Why do you need MAWP? and Disadvantages of MAWP 11

[mechengineer]

Why do you need MAWP?
A customer needs a pressure vessel to operate in an operating pressure & temperature. After the consideration of the design margin to get the design pressure & temperature to design the pressure vessel by a vessel manufacturer. It should be a very simple thing without MAWP. However, MAWP makes a design of pressure vessel became two designs, one is based on the design pressure and another is based on the MAWP.
I disagreed with the comment that MAWP is the way for optimal design of pressure vessel to save material. Design engineer will be unlikely to increase the thickness if the thickness is sufficient with the design pressure, no need using MAWP to prompt for the overdesign.
Disadvantages of MAWP in pressure vessel design
• If any thickness change of the pressure vessel components will affect the MAWP and further affect the pressure of the hydraulic test. The hydro-test pressure in GA may have to change in very revisions. If use design pressure, you would not be worry about the hydro-test change due to a thickness change caused by the material availability in work shop or any other reasons.
• PV Elite does not use MAWP, but use the design pressure to calculate the saddle support, the skirt support and nozzle external load analysis by WRC297 or WRC107. You have to use MAWP as a ’design pressure’ (set the design pressure =MWAP in PV Lite) to re-run the all programs.
• Double engineering work.
• Misleading and catering to customers' mentality of higher design pressure and more safety. ‘The MAWP to be calculated…, the MAWP shall not be limit by flanges ….’ is a typical in most of client specifications.
Solution:
Always makes a design pressure as MAWP in your design if possible.
I sincerely appeal to EPC engineers or users, according to the contract and data sheet, the manufacturer should only be responsible for the safety of the pressure vessel under design pressure. If the working pressure of the vessel has never been upgraded during the entire service life, such situation (pressure upgrading) is rare in practice, the MAWP design may be meaningless and waste engineering man-hours. You also spend engineering man-hours to review MAWP for nothing.

 

[david339933]

Lol...well I will give you a star for that. MAP other than MDMT is also known as "new and cold" which includes CA.

 

[SnTMan]

DandB, if your cited standard, or a client specification with this language is accepted as part of the vessel contract you are bound by it. It is "true in reality", and you may not ignore the nozzles..

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Christine74]

The MAP is defined as the highest permissible pressure as determined by the design equations for a component using the nominal thickness LESS CORROSION ALLOWANCE and the maximum allowable stress value

That's certainly not how MAP is typically defined in the industry. MAP is sometimes called the "new and cold" maximum allowable pressure because it's calculated based on the allowable stress values taken at ambient temperature with the corrosion allowance set to zero.


-Christine

 

[Christine74]

"The basis for calculated test pressure
in either of these paragraphs is the highest permissible internal
pressure as determined by the design equations,
for each element of the vessel using nominal thicknesses
with CORROSION ALLOWANCES INCLUDED."

"Corrosion allowances included" is very imprecise language. One engineer could interpret this to mean that the design corrosion allowance should be incorporated into the calculations by subtracting it from the nominal thicknesses, and another could interpret it as meaning that the material thicknesses used for the calculations should NOT subtract off the corrosion allowance.

Better to use terms like "uncorroded condition" or "fully corroded condition."


-Christine

 

[r6155]

Christine74
These definitions are in ASME VIII Div 1 , you can make the claim to ASME.

Regards

 

[Christine74]

Sorry r6155 I didn't realize that you were quoting straight from the Code but yes, the wording is ambiguous and should be rewritten.

I also think that ASME must have made an error in their definition of MAP. Why would you use the corroded thicknesses for the calculations when the equipment is brand new?


-Christine

 

[r6155]

@ Christine74
See ASME VIII Div.1 UG-16 (e) Corrosion Allowance in Design Formulas. The dimensional symbols used in all design formulas throughout this Division represent dimensions in the corroded condition.

Regards

 

[KevinNZ]

In 35 years of purchasing pressure vessels we have never asked for your got any anything other than design pressure. A higher pressure is pointless when all the other parts and piping and pressure protection is to design pressure.

If max allowable nozzle loads have been calculated at design pressure then any higher shell pressure reduces the max allowable nozzle loads and you can not operate the vessel at a higher pressure.

 

[r6155]

@ KevinNZ
It may be that something was wrong in these 35 years.

Regards

 

[SnTMan]

If max allowable nozzle loads have been calculated at design pressure then any higher shell pressure reduces the max allowable nozzle loads and you can not operate the vessel at a higher pressure.

Some truth to this, but a different can of worms altogether

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

SnTMan,

If that can of worms operates at more than 15 psig you'ld better do something about it. Contingent upon jurisdiction of course.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[SnTMan]

IM, always a topic of contention as to whether or not to run fictitious nozzle loads at DP or MAWP. Same for Zick and other structural type stuff. Implies a greater degree of confidence in the data and computational accuracy than exists.

Ask two people, get three opinions

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

Puts me in mind of a great quote from Groucho Marx:

​

“Those are my principles, and if you don't like them...well I have others.”

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[SnTMan]

The Master

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[mechengineer]

Since it cannot let MAWP 'disappear', the best design is that make MAWP to be equal or close to design pressure as possible as you can. Don't leave a chance to attract a customer to argue with you for such little additional "benefit" and make the 'additional bebefit' becaome a main problem in your design.