MAWP on Nameplate 1

[ElCidCampeador]

Hi,

I've just received an order for building 1 vessel acc. to ASME VIII Div.1, Stamp U.
It's a very simple vessel, sort of filter, made from pipe and caps, carbon steel.
On the drawing it's written 10 bar as design pressure, and 23 bar as MAWP. Hydrostatic test is according to UG-99(b).
Calcs (given me by the client) were made using design pressure as reference for estimating minimum thickness of components. However, in one page is reported the MAWP as written on the drawing.
My question is: what MAWP do I have to stamp on the nameplate? Usually my AI advises me to put DP=MAWP, but in this case? if I wrote 23 bar on the nameplate, wouldn't this imply that the vessel is verified for an higher pressure, higher than the real one (10 bar)? Do I have to redo calcs at 23 bar?

Thank you

 

[david339933]

Often times the thickness used in fabrication will exceed the minimum required thickness for design. Calculations can then be done to see what the MAWP would be at that increased thickness. Either one can be put on the nameplate, but you need to show that 23 bar is indeed the MAWP.

 

[ElCidCampeador]

Thicknesses in the calculations are verified for design pressure of 10 bar. Obviously adopted thicknesses are higher than minimum; but if i didn't want to redo calcs with other pressure, what MAWP should i have to put on the nameplate? tnks

 

[LittleInch]

I find it very difficult to believe any design code would allow any pressure higher than the Design Pressure ( the key is in the name) to be marked, stamped or otherwise acknowledged in any documentation whatsoever.

If the thickness is indeed higher than the minimum required for design, that's irrelevant. It's what the design is that matters.

Does this thing have any flanges or other pressure rated components?

If you want to stamp it 23barg @ X deg C, then design it for 23 barg at X deg C. Simple.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[david339933]

See 3-2 MAWP Definition: "The design pressure may be used in all cases in which calculations are not made to determine the value of the maximum allowable working pressure".

 

[Jano6924]

As david339933 states, and per 3-2, the design pressure can be used as the MAWP if you don't make calculations to find the real MAWP. However for this case I would prefer to make sure is your client's MAWP is correct. Since you say this is a simple vessel it will not take long for you to find if your client's MAWP is correct or not, it's a matter of safety. Or you can ask your authorized inspector.

Regards.

 

[Jano6924]

Sorry, I forgot to mention that you can see UG-116 and note 36 in the End Notes section of ASME VIII-1.

I apologize.

 

[GenB]

I think the Eng made the mistake of swapping the numbers. MAWP 10 n design 23. Why not asking I stead of breaking your head...

 

[Afox88]

GenB,
MAWP has to be higher than design pressure, otherwise the unit would not pass CODE.

ElCid,
You can either stamp 10 bar or 23 bar on the nameplate, but have to provide calculations showing that the MAWP that you mark on the nameplate passes CODE.

 

[ElCidCampeador]

ElCid,
You can either stamp 10 bar or 23 bar on the nameplate, but have to provide calculations showing that the MAWP that you mark on the nameplate passes CODE.

For "passes" do you mean, for example, to substitute the pressure in the calculation of minimum thickness of all components with the MAWP, and then verify that this "new" min.thk is < adopted thk (considering undertolerance, corrosion, etc...)?

 

[LittleInch]

Ok, I think this highlights the difference between Pressure vessels and piping design codes.

This post gives most of the answers.

http://www.eng-tips.com/viewthread.cfm?qid=229469

Basically as far as I can figure out - to start with MAWP is pressure at the top of the vessel and design pressure the pressure at the bottom including any liquid head. This is used to design the vessel with min thicknesses of plates etc.

However if you then build it with thicker components, you can take the corroded thickness ( assumed of course at the start of design) and back calculate the MAWP of each main component. The lowest MAWP becomes the MAWP of the vessel.

For small diameter simple vessels you can easily need to use thicker pipe than you need for practical purposes.

However you can only stamp MAWP of any figure if you've got the design data to prove it.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jte]

Little Inch-

Welcome to pressure vessel design! Yes, it's a tad different than piping, and even more removed from pipeline work.

You almost got it right in your summary above: The Design Pressure of the vessel is what the process engineer specifies at the top of the vessel. The design pressure of a given component takes the process DP and adds any hydrostatic head and then the component is designed. That's all well and good. The code, since the original Boiler Code of 1914, has included the premise of optimizing the design. In most cases when optimization is performed, it takes the form of MAWP which is the back-calculated maximum pressure of the vessel as designed. You might think of this as a "pre-stamping rerate". However, as others have noted above, Section VIII Div. 1 has allowed for a lack of optimization of the steel, specifically in that there is no requirement to go back and rerate the vessel to determine the MAWP.

One crucial point you miss in your post above is that all components, not just the main components must pass muster with VIII-1 design requirements and the one with the lowest pressure rating then sets the MAWP for the entire vessel. Many company spec's have a statement to the effect of "minor components shall not govern" but this is a (good, in my opinion) user's decision, not a code mandate.


ElCid-

It is not clear to me what your contractual obligations are in this case. From my perspective, if I were to place an order with you and state in the data sheets that I want or expect a 23 bar MAWP, then that's what I expect to see on the nameplate. The 10 bar design pressure is of value only in that it indicates to future engineers what the process engineer decided that they need. This may be what the relief system is designed around, but it doesn't change the fact that I asked for a 23 bar MAWP. As Afox88 points out above, it is the manufacturer's (i.e. your) responsibility to provide design calc's that back the 23 bar MAWP.

 

[LittleInch]

jte,

Glad I got it nearly correct, most of the time I don't get involved in PV discussions for obvious reasons, but this one I couldn't help myself - it was useful to understand how it works though and that's the value of this site - it forces you to go and find something out that you didn't know before.

for me normally Design pressure is King, nothing should normally exceed it (hydro and intermittent excepted). Clearly PVs work a little differently.....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[GenB]

JTE. We're are talking here of two types of design pressures. You and I are taking about ASME Code design pressure.
I. Chemical as Afox88 design pressure is their used pressure. That's the controversy. I stand that the specifier shall be contacted to clarify the MAWP.
But it seems that the order came from Chemical so the calculations should be to the higher pressure. When the poster, ASME engineer does the required re,calc then he can see if the vessel Comply to the higher pressure or not. Real simple.
Regards to all. Gen blr.

 

[jte]

LittleInch-

Yep, the very same reason I try to stay out of pipeline issues. Different environment, different code philosophies.

jt

 

[ElCidCampeador]

I ask AI about this problem

In my case, as he says, stamping on the nameplate 23bar as MAWP is correct because:

1) 23 bar is the lowest MAWP among all the items (as ASME required)
2) DP is lower than MAWP, which means that the vessel is built with "extra-tickness", not only with the strictly minimum required. Using DP as MAWP would mean undervaluing the "potentiality" of the vessel.

 

[loilfan]

What I have seen happen is the owner will provide the designer with the required design information (number of nozzles, size of nozzles, design pressure, design temperature, MDMT, etc.) In your case, the owner likely specified that they need a vessel designed for 10 bar and the designer came back with a design that is certified for 10 bar, but it has an MAWP of 23 bar. Many programs automatically calculate this and you need to actually uncheck something for it to not tell you.

23 bar is significantly higher than 10 bar, so this vessel is likely way overdesigned. But if that's the case, then so be it. The nameplate should be stamped based on what the hydrotest pressure was assuming there are calculations to support that pressure. If it was hydrotested at 1.3*10bar, then you can not stamp the MAWP at 23 bar into the nameplate.

 

[ElCidCampeador]

Hydrotest pressure is 1,3*MAWP, so I think that the analysis of AI is correct.

 

[eduardoeh]

Agreed with the AI. MAWP should be the lowest from any element in the Vessel. One may have chosen a thickness that withstand 300 psi at 80 deg F, but if you have a #150 flange then your MAWP will be 285 psi due to the flange.

 

[GenB]

The whole thing does not make sense. You all know wat it is. The design and dwgs are wrong so let them correct the mess. All the answera that I see above are based on opinion. The Code is clear and cannot be deviated. You all know that stamped pressure cannot be higher than designed or calculated pressure. Good luck to all.