According to working principle, the safety valve or relief valve can be divided into direct-acting safety valve or relief valve and non-direct-acting safety valve or relief valve.
1. Direct-acting safety valve or relief valve
Direct-acting safety valve or relief valve depends on the acting of working medium to open the disc. In other words that depends on the working medium pressure to overcome the mechanical load on the disc which is loaded by loading mechanism to open the disc. Direct-acting safety valve or relief valve has the advantages of simple structure, prompt action and reliable performance. But because of depending on the structure loading, its loading is limited. Direct-acting safety valve or relief valve cannot be applied to high pressure or large sized working conditions.
2. Non-direct-acting safety valve or relief valve
Non-direct-acting safety valve or relief valve is divided into pilot operated safety valve or relief valve and safety valve or relief valve with auxiliary power device.
3. Pilot operated safety valve or relief valve depends on the mediums discharged from pilot valve to drive or control. And pilot valve is just one type of direct-acting safety valve or relief valve normally, although sometimes that it is other type of valve. Pilot operated safety valve or relief valve is applied to working condition of high pressure and large sizes. The main valve of pilot operated safety valve or relief valve can be also designed to depend the working mediums to achieve in sealing, or bring much bigger mechanical loading than direct-acting safety valve or relief valve to the valve disc. So, pilot operated safety valve or relief valve has excellent sealing. Meanwhile, the action of pilot operated safety valve or relief valve is rarely effected by back pressure. The disadvantage of the pilot operated safety valve or relief valve is that the action of it is not as quick or reliable as direct-acting safety valve or relief valve, because its reliability is related to the main valve and pilot valve. And even more, the structure of pilot operated safety valve or relief valve is more complex.
The safety valve or relief valve with auxiliary power device depends on one auxiliary power device to enforce the safety valve or relief valve to open in the condition of under the normal discharge pressure. The safety valve or relief valve with auxiliary power device is applied to working condition of the safety valve or relief valve discharge pressure is close to its working pressure; or needing open the safety valve or relief valve termly to take a check or remove the adhesive mediums or frozen mediums. At the same time, it is also one way of opening the safety valve or relief valve compulsively in case of emergency.