MAWP vs. Design Pressure
MAWP vs. Design Pressure
(OP)
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
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





RE: MAWP vs. Design Pressure
RE: MAWP vs. Design Pressure
"The design pressure may be used in all cases in which calculations are not made to determine the maximum allowable working pressure"
In this case (as explained above), calculations were made and the MAWP was determined. Are you referring to something else? The rest of the MAWP definition is in accordance with my previous understanding of vessel MAWP.
Regards,
Jproj
RE: MAWP vs. Design Pressure
To have a vessel MAWP that is almost 3 times the design pressure indicates a very conservative design and might mean that money could've been saved in the construction. But there's nothing really wrong with hell-for-stout.
The real issue here for stamping the nameplate is what is the test pressure based on? Consider a hydrostatic test. Per UG-99(b) the test pressure is based on the chamber MAWP or the design pressure if the MAWP is not calculated.
If the test is performed as based on the chamber MAWP (eg: 1.3*MAWP*stress ratio) then the nameplate may legitimately be stamped with the chamber MAWP. It would be inconsistent to require a test based on the MAWP but allow the name plate to reflect only the design pressure.
RE: MAWP vs. Design Pressure
Doug
RE: MAWP vs. Design Pressure
The difference between a MAWP of 140psig and a design pressure of 50psig may seem to be excessive but sometimes the practicalities of having materials of a thickness that can be handled in the works takes precedence.
That's what I remember from my days as a pressure vessel designer, although it was 40 years ago.
athomas236
RE: MAWP vs. Design Pressure
The 140psig compared to 50psig seems a bit odd, however, is most likely to be the rating of a flange, rather than a shell section rated for the 50 psig design pressure (or any other weaker component).
However, a carefully calculated MAWP for all the vessel components is unlikely to be much higher than the design pressure and I'd be worried with a high one (incomplete calculations, over-design, incompetence???) picked-up from blue sky. Sometimes, the fabricator is too lazy to do the calcs, particularly the manual ones and as noted above, will nominate, incorrectly, the design pressure as MAWP. Please note that the up-rating of the vessel for any reason, without correctly calculated MAWP at the design stage will be very difficult, if not impossible.
Cheers,
gr2vessels
RE: MAWP vs. Design Pressure
RE: MAWP vs. Design Pressure
Regarding concerns about the MAWP being much larger than the design pressure, it is a combination of several other design factors (full vacuum, allowable nozzle loads, MAWP limited by head / shell, etc.).
It seems like the consensus is in accordance with the Appendix 3 definition of "MAWP"... if calculations are made, show MAWP, if not, show the design pressure. This makes sense to me, but I'm still confused at their AI's reasoning (since it seamingly contradicts the above referenced definition).
Regards,
jproj
RE: MAWP vs. Design Pressure
This is exactly what their AI said. Can you explain why the caluclations must be run twice? It doesn't make sense to me. Using the same equations to calculate different terms should give you the same result (if they don't I think we have bigger problems...).
The way I see it, entering the actual vessel components into the calculations and calculating the MAWP is the same as entering the MAWP as the design pressure and calculating the minimum thickness required.
If I'm not looking at this correctly, please correct me.
Regards,
Jproj
RE: MAWP vs. Design Pressure
Necessary design pressure and design temperature are determined by a process engineer.
This DP/DT is used as a basis for the design by the mechanical engineer for the major components: The heads, shell, and perhaps other large cost items such as body flanges.
The MAWP of the major components is determined. (One could actually optimize one or a combination of the DP, DT, or corrosion allowance. I have a bias towards optimizing corrosion allowance based on my experience.)
The chosen MAWP (and corresponding DT and CA) is used as the new design basis. The major components are re-run with the MAWP as the design basis and the minor components such as nozzles are added to the calculation at this point.
The design is evaluated one more time for MAP (max allowable pressure, new and cold) which is then used as the basis of the hydrotest pressure.
jt
RE: MAWP vs. Design Pressure
It is my understanding that (in general, with COMPRESS at least) the calculations are run to determine the minimum required thickness (for internal / external pressure). Based on the results, head / shell thickness are chosen (next largest commercially available thickness). Next, the actuall shell / head thickesses are used to check the rest of the vessel (supports, nozzles, etc.). The final results (design pressure, MAWP, MAP, etc) are detailed in the "Pressure Summary". Obviously, one can run the calculations using minimum calculated thicknesses, but the pressure summary in this case would just show a MAWP equal to the DP, right?
Regards,
Jproj
RE: MAWP vs. Design Pressure
Yes, I believe that in the scenario that you describe - a once thru evaluation - that the MAWP would be very close to the DP since the nozzle reinforcement calc's will most likely take full credit for the excess thickness in the head/shell. Thus a minor component such as a repad will wind up governing the MAWP since its design is optimized and not the shell or head which it is reinforcing. This kind of approach frustrates me as an engineer who then has to deal with the inevitable rerate and simply since I've been brought up as an engineer to optimize resources.
A once thru design optimizes the fabricator's resources by saving them perhaps 15 minutes (I'm being generous...) of engineering time.
jt
RE: MAWP vs. Design Pressure
As the designer enters the information into COMPRESS, the software determines the minimum thickness required by Code rules. The designer can enter this thickness or some larger value as the "nominal thickness" of the component. In COMPRESS, if a thickness needs to be increased to meet changing design conditions the software increases it automatically.
Depending on the calculation/reporting options selected, COMPRESS reports the minimum thickness required for design pressure (plus static head, if any), and optionally reports the component's MAWP and MAP. MAWP is defined in Section VIII Division 1, Appendix 3. MAP (maximum allowable pressure) is not defined in this Code but is usually taken (as by COMPRESS) to be the maximum pressure that the component may withstand in the new (uncorroded) condition at ambient temperature, with no operating liquid.
These principles apply to the major "shell" components of the vessel; eg: formed heads, cylinders and transitions. Finding the MAWP for such components is usually very simple.
For example, for an ellipsoidal head there is only a single formula relating the thickness of the head to internal pressure. Solve for required thickness as a function of pressure, or maximum pressure as a function of pressure using only one formula.
A cylindrical shell has two different Code requirements for thickness: one based on circumferential stress and one based on longitudinal stress. In most cases the circumferential stress will govern. But longitudinal stress may govern for cases of tall towers under high wind. Finding the MAWP of the cylinder requires that both formulas be investigated, the lower value will govern.
Things get more complicated with a cone. Again there are two different formulas required to consider required thickness based on circumferential and longitudinal stress, and these must be checked at both the small and large diameters of the cone (of course, you can make a priori conclusions that one end or the other will govern). So now we have 4 formulas required to check the MAWP of the cone. But wait, we're not done yet! For cones we also have to check the cone-cylinder juncture requirements of Appendix 1-5. Strength of either of the junctions may also govern the cone MAWP. It's at this point that obtaining the MAWP is not a trivial task involving only 1 formula or several independent formulas.
Determining MAWP of nozzles gets even more complicated. There is no single formula that you can express to get the nozzle MAWP. For a given nozzle construction the MAWP may be limited by available reinforcing area, minimum nozzle neck thickness, minimum weld sizes, weld path strength requirements, rating of attached flange, stresses in shell or nozzle neck if there are external loads (WRC-107) on the nozzle, stresses per Appendix 1-7(b) if a large opening, MDMT rating, and likely many, many more criteria. These many unrelated criteria must all be met at the MAWP. Because they are generally unrelated it is impossible to define an algebraic system (such as an 8 x 8 matrix, etc) of equations to solve for the nozzle MAWP. Instead, each Code requirement must be checked one by one for each possible "candidate" MAWP. Using computer software makes the job of finding the nozzle MAWP tractable; by hand it would be very laborious and not practical. This explains why in the olden days vessel owners’ would indicate in their specifications things like "full nozzle area replacement", or "nozzles shall not limit MAWP", etc, so that a nozzle or some inconsequential part on a vessel would limit the vessel MAWP.
Tom Barsh
Codeware Technical Support
RE: MAWP vs. Design Pressure
Thanks for the insight, your last paragraph on the nozzle MAWP was especially enlightening (particularly the part about prohibiting nozzles limiting the MAWP).
I understand that these are very detailed and lengthy calculations. If, however, the MAWP was determined, shouldn't all component parts "pass" if the MAWP of the limiting component was entered into the program as the design pressure (keeping all materials & thicknesses the same)?
Regards,
Jproj
RE: MAWP vs. Design Pressure
Yes, if the limiting MAWP (the "chamber MAWP") is re-entered into COMPRESS there should be no change to the other components, all components should "pass" at this pressure (this is the same as what I would expect if I were doing the calculations by hand). By definition the MAWP of the pressure vessel will be the lowest of the MAWPs calculated for each component of the vessel.
I accidentally omitted in the list above of possible limiting factors for nozzle MAWP that the thickness of the shell to which the nozzle is attached may also limit the nozzle MAWP.
Note that, as jte alluded to, when using COMPRESS it really costs your fabricator no more time to include MAWP calculations for the vessel than not. The reports will be longer and may lead to more questions from a knowledgable vessel owner or their representative. Additional engineering time may be required to permit the designer/fabricator to optimize the construction as per jte's valuable comments.
Tom Barsh
Codeware Technical Support
RE: MAWP vs. Design Pressure
What still confuses me is why an AI would insist on new calc's using the MAWP as the DP when the current calc's (with the actual DP) already show the MAWP. Is this just an incompetent AI or just one that is unwilling to stamp anything with less than a 20x overall safety factor?
Regards,
Jproj
RE: MAWP vs. Design Pressure
RE: MAWP vs. Design Pressure
I can't disagree with you, it doesn't necessarily make sense, and I don't see how it differs from 'solving for thickness' vs 'solving for pressure', just stating my experience which seems to be the same experience as you. The long and short of it is, the AI's want to see everything calculated on a set of 'control' parameters. Which for me basically means determining MAWP on my weakest componenet (or as set out by jte) and then rerunning all calc's based on the 'new' pressure as indicated in MAWP report. And as Tom indicated, the AI gets the final say. But at least they seem to be consistent with their wants.
RE: MAWP vs. Design Pressure
RE: MAWP vs. Design Pressure
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
RE: MAWP vs. Design Pressure
- 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
RE: MAWP vs. Design Pressure
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
RE: MAWP vs. Design Pressure
RE: MAWP vs. Design Pressure
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
RE: MAWP vs. Design Pressure
Although I might understand the rationale behind this approach by AI, I don't concur.
Any thoughts?
Putting Human Factor Back in Engineering
RE: MAWP vs. Design Pressure
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
RE: MAWP vs. Design Pressure
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
RE: MAWP vs. Design Pressure
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
RE: MAWP vs. Design Pressure
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
RE: MAWP vs. Design Pressure
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
RE: MAWP vs. Design Pressure
... a man's got to do what a man's got to do...
RE: MAWP vs. Design Pressure
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RE: MAWP vs. Design Pressure
rjoaks
RE: MAWP vs. 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.
RE: MAWP vs. Design Pressure
The "design pressure" should be the pressure at the top of the vessel, with pressure adjusted at various points downward to include static head.
RE: MAWP vs. Design Pressure
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...
RE: MAWP vs. Design Pressure
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.
RE: MAWP vs. Design Pressure
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$$$