The safety device of the boiler and the pressure vessel is specifically an auxiliary device that is mounted on the equipment for the safe running surface of the pressure vessel, and is often referred to as a safety accessory. Boiler and pressure vessel safety devices can be divided into the following four types according to their performance or use: (1) The interlocking device refers to a control mechanism provided to prevent an operation error, such as an interlock switch, a linkage valve, and the like. (2) The alarm device refers to an instrument that can automatically emit an audible or other obvious alarm signal, such as a pressure alarm and a temperature monitor, when the device is in an unsafe condition during operation. (3) Metering device refers to an appliance that can automatically display the safety-related process parameters during operation of the equipment, such as pressure gauges, water gauges, thermometers, etc. (4) Devices that can automatically discharge pressure when the pressure relief device is overpressured, such as safety valves and rupture discs. Boilers and pressure vessels shall be equipped with corresponding safety devices according to their structure, size and purpose. The safety relief device is an appliance that prevents overpressure of boilers and pressure vessels. Its function is: when the pressure in the boiler and pressure vessel exceeds the normal working pressure, it can be automatically opened, and the medium in the container is discharged, so that the pressure in the boiler and the pressure vessel is always kept within the maximum allowable pressure range. Safety relief devices are divided into four types according to their structural type: (1) Valve type valve type safety relief device is a commonly used safety valve, which reduces the pressure inside the device by discharging the internal medium through the opening of the valve. The safety relief device of this type is that only the part of the pressure in the drain is higher than the specified part of the pressure, and when the pressure in the unit drops to the normal working pressure, it automatically shuts down the equipment and can continue to operate. The device itself can be reused many times, and the installation adjustment is relatively easy, but its sealing performance is poor, the pressure relief reaction is slow, and the valve port is blocked or the valve flap is stuck. The valve type pressure relief device is suitable for equipment with relatively pure media, and should not be used for equipment with high toxicity of the medium and equipment whose pressure inside the device may rise sharply. (2) Fracture type fracture type safety relief device, which is commonly used as a rupture disc. It discharges gas through the breakage of the rupture disc. The characteristics of the device are: good sealing performance and quick pressure relief reaction, but after the pressure is released, the rupture disc cannot be used continuously, and the container has to be stopped. The fracture type pressure relief device is preferably used for containers with highly toxic medium and a container whose pressure is sharply increased due to chemical reaction, and is not suitable for use in a liquefied gas storage tank. (3) The molten type melting type safety relief device is a commonly used fusible plug. It uses the low melting point alloy in the device to melt at a higher temperature and open the channel to relieve pressure. The characteristics of this device are: simple structure and easy replacement, but it cannot be used after the pressure reduction, and the discharge area is small. It can only be used in small containers where the pressure inside the unit is completely dependent on temperature, such as gas cylinders. (4) The combined type of common type safety relief device is a series combination of valve type and fracture type, and it has the characteristics of valve type and fracture type at the same time. It is generally used for containers with highly toxic or rare gases in the medium and cannot be used in reaction vessels with extremely fast boosting speed.