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.

 

[r6155]

You are some confused
Read again UG-98, UG-99 and "calculated test pressure" as defined in 3-2.

Regards

 

What is the purpose of MAWP for?

Are you saying it is possibly redundant, like your sentence?

Always makes a design pressure as MAWP in your design if possible.

I'm not a designer, but that sounds inherently very hazardous. Follow r6155.

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

 

[0707]

IMO Design Pressure is based on the code of construction of a new vessel.
Maximum Allowable Working Pressure is the pressure calculated along the remaining life of a vessel.

luis

 

[david339933]

I agree with some of what you stated...can't make heads or tails of most of it.
You do understand that the design pressure and MAWP can be the same thing? If you don't calculate the actual MAWP, your design pressure is the MAWP.

 

[marty007]

In some ways I agree with your general idea of keeping the MAWP equal to the design pressure, and also share your frustration with having to calculate and often adjust the MAWP and hydrotest pressure every time the customer makes a change request during fabrication.

I have worked for a smaller EPC in the past, and don't really understand it from their perspective either. Let's say you have a process unit that is made up from multiple pieces of process equipment. During the system design, an appropriate design pressure is selected and applied to all associated equipment. Now the equipment goes out for fabrication, and separate MAWPs are calculated for each individual piece of equipment. If two pieces of equipment have different MAWP values stamped on their nameplates, but are protected by a single relief valve, then you're not getting any benefit from the slightly higher calculated MAWP on one of the pieces of equipment. The system will still be limited by the lowest MAWP in the system. What benefit is there to the end user?

I've often wondered if the more useful approach would be to set the MAWP equal to the design pressure, but calculate the maximum allowable corrosion allowance. This would leave all pieces of equipment in the system with the same MAWP (= design pressure), but would provide a benefit to the end user as they would get slightly higher corrosion allowances for some equipment based on selected material thicknesses. This might be a very slight increase (eg. 0.125" -> 0.13"), but this could allow them to potentially delay equipment replacements in the future by a year or two, which could be a real benefit to plant operations in certain situations.

Cheers,
Marty

 

[marty007]

You do understand that the design pressure and MAWP can be the same thing? If you don't calculate the actual MAWP, your design pressure is the MAWP.

Yes you can by code, but many customer standards/specs require the fabricator to calculate the MAWP. Contract requirements from the EPCs and owners are driving the requirement.

 

[TGS4]

marty007 mentioned the concept of MACA (Maximum Allowable Corrosion Allowance). This concept is gaining credibility and acceptance in the industry and should help to deal with some of the concerns listed here. This concept was introduced in 2016 here and here.

 

[david339933]

Thanks Marty...now I grasp what the OP is stating, and tend to agree. I could never understand the advantage of calculating MAWP, especially when there is a known operating pressure for a complete system.

 

[r6155]

The purpose of pressure test is to verify the structural integrity. This is reached when the stress is 1,3 x allowable stress.
Example: seamless shell, allowable stress 20,000 psi , corrosion= 0.
Required thickness= 0.315” selected thickness: 0.375”
Now with this 0.375” the actual stress is 16,800 psi this is far than allowable stress 20,000 psi
Hence we need to define the pressure that correspond with actual thickness 0.375". This is the MAWP.
Now pressure test is MAWP x 1,3

My specification require UG-99(c)

Regards

 

[david339933]

And then on the first inspection, you have to de-rate your vessel because it is no longer .375".

 

[SnTMan]

Maybe kind of circular logic, but MAWP calculation has typically been performed because the owners asked for it. They must have seen some benefit. Systems were sometimes up-rated with respect to pressure. I suppose this may provide benefits in some processes, perhaps not in others.

Most software calculates and tabulates component MAWP and MAP and the better ones will report the limiting MAWP / MAP. When calculating MACA with software design to calculate MAWP, care must be exercised that the original design is not changed.

When sufficient buyers require MACA rather than MAWP calculations be made the software can be changed. Then someday we can have this conversation again, perhaps with respect to temperature

EDIT: How are standard flanges and so forth to be treated?

Regards,

Mike

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

 

[marty007]

Most software calculates and tabulates component MAWP and MAP and the better ones will report the limiting MAWP / MAP. When calculating MACA with software design to calculate MAWP, care must be exercised that the original design is not changed.

When sufficient buyers require MACA rather than MAWP calculations be made the software can be changed.

Very very true. We had a customer a while back asking for MACA and it was a pain using currently available software. You'd have to design the vessel for the design pressure, then gradually increase the corrosion allowance until the software automatically changes something, then take a step back. On small vessels it's not too bad, but on bigger vessels with lots of nozzles, it was a real pain!

 

[SnTMan]

Body flanges, tubesheets, floating heads

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

 

[marty007]

The purpose of pressure test is to verify the structural integrity. This is reached when the stress is 1,3 x allowable stress.
Example: seamless shell, allowable stress 20,000 psi , corrosion= 0.
Required thickness= 0.315” selected thickness: 0.375”
Now with this 0.375” the actual stress is 16,800 psi this is far than allowable stress 20,000 psi
Hence we need to define the pressure that correspond with actual thickness 0.375". This is the MAWP.
Now pressure test is MAWP x 1,3

My specification require UG-99(c)

Regards

Yes, we could still hydrotest in a way to test the structural integrity of a vessel. This could be done by calculating the MAP of the vessel in the cold and new condition with 0" corrosion allowance per UG-99(c). At the same time, we could calculate the MACA for the vessel, leaving the MAWP equal to the design pressure, providing the end user with additional corrosion allowance.

 

[r6155]

@ david339933
corrosion= 0 in my example, then the original thickness 0.375" remain the same

Regards

 

[marty007]

Body flanges, tubesheets, floating heads

Good point... If software were to start providing a means to automatically calculate the MACA, how should it handle common elements on heat exchangers? If your tubesheet is the limiting component, should it give the corrosion allowance to the tube side or shell side? Or... do we start listing MACA for each individual part on a pressure vessel (ugh... please no!)

 

[SnTMan]

Or... do we start listing MACA for each individual part on a pressure vessel (ugh... please no!)

Listed or not, it seems MACA would need computed for each component to find the limiting one. How common elements would be handled I don't know. I don't actually know if there is an accepted method to calculate MAWP of a flange pair (2-5(2)(2)) (2-5(a)(2)) .

Can 'O Worms

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

 

[DriveMeNuts]

ASME VIII-1 End Note 35 deals clearly with this:

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.

So, calculating MAWP is optional and therefore there is no need for double engineering work or re-working calcs because of a thickness change.
For my work, a process engineer typically provides the DP, from which the vessel is designed. The true MAWP is not calculated as it is irrelevant. It is very irritating when a customer asks for a separate MAWP calc "after" placement of PO. The resulting dispute about who pays for the unplanned engineering hours does not look pretty.

 

[SnTMan]

Buyer asks, buyer gets. At whose expense is a matter for the project managers...

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