Safety valve in system safekeeping of security function. When the system pressure surpasses the rating, the safety valve opens, disperses into outside the atmospheric/pipeline a system's in part of gas/fluid, causes the system pressure not to surpass the permissible value, thus the guarantee system excessively is not high because of the pressure has the accident. The safety valve calls the overflow valve, or decompression valve, but it and the overflow valve also have some differences. The graphical representation is safety valve's several kind of modular design form. Chart a is the plunger-type safety valve, the valve core is a plate. Is mad the source pressure function on piston A, when the pressure surpasses by the spring strength definite security value, piston A goes against, partial compaction air namely disperses into the atmosphere from the valve port; When was mad when the source pressure is lower than the security value, the spring actuates under the piston to move, closure valve port. This valve s