I: safety valve calibration
There are two main methods for calibrating safety valves: on-site calibration (also known as on-line calibration) and calibration bench calibration.
Field calibration is preferred whenever possible because it is closer to the actual operating conditions and is therefore more reliable. The advantages of on-site calibration are that it is particularly suitable for welded safety valves and can accurately measure the return pressure. However, there are some disadvantages to this method, such as long calibration times, the need to repeatedly pressurize the system, which is both uneconomical and risky, and the impossibility of carrying out a sealing test.
Bench calibrations are usually carried out before the product leaves the factory, using an ambient temperature safety valve calibration bench for opening pressure setting and sealing tests on air media. Please note, however, that the function of the ambient temperature calibrator is limited to opening pressure calibration and sealing test.
II: Safety Valve Adjustment
Adjustment of safety valve opening pressure
The opening pressure of the safety valve will be adjusted to the calibration value required by the user before leaving the factory. If the user specifies a spring-loaded operating pressure stage, the adjustment is normally made to the lower limit of that pressure stage.
Before installing the safety valve on the protected equipment, the user must readjust its opening pressure at the installation site to ensure that its setting pressure value is in accordance with the requirements.
The opening pressure is adjusted by rotating the adjusting screw to change the amount of spring compression within the spring operating pressure stage specified on the nameplate.
Before turning the adjusting screw, it is necessary to reduce the valve inlet pressure to less than 90% of the opening pressure to prevent damage to the sealing surfaces.
In order to ensure the accuracy of the opening pressure value, the media conditions (e.g., media type, temperature, etc.) at the time of adjustment should be as close as possible to the actual operating conditions. The opening pressure usually changes when the medium type changes, especially from liquid phase to gas phase. At the same time, as the operating temperature increases, the opening pressure will generally decrease. Therefore, when adjusted at room temperature and used in high temperature environments, the set pressure value at room temperature should be slightly higher than the required opening pressure value, and the exact degree of increase depends on the valve structure and material, and should be subject to the manufacturer’s instructions.
When the safety valve is used in the occasion of fixed additional back pressure, when the opening pressure is adjusted after inspection (at this time the back pressure is atmospheric pressure), its setting value should be the required opening pressure value minus the additional back pressure value.
Adjustment of safety valve discharge pressure and return pressure
To adjust the discharge pressure and seat pressure of the valve, it is necessary to carry out an action test with the valve at full opening height. This can usually only be done on a high-capacity test rig or after the safety valve has been installed in the protected equipment. The method of adjustment varies according to the valve construction.
For constructions with a recoil disk and seat adjustment ring, adjustment can be made using the seat adjustment ring. The vent pressure and seat return pressure are adjusted by rotating the adjusting ring to change the extent to which pressure builds up in the chamber between the flapper and the adjusting ring when the valve is initially opened.
For structures with upper and lower adjusting rings (one on the guide sleeve and one on the seat), the adjustment process is more complex. The seat adjusting ring is mainly used to change the size of the passage between the valve flap and the adjusting ring, thus adjusting the degree of pressure buildup during the initial opening of the valve. The upper adjusting ring is used to adjust the return pressure by changing the angle at which the fluid is reflected on the lower side of the flap, thereby adjusting the fluid force.
Please note that before making any adjustments, the inlet pressure to the safety valve should be appropriately reduced to prevent accidents caused by sudden opening of the valve.
How to adjust and calibrate safety valves
