Operating pressure higher then MAWP 1

[Vyncke]

Hello,

Our boilers consist of different parts.
Is the following situation allowed in ASME:
Entering part 1 of the boiler with water with an operating pressure of 2.5 barg - design pressure 3 bar.
Then entering an upper part 2 of the boiler with an operating pressure of 1.5 barg (= less than part 1 because of pressure loss and static head) - design pressure 2 bar.
The two parts make one assembly without valves in between.
The relief pressure is MAWP = 2 barg.

Can we design one assembly with two design pressures?
Is it possible then to have in part 1 a higher operating pressure than the final MAWP?
Is this allowed in ASME (We design according Section IV).

 

[davefitz]

For teh case you described, the answer is yes.

Normally in a boiler, the pressure relief valves are on the steam drum, and this drum is at the highest elevation of the boiler cicuitry. These relief valves must be set at or below the MAWP.

The waterwall and downcomers and economizer are located below the steam drum, and therefore they are operating at a pressure above the steam drum , so they are above MAWP during a relief valve event. The boiler external piping design pressure is greater than MAWP by the effect of tehis gravity head, and this is also required in the B31.1 rules for design of boiler external piping.