. small S&T, aerial coolers, etc), the Design Pressure is equal to MAWP.

So, just the naming convention has been modified, but it created some confusion.

 

[JStephen]

And what is your jurisdiction?

The "design pressure" should be the pressure at the top of the vessel, with pressure adjusted at various points downward to include static head.

 

[twistobar]

By the definition, a static head is a part of the design pressure. (Please see ASME VIII-1 App.3).
The static head application is "0/1" situation.
Whether one has it or has not.
Example: if "non-appreciable" static head of 0.3 psi makes design pressure 285.3 psi, the ASME B16.5 150# carbon steel flanges suddenly are not good anymore, and one of the things or their combinations has to happen:
-design pressure reduced;
-design temperature reduced;
-flange rating increased...
Regards,

... a man's got to do what a man's got to do...

 

[thermmech]

JStephen: I am in Alberta, Canada.

twistobar: I agree with you - in principal. However - the approach here is such that the static head is included in the calculations of components' thicknesses only when it creates appreciable effect on the thickness.

The error that is introduced by ignoring static head in S&T/AC HEx and boilers is well within the "noise" introduced by other uncertainties such as actuall thickness of the components, mechanical properties of materials, etc. It will not make a difference in most of the exchangers/boilers.

I did use static head in calculations of low pressure tubesiedes of sulphur condensers which have DP = 14.9 psig and are usually of larger diameter. I assume that in design of tall/large low-pressure vessels 0.3 psi is significant.

 

[DoubleARon]

jproj,

my question would be why do you care if it was stamped with design pressure vs MAWP? If the stamped design pressure is sufficient for operating pressure and temperature, the lower stamped pressure is a positive. When your inspectors calculate the required thickness for remaining life calculations during their internal inspections your remaining life will be maximized, hence longer inspection intervals, and cost savings$$$