Calculation of the setting pressure for safety valves

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It opens and closes automatically based on the operating pressure of the pressure system, and is generally installed on equipment or pipelines in closed systems to protect the safety of the system. When the pressure inside the equipment or pipeline is at the normal operating pressure, the safety valve remains airtight and leak-free ； When the pressure inside a device or pipeline exceeds the set pressure of the safety valve, it opens automatically to release pressure, ensuring that the pressure of the medium within the device or pipeline remains below the set level. This helps to maintain the proper functioning of the device and pipeline, prevents accidents, and reduces losses. Lever-type safety valve: A type that uses the force exerted by a weight through a lever on the valve disc to control the opening and closing of the valve, thereby providing protection against excess pressure in the tank. Spring-loaded safety valve: A type of safety valve that uses the compressive force generated by a compressed spring to act on the valve disc, thereby controlling the opening and closing of the valve and providing protection against excessive pressure in the tank. Spring-loaded safety valves are widely used in petrochemical plants. The loading mechanism of a spring-type safety valve is a spring; by adjusting the nut, it is possible to change the degree of compression of the spring, thereby adjusting the force with which the valve disc presses against the valve seat and determining the value of its opening pressure. The spring-type safety valve is compact in size, small in volume, and responds quickly; it is not very sensitive to vibrations. It can be installed on mobile containers. The downside is that the elasticity of the spring inside the valve decreases when exposed to high temperatures. II. The concept of setting pressure: Setting pressure, also known as opening pressure, is an important parameter in the operation of safety valves. The value of this setting pressure has a direct impact on the safety of equipment operation. III. Determination of the setting pressure: The setting pressure of a safety valve is generally not greater than the design pressure of the pressure vessel or pipeline. Unless there are special process requirements, the set pressure of the safety valve is 1.10 times the normal operating pressure, with a minimum of 1.05 times. IV. Adjustment of the set pressure: Within the specified operating pressure range, the set pressure can be adjusted by rotating the adjustment screw, thereby changing the amount of compression of the spring preload. Remove the valve cover, loosen the lock nut, and then the adjustment screw can be adjusted. First, increase the inlet pressure to cause the valve to open once; if the opening pressure is too low, tighten the adjustment screw in a clockwise direction ； If the operating pressure is too high, loosen it in a counterclockwise direction. After adjusting to the desired set pressure, tighten the lock nut and install the cover cap. If the required setting pressure exceeds the operating pressure range of the spring, it is necessary to replace it with another spring that has an appropriate operating pressure range, and then make the adjustments. After replacing the spring, the corresponding values on the nameplate must be changed.  When adjusting the set pressure, the following points should be taken into consideration: (1) When the medium pressure approaches the set pressure (reaching over 90% of the opening pressure), the adjustment screw should not be turned, to prevent the valve disc from rotating as well and thus damaging the sealing surface. (2) To ensure the accuracy of the set pressure value, the medium conditions used for adjustment, such as the type of medium and its temperature, should be as close as possible to the actual operating conditions. The set pressure often changes when the type of medium changes, especially when it transitions from a liquid phase to a gas phase. As the operating temperature increases, the set pressure decreases. Therefore, when it is adjusted at normal temperature for use at high temperatures, the set pressure value at normal temperature should be slightly higher than the required opening pressure value. V. Determination of the special structure of safety valves ★ For safety valves used with steam at a discharge pressure greater than 3 MPa, or for safety valves used with gases at temperatures exceeding 320°C, safety valves equipped with radiators (fins) should be selected. ★For safety valves that are subjected to additional back pressure, and when the change in this back pressure exceeds 10% of the set pressure, diaphragm safety valves should be used ； Furthermore, for safety valves used with corrosive media, in order to prevent the springs and guiding mechanisms from being corroded by the medium, diaphragm safety valves should also be selected. ★For flammable media with extremely or highly hazardous toxicity, closed safety valves must be used; if a valve equipped with a lifting mechanism is required, a closed safety valve with a wrench should be employed. ★For non-hazardous media such as air, hot water above 60°C, or steam, safety valves equipped with wrenches should be used. ★Liquefied gas tankers should be equipped with built-in safety valves. ★For conditions with high discharge rates, a fully open type should be selected ； For operating conditions with stable working pressure and low discharge volume, a slightly open type is advisable ； For high-pressure conditions with large discharge volumes, a non-direct-acting type, such as a pulse safety valve, is recommended ； For containers with a length exceeding 6m, two or more safety valves should be installed. ★For fixed containers with low operating pressure, gravity-type (pressure cookers) or lever-weight type safety valves can be used ； Mobile devices should be equipped with spring-loaded safety valves. ★For media that are thick and prone to clogging, a discharge device consisting of a safety valve in series with a burst disc is recommended.