Bobby, Art.
Thank you very much for you insight.
Bobby: there are hydraulic let down stations that tracks the steam usage and extraction of the turbines, have been characterized and take action to maintain the pressure of the intermediate pressure header at 45+ bar. That part is already covered.
The main issue here is that when the PSV pops at the steam generation drum and do not re-seat, a total shutdown is needed. Today the setup is to vent the excess steam to atmosphere by a control valve. However, it has been long discussed how fast can that control valve needs to be to avoid the main relief valves from acting. All the discussion lead to an hydraulic operated system with high speed signal polling. We have gone thru many discussion about anticipating action triggered from the compressor trip, changing the actuator size, installing an air reservoir near the valve, etc, etc.
Art:
Agree that for a control function, you need a device designed for that fuction. So let me rephrase.
The purpose of the the vent valve is to remove the excess steam to avoid the actuation of the relief devices on the steam generator. That is, keep the pressure below MAWP.
If those valves were ASME VIII devices we would be looking into the exceptions of appendix M to maintain the valves whithout shutting down the unit. This is not possible on ASME I devices. We need a quick acting device to prevent pressure for rising or tolerate the consequences of the PSV popping.
We have received the proposal of of using a relief device for this function with a good deal of questioning and doubts. However there are people arguing that the ASME code does not forbids the usage of a device below the MAWP, nor set any restriction on those (like prohibiting block valves) as long as the main relief valves are rated for the full relief requirements and comply with all the code mandates.
The code, the argument goes, does not dictates how to control the pressure below the MAWP. It only states that when the pressure reachs the MAWP there should be relief valves with a set of requirements that must open.
In a way, the PSV with a lower set point would be a "control" device and not subjected to the code requirements.
The consequences, the argument keeps going, would not be worse that the current opening of the main PSVs. The alternative is installing an hydraulic valve on a elevated point or bringing the pipig to grade.
My gut feeling is that this is not a good solution, I have not seen this arrangment anywhere before, but I cannot find a good argument to say "no, because we are violating (whatever) we need to go with the more expensive option".