Given mawp as a design parameter rather than internal pressure 1

[rab1944]

How do I set up Compress when given mawp rather than internal pressure as the design parameter? Thanks

 

[jte]

Just out of curiosity, how can you be "given" an MAWP?

The MAWP is determined by either designing the vessel to a given DP, then back-calculating based on the chosen thicknesses of the components or the MAWP is simply set to the DP (poor practice in my opinion).

jt

 

[rab1944]

Thanks. What I ended up doing was to set my DP at 0 and the making sure the MAWP was greater than that required by the customer. If that is not good practice, please let me know.

 

[vesselfab]

never heard of going about a design in this manner.

good luck

 

[rab1944]

First time for me too. But I set all thickness at the code minimum and gave the the customer what they asked for.

Thank all for your perspectives.

 

[jte]

rab1944

Out of curiosity, is this for a replacement vessel? I could see putting a limit on a MAWP as a misguided attempt at not changing anything system wide if only a vessel is being replaced. Some young engineer may have interpreted relief system requirements as being rigid – if the MAWP is raised, then the pressure relief device set point must also be correspondingly raised – and you don’t want to mess with the whole relief system, do you? So the engineer places an order for a vessel with a “not to exceed” MAWP so they won’t have to change the relief system.

The reality, of course, is different: Just because the MAWP of a replacement vessel is higher (say it’s physically identical, but takes advantage of the new Div. 1 allowable stresses) doesn’t mean you can’t leave the relief system at the old settings thus providing relief to the vessel at a lower pressure than necessary.

If I was buying the vessel, I’d order it with a statement that the DP of the replacement shall be the same as the MAWP of the existing (soon to be retired) vessel, and that the replacement shall have at least as much CA as the existing. The CA of the replacement vessel shall be increased such that the MAWP matches the DP. This requires a few iterations of the design – perhaps adding 15 minutes of labor. But with a 10 mils per year corrosion rate, how many years added life can you add with this type of optimization?

jt

 

[rab1944]

Nope- new vessel, but very small and the actual MAWP far

exceeded the required value.

You make some good points. The could be a replacement unit.

We do, however, make the customer the boss whenever

possible and responsible.

Thanks all.