Working principle and classification of self operated control valves

Self operated control valve, also known as self operated regulating valve, is a regulating valve that relies on the pressure and temperature of the medium flowing through the valve as the energy source to drive the valve to work automatically, without the need for external power supply and secondary instruments.

A self operated control valve usually consists of a main valve body, a throttle valve, a controller, an actuator, and a pressure regulating spring. As shown in the following figure.

Direct acting pressure regulating valve

A self operated control valve with a controller is similar to a direct acting self operated control valve, and its setting signal is set by a pressure regulating spring. The pressure required for the opening and closing of the actuator of a self operated control valve with a controller is regulated by the controller connected to the medium. As shown in the above figure, when the pressure in front of the valve is higher than the set pressure, the commander opens, the pressure of the actuator decreases, and the valve opens; When the pressure in front of the valve is lower than the set value, the commander closes, the actuator pressure increases, and the valve closes.

There is another type of self operated pressure control valve that uses a heavy hammer as the setting. The heavy hammer is connected to the valve stem through a lever, and its gravity is amplified to act on the valve stem. The pressure before or after the valve is converted into thrust through the membrane, which also acts on the valve stem. Only when the two are balanced, the valve stem does not move. Due to its simpler structure and less maintenance, it is used in some unimportant or primary pressure control systems, such as pressure control of steam pressure cylinders.

Classification of self operated control valves

Self operated control valves are mainly divided into self operated pressure regulating valves, self operated pressure differential regulating valves, self operated flow regulating valves, self operated temperature regulating valves, etc.

1. Self operated pressure regulating valve

Self operated pressure control valves are divided into two types based on the position of the pressure tapping point: front and rear. When the pressure tapping point is in front of the valve, it is used to control the constant pressure in front of the valve; When the pressure tap is located behind the valve, it is used to control the constant pressure behind the valve. For steam medium, a condenser can be used to condense the medium steam, protecting the temperature of the medium entering the membrane head under the on-site environmental temperature to avoid damage to the membrane. For gas media, the controlled gas can be directly fed to the membrane head.

2. Self operated differential pressure regulating valve

When the pressure before and after the valve is simultaneously introduced to both sides of the air chamber of the actuator, the self operated pressure difference control valve can control the pressure difference at both ends of the valve to be constant.

3. Self operated flow regulating valve

Introduce the pressure difference between the two ends of the orifice plate (flow meter) installed on the pipeline into the two sides of the air chamber of the pneumatic membrane actuator to form a self operated flow control, or use a self operated pressure difference control valve to achieve flow control after flow detection in other ways. The flow setting adjustment is used to adjust the flow resistance. Setting adjustment is used to change the initial preload of the spring and adjust the set value of the pressure difference. The pressure difference is achieved by comparing the output thrust of two thin film actuators, or by using only one thin film actuator. When the signal from the pressure tapping point comes from both sides of the orifice plate installed on the same pipeline, it can be used to control flow.

4. Self operated temperature regulating valve

The self operated temperature regulating valve utilizes the principle of liquid thermal expansion and incompressibility to achieve automatic regulation. The liquid expansion inside the temperature sensor is uniform, and its control function is proportional adjustment. When the temperature of the controlled medium changes, the volume of the temperature sensitive liquid inside the sensor expands or contracts. When the temperature of the controlled medium is higher than the set value, the temperature sensing liquid expands and pushes the valve core downward to close the valve, reducing the flow rate of the heat medium; When the temperature of the controlled medium is lower than the set value, the temperature sensing liquid contracts, and the reset spring pushes the valve core to open, increasing the flow rate of the heating medium.