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MAWP vs. Design Pressure
5

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

RE: MAWP vs. Design Pressure

See SecVIII Div1 Appendix 3

RE: MAWP vs. Design Pressure

(OP)
Please elaborate.  Per the last sentence of the MAWP definition (sorry if this is old, but I only have the 1995 edition to reference):

"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

2
Due to the practicalities of construction and fabrication, the vessel MAWP will likely be somewhat higher than the design pressure. eg: if the required shell thickness for design pressure plus static head is 0.6123", it wouldn't be unusual to specify 5/8" (0.625") plate unless you are placing a mill order to 1/100ths of an inch. This slight thickness over the required thickness then provides a higher MAWP, if analyzed. Of course, all components in the vessel would have to have their MAWP determined, including nozzles, cone-cylinder junctions, etc, which are not as elementary to determine MAWP for as are cylinders and heads.

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

Vessel fabricators, in my opinion, are prone to balk when asked for a vessel's MAWP. To some degree, that makes sense because the MAWP is sort of a "guarantee point". The fabricator warrants that the vessel meets code requirements for the stated MAWP pressure. Typically, they don't want to guarantee any more than they need to (would you?). The buyer is typically only concerned with a vessel capable of withstanding the pressure he/she has specified - the design pressure. In times past, there were extra efforts in calculating the MAWP, and the fabricators wanted to be paid if you required those extra steps. With modern vessel design software, the extra work is minimal, and the fabricators should be able to easily extract the MAWP. If they feed you back the design pressure and call it the MAWP, that means they are NOT supplying you with the real MAWP; there is always some difference between the two.
Doug

RE: MAWP vs. Design Pressure

In my opinion the MAWP is the design pressure unless calculations are made to determine the MAWP. If calculations are made then clearly whatever the calculations show is the MAWP.

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 MAWP is the lowest pressure which one or several pressure vessel components can withstand under the basic design conditions.Typically must be higher than the design pressure, because of reasons listed by TomBarsh above.
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

As it works in my area (I will assume it to be the same everywhere) the pressure stamped on the nameplate is the presure used for "design" during calculations.  If the calculation package indicates 50 psig as "design", then it is 50 psig that goes on the nameplate.  If someone would like the unit rated to 140 psig (or other calc'd MAWP), all calculations must be done again using 140 (or calc'd MAWP) as "design".  Using a design of 50 psig, and having the calc report a MAWP of 140 psig does not necessarily mean you can stamp the vessel to 140.  This is why most fabricators don't like registering to MAWP, it means they have to do their work twice, and it also erases any safety factors they have built in beyond code requirements.

RE: MAWP vs. Design Pressure

(OP)
Thank you all for the responses.  

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

(OP)
chaulklate,

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

The hazard I see with not running the calc's twice is that some of them are interdependent. If, for example, you run an opening reinforcement calc and take credit for the excess area in the shell based on the DP of the vessel, without checking for adequate reinforcement based on the MAWP, you could find yourself in a trap. Here's how I usually see it done when the design process is done well:

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

(OP)
Thanks for the details, jt.  I know I am making certain assumptions that do not necessarily apply to all cases... I didn't think about it until I read your reply, but it probably matters that our fabricator is using COMPRESS for their code calculations.  

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

jproj-

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

Using COMPRESS software (and likely other software as well), it is actually a pretty simple, one-step process. Or perhaps several of your steps are combined...think of tying your shoes, exlaining this to someone in detail may give them the impression that it is more complicated than it really is.

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

(OP)
Tom,

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

jproj,

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

(OP)
That's pretty much what I expected (basic logic the way I look at it... 2+2=Y is the same as Y-2=2).  

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

That's a good question. UG-116 "Required Marking" for the nameplate even shows that the MAWP (not "design pressure") is to be on the nameplate (see UG-116(a)(1)(3)). A footnote to this paragraph states "The maximum allowable working pressure may be assumed to be the same as the design pressure when calculations are not made to determine the maximum allowable working pressure." Sounds like the AI doesn't have much of a leg to stand on. But he's the "judge, jury, and executioner" in this case. The work required to produce the COMPRESS calculations for design pressure = chamber MAWP would be minimal; the hardest part is printing them, getting them distributed, approved, etc.

RE: MAWP vs. Design Pressure

jproj,

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

I fail to understand your comments about "rerunning the calcs". As powerful as software like COMPRESS is, are you saying that significant extra efforts are required to go from the vessel design mode to (essentially) a vessel rating mode. All the data is already in the program, so I'd think very little effort would be needed. However, I admit my background is not in vessel mechanical design, and I've never personally run COMPRESS. I more looking to be educated on this subject. Thanks.

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

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.

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

(OP)
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

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

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
 

RE: MAWP vs. Design Pressure

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...

RE: MAWP vs. Design Pressure

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

RE: MAWP vs. Design Pressure

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.

RE: MAWP vs. Design Pressure

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.

RE: MAWP vs. Design Pressure

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...

RE: MAWP vs. Design Pressure

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.

RE: MAWP vs. Design Pressure

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$$$

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