Safety valve calibration and management

Thread Content

1. Basic knowledge of safety valves (1) Valves: A general term for mechanical devices equipped with moving parts, 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 medium it is used with. (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 return 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/closing pressure difference △Pbl: The difference between the set pressure and the reseating pressure of the safety valve. It is usually expressed as a percentage of the set pressure; only when the set pressure is very low is it expressed in MPa. (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 regarding 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°C. 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: Flanged safety valve. b: Threaded connection safety valve. c: Weld the safety valve. (5) Classified by opening height: a: Low-lift 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 opened safety valve: the opening height is between that of a slightly opened valve and a fully opened 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 open safety valve. e: Fully open safety valve. f: Fully closed safety valve. g: Semi-closed 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 their operation, they must exhibit: prompt opening, sufficient discharge capacity, timely reseating, and 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 used for air or other gases at temperatures above 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, this wrench can be used to lift the valve disc, and it should 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 leakage at the valve cover and protective cover gaskets 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 fluctuations of that pressure. (8) For pilot-operated safety valves, both the pilot valve and the main valve shall be tested for sealing performance and operating capability, and both 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 exerted by the medium, Po.S; the other part of this force generates the 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. In the early days, most of China’s safety valve standards were modeled after those of the former Soviet Union. Currently adopted safety valve standards include: GB/T12241 (General requirements for safety valves), GB/T12242 (Test methods for the performance of safety valves), GB/T12243 (Direct spring-loaded safety valves), ZB-J98-013 (Technical specifications for power plant safety valves), and API520/API526/API527 (American standards). 8. Calibration and maintenance of safety valves: To comprehensively evaluate all technical performance indicators of safety valves, full-performance tests must be conducted. The most fundamental requirement for such tests is that they should be carried out under actual operating conditions, or even under conditions exceeding those of normal operation. 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 testing. Before leaving the factory, safety valve calibration rigs at room temperature are generally used to set the opening pressure for air media and to conduct leakage tests. The normal-temperature calibration bench for safety valves can only be used for setting the opening pressure and conducting seal 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) Severely damaged and corroded valves. 4) Valves with missing nameplates. 5) Valves with damaged lead seals. 10. Significance and methods of adjusting the reseating pressure: The reseating pressure defined in national standards refers to the static pressure at the valve inlet when, after the safety valve has reached its discharge state, the medium pressure drops to a certain level, causing the valve disc to come into contact with the valve seat once again—that is, when the opening height becomes zero. It is undesirable for the reseating pressure to be either too low or too high. If it is too low, it results in losses of medium and energy; if it is too high, the required discharge rate cannot be achieved, leading to frequent valve cycling. In principle, the reseating pressure should be set as high as possible while still meeting the required discharge rate, thereby minimizing 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 harder 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, moving the adjusting ring upward reduces the reset pressure, while moving it downward increases the reset pressure ； For safety valves with upper and lower adjustment rings, as the distance between these rings decreases, the reset pressure decreases; whereas as the distance between them increases, the reset pressure rises. 11. Verification methods and their advantages/disadvantages There are two methods for verifying safety valves: on-site verification (online verification) and verification on a test bench. Whenever conditions permit, on-site verification should be preferred, as it better reflects 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 narrow 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 is the seal integrity test. 15. Repair of safety valves: The repair of safety valves mainly involves the maintenance of 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 endure the shocks associated with 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 able to withstand high temperatures, so as to function properly with media such as steam without becoming soft due to heat; and finally, they must be corrosion-resistant, able 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. Abrasive agents: 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 process results in a high degree of flatness and a high level of surface finish for the workpiece, and this type of processing 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 motion, the abrasive particles slide and roll between them, resulting in cutting and compressive forces that remove protruding peaks from the workpiece surface. Meanwhile, the abrasive agent exerts a chemical effect, quickly forming an oxide layer. (4) Contents that should be included on the metal nameplate of the safety valve: 1) Model and specifications (nominal diameter). 2) Pressure rating. 3) Flow channel diameter. 4) Nominal pressure. 5) Operating temperature and medium. 6) Rated displacement coefficient. 7) Manufacturer, manufacturing approval number. 8) Date of manufacture. 9) Product number. 10) Inspection conformity mark. 11) Opening pressure. (5) Characteristics and precautions for high-pressure calibration 1) When the high-pressure safety valve opens, it makes a very loud noise, especially in the case of large-diameter valves. 2) Vibration is relatively high 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 often operate under high temperature and pressure; there may be discrepancies between their performance during room-temperature calibration and actual operating conditions (the spring becomes softer at high temperatures). 6) The airflow and water flow used for verification must be filtered.