Pilot operated PSVs are typically designed to use the inlet gas pressure to keep the valve closed. The inlet gas pressure is routed through the pilot onto the top of the piston. Since the top area of the piston is designed greater than the inlet, there is a constant force difference keeping the valve closed. For a conventional spring PSV, as the inlet pressure increases, the force against the spring is increasing reducing the net force keeping the valve closed, hence the lower % of set-pressure recommended to avoid simmering.
Back to pilot PSVs. Pilot PSVs can be either snap opening or modulating. Snap opening is just what it sounds like. Once the pilot reaches set pressure, it dumps the gas above the PSV piston to atmosphere and the valve completely opens. Once the inlet pressure drops to its blowdown setting, the pilot closes and allowed process gas to flow back onto the top of the piston closing it off.
Modulating pilots only vent off enough gas above the piston to allow it to open as needed to maintain the pressure to the pilot at/slightly above set pressure (think of it as a control valve). AGCO's modulating PSVs achieve full lift with about 2 to 3% overpressure, well below the 10% allowed by code for a single PSV in other than fire exposure so the modulating action from virtually zero flow to full flow occurs over a relatively narrow pressure range. This is because the pilots are small and springs can be made to finer tolerances allowing closer approaches.
Money - the price of a spring-loaded PSV is largely governed by the price of the spring and the spring housing (due to the fact that the spring is often made from expensive materials such as inconel). With high pressure valves, the break even size can be as low as 3" inlet. Even for low pressure valves (#150) size Q and above might be cheaper.
Dirty Service - need to get fancier to use pilot-operated valves, keeping dirt out of the pilot assembly
Code - some won't permit pilot operated valves in particular services
Capacity There ISN'T anything sacred about the API nozzle sizes and you can get much higher capacities from non-API pattern pilot op valves.
Control A modulating pilot will only discharge as much gas as there is to maintain setpressure - a spring op valve may discharge twice as much as you need or want.
I have taken the printed above from various mail re. the question because i think they are correct and i couldnt have put it better myself