Superheater PSV Requirement (ASME PG-68.1,2) 1

[Schabot]

Reference: thread292-80762

I am working on an existing heater that has 2 PSV's installed on the boiler that are designed to releif 100% of the steam rate (70,000 lb/hr at a set pressure of 725 psig). However, there are currently no PSV installed downstream to the superheater, which is not in-lne with ASME Sec I PG-68.1 “…every attached superheater shall have one or more safety valves in the steam flow path between the superheater outlet and the first stop valve.”

My Question is: For which flow should I size the new PSV I will install at the outlet of my superheater, and at what set pressure?

The superheater (and line) pressure drop is 15 psig. Based on my understanding of the code, since this new PSV is to protect the superheater against a high temperature only, the setpressure needs to be at a maximum of 710 psig (725 - 15). I would guess that I could size it to discharge a flow of 25% * 70,000 lb/hr = 17,500 lb/hr. But this is my guess only, I do not beleive the ASME states how much flow needs to go through this PSV in the case the PSV's installed on the boiler already takes care of 100% of the required releiving capacity.

If someone can help, I would appreciate.

 

[metengr]

Assuming that PG-58.3.1 does not apply for your installation, your selection of a PSV for the SH outlet and set pressure appear correct per PG-68.1 where set pressure = design pressure - pressure drop. You want to allow for steam flow thru the SH outlet PSV in the event steam is bottled up downstream of the SH, while still firing the boiler.

Regarding the relieving capacity of the added valve, per PG-68.2, the PSV's on the SH could be used in conjunction with the PSV's on the boiler for total relieving capacity. Since you mention that with the current PSV's on the boiler you have 100% relieving capacity, the remainder of PG-68.2 goes on to state that if you were to include the SH PSV's and PSV's on the boiler, the aggregate relieving capacity of the boiler PSV's must be 75%. So, your selection of 25% (100%-75%) seems reasonable for this added valve on the SH outlet.