Pressure relief valves are commonly used for protection of equipments against internal over pressure. Following are the main relief valve types commonly used in the industry. Before getting into the relief valve types, some terms need to be described.
Superimposed back pressure
Superimposed back pressure is the static backpressure that exists on the outlet of the pressure relief valve, when the valve is closed. This pressure can be constant or variable depending on the conditions in the flare system before the relief valve can discharge.
Built-up back pressure
Built-up back pressure is the backpressure generated due to pressure losses at the outlet of an open relief valve when it is discharging. This pressure depends on the downstream pressure in the flare header to which the relief valve is discharging and the relieving flowrate which is being discharged.
When the relief valve is discharging, effects of superimposed and built-up back pressure exist together and felt as the combined back pressure.
Conventional pressure relief valves (PRVs)
Operational characteristics of this pressure relief valve are directly affected by back pressure changes. For conventional safety relief valve, only the superimposed back pressure affects the opening characteristic and set pressure value, but the combined back pressure (superimposed backpressure plus built up backpressure) affects the blowdown characteristic and re-seat pressure value. A conventional pressure relief valve is not used when the built-up backpressure is greater than 10% of the set pressure at 10% overpressure. A higher maximum allowable built-up backpressure may be used for overpressure greater than 10%.
Advantages of conventional relief valves are reliability and versatility. These relief valves are most reliable when sized properly and can be used in a wide range of services.
Disadvantage of these relief valves is the effect of backpressure on the relieving pressure of valve and hence pressure accumulation in the protected equipment. Also for high built up backpressure values generated by higher pressure loss in the relief valve discharge line, chattering can occur in these relief valves.
Balanced Bellows relief valve
Effect of back pressure on the operational characteristics of the valve is minimized by incorporating bellows. The bellows encircle an area equal to the inlet orifice area. This area is maintained free from the effect of back pressure from the discharge side of the relief valve. The space enclosed by bellows is freely vented to air. Thus the opposing pressure on the inlet fluid is generated by the spring alone without contribution from any sort of backpressure. For these relief valves allowable back pressure is 10 - 50% of the set pressure.
Advantage of using balanced bellows relief valves is no effect of back pressure on the relieving pressure and pressure accumulation. The spring is isolated from discharge fluid from the bellows, hence risk of corrosion mitigated. These relief valves get special consideration when there is high combined backpressure on the relief valve.
Disadvantage can be said to be the susceptibility of bellows to fatigue and failure and possibility of release of flammable or toxic process fluids being discharged to atmosphere through bellows vent.
Pilot operated relief valve
In pilot operated setup, main relief is combined with and controlled by a smaller self actuated pilot valve. This relief valve valve uses the process fluid itself, circulated through a pilot valve, to apply the closing force on the safety valve disc. The pilot valve is itself a small safety valve with a spring. The main valve does not have a spring but is controlled by the process fluid from pilot valve. This arrangement allows operation of pilot operated valves with a very narrow margin between set pressure of the relief valve and operating pressure of the protected equipment. Hence these relief valves are particularly used for services where relief valve inlet line pressure drop is high (typically higher than 3% of set point) or when back pressure is high. Allowable back pressure is typically more than 50% of the set pressure.
Disadvantage of using pilot operated relief valves can be blockage of the pilot valve inlet outlet tubing by foreign matter such as hydrate, ice, wax etc.
This valve is designed to admit external fluid to prevent an excessive internal vacuum.