Design Pressure as MAWP 1

[tabbymulla]

Hi All,

I was searching ASME Section VIII Div 1 for a provision that MAWP can be taken as Design Pressure i.e

MAWP = Design Pressure

Even if MAWP is calculated during design calculations (Obviously its value is higher than DP). However, I coudn't find one. So, I thought someone here could guide me. Please

Regards,

Tabby

 

[SJones]

http://www.eng-tips.com/search.cfm?q=MAWP&action=search

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[XL83NL]

For the 2007 edt 2008a addenda; see UG-99 note 34. It's there.

 

[tabbymulla]

Well it says that MAWP can be assumed to be equal to DP if calculations are not performed.

But problem is,

Calculations have been performed for all parts and still the DP is marked as MAWP. Does it violates ASME VIII DIV 1 or is it allowed somehow or by some para of the code?

Regards,

Tabby

 

[SnTMan]

tabbymulla, I assume your software had calculated MAWP's per component automatically, and maybe tabulated them.

If you have not confirmed them and selected the limiting MAWP(s) for the vessel, I would interpret that UG-99 note 34 is met, and you may use DP.

Regards,

Mike

 

[tabbymulla]

Wow Mike. It is exactly as you described. The MAWP has been calculated by the software and also tabulated later in report. I could'nt have guessed that this also has to do something with Code requirements or I wud have mentioned this at the beginning.

So now I conclude that if MAWP calcs are done by designing software on default basis then according to CODE MAWP calcs have not been made and so MAWP=DP.

BTW, as you mentioned to confirm them, what form of confirmation is required by the CODE (ASME VIII Div 1) to rate vessel as per actual MAWP. Hand calculations?

While reviewing the design calculations of vendor we simply verified the actual MAWP using another software (PVElite) but if we require vendor to confirm the MAWP calcs, what they must do additionally? They provided their software the actual thickness they are using for fabrication on basis of which their software showed the actual MAWP.

I think, for confirmation, they will again use the same software for calculation of MAWP which will use the same procedure as it did before.

Regards,

Tabby

 

[SnTMan]

tabbymulla, by confirm I did not mean to imply that they necessarily be checked with other software, hand calcs and so forth.

What I meant is that the designer examine them, feel good about them, perform addtitonal calcs, etc as required. As opposed to blindly accepting what the software gives.

For example, I generally use Codecalc (PVE) for my shell & tube calcs. It calculates MAWP's. It has limitations. I don't trust everything it does. If I am going to assign limiting MAWP's and components, I had better have a look.

Additionally, as my practice, I will not assign MAWP's in excess of design pressures to fixed tubesheet exchangers, the more so as Part UHX became effective. The calculations are sufficiently complicated that I cannot readily confirm / trust them. I don't care what the software tells me.

How your vendor handles this is his choice. Code makes no special requirements on these things, MAWP's are normally determined just be solving the equations for pressure rather than thickness. Easy for a single component. Not always so straight forward for flange pairs, and so forth.

Regards,

Mike

 

[tabbymulla]

You mean even if the calculations for actual MAWP are made by software or by someone, designer or someone must endorse them for the AI to consider stamping vessel against actual MAWP.

Obviously someone who would endorse them would demand more. So extra cost must be spent to rate the vessel against actual MAWP even at the time of fabrication of the vessel.

So in order to decide whether to ask vendor for actual MAWP rating or not, I would have to consider possible future possibilities of vessel operation at higher pressures and compare with "Re-rating Cost of Vessel".

Did I get that right?

Regards,

Tabby

 

[SnTMan]

tabbymulla, yes, someone, typically the vessel designer, has to take responsilility for and properly document them, same as any of the other calculations, design details and so forth.

Yes, as it is a bit more work to do, it may cost extra, depending on the complexity. Typically less expensive to specify it at time of inquiry rather than later.

The vendor then has the responsibility to take exception, or comply and price accordingly.

Regards,

Mike

 

[Duwe6]

Tabby, it is essentially impossible to obtain an ASME "U" stamp without those aforementions calculations. They are an essential, integral part of obtaining the certification and Stamp. It is also impossible to obtain a MAWP higher than the Design Pressure. But, if you have the Project people I have had, you can get an MAWP much, much lower than Design.

If you request a carbon steel Exchanger 14-inches in diameter, the Hx shop will use a piece of 14"STD pipe [pipe is cheap, and 14" is too small to roll]. The Design Pressure, with a Corrosion Allowance of 1/16" and a Joint Eff. factor of 85% is 918 psig. If the Project folks ask for a MAWP of 125 psig, that is what the Hx will be hydro'd to, and that is what it gets stamped on the Code Data Plate and entered on the U-1 form. Without a Code-stamp-holding firm completing an Alteration on an R-2 form, that 918 psi Hx cannot be allowed to exceed the 125# MAWP.

 

[josenieto22]

The MAWP is in the top Of pressure vessel, the pressure desing is by component, you need add the static head by component

 

[tabbymulla]

Thnx Mike. It is all clear to me now.

Duwe, I think the value of 918 psig u r referring to as Design Pressure is acutally the MAWP for the pipe and 125 is the DP which is also allowed to be marked MAWP for the HX. HX shop may take higher thickness pipe if it costs less but they wont be fabricating nozzles and tubesheets for more than asked design pressure. I mean our fab shop does the same when we fabricate HX ourself. And, I have seen MAWP less than DP only in cases where there is a corroded vessel with FFS assessment after years of operation.

Regards,

Tabby