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1. Depending on their overall structure and loading mechanism, safety valves can be classified into three types: weight-lever type, spring type, and pulse type. Among the three types of safety valves, the spring-type safety valve is the most commonly used. ①The weight-lever type safety valve uses a weight and a lever to balance the force acting on the valve disc. Based on the principle of levers, it can use a weight with a smaller mass to generate a greater force through the amplifying effect of the lever, and adjust the opening pressure of the safety valve by changing the position of the weight (or its mass). Advantages: The weight-lever type safety valve has a simple structure, is easy and relatively accurate to adjust, and the load applied does not increase significantly as the valve disc rises. It is suitable for high-temperature environments and was commonly used in the past, especially in boilers and pressure vessels operating at high temperatures. Disadvantage: However, the structure of the weight-lever type safety valve is relatively bulky; the loading mechanism tends to vibrate, and leakage often occurs as a result of these vibrations ； It has a low seating pressure, and once opened, it is difficult to close and maintain a tight seal. ②Spring-loaded safety valve: A spring-loaded safety valve uses the force of a compressed spring to counteract the force acting on the valve disc. The compression amount of the helical coil spring can be adjusted by turning the adjustment nut on it; this mechanism allows the opening (setting) pressure of the safety valve to be corrected as needed. Advantages: The spring-type safety valve is lightweight and compact, has high sensitivity, and its installation location is not restricted. Additionally, due to its low sensitivity to vibrations, it can be used in mobile pressure vessels. Disadvantage: The applied load changes as the valve opens; that is, as the valve disc rises, the amount of compression of the spring increases, and the force acting on the valve disc also increases accordingly. This is detrimental to the rapid opening of the safety valve. Furthermore, the spring on the valve may lose its elasticity due to prolonged exposure to high temperatures. When used on containers with high temperatures, the insulation or heat dissipation of the spring often needs to be considered, which complicates the design. ③Pulse safety valve: A pulse safety valve consists of a main valve and an auxiliary valve; the pulse action generated by the auxiliary valve drives the operation of the main valve. It has a complex structure and is typically used only in boilers and pressure vessels that require a large amount of safety discharge. 2. Depending on the method of medium discharge, safety valves can be classified into three types: fully enclosed, semi-enclosed, and open type. ①Fully enclosed safety valve: When a fully enclosed safety valve releases gas, all of the gas is discharged through the exhaust pipe, preventing any leakage of the medium; it is mainly used in containers containing toxic or flammable gases. ②Semi-enclosed safety valve: The gas discharged by a semi-enclosed safety valve passes partly through the exhaust pipe, while some of it leaks out through the gap between the valve cover and the valve stem; it is commonly used in containers containing gases that do not pose a risk of contaminating the environment. ③Open-type safety valve: The bonnet of an open-type safety valve is open, allowing the spring chamber to be in contact with the atmosphere; this helps to reduce the temperature of the spring. It is mainly suitable for containers containing steam or other high-temperature gases that do not cause pollution to the atmosphere. 3. Based on the ratio of the maximum opening height of the valve disc to the flow channel, and more specifically on the ratio of the maximum opening height of the valve disc to the diameter of the safety valve’s flow channel, safety valves are mainly divided into slightly open-type safety valves and fully open-type safety valves. ①Slightly opening safety valve: The opening height of a slightly opening safety valve is less than 1/4 of the diameter of the flow channel, usually ranging from 1/40 to 1/20 of the flow channel’s diameter. The operating mechanism of a slightly opening safety valve is proportional, and it is mainly used in liquid applications; it is sometimes also used in gas applications with very low discharge rates. ②Full-open safety valve: The opening height of a full-open safety valve is greater than or equal to 1/4 of the diameter of the flow channel. The discharge area of a fully open safety valve is the minimum cross-sectional area at the valve seat throat. Its operating process consists of two stages, and a lifting mechanism is required to achieve full opening; full-opening safety valves are mainly used in applications involving gaseous media. 4. Classified by operating principle, safety valves can be divided into directly-acting safety valves and indirectly-acting safety valves. ①Direct-acting safety valve: A direct-acting safety valve opens under the direct action of the working medium; that is, it relies on the pressure of the working medium to overcome the mechanical load applied to the valve disc by the loading mechanism, thereby causing the valve to open. This type of safety valve has advantages such as a simple structure, rapid operation, and good reliability. However, due to reliance on structural loading, its load capacity is limited, so it cannot be used in high-pressure or large-diameter applications. ②Non-direct-acting safety valves: These types of safety valves can be divided into pilot-operated safety valves and safety valves equipped with power-assist devices. Pilot-operated safety valve: It is driven or controlled by the medium discharged from the pilot valve, which itself is a directly-acting safety valve; sometimes other types of valves are also used. Pilot-operated safety valves are suitable for high-pressure and large-diameter applications. The main valve of a pilot-operated safety valve can also be designed to be sealed by the working medium, or a much greater mechanical load can be applied to the valve disc than in a directly-acting safety valve, thereby achieving excellent sealing performance. At the same time, its operation is little affected by back pressure. The disadvantage of this type of safety valve is that its reliability depends on the main valve and pilot valve; it does not operate as quickly or reliably as directly-acting safety valves, and its structure is more complex. Safety valve with power assist device: It uses a power assist device to force the safety valve to open at a pressure lower than the normal opening pressure. This type of safety valve is suitable for applications where the opening pressure is very close to the operating pressure, or in situations where the safety valve needs to be opened regularly for inspection or to remove adhered or frozen media. At the same time, it also provides a means to force the safety valve to open in emergency situations.