Maintenance and calibration of pressure vessel safety valves

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Selection of safety valves 1. Pressure rating The operating pressure of a safety valve has a completely different meaning from the nominal pressure and the operating pressure of the spring. The setting pressure of the safety valve can be adjusted by changing the pre-compression amount of the spring, and the various performance characteristics of the safety valve are also controlled by the spring. Each spring can only operate within a certain specified pressure range; if this range is exceeded, a spring with an appropriate pressure rating must be replaced. When replacing the spring, it is also important to ensure that the pressure does not exceed the value indicated by the hard markings on the valve seat. If the marking on the side of the valve seat is 1.6 Mpa, then it is not allowed to use a spring with a pressure rating higher than 1.6 Mpa. 2. Selection and Installation: When selecting a safety valve, it is necessary to take into account not only the properties of the medium used, as well as the operating temperature and pressure, but also various factors such as the processability of the material and its cost, in order to choose a safety valve with an appropriate design. For example, the safety valves of types A48Y, A27W, and A28H with levers are suitable for steam and air ； A41H and A42Y hermetic safety valves are commonly used in equipment handling fluid media. For flammable, explosive, and toxic liquids and gases, safety valves with high sealing performance should be selected. For example, natural gas, liquefied petroleum gas, and chlorine should use safety valves of type A42F and A21F. When the operating temperature of closed safety valves exceeds 300°C and that of open safety valves exceeds 350°C, safety valves equipped with radiators should be selected. When an additional back pressure is applied and the change in back pressure exceeds 10% of the set pressure, and the medium is corrosive, a bellows safety valve should be used. 3. Installation of safety valves (1) Safety valves should be installed vertically at the top of the boiler, that is, on the drum, at the top of the steam vessel, on the main pipelines for superheated steam, as well as in those parts of pressure vessels and protected equipment that are most sensitive to pressure changes. (2) When the pressure source of the protected system is a device with pulsation generation (such as an air compressor), and the upper limit of this pressure fluctuation is very close to the opening pressure of the safety valve, the safety valve should be installed at an appropriate distance from the pressure source or the stabilization device in order to prevent abnormal frequent opening of the safety valve. (3) The safety valve should be installed in a location that facilitates disassembly, regular maintenance, and calibration, and its discharge outlet should be led to a safe place. Maintenance and Calibration 1. Visual inspection: Check whether the manufacturing nameplate and manufacturing license number of the safety valve submitted for inspection are intact, whether the valve body and springs are corroded or damaged, and whether any components are missing ； Are there issues such as mechanical damage? Disassembly, assembly, and maintenance (taking a spring-loaded safety valve with a wrench as an example): (1) Loosen the screws that hold the protective cover in place, then remove the pin, crossbar, and protective cover. (2) Unscrew the lifting nut from the valve stem. (3) Loosen the lock nut and adjustment rod of the adjustment screw to remove the pre-tension load of the spring. (4) Unscrew the nut that connects the valve cover to the valve body, remove the assembly consisting of the valve cover, valve stem, spring, and other components, and then take out the guide sleeve and valve disc. (5) In the case where the valve core of a fully open safety valve is rusted inside the rebound disc, methods such as rust removers, lubricants, baking and water cooling, or tapping can be used to remove it; care should be taken not to tap the sealing surface. (6) Grind the valve disc (valve core) and the valve seat; it is a very important task to ensure that the sealing surfaces are finely ground. By performing rough grinding, medium grinding, and fine grinding on the sealing surfaces of the valve disc and valve seat, these surfaces can be brought to a mirror-like condition, meeting the requirements for proper sealing. 2. Calibration of safety valves (1) The medium used for calibrating safety valves is generally air or nitrogen. When testing a safety valve, place it firmly in the automatic clamping chuck, and gradually increase the inlet pressure of the valve in order to adjust its set pressure. Once the pressure reaches 90% of the set pressure, the rate of pressure increase should not exceed 0.01 Mpa/s. When a continuous and steady flow of the test medium can be heard coming out, the inlet pressure at that point is considered to be the set pressure for that safety valve. Generally, the set pressure test should be conducted at least 3 times. (2) After the set pressure of the safety valve has been adjusted to the proper value, the pressure should be reduced and the inlet pressure of the safety valve adjusted. When the set pressure is less than or equal to 0.3 Mpa, the sealing pressure should be 0.03 Mpa lower than the set pressure ； When the set pressure is greater than 0.3 Mpa, the sealing pressure is 90% of the set pressure ； Both the set pressure and the sealing test pressure data must be entered into the verification record in a timely manner. (3) There is always a certain margin of error in the set pressure of safety valves, but as long as it meets the usage requirements and the allowable error range, the actual opening pressure of such a safety valve is considered to be its set pressure. When the set pressure is less than or equal to 0.5 Mpa, the allowable error between the measured set value and the required set value is ±0.014 Mpa ； When the set pressure is greater than 0.5 Mpa, the allowable error is ±3% of the set pressure. (4) When the inlet of the safety valve is under the set pressure, the discharge outlet should be positioned in the opposite direction to the person performing the calibration; the calibrator’s head should be kept away from the top of the safety valve to avoid any potential injuries. Several issues: (1) The using units lack awareness of the importance of safety valves and fail to manage them properly, resulting in severe rusting and failure of these valves in certain environments ； Bird nests, dead rats, insects, and similar things can be found at the discharge outlet of the safety valve. (2) The user unit did not handle the safety valve properly during transportation, resulting in vibrations that caused the valve stem and valve core of the safety valve to shift. This led to a change in the valve’s set pressure, thereby causing inaccurate pressure readings during use. (3) During the installation of the safety valve, improper handling by the installers can result in a change in the set pressure of the valve, or even leakage. For example, packing tape used during installation may end up inside the valve inlet, or debris may be carried into the valve chamber along with the fluid, accumulating on the sealing surface and preventing it from sealing properly, thus causing leakage. (4) Safety valves may come out of the factory with inaccurate pressure ratings. For example, a safety valve with a pressure rating of 0.3–0.7 Mpa might, upon calibration, be found to have a minimum setting of only 0.4 Mpa or a maximum setting of only 0.6 Mpa. (5) Some manufacturers, in order to reduce production costs, replace components such as guide sleeves, recoil discs, and valve stems, which should be made of stainless material, with cheaper materials that are prone to rusting, resulting in the rusting and failure of safety valve parts. For example, in safety valves of model A27W-10T, the adjustment screw, lock nut, and valve stem material are supposed to be made of copper; however, some manufacturers use alloys or iron as substitutes, which reduces their service life.