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.

 

[DriveMeNuts]

Nope, the EPC placed the order without providing any end-user customer specs, and therefore agreed to our specs.
Our specs provided in the quote specify DP design basis only and that the customer must request MAWP before purchase.
Making request after PO placement means the EPC pays (plus a 300% markup for the extra work, of course ).

 

[SnTMan]

Nice add if you can get it. Congrats

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

 

[Trestala]

MAWP is useful for HAZOP as it may be referred for potential abnormal operation or trip cases so major EPC's and end-users typically require it to be calculated.

 

[marty007]

MAWP is useful for HAZOP as it may be referred for potential abnormal operation or trip cases so major EPC's and end-users typically require it to be calculated.

In my experience at least the preliminary HAZOP has already been completed by the time equipment is purchased. If having an MAWP over the design pressure is what makes the HAZOP work, then isn't there is a problem with the design process?

 

[Morcuse]

MAWP is asked to be calculated by users (and it is also asked that shells and heads govern this calculation) in order to take full advantage (strength-wise to the allowed extent of div. 1) of the main components of the vessel.

Reinforcing small details that could also govern the vessel MAWP such as nozzles result usually in such a small increase of cost, that it's worth having the workshop quote the additional 5 minutes of software preparation required when I submit my design specifications

 

[mechengineer]

marty007 - I am not only frustrated with MAWP or MACA. 20 to 30 years ago, I used 10~20 pages of hand-calculations to complete a design of pressure vessel. Now, I see a common pressure calculation containing more than 600 pages by using PVElite, NozzlePro, StarrdPro and Spreadsheet. So many new requirements, such as MAWP, MACA, nozzle external load, a pressure thrust load..., regardless of whether it is applicable or has analytical value all put into the calculation. It is really pain. I hope a pressure vessel design to be simple and practical.

 

[mechengineer]

In general, the pressure vessel design & fabrication is according to design pressure, corrosion allowance and design service life for 20 years. If the service life is beyond 20 years, no matter what is MACA, the pressure vessel must be fully reassessed, not to say that if the MACA is greater than the CA, the pressure vessel is allowed to continue to be used. Reassess the pressure vessel in servicing is another professional field, it is different with a new pressure vessel design. I think that a difficult point is how to assess the material properties after 20 years-service. For example, the vessel service is in sour service and HIC corrosion condition, even if MACA is larger than CA, the vessel may not be allowed to continue to be used after 20 years of service. MAWP and MACA have a same characteristic that are not sure whether it will work in the future. But I believe that 90% of pressure vessels are used in design pressure and CA conditions. Why should our design consider an almost impossible situation and increase the design cost?

 

[0707]

Diferences between Design pressure and MAWP

luis

http://www.differencebetween.net/science/difference-between-mawp-and-design-pressure/

 

[Morcuse]

Why should our design consider an almost impossible situation and increase the design cost?

Two reasons:

1. A MAWP calculation on a "regular" div. 1 vessel will not result in a significant time increase, when done properly from day one. In reality with the current software capabilities all you need to do is set up your commercial tools to backcalculate all your elements to a MAWP, provided you spend a small ammount of time proofchecking it.
2. A small increase in pressure-retaining capabilities corrected provides very valuable operation flexibility for the equipment owner, should the company decide in the future to change the original service parameters (whether it is changing all its system existing operation philosophy, or moving the equipment to another subprocess).

The requirement is in the design specification for a reason. I would suggest to comply with it.

 

[r6155]

I have worked with many reputable manufacturers and several of them use MAWP x 1.3 for hydraulic testing. But the nameplate says design pressure.
For stainless steel equipment users require MAWP because it has a better sale price in the used market.

I insist: my specification require UG-99 (c)

Regards

 

[david339933]

Nameplate is not allowed to say design pressure, per UG-116.
UG-99(c) doesn't use MAWP, it uses MAP.

 

[marty007]

I am not only frustrated with MAWP or MACA. 20 to 30 years ago, I used 10~20 pages of hand-calculations to complete a design of pressure vessel. Now, I see a common pressure calculation containing more than 600 pages by using PVElite, NozzlePro, StarrdPro and Spreadsheet. So many new requirements, such as MAWP, MACA, nozzle external load, a pressure thrust load..., regardless of whether it is applicable or has analytical value all put into the calculation. It is really pain. I hope a pressure vessel design to be simple and practical.

I think the days of 10-20 page calculations are long gone, and won't be coming back.

Working at a fabrication shop, I have built a lot of replacement vessels for those built 20-30 and up to 70 years ago. In doing so, I have also found a lot of errors in the original designs. Fortunately I think the higher design margins used in the past (5.0 pre 1950s and 4.0 pre 1999 vs. 3.5 today) worked to cover up some of these errors at the expense of heavier vessels.

To justify lower design margins and therefore thinner walled vessels going forward, we need to analyse the vessels more carefully and work to reduce the number of unknowns. Back when the design margins were 5.0 and 4.0, there wasn't the same focus on looking at nozzle loads on vessels for example. Fortunately because the design margin was higher, vessels were built heavier and could generally withstand piping loads.

Food for thought...

 

[r6155]

@ david339933
MAWP is used by the manufacturer only for pressure test and is not included in any document for the user

Only I mentioned UG-99 (c) and not mentioned MAWP in it.
MAP is not mentioned in UG-99 (c), “calculated test pressure” is mentioned, see Definitions in 3-2
Also see definition of MAP.

Regards

 

[marty007]

@ david339933
MAWP is used by the manufacturer only for pressure test and is not included in any document for the user

I think you may have this mixed up... MAWP is included on all documentation for the user. It is a required field on Form U-1 Manufacturer's Data Report on line #9 and it is required to be stamped on the nameplate (See Figure UG-118).

The question being debated in this thread is whether there is a benefit to have the MAWP higher than the design pressure. Footnote #35 states that it is not required to have the MAWP exceed the design pressure, however many customer specs require the calculated MAWP to be reported on the U-1/nameplate.

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.

 

[r6155]

@ marty007
Some manufacturers do the hydraulic test that I mentioned off the record, before the inspector comes.

Regards

 

[Trestala]

In my experience at least the preliminary HAZOP has already been completed by the time equipment is purchased. If having an MAWP over the design pressure is what makes the HAZOP work, then isn't there is a problem with the design process?

It happens but it's few and far in between. Overall plant design are iterative and changes happen even after placing PO. However, I am not saying that is the main reason MAWP is requested. I had one recently for a HAZOP case of level instrument failure and system is flooded. It is low probability and not treated as design case and there are some protections present. It was only for study.

Oil majors have in their design practice for nozzle reinforcement and shell stresses at saddle to not limit the MAWP. I guess it is related on how those are calculated (area replacement and LP Zick).

EPC's and end-users are essentially paying for the MAWP if the fabricators will end up quoting based on standard plate thicknesses, pipe schedule, flange classes, etc. Additional cost for calculating the MAWP which can be done quickly by software is negligible.

 

[mechengineer]

MAWP is random and sensitive to many factors in pressure vessel design. As a design result, MAWP of pressure vessel will be confirmed only by AS-built of pressure vessel. HAZOP may be not possible to use MAWP for HAZOP before the vessel as built.

 

[david339933]

MAP is not mentioned in UG-99 (c), “calculated test pressure” is mentioned, see Definitions in 3-2
Also see definition of MAP.

MAP is not defined in the Code...but please enlighten me as to the difference between calculated test pressure and MAP.

 

[r6155]

david339933

See Figure UCS-66.2
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 from the Section


calculated test pressure: the requirements for determining
the test pressure based on calculations are outlined in
UG-99(c) for the hydrostatic test and in UG-100(b) for
the pneumatic test. 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.

Regards

 

[DandB]

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.

The mentioned text (PVP2016-63075) indicates that "minor components such as nozzles..." shall not limit the MACA. Is this true in reality? For those who have determined the MACA for a vessel, did you ignore the nozzles?