Knowledge of safety valves

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The website http://bbs.hcbbs.com/viewthread.php?tid=284542 already covers some information on safety valves; I will add a few additional points. Please feel free to point out any mistakes. 1. Overview of safety valve knowledge: Safety valves are important safety accessories installed on boilers, pressure vessels, and other pressure-bearing equipment. The reliability of its operation and its performance have a direct impact on the safety of equipment and people, and are closely related to energy conservation and environmental protection. II. Definition of safety valves Generally speaking, safety valves include relief valves. From a regulatory perspective, they are valves that are installed directly on steam boilers or certain types of pressure vessels; a necessary condition for such valves is that they must be approved by the technical supervision authorities. In a narrow sense, these valves are referred to as safety valves, while others are generally called relief valves. Safety valves and relief valves are very similar in structure and performance; both discharge the fluid inside automatically when the set pressure is exceeded, in order to ensure the safety of the production equipment. Due to this inherent similarity, people often confuse the two when using them; furthermore, some manufacturing equipment also specifies that either one can be used. Therefore, the differences between the two are often overlooked. As a result, many problems have arisen. If a more precise definition for both is to be provided, it can be understood in accordance with the definition outlined in Part 1 of the ASME Boiler and Pressure Vessel Code: 1. Safety Valve: An automatic pressure relief device driven by the static pressure of the medium in front of the valve. It is characterized by a sudden, full-open starting action. Used for gas or steam applications. 2. Relief Valve, also known as a pressure relief valve, is an automatic pressure release device driven by the static pressure of the medium in front of the valve. It opens proportionally as the pressure exceeds the opening force. It is mainly used in fluid applications. 3. Safety relief valve, also known as a safety overflow valve, is an automatic pressure-release device driven by the pressure of the medium. It can be used as either a safety valve or a relief valve, depending on the application. Taking Japan as an example, there are few clear definitions for safety valves and relief valves. Devices used as safety mechanisms in large pressure vessels such as boilers are generally referred to as safety valves, while those installed on pipes or other equipment are called relief valves. However, according to the \"Technical Standards for Thermal Power Generation\" set by Japan’s Ministry of International Trade and Industry, safety valves are specified as an essential component for ensuring safety in equipment such as boilers, superheaters, and reheaters. Where the lower side of the pressure relief valve needs to be connected to a boiler and a turbine, a drain valve or safety valve must be installed. From this perspective, safety valves are required to be more reliable than relief valves. Furthermore, based on the regulations regarding the management of high-pressure gases set by Japan’s Ministry of Labor, as well as the rules established by the Ministry of Transport and various ship associations concerning the acceptable levels of emissions, we refer to valves that ensure such controlled emissions as safety valves, while those that do not guarantee controlled emissions are called relief valves. In China, both fully open and slightly open types are collectively referred to as safety valves. III. Selection of Safety Valves 1. Classification of Safety Valves 　　 At present, the safety valves produced in large quantities fall into two main categories: spring-type and rod-type. There are also impulse-type safety valves, pilot-operated safety valves, safety switching valves, safety pressure relief valves, gravity-type safety valves, and others. Spring-loaded safety valves operate primarily due to the force exerted by springs. There are also closed and open types of spring-loaded safety valves; for media that are flammable, explosive, or toxic, closed-type valves are generally preferred, while open-type valves can be used for steam or inert gases. Additionally, there are spring-loaded safety valves with levers and those without levers. The primary function of the wrench is to check the flexibility of the valve disc; it can also be used for manual emergency pressure relief. Lever-type safety valves operate primarily by the force of a lever and weight, but their large size often limits their range of application. Use a safety valve with a radiator at higher temperatures. The main parameter of a safety valve is its discharge capacity, which is determined by the diameter of the valve seat and the opening height of the valve disc. Depending on this opening height, they are classified into two types: slightly open type and fully open type. Slightly opening type refers to a valve disc opening height of 1/40 to 1/20 of the valve seat throat diameter. Full opening means that the opening height of the valve disc is 1/4 of the throat diameter of the valve seat. 2. Selection of safety valves: The nominal pressure of the safety valve is determined by the operating pressure, while its operating temperature range is determined by the operating temperature. The pressure setting range for the spring or lever is determined based on the calculated pressure setting value of the safety valve. The material and structural design of the safety valve are chosen according to the medium used, and the throat diameter of the safety valve is calculated based on its discharge capacity. The following are the general rules for selecting safety valves. (l) Hot water boilers generally use unenclosed safety valves with a wrench for slight opening. (2) Steam boilers or steam pipes generally use unenclosed, full-opening safety valves with levers. (3) Incompressible media such as water generally use closed micro-tilt safety valves, or safety relief valves. (4) Closed full-opening safety valves are generally used for high-pressure feed water, such as in high-pressure feed water heaters and heat exchangers. (5) Compressible media such as gases generally use closed full-opening safety valves, such as in gas storage tanks and gas pipelines. (6) Class E steam boilers generally use deadweight safety valves. (7) Pulse-type safety valves are generally used in large-diameter, high-displacement, and high-pressure systems, such as temperature and pressure reduction devices and power plant boilers. (8) Tank cars, road tankers, and storage tanks used for transporting liquefied gas generally employ internal safety valves. (9) Hydraulic safety valves are generally used at the top of oil tanks, and they need to be used in conjunction with breather valves. (10) Pilot-operated safety valves are generally used for underground drainage or gas pipelines. (11) A safety return valve is generally used on the liquid-phase return pipeline at the outlet of the tank pump in LPG stations. (12) Systems that may experience negative pressure or negative pressure during operation generally use vacuum pressure relief valves. (13) Bellows safety valves are generally used for systems with large backpressure fluctuations and for containers or piping systems that are toxic or flammable. (14) For systems with a low freezing point of the medium, insulated jacketed safety valves are generally used.