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1. Basic knowledge of safety valves (1) Valves: A general term for mechanical devices equipped with movable components, which are installed on pressure vessels, pressurized equipment, and connecting pipes to control the flow direction of the medium. (2) Nominal diameter of the valve: refers to the nominal diameter of the pipe at the connection point between the valve, the pipe, and all other accessories, measured in millimeters (denoted as DN). (3) Safety valve: It is an automatic valve that can discharge a specified amount of fluid without any external force, relying solely on the force of the fluid itself, in order to prevent the pressure within the system from exceeding a predetermined safe level. A valve that closes on its own and prevents the medium from continuing to flow once the pressure returns to normal. (4) Operating pressure P: The pressure of the valve at the temperature of the medium it handles. (5) Operating temperature T: The temperature of the valve under the operating medium. (6) Setting pressure Ps: The predetermined pressure at which the safety valve begins to open under operating conditions. At this pressure, the force that opens the valve disc balances the force that keeps the valve disc in place on the valve seat. (7) Seat-back pressure Pr: Refers to the static pressure at the valve inlet when, after the safety valve reaches its discharge state and the medium pressure drops to a certain value, the valve disc comes into contact with the valve seat again, that is, when the opening height becomes zero. (8) Opening and closing pressure difference △Pbl: The difference between the set pressure of the safety valve and its return pressure, usually expressed as a percentage of the set pressure; it is expressed in Mpa only when the set pressure is very low. (9) Discharge pressure Pb: Set pressure plus excess pressure (excess pressure refers to the pressure increase beyond the set pressure of the safety valve, usually expressed as a percentage of the set pressure). (10) Opening height h: The actual lift of the valve disc from the closed position. (11) Flow area A: Refers to the minimum cross-sectional area of the flow path from the valve inlet to the sealing surface of the closing element, and is used to calculate the theoretical displacement in the absence of any resistance effects. The channel diameter corresponding to the channel area A is do (throat diameter). (12) Displacement coefficient Kd: The ratio of the actual displacement of the valve to its theoretical displacement. (14) Sealing test pressure Pt: The pressure specified for conducting the sealing test, at which the leakage rate through the sealing surface of the closing element is measured. Generally, 90% of the set pressure is used as the sealing test pressure. (15) Factory test ； Tests conducted on the product prior to leaving the factory, including shell strength, sealing performance, and opening pressure. 2. Basic structure of the safety valve (1) Valve body. (2) Valve seat. (3) Valve disc. (4) Adjustment ring. (5) Spring. (6) Valve stem. (7) Valve cover. (8) Valve cap. 3. Classification of safety valves (1) By operating medium: a: Safety valves for steam. b: Safety valves for air and other gases. c: Safety valve for liquids. (2) Classified by nominal pressure: a: Low-pressure safety valves: nominal pressure PN ≤ 1.6 Mpa; b: Medium-pressure safety valves: nominal pressure PN 1.6–6.4 Mpa. c: High-pressure safety valve: Nominal pressure PN6.4～80.0 Mpa; d: Ultra-high-pressure safety valve: Nominal pressure PN＞100 Mpa. (3) Classified by applicable temperature: a: Ultra-low temperature safety valves: t≤-100℃. b: Low-temperature safety valve: -100°C to -40°C. c: Normal temperature safety valve: -40 ℃～120 ℃. d: Medium-temperature safety valve: 120 ℃～450 ℃. e: High-temperature safety valve: t > 450 ℃. (4) Classified by connection method: a: Flange-connected safety valve. b: Threaded connection safety valve. c: Weld the safety valve. (5) Classified by opening height: a: Slightly opening safety valve: the opening height is between do/40 and do/20. b: Fully open safety valve: the opening height is not less than do /4. c: Slightly open safety valve: the opening height is between that of a slightly open valve and a fully open valve. (6) Classified by structural form: a: Lever-weight type safety valve. b: Spring-loaded safety valve. c: Pulsating safety valve (also known as pilot-operated safety valve). d: Slightly opening safety valve. e: Fully open safety valve. f: Fully enclosed safety valve. g: Semi-enclosed safety valve. h: Open-type safety valve. 4. Basic requirements for safety valves Safety valves must meet two key requirements: operational safety and economic efficiency. During operation, they must be able to open promptly, discharge sufficient volume of fluid, return to their original position in a timely manner, and provide reliable sealing. 5. Basic technical requirements for safety valves: (1) Safety valves for steam with a setting pressure greater than 3.0 Mpa, or those for air or other gases with a temperature exceeding 235°C, should be capable of preventing the discharged medium from directly eroding the spring. (2) The safety valve for steam must be equipped with a wrench; when the pressure of the medium reaches 75% or more of the set pressure, the valve disc can be lifted using this wrench, and such wrench shall not interfere with the operation of the valve. (3) For safety valves used with toxic or flammable media, closed-type safety valves must be employed, and leaks at the valve cover and the gasket of the protective cover must be prevented. (4) To prevent the mechanism for adjusting the spring compression from loosening, as well as to avoid arbitrary changes to the set pressure, a anti-loosening device must be installed along with a seal. (5) The valve seat shall be fixed to the valve body and must not become loose; full-open safety valves shall be equipped with a mechanism to limit the opening height. (6) Even if the safety valve is partially damaged, it should still be able to achieve the rated discharge volume; when the spring is damaged, components such as the valve disc will not fly out of the valve body. (7) For safety valves with additional back pressure, a back pressure balancing mechanism should be installed based on the magnitude and variation of that pressure. (8) For pilot-operated safety valves, both the pilot valve and the main valve shall be tested for sealing performance and operation, and they must meet the performance requirements specified in the standards. 6. Operating principle of the safety valve: The spring pre-tension force Fd acting downward on the valve disc is used in part to counteract the force Po.S exerted by the medium, and in part to create a compressive force f between the valve seals. As the pressure of the medium increases, this compressive force between the seals decreases; when the medium pressure reaches the opening pressure Ps, this compressive force becomes zero and the safety valve opens. At this point, the following relationship holds: Fd = Ps.S. 7. Standards related to safety valves: Due to the importance of safety valves, countries around the world have established numerous relevant standards and regulations for their design and manufacture. Most of the early safety valve standards in our country were based on those of the former Soviet Union. The current standards for safety valves include: GB/T12241 (General requirements for safety valves), GB/T12242 (Methods for testing the performance of safety valves), GB/T12243 (Safety valves with direct spring loading), ZB-J98-013 (Technical specifications for safety valves in power plants), API520/API526/API527 (American standards). 8. Calibration and maintenance of safety valves: To thoroughly examine all the technical performance parameters of a safety valve, it is necessary to conduct comprehensive performance tests. The most basic requirement for such tests is that they should simulate actual operating conditions, or even exceed them. In addition to equipment such as high-temperature and high-pressure vessels, such testing facilities also require instruments for the rapid measurement of various parameters, as well as a high-temperature and high-pressure steam source with a large flow rate, which results in enormous costs. Verification is part of the safety valve testing and a key item in the manufacturer’s factory tests. Before leaving the factory, a safety valve calibration bench at room temperature is generally used to set the opening pressure for air media and to conduct seal integrity tests. The normal-temperature calibration bench for safety valves can only be used for setting the opening pressure and conducting seal integrity tests. 9. Under what circumstances does a safety valve need to be calibrated? 1) Before long-term storage or before first use. 2) Regular calibration. 3) Valves with severe damage and rust. 4) Valves with missing nameplates. 5) Valves with damaged seals. 10. Significance and methods of adjusting the seating pressure: The seating pressure as defined by national standards refers to the static pressure at the valve inlet when, after the safety valve has reached its discharge state and the medium pressure drops to a certain level, the valve disc comes into contact with the valve seat again, that is, when the opening height becomes zero. Neither too low nor too high a seating pressure is desirable; too low a pressure leads to losses of medium and energy, while too high a pressure prevents adequate discharge and results in frequent valve operation. The principle is to increase the seating pressure as much as possible while still ensuring adequate discharge, thereby reducing losses of medium and energy. The return pressure is adjusted through an adjusting ring, based on the principle of clearance. The smaller the clearance, the greater the resistance during spraying, and the greater the force that holds the valve core in place, making it more difficult for it to return to its original position. Conversely, the larger the clearance, the easier it is for the valve core to drop back, resulting in a higher return pressure. For safety valves that have only a lower adjusting ring, when the adjusting ring is moved upward, the reset pressure decreases; when it is moved downward, the reset pressure increases ； For safety valves with upper and lower adjustment rings, reducing the distance between these rings lowers the return pressure, while increasing that distance raises the return pressure. 11. Verification methods and their advantages and disadvantages: There are two methods for verifying safety valves: on-site verification (in-line verification) and verification on a testing bench. Whenever possible, on-site verification should be used, as it better reflects the actual operating conditions and is therefore more reliable. Advantages of on-site calibration: It facilitates the calibration of welded safety valves, allows for the determination of the reset pressure, and provides accurate measurements. The disadvantages are long verification time, the need for the system to be pressurized repeatedly, which is uneconomical and dangerous; moreover, it cannot be used for leak testing. 12. Advantages and disadvantages of the normal temperature calibration bench for safety valves: a. It enables the adjustment and leak testing of safety valves used with media at normal temperatures as well as at working temperatures below 250°C. b. Determining a narrower range of allowable errors for the safety valve’s opening pressure saves time required for adjusting newly installed safety valves, reduces labor intensity, lowers energy consumption, and decreases work-related risks. c. There is a deviation between the operating temperature and room temperature (the spring softens at high temperatures), making it impossible to verify the return pressure. 13. Media used for calibration at room temperature: For calibration at room temperature, air, nitrogen, and water are generally used (with pressures above 20 Mpa). Flammable, oxidizing, or toxic and harmful media such as oxygen, hydrogen, and acetylene gas cannot be used as medium sources for calibration. Kerosene, gasoline, diesel, etc., cannot be used as medium sources either. Although carbon dioxide gas is non-toxic and non-flammable, it freezes easily and can block pipelines. 14. Principle of safety valve calibration: A pressurized medium is introduced at the inlet of the safety valve to be calibrated. Once the pressure of the medium rises to the level at which the safety valve opens, the pressure at that point is measured; this value represents the opening pressure. It is then adjusted to the specified opening value, thereby completing the calibration of the opening pressure. Then, when the pressure drops to the specified value (90% of the operating pressure), a pressure gauge or other approved methods are used to check for any leakage of fluid, which constitutes a seal integrity test. 15. Repair of safety valves: The repair of safety valves primarily involves fixing their sealing surfaces; therefore, high requirements are placed on the materials used for these sealing surfaces. The basic requirements are as follows: they must have high strength to withstand high sealing pressures; they need to possess certain impact resistance to cope with the shocks resulting from the rapid opening and closing of the valve core; they should have high hardness to resist erosion by the medium flowing through them; they must not rust, as this would lead to leakage; they need to be heat-resistant so that they can function with media such as steam without becoming soft due to high temperatures; and they must be corrosion-resistant to withstand the effects of acids and alkalis. The main types of hard seals for safety valves include flat seal, conical seal, spherical seal, etc., among which the flat seal is the most commonly used. (1) Repair of the safety valve sealing surface: There are two types of sealing surfaces for safety valves – metal and non-metal. Non-metallic seals are generally used for media at normal temperatures, while metallic seals are more commonly used for media under high temperature and pressure conditions. When the sealing surface made of non-metallic material is damaged, the gasket (sealing ring) is usually removed and replaced; if it cannot be removed or no spare parts are available, machining is the only option. When the metal sealing surface is not severely damaged, grinding is the method used. Grinding is divided into rough grinding and fine grinding. The valve seat and the valve core must be ground apart using flat grinding tools; they cannot be ground against each other, as grooves will form as a result, and leakage will occur once the valve is opened and then closed again. (2) Grinding tools and materials: Grinding plates: a. Cast iron (used for rough grinding). b. Cemented carbide (used for fine grinding). Grinding head (used for grinding valve seats). Abrasives: chromium oxide, white corundum, 280#, 360#, 600#, W4, W2.5, W1.5. Abrasives: oleic acid, machine oil, rapeseed oil, lard. (3) Grinding of the sealing surface: Grinding involves the use of grinding tools and abrasives; under a certain pressure applied by the tools, the working surface is slightly abraded, thereby removing an extremely thin layer of metal from the surface of the workpiece. This results in a high degree of flatness and a high level of surface finish for the workpiece. This processing technique is known as grinding. The principle of grinding ； During grinding, the abrasive applied to the grinding tool is subjected to certain pressure from the workpiece and the tool, causing some of the abrasive particles to be embedded within the tool. When the grinding tool and the workpiece undergo complex relative movements, the abrasive slides and rolls between the workpiece and the tool, resulting in cutting action and pressure; this causes a layer of protrusions on the workpiece surface to be removed. At the same time, the abrasive exerts a chemical effect, leading to the rapid formation of an oxide film. (4) Contents that should be included on the safety valve’s metal nameplate: 1) Model, specifications (diameter). 2) Pressure rating. 3) Channel diameter. 4) Nominal pressure. 5) Operating temperature, medium. 6) Rated displacement coefficient. 7) Manufacturer, manufacturing approval number. 8) Manufacturing date. 9) Product number. 10) Pass mark. 11) Opening pressure. (5) Characteristics and precautions for high-pressure calibration 1) High-pressure safety valves make a loud noise when they open, especially those with large diameters. 2) Vibration is relatively large when the high-pressure safety valve opens. 3) The recoil force is very large when it is activated, so the material at the flange connection must be of high quality. 4) The spring adjustment cannot be large; only fine tuning is possible (the spring of the high-pressure safety valve is thicker). 5) High-pressure safety valves are often exposed to high temperatures and pressures; there is a discrepancy between testing them at normal temperature and their actual operating conditions (the spring becomes softer at high temperatures). 6) The airflow and water flow used for verification must be filtered.