Pressure tapping method for self-acting control valves

A self-acting pressure control valve does not require any external power source; it uses the energy of the medium being regulated as its power source. An actuator is used to control the position of the valve core, thereby changing the pressure difference and flow rate between the two ends and maintaining stable pressure before (or after) the valve. It features rapid response, good sealing performance, and minimal fluctuations in pressure set points, and is widely used in the automatic control of pressure reduction, stabilization, or relief for gas, liquid, and steam media. This series of products is available in three configurations: single-seat (ZZYP), sleeve-type (ZZYM), and double-seat (ZZYN). The actuators come in diaphragm type and piston type. The operating modes include pressure regulation after the valve for pressure reduction (Type B) and pressure regulation before the valve for pressure relief (Type K). The nominal pressure grades of the product are PN1.6, 4.0, and 6.4 ； Valve body diameter range: DN20~300 ； The leakage level categories are II, IV, and VI ； The flow characteristic is quick opening ； The pressure can be adjusted in steps from 15 to 250 MPa. The first image posted by the original poster shows media such as gases and liquids at room temperature, while the second image depicts media at a certain temperature, usually steam, as it is equipped with a condenser to prevent excessive temperatures from damaging the diaphragm during pressure measurement. The principle is roughly the same: in one case, pressure is measured and regulated before the valve to achieve automatic control of pressure maintenance and release

Self-acting pressure control valves are divided into two types based on the location of the pressure sampling point: those with the sampling point before the valve, and those with it after the valve. When the sampling point is before the valve, such valves are used to maintain a constant pressure before the valve； When the pressure measurement point is located behind the valve, it is used to maintain a constant pressure behind the valve. For steam-based media, a condenser can be used to condense the steam, keeping the temperature of the medium entering the diaphragm head at the ambient temperature and thus preventing damage to the diaphragm. For gaseous media, the controlled gas can be directly fed into the membrane head. Self-acting control valve for the pressure before the control valve; Self-acting control valve for the pressure after the control valve. When the pressures before and after the valve are introduced to both sides of the actuator’s chamber, a self-acting differential pressure control valve can maintain a constant pressure difference across the control valve. It is also possible to use the differential pressure across orifice plates installed in the pipeline on either side of the pneumatic diaphragm actuator to create a self-acting flow control valve, or to use other methods to detect flow rates and then employ a self-acting control valve to regulate that flow. The diagram on the right shows a self-acting differential pressure control valve. In the diagram, the flow rate setting adjustment is used to regulate the flow resistance. The setting adjustment is used to change the initial pre-tension of the spring and to adjust the set value of the pressure difference. The pressure difference shown in the figure is achieved by comparing the thrusts generated by two diaphragm actuators; it is also possible to use just one diaphragm actuator. When the signals from the pressure measurement points come from both sides of an orifice plate installed on the same pipeline, it can be used to control flow. A pilot-operated self-acting control valve is similar to a directly acting self-acting control valve, with its set signal being determined by the setting spring of the pilot. There is another type of self-acting control valve that uses a weight as the setting element. A weight is connected to the valve stem via a lever; its gravity is amplified and exerted on the valve stem. The valve stem remains stationary only when these two forces are in balance. Due to its simple structure and low maintenance requirements, it is used in some less critical or basic pressure control systems, such as the pressure control of steam decompression cylinders.

It depends on whether it’s before or after the control valve… A pressure measurement point taken after the control valve cannot be inside the valve; it must be several times larger in diameter than the valve, otherwise there will be significant pressure fluctuations. In principle, for a pressure measurement point taken before the control valve, it’s better to place it as close to the valve as possible. There isn’t much difference between taking measurements inside the valve and at the flange on the side of the valve before it·