Design Pressure Vs MAWP (API, ASME)

On the drawing Pdesign: 16kg/cm2 and MAWP: 26 kg/cm2
User defines Pd: as above. t calculated: 4mm
T selected: 12mm and second trial for formula founded P: 26 kg/cm2.
Then designer says it is MAWP and equals to 26, which is > P design!
Is that right?
Is MAWP BE > THAN P design?

First: Please distinguish Design Pressure of the Vessel from Design Pressure of the shell, head, nozzle or other individual part of the vessel.

The same applies to MAWP of the vessel versus MAWP of the head, shell, etc.

Suddenly it all becomes clear: (I use MPa, rather than kg/cm², which is too often confused with bar =1,02 kg/cm²)

1) Customer purchases design pressure of the vessel 2 MPa. Vessel for Efes Pilsen density 1000kg/m³ and 10 m high.

2) Designer plans the design and calculates the vessel parts with 2MPa PLUS hydrostatic head.
Bottom head and shell will be designed with design pressure of the head of 2.1 MPa.
Top head will be designed with design pressure of the head of 2.004 MPa.

3) Resulting thickness: Bottom head: 9 mm, shell: 9 mm, top head: 8,75 mm. All minimum thickness, plus allowances and tolerances.

4) Actual thickness: Bottom head: 10,3 mm, shell: 12 mm, top head: 9,5 mm. All actual thicknesses after forming.

5A) Now the designer may say the MAWP of the vessel is 2MPa. Customer is happy, and the fabrication may start
OR:
5B) The designer may determine the MAWP of the bothead 2,5MPa, MAWP of the shell 2,8MPa, MAWP of the top head2,22MPa.
Now he must consider the hydrostatic heads and determine the MAWP of the vessel based on the actual thickness. The result in my example would be 2,216 MPa, based on 0,004 MPa hy-head. This could be stamped on the vessel as MAWP.

I hope my example has made it clear. Both ways are permitted. Some customers are famous to require 5B in order to get the highest possible pressure on the nameplate. Of course they pay only for 2MPa.

Be careful with the 5B. As a principle, I always take an exception if it’s required in the specs. If they insist, I propose a hefty markup for this, to make sure I don’t have to do it, or I have enough time and resources if I do.

The problem is, many users believe that MAWP as defined in SecVIII-1 is the pressure at which they can safely use the vessel because it has been designed to withstand it, which is simply not true.

MAWP calculated in such way is only the lowest value out of all MAWPs separately calculated for all components. What is missing in the evaluation procedure being followed by absolute majority of automatic PV design packages, is what happens in interfaces between components. In your example, you’ve got a step change in thickness between parts, and your 5B does not include checking stress concentrations in these areas, because it’s simply not required by SecVIII-1. To do your 5B the decent way, one would have to run the entire pressure vessel in an FEA, considering all gross structural discontinuities (which excludes any shell elements in the model, you can do it properly only with bricks, tets or otherwise volumetric elements), and check all interactions between parts, including nonlinearities at local spots of yielding. Particularly when there is cyclic loading, creep or SCC involved, as these are stress risers likely to be the driving factors. Do you expect an average SecVIII-1 design engineer capable of doing this?

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