Pressure relief valves are commonly used for protection of equipments against internal over pressure. Following are the main pressure relief valve types commonly used in the industry. Before getting into the pressure relief valve types, some terms need to be described.
叠加的背压
叠加的背压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
积压的背压是由于排放时的开放救济阀出口处的压力损失而产生的背压。这种压力取决于浮雕阀要排出的耀斑头的下游压力以及正在排放的缓解流量。
When the relief valve is discharging, effects of superimposed and built-up back pressure exist together and felt as thecombined 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%.
常规浮雕阀的优势是可靠性和多功能性。这些浮雕阀尺寸正常时最可靠，可以在各种服务中使用。
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
通过掺入波纹管，可以最大程度地减少背压对阀门操作特征的影响。波纹管环绕着等于入口孔口区域的区域。该区域没有从浮雕阀的放电侧产生的背压影响。波纹管封闭的空间被自由排放到空中。因此，仅春季就不会产生对入口流体的相对压力，而没有任何形式的背压。对于这些浮雕阀，允许的背压为设定压力的10-50％。
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.
真空缓解
这种压力阀类型用于接收外部液体以防止过度的内部真空。
