Relief valve setpoint to close to operating pressure

[dreve]

I'm the "Fuel Pool Cooling System Engineer" at a nuclear plant. We expecienced problems with the relief valve on the heat exchangers lifting and remaining open due to the narrow margin between the operating pressure (180 to 190 psig) and the design pressure (200 psig). The possible solutions I see are:

-Live with the problem (not a good option).

-Perform a study to pull margin out of the original design analysis. The cost to re-rate the heat exchanger and associated piping will be in the tens of thousands of dollars.

-Find a relief valve that can operate within the tight prameters mentioned. I've been told that the noraml blowdown for 200 psig PRVs is about 15%.

So the question is, does anyone know if there are reliefs available to operate as I've described? Or, is there any other reasonable solutions I might entertain?

Thanks, Vince

 

[TD2K]

Look at a modulating pilot pressure relief valve, Anderson Greenwood makes them. With a soft seat, they should be bubbletight to 95% of set pressure or better (in some cases, up to 98%).

The other main advantage of them is that there is no large blowdown like with conventional relief valves in liquid service. As the pressure reaches the set pressure, the pilot causes the PSV to start to open to maintain inlet pressure, the PSV does not 'pop' fully open. Full lift is achieved at about 3% over set pressure. As the inlet pressure drops, the valve closes off. In effect, the PSV acts like a pressure regulator. Nice valves.

I don't know the size of your current relief valve so these might require some piping rework.

 

[KernOily]

As TD2K sez, the pilot-operated PSV is a good choice when operating pressure is within about 10% of set pressure. Most direct-acting spring-operated valves will start to 'chatter' at the conditions you described.

The pilot-operated soft-seated valve is a good choice IF you have nice clean fluid service and IF your maintenance organization is up to snuff, which yours should be in a nuke plant. I work in the oil patch and, well, the maintenance here is from the 50's. But it is improving. I digress though...

Pilot-operated valves do not like small solid particles in the process fluid. The pilot orifices will stop up and render your valve non-operational, or it will go off unexpectedly. Been there done that a few times... Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com