Pilot-operated safety valve

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Pilot-operated safety valve: A pilot-operated safety valve is a type of safety valve that relies on the medium discharged from a pilot valve to drive or control it. A safety valve controlled by a pilot valve has its set pressure regulated by the pilot valve, and its operation is essentially unaffected by back pressure. Safety valves with pilot valves are further divided into quick-opening (full-opening) and regulating (gradual-opening) types. Image a. The closing force of the safety valve seat. The leakage of safety valves is often influenced by the closing force of the valve; in terms of its operating principle, the higher the pressure in front of a pilot-operated safety valve, the more tightly it will close ； In contrast, ordinary spring-loaded safety valves work in the opposite way: the closer the pressure in front of the valve is to the set pressure, the weaker the closing force of the valve, and the more likely it is to leak. A pilot-operated safety valve uses a pilot valve to control the main valve. Since the pilot valve features a diaphragm structure with a large diaphragm area, it can still generate a certain force even at very low operating pressures. Thereby controlling the operation of the main valve. Furthermore, before the pilot valve opens, the main valve is not affected by the control flow and remains tightly closed; therefore, the pilot-operated safety valve can achieve a bubble-level seal. b. Soft seal. The temperatures in most chemical production processes are not very high. With the development of modern industry, various temperature-resistant elastomeric materials have been developed as sealing materials for valves; for example, polytetrafluoroethylene has a suitable temperature range of –253 to 204°C. Pilot-operated safety valves use soft seals for their valve components, which ensures good sealing performance ； Most spring-type safety valves use hard-sealing materials, resulting in poor sealing performance. Furthermore, the pilot-operated safety valve exerts a high closing force under the pressure acting in front of it, which ensures its excellent sealing performance. c. Leakage before the safety valve opens. Spring-loaded safety valves begin to leak when the pressure in front of the valve reaches 90%~95% of the set pressure, and as the pressure further increases, the leakage from the valve increases as well. When the pressure before the valve reaches a set value, the valve opens ； The valve opens fully when the pressure before it rises to 110% of the set pressure. As the pressure is released, the pressure in front of the valve drops, and the valve closes when it falls to 95% of the set pressure. It is a quick-opening pilot-operated safety valve that begins to leak when the pressure in front of the valve reaches 98% of the set value; once the pressure reaches that set value (without any overpressure), the valve opens fully ； The valve closes when the pressure before it drops to 95%~98% of the set pressure. It is a adjustable pilot-operated safety valve that begins to leak when the pressure in front of the valve reaches 98% of the set value; the valve opens once the pressure there reaches the set value ； As the pressure before the valve rises to 110% of the set pressure, the valve opens fully ； When the pressure before the valve decreases, the valve opening decreases ； The valve closes once the pressure before it drops to 98%~99% of the set pressure. It can be seen that the pilot-operated safety valve starts to leak later and at a lower rate before the pressure in front of the valve reaches a certain level ； When the pressure before the valve drops below a set value, it can close earlier, thereby reducing leaks and excessive emissions. d. Maintenance. After the pilot-operated safety valve releases pressure, the valve core can return to its seat smoothly and continue to provide effective overpressure protection, thus eliminating the need to remove the safety valve for readjustment. e. The operating characteristics are not affected by back pressure. In a general-purpose spring safety valve, a back pressure that exceeds 15% of the inlet pressure will affect the valve’s discharge capacity; therefore, in engineering design, it is common to limit the back pressure of such valves to no more than 10% of the inlet pressure. Although balanced spring safety valves can withstand a certain amount of back pressure, when the back pressure exceeds 30% of the inlet pressure, it will also affect the valve’s discharge capacity. Pilot-operated safety valves can withstand a much higher back pressure than the two spring-operated safety valves mentioned above. f. Release volume. The nozzles of spring safety valves are mostly semi-nozzles, and the diameter of these semi-nozzles is smaller than the inner diameter of the flange at the inlet of the safety valve ； The nozzle of a pilot-operated safety valve is a full nozzle, and for inlet flanges of the same size, its diameter can be larger than that of a half-nozzle. In other words, when the inlet flange dimensions are the same, the nozzle size of a pilot-operated safety valve can be larger than that of a spring-loaded safety valve, resulting in a higher discharge capacity for the pilot-operated safety valve. g. Pressure drop in the safety valve inlet pipe. The pressure drop in the inlet pipe of the spring safety valve must not exceed 3% of the inlet pressure; otherwise, it may affect the proper functioning of the spring safety valve ； In the case of pilot-operated safety valves, the pressure guide tube of the safety valve can be extended to be connected directly to the pressure source container or pipeline for pressure measurement. This allows for the direct detection of the actual pressure in the system being protected, while also preventing the safety valve from malfunctioning due to excessive pressure drops in the pipeline leading to it. h. Pilot-operated safety valves are suitable not only for low-pressure, normal-temperature storage tanks containing general chemical materials, but also for low-temperature storage tanks used for natural gas, synthetic ammonia, propane, butane, oxygen, and similar substances. i. Price of safety valves. For pilot-operated safety valves, regardless of their diameter, the pilot valve used within a certain pressure range is always of the same size. Therefore, the manufacturing cost of large-diameter, high-pressure pilot-operated safety valves is lower than that of spring safety valves, but at low pressures and small diameters, the price of pilot-operated safety valves is higher than that of spring safety valves of the same specifications. From the above comparison, it can be seen that although the directly-loaded spring-type safety valve is a commonly used type of safety valve in pressure vessels, its performance is inferior to that of the pilot-operated safety valve. When a pilot-operated safety valve is used, some of the contradictions mentioned in the discussions regarding the pressure of the pressure vessel in relation to the safety valve and its maximum allowable operating pressure can essentially be resolved; in other words, the use of a pilot-operated safety valve allows for the solution of certain challenging issues in the design of chemical processing equipment at the process system design level. However, it must be ensured that when non-direct load-type safety valves are used, the main valve will still be able to open on its own at the specified opening pressure and discharge its rated amount of flow, even in the event of a failure of the valve mechanism.