The nominal pressure of the safety valve is determined by the operating pressure, the operating temperature range of the safety valve is determined by the operating temperature, and the constant pressure range of the spring or lever is determined by the calculated constant pressure value of the safety valve. The medium determines the material and structural type of the safety valve, and then the throat diameter of the safety valve is calculated based on the discharge volume of the safety valve. The following are general rules for selecting safety valves. Figure d is a pilot-operated breathing valve. A small direct-acting valve provides control pressure to act on the diaphragm. The hard core on the diaphragm is the valve core, which is pressed on the valve seat. When the air source pressure A is greater than the safety pressure, the valve core opens and the compressed air is discharged into the atmosphere from the left output hole. Diaphragm safety valves and pilot-operated safety valves have better pressure characteristics and sensitive action; however, the maximum opening force is relatively small, that is, the flow characteristics are poor. In actual application, the type of safety valve should be selected according to actual needs, and its diameter should be selected according to the maximum exhaust volume.
The fire retardant breathing valve plays a safety protection role in the system. When the system pressure exceeds the specified value, the safety valve opens and discharges part of the gas in the system into the atmosphere so that the system pressure does not exceed the allowable value, thereby ensuring that the system does not cause an accident due to excessive pressure. Safety valve is also called relief valve. The figure shows several typical structural forms of safety valves. Figure a shows a piston safety valve, the valve core is a flat plate. The air source pressure acts on piston A. When the pressure exceeds the safety value determined by the spring force, piston A is pushed open, and a part of the compressed air is discharged from the valve port into the atmosphere; when the air source pressure is lower than the safety value, the spring drives The piston moves down, closing the valve. Pictures b and c are ball valve type and diaphragm type breathing valves respectively. The working principle is exactly the same as that of the piston type. These three types of breathing valves are springs that provide control force. By adjusting the spring preload, the safety value can be changed, so they are called direct-acting fire-retardant breathing valves.