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I. Classification of safety valves: 1. Based on differences in overall structure and loading mechanism, safety valves can be divided 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. ①Weight-lever type safety valve: A 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. Disadvantages: However, the structure of the weight-lever type safety valve is relatively bulky; the loading mechanism tends to vibrate, and leaks often occur as a result of these vibrations ； Its seat-back pressure is low, making it difficult to close and maintain a tight seal once opened. 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. Disadvantages: The applied load changes as the valve opens; that is, as the valve disc rises, the compression of the spring increases, and the force acting on the valve disc also rises accordingly. This is unfavorable for 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 main valve to operate. It has a complex structure and is typically used only in boilers and pressure vessels that require a large amount of safety venting capacity. 2. Based on the method of medium discharge, safety valves can be classified into three types: fully enclosed, semi-enclosed, and open, depending on the way in which the medium is discharged. 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. Classification based on the ratio of the maximum opening height of the valve disc to the flow passage: Based on the ratio of the maximum opening height of the valve disc to the diameter of the safety valve’s flow passage, safety valves are mainly classified into two types: low-lift safety valves and full-lift 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 diameter of the flow channel. The operating mechanism of a slightly opening safety valve is proportional, and it is mainly used in liquid applications; sometimes it is 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 mechanism is of two-stage type; a lifting mechanism is required to achieve full opening. Full-opening safety valves are mainly used in applications involving gaseous media. 4. By operating principle, safety valves can be classified into directly-acting safety valves and indirectly-acting safety valves. ①Direct-acting safety valves are valves that open under the direct action of the working medium. In other words, the pressure of the working medium overcomes the mechanical load imposed on 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, the magnitude of the load it can handle 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 direct-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. II. Selection of safety valves 1. Determination of various parameters of safety valves ① Determine the nominal pressure of the safety valve. The nominal pressure is selected based on the valve material, operating temperature, and maximum operating pressure. ②Determine the operating pressure rating of the safety valve. The operating pressure class is determined based on the design pressure and design temperature of the pressure vessel; the operating pressure of a safety valve has a different meaning from the pressure class of its spring. The operating pressure of a safety valve refers to the static pressure prevailing upstream of the valve when it operates normally; this pressure is the same as the operating pressure of the system or equipment being protected. The operating pressure range of a spring refers to the range of working pressures for which that particular spring can be used; within this range, the opening pressure of the safety valve (i.e., the set pressure) can be adjusted by changing the degree of pre-compression of the spring. Safety valves with the same nominal pressure can be divided into various operating pressure classes depending on the requirements of their spring design. When selecting a safety valve, the operating pressure class of the valve should be determined based on the required opening pressure value. ③Determine the discharge pressure Pd of the safety valve. The discharge pressure of a safety valve is generally 1.1 times the set pressure (opening pressure), while for steam boiler safety valves, it is 1.03 times the set pressure. ④Determine the diameter of the safety valve based on the required discharge volume; the discharge capacity of the safety valve must be ≥ the required discharge volume. The required discharge volume for a protected system refers to the amount that must be discharged to prevent overpressure when an abnormal overpressure occurs in the system. This volume is determined by factors such as the operating conditions and capacity of the system or equipment, as well as potential causes of overpressure. ⑤Material selection: When choosing the material for a safety valve, various factors must be taken into account, including the operating temperature and pressure of the medium, the properties of the medium itself, as well as the processability and cost-effectiveness of the material. 2. Determination of the special structure of safety valves ① For safety valves used with steam at a discharge pressure greater than 3 MPa, or for safety valves used with gases at a temperature exceeding 320°C, safety valves equipped with radiators (fins) should be selected. ②For safety valves subjected to additional back pressure, and when the change in this back pressure exceeds 10% of the set pressure, diaphragm safety valves should be used. Furthermore, for safety valves used with corrosive media, in order to prevent the springs and guiding mechanisms from being corroded by the medium, diaphragm safety valves should also be selected. ③For flammable media with extremely or highly hazardous toxicity, closed safety valves must be used; if a valve equipped with a lifting mechanism is required, then a closed safety valve with a wrench should be employed. ④For non-hazardous media such as air, hot water above 60°C, or steam, a wrench-operated safety valve should be used. ⑤Liquefied gas tank trucks shall be equipped with built-in safety valves. ⑥ For conditions with high discharge rates, a fully open type should be selected ； For operating conditions with stable working pressure and low discharge volume, a slightly open type is advisable ； For high-pressure conditions with large discharge volumes, a non-direct-acting type, such as a pulse safety valve, is recommended ； For containers with a length exceeding 6m, two or more safety valves should be installed. ⑦For fixed vessels with low working pressure, deadweight-type (pressure cooker) or lever-weight-type safety valves may be used. Mobile devices should be equipped with spring-loaded safety valves. ⑧For media that are thick and prone to clogging, a discharge device consisting of a safety valve in series with a burst disc is recommended. III. Installation Requirements 1. Installation Location ① Install vertically upward. ②Installation location: as close as possible to the equipment or pipeline to be protected. ③Install it in a location that is easy to maintain and adjust, with sufficient space around it. ④The safety valve of a pressure vessel is installed in the gas space above the liquid level within the vessel body, or it can be installed on a pipe whose connection point is located in the gas space of the pressure vessel. ⑤For containers and equipment used to hold flammable, toxic, or viscous substances, a globe valve can be installed in front of the safety valve. However, the flow area of this globe valve must not be smaller than the minimum flow area of the safety valve, and it should be sealed with lead to ensure that it remains in an open position at all times. ⑥The safety valve should be installed in the opposite direction to the stop valve, in order to reduce the influence of the valve’s own weight and prevent stress fatigue as well as vibration-induced fatigue during discharge. ⑦For safety valves that may be blocked or corroded by materials, a burst disc is installed in front of their inlet, and a check valve is placed between the safety valve and the burst disc. Anti-clogging measures such as back flushing, heating, or insulation are applied to the inlet pipes. ⑧Safety valves installed on pipelines should be placed in areas where the fluid pressure is relatively stable and at a certain distance from sources of pressure fluctuations; they must not be installed in dead corners of horizontal pipelines. ⑨For pipelines, heat exchangers, or pressure vessels that use liquid media, when valves are closed thermal expansion may occur, leading to high pressure buildup; in such cases, the safety valve can be installed horizontally to allow the liquid to flow out directly downward. ⑩For emergency venting to the atmosphere, the pipe outlet must be smooth and free of sharp edges or burrs, to prevent static discharge. The installation point of the safety valve must not subject it to excessive back pressure; it should remain within the specified allowable range. The valve body of the safety valve must be supported stably. For containers containing extremely hazardous or highly hazardous, as well as flammable and explosive media, the discharge outlet of the safety valve should be led to a safe location and properly managed. When two or more safety valves share a single discharge pipe, the cross-sectional area of that pipe must be at least equal to the sum of the cross-sectional areas of the outlets of all the safety valves. However, oxygen or flammable gases, as well as any two gases that can react with each other chemically, cannot share a single discharge pipe. 2. Installation requirements: ① For corrosive media, the safety valve should be installed in combination with a burst disc. ②For highly toxic media, a safety valve type with good sealing performance should be selected. ③For safety valves used with high-temperature media, high temperatures have a significant impact on the springs; therefore, spring-type safety valves should be avoided. ④Sprinkler protection should be provided for important safety valves, or those that are susceptible to fire. ⑤For safety valves used on spherical tanks, two safety valves should be installed, and the pressure relief capacity of either of these valves must be sufficient to meet the tank’s safety pressure relief requirements. ⑥An electrostatic bypass wire should be installed on the safety valve. ⑦To prevent the safety valve from opening and closing repeatedly, which can cause fluttering and damage to the valve, it is necessary to reduce the pressure drop in the inlet pipe of the safety valve, that is, by increasing the diameter of the inlet pipe and shortening its length.