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http://www.shinjovalve.com/news/how-to-choose-between-conventional-safety-relief-valve-and-pilot-operated-safety-relief-valves.html

Conventional Safety Valve Pilot Operated Safety Valve
(L) Conventional Safety Valve and (R) Pilot Operated Safety Valve
Safety relief valves (PRVs)can be categorized into two primary groups – conventional spring-operated safety valves and pilot operated safety valves (POSVs). This article will showcase the idea of their operation and the pros and cons to consider when selecting which type may be best suited for the application.
conventional spring-operated safety valve​​
A conventional spring-operated safety valve​ uses a spring to maintain the PRV closed until the fluid reaches the set pressure. The spring force pushes the valve’s disc against the nozzle seat keeping the valve closed. Fluid pressure exerts an opposing force on the disc. When the fluid pressure force on the disc overcomes the opposing spring force, the disc lifts and the valve automatic to open and relieve pressure to protect the pipeline.
pilot operated safety valves​
On the other hand, a pilot operated safety valves​ uses liquid pressure to maintain the valve closed until set pressure is reached. POSVs consist of a main valve that relieves the required capacity of process media and a pilot valve that controls the main valve. Until set pressure is reached, both the top and bottom of the valve’s piston are exposed to liquid pressure forces. The surface area on top of the piston is larger than the surface area of the bottom of the piston at the nozzle.
Because of the difference in area and pressure being equal, the force on the top of the piston is higher than the force on the piston at the nozzle. Because of that, as liquid pressure increases, the sealing force of the piston increases at the same time. When set pressure is reached, the pilot valve relieves the pressurized fluid from the dome area atop the piston, allowing process pressure at the nozzle to force the piston upward from its seat, which allows the valve to open and relieve pressure in the pipeline.

Conventional spring-operated safety valve is most likely to be the best for the applications that will expose the valve to high temperatures, highly viscous, or dirty services. In smaller sizes they are greatly cheaper to use than POSVs, and most designs will follow standard API 526 center-to-face flange dimensions.

Conventional spring-loaded safety valve may not be the good choice for applications requiring maximum seat tightness / sealing, minimal simmer, or applications with high or variable back pressures. Measures can be taken to mitigate these shortcomings; however they bring limitations of their own. For example, a conventional valve can be constructed with a soft seat to improve sealing, but this will generally introduce temperature limitations. To reduce the effects of back pressure a conventional PRV can be outfitted with a bellows. However, a bellows has lower back pressure tolerance than a POSV and can be more costly to maintain.
Comparison of closing forces
Comparison of closing forces: POSV vs Spring Loaded (Conventional) PRV

The POSV is designed to mitigate many shortcomings of a conventional valve but has its own limitations. POSVs are best suited to applications requiring maximum seat tightness when approaching set pressure, applications with superimposed back pressure and built-up back pressure, and high relieving capacity applications with physical size limitations, to name a few. POSVs can also have set pressure capabilities that go beyond API 526 set pressure limitations associated with conventional PRVs.