Pressure adjustment and operating procedures for safety valves

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1. Adjusting the opening pressure: ① Before leaving the factory, the safety valves should have their opening pressure adjusted individually to the set value required by the user. If the user specifies the spring operating pressure class, it is generally adjusted at the lower limit of that pressure class upon leaving the factory. ②Before installing the safety valve on the equipment to be protected, or prior to installation, it must be readjusted at the installation site to ensure that its set pressure value meets the required specifications. ③Within the spring operating pressure range specified on the nameplate, the opening pressure can be adjusted by rotating the adjustment screw to change the degree of spring compression. ④Before rotating the adjustment screw, the valve inlet pressure should be reduced to below 90% of the opening pressure, to prevent the valve disc from being rotated when the adjustment screw is turned, which could damage the sealing surface. ⑤To ensure the accuracy of the opening pressure value, the medium conditions during adjustment, such as the type of medium and temperature, should be made as close as possible to the actual operating conditions. The opening pressure often changes with the type of medium, especially when the aggregation state of the medium differs (for example, from liquid to gas phase). As the operating temperature increases, the opening pressure generally 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. The extent to which it is high depends on both the valve design and the material chosen; it should be based on the manufacturer’s instructions. ⑥Conventional safety valves are used in applications where an additional back pressure is applied. When the opening pressure is adjusted after testing (with the back pressure at atmospheric level), its set value should be the required opening pressure minus the value of the additional back pressure. 2. Adjustment of discharge pressure and return pressure ① To adjust the valve’s discharge pressure and return pressure, it is necessary to conduct a test in which the valve is opened to its full extent; therefore, this can only be done on a large-capacity testing setup or after the safety valve has been installed on the equipment it is meant to protect. The adjustment method varies depending on the valve structure. ②For structures with a recoil disc and a seat adjusting ring, the adjustment is carried out using the seat adjusting ring. Unscrew the fixing screw of the adjustment ring, and insert a tool such as a thin iron rod into the exposed screw hole; this will allow you to move the teeth on the adjustment ring, thereby causing it to rotate left and right. When the adjusting ring is rotated clockwise to the left, its position rises, and both the discharge pressure and the return pressure decrease. Conversely, when the adjusting ring is rotated clockwise to the right, its position drops, and both the discharge pressure and the return pressure increase. With each adjustment, make sure that the amount of rotation of the ring is not too large (usually just a few teeth’ worth of rotation is sufficient). After each adjustment, the set screw should be tightened so that its end lies in the groove between two teeth of the adjustment ring; this prevents the adjustment ring from rotating without exerting any radial pressure on it. For safety reasons, before turning the adjustment knob, the pressure at the inlet of the safety valve should be reduced appropriately (generally to less than 90% of the opening pressure), in order to prevent the valve from opening suddenly during adjustment and causing an accident. ③For a structure with upper and lower adjustment rings (one adjustment ring on each of the guide sleeve and the valve seat), the adjustment is somewhat more complex. The valve seat adjusting ring is used to change the size of the passage between the valve disc and the adjusting ring, thereby altering the extent to which pressure accumulates in the chamber between the valve disc and the adjusting ring at the initial opening of the valve. When the valve seat adjustment ring is raised, the degree of pressure buildup increases, thereby reducing the phase of proportional opening of the valve and leading to a sudden, rapid opening. Therefore, raising the valve seat adjustment ring can reduce the discharge pressure. It should be noted that the valve seat adjustment ring must not be raised too close to the valve disc either. In that case, leakage at the sealing surface can cause the valve to open suddenly and prematurely; however, since the pressure of the medium is not yet sufficient to keep the valve disc in the open position, the valve disc closes again immediately, resulting in frequent opening and closing of the valve. The upper adjusting ring is used to change the angle at which the flowing medium is deflected after reflecting off the underside of the valve disc, thereby altering the magnitude of the fluid force and allowing for the adjustment of the reseating pressure. When the upper adjusting ring is raised, the bending angle decreases, the fluid force accordingly decreases, which in turn increases the reseating pressure. Conversely, when the upper adjustment ring is lowered, the reseating pressure decreases. Of course, while changing the return pressure, the upper adjustment ring also affects the discharge pressure – increasing the upper adjustment ring raises the discharge pressure, while decreasing it lowers it – although the degree of this effect is not as significant as that on the return pressure. 3. Sealing: Once the safety valve has been adjusted, it should be sealed to prevent any unauthorized changes to the settings that have been made. When servicing a safety valve, the positions of the adjusting screw and adjusting ring should be noted before disassembling the valve, to facilitate readjustment after the servicing. After readjustment, it should be resealed with lead.