ASME I and drum safety valves

In no case can you install all relief valves at the superheater outlet . On most boilers, there is a 10-15% pressure drop thru the superheater at full load, and installing all relief valves there at a lifting pressure equal to the boiler MAWP would imply a 10-15% drum overpressure plus 3% accumulation for a possible overpressure of 18%. Section I does not permit the drum overpressure to exceed 7% total , including 3 % accumulation. This problem is worsened by the new code allowable stresses based on 67% of yield stress.
Usually 85%-90% capacity is installed on the drum and 10-15% on the superheater outlet.

Per Sect I PG-68.2, at least 75% PSV capacity must be on the boiler. Hence, the typical capacity splits already quoted.

Thanks guys for you help
athomas236

Just to add a bit of knowledge. The ~15 to 25% flow to the superheater outlet is there to maintain a flow through the superheaters to prevent overheating that would occur if the full relief flow was through the drum.

Thanks Benjym,

The proposal is to put safety valves only on the superheater outlet which will be rated for 100% of boiler MCR steam flow.

I will be visiting plant in Poland over the next two weeks that do not have drum safety valves and only have superheater outlet safety valves.

This is allowed by Polish boiler regs. Should be interesting.

athomas236