Spring type safety valve means to lock the valve or plunger and other seals depending on the elastic pressure of the spring. Once the pressure of the pressure vessel is abnormal, the high pressure will overcome the spring pressure of the safety valve, so the locking device is jacked open to form a pressure relief channel to release the high pressure. According to the opening height of the valve disc, it can be divided into two types: full lift and micro lift. The full lift type has large discharge capacity and good resilience, which is suitable for liquid and gas medium, while the micro lift type is only suitable for liquid medium. According to the analysis of the opening action characteristics of spring safety valve, it can be concluded that: under the action of external force FW, the change rule of external force FW in the whole process of a safety valve from closing to opening, and then from opening to closing. When the additional force gradually increases from zero and the sum of internal pressure pl × s is exactly the spring preload, the valve slightly opens, increasing the medium action area s, so that the In order to overcome the spring preload, the internal pressure force increases sharply, and the external force decreases in a moment. The first characteristic peak a appears. When the external force decreases gradually and reaches the closing point, due to the sudden decrease of the acting area of the medium, in order to maintain the balance of the force, the external force will rise suddenly, that is, the second characteristic peak point B. The above two characteristic peaks A and B are the technical basis for detecting the opening pressure and reseating pressure of the safety valve under the online condition.

The pilot safety valve is a kind of safety protection valve. Its opening and closing parts are normally closed under the action of external force. When the medium pressure in the equipment or pipeline increases and exceeds the specified value, it will open automatically. By discharging the medium outside the system, the medium pressure in the pipeline or equipment will be prevented from exceeding the specified value.

Safety valve belongs to automatic valve category, which is mainly used in boiler, pressure vessel and pipeline. The control pressure does not exceed the specified value, which plays an important role in protecting personal safety and equipment operation. According to the opening height of safety valve disc, it can be divided into micro open safety valve and full open safety valve. The opening stroke height of micro open safety valve is ≤ 0.05d0 (minimum discharge throat diameter); the opening height of full open safety valve is ≤ 0.25d0 (minimum discharge throat diameter). The pilot type safety valve can be divided into vertical hammer type, lever type, spring type safety valve and pilot type (pulse type) according to the structure of the valve body, which can be divided into closed type and non closed type. The closed safety valve means that the discharged medium does not leak out. It is discharged to the designated place along the outlet. It is generally used in toxic and corrosive medium. For air and steam safety valve, non closed safety valve is often used. The selection of safety valve products shall be determined according to the actual sealing pressure. For spring type safety valve, in the range of one nominal pressure (PN), there are several kinds of springs with working pressure level. When ordering, besides the safety valve model, name, medium and temperature, the valve body sealing pressure shall also be indicated, otherwise the maximum sealing pressure shall be used for supply.