Question about settings for our thermal relief PRVs around valves in tank farms. Nornmally we have the PRvs set at 25 psi in the piping and 75 psi at the valve directly off the tank (our tanks are not enormous -max size 460,000 l). The thermal bypass lines go around the valve in all cases-(ie not directly back into tank). We have had problems (sample barels filling too fast) further on in the lines past the PRVs and I think our settings are the problem. In worst case we have about 15 psi  headon system (due to height of tanks) with pump not running-pump head about 25 psi (includes hydraulic head)-so sounds as though 25 psi is not enough..Should we be seeting the PRV at 10 % more?  We are using Swagelock PRVs which have allen key adjustment an nominal seeting stamped on side but vatiable range. The service staff adjust "as required" but as they do not have proper test set its hard to know what pressure seeting we are getting til the PRV is in the line..

The swagelok relief valves in your service do not open up at the 25 psig set point, they will relieve when the differential pressure between the inlet and outlet are 25 psi apart.  The set pressure should be the pressure differential of the pump plus losses in the outlet and pump diffeential at maximum heat (lowest flow).  

What is the MAOP of the lines in the yard, I'd suspect at least 150 psig.  I'd run the thermal relief (actually thermal check valves) up to about a 100 psig differential.

Normally we set our thermal relief valves at the design pressure of the pipe in question. With that being said though, if you have multiple PSVs on one line you need the valves to cascade back to the tank. For example, the furthest PSV may be set at 275 psig. The next PSV set at 225, ect. A set pressure of 25 psig sounds way too low - the valve is probably always open.

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