My understanding is that modulating valves can be used to avoid PSV chatter due to oversizing. In one case, a client of mine bought a vessel with a large orifice (16 sq inch) relief valve. The vessel needed only a 4 sq in orifice and I determined the cost of switching to a modulating pilot was actually less than the cost of purchasing a more moderately-sized relief valve.
One case where a modulating would not be recommended is if you have a high Joule-Thompson effect during discharge. In some cases, ice could potentially form (if discharging vaporizing water) and block the discharge.
There are certainly other reasons to choose one over the other too but those are my thoughts.
As another consideration, typically you will not want a pop open pilot valve for liquid relief scenarios in order to avoid liquid, hydraulic hammer issues. You should consider the modulating design for liquid relief.
One of the main reasons in choosing valve action type is, modulating will keep constant discharge, pop (or sometimes known as "snap acting") give fast relief & reseating pressure well below set pressure.
If your system is constantly working over set pressure, a pop acting will keep blowing off, reseating, blowing off, reseating.....a modulating valve will keep constant discharge..(think of it a a pressure regulator action....working on discharge)...I would get my knuckless whacked by my old chargehand should he read this last bit.
Ciao,
