How do Self-operated Control Valves Control Pressure
I.Introduction of self-Operated control valve
Self-operated control valves, also known as self-operated control valves, rely on the pressure and temperature of the medium flowing through the control valve itself as an energy source to drive the valve to work automatically, without needing an external power supply and secondary instruments.
Self-operated control valves are divided into self-operated pressure control valves, self-operated differential pressure control valves, self-operated temperature control valves, self-operated flow control valves.
Self-operated control valves use the feedback signal (pressure, differential pressure, temperature) at the output of the valve to drive the valve flap through the signal pipe to the actuator to change the opening of the valve to achieve the purpose of adjusting the pressure, flow and temperature. II.Method of pressure extraction for self-regulating valves
Self-operated pressure control valves are divided into two types, pre-valve and post-valve, depending on the location of the pressure point. When the pressure point is in front of the valve, it controls a constant pressure in front of the valve. The pressure point is behind the valve contains the continual strain after the valve. For steam media, a condenser can condense the media vapor. This protects the temperature of the medium entering the diaphragm head at the ambient temperature on-site and prevents damage to the diaphragm. The controlled gas can be fed directly into the diaphragm head for gas media. When the pressure before and after the control valve is simultaneously introduced into both sides of the gas chamber of the actuator, the self-acting differential pressure control valve can control the constant differential pressure on both sides of the control valve, or the differential pressure at both ends of the orifice plate installed on the pipeline can be introduced into both sides of the gas chamber of the pneumatic film actuator to form a self-acting flow control valve, or the flow can be detected in other ways and then the self-acting control valve can control the flow.
Downstream pressure control
The pre-valve pressure P1 of the working medium becomes the post-valve pressure P2 after throttling behind the spool and seat. P2 is fed into the lower diaphragm chamber of the actuator through the control line. It acts on the top disc, generating a force balanced by the reaction force of the spring, determining the relative position of the spool and seat, and controlling the post-valve pressure.
When the post-valve pressure P2 increases, the force acting on the top disc by P2 also increases. At this point, the strength of the top disc is greater than the spring reaction force, so that the position of the spool off to the valve seat until the power of the entire disc and the spring reaction force is balanced. At this point, the flow area of the spool and the valve seat is reduced, and the flow resistance becomes more extensive, thus reducing P2 to the set value. Similarly, when the pressure after the valve P2 is reduced, the direction of action is the opposite of the above, which is the working principle of the self-acting (after the valve) pressure adjustment valve. This kind of valve usually is to be horizontally mounted inside the pipeline.
The adjusting nut can be adapted when the back valve pressure P2 needs to be modified.
Upstream pressure control
The pre-valve pressure P1 of the working medium is changed to the post-valve pressure P2 after throttling behind the spool and seat. At the same time, P1 is fed through the control line into the upper diaphragm chamber of the actuator and acts on the top disc, and the resulting force is balanced with the reaction force of the spring, which determines the relative position of the spool and seat and controls the pre-valve pressure.
When the pressure in front of the valve P1 increases, the force acting on the top disc by P1 also increases. At this point, the emphasis on the whole disc is greater than the spring reaction force so that the spool moves in the direction of leaving the valve seat until the pressure on the top disc is balanced with the spring reaction force. At this point, the flow area between the spool and the valve seat increases, and the flow resistance becomes smaller, thus reducing P1 to the set value. Similarly, when the pressure in front of the valve P1 is reduced, the direction of action is the opposite of the above. This is the working principle of the self-operated pressure control valve. The adjusting nut can be regulated when the setting value of the front valve pressure P1 needs to be revised.
III.Selection of pressure point for self-operated control valves
A self-operated control valve pressure point should be taken at the appropriate location of the regulator.
The pressure regulation before the valve should be more significant than two times the diameter of the pipe, and the pressure regulation after the valve should be greater than six times the diameter of the tube.
According to the calculated diameter, self-operated valves can be smaller than the pipeline’s diameter. In comparison, stop valves, cut-off ball valves, bypass valves, filters can not be smaller than the pipeline’s diameter.
Self-operated pressure control valves are self-contained pressure control valves. Generally speaking, the medium pressure varies with the medium flow rate. Therefore, when the medium flow rate changes, to ensure that the pressure is constant, a self-operated control valve is required to control it.
- ZZY type self-operated pressure control valves are typically positioned upright onto a horizontal pipeline when the valve is used to measure gas or low viscosity liquid media. Only if there is no space available will the valve be installed backward or diagonally.
- Appropriate space should be left around the regulator to facilitate on-site maintenance and operation.
- A pre-valve filter can be installed when the medium is a pure or clean gas or fluid.
- A fixed bracket should ensure that the medium is clean if the regulator is too large in diameter (DN≥100).
- The bypass valve (manual) can be omitted if the position does not allow it (we do not recommend it).
- After the valve is set, the user can choose check valves, safety valves, etc., as required.
- When the valve is used in steam or high viscosity liquid media, the ZZY type self-operated pressure regulating valve is usually installed upside down on a horizontal pipe. The condenser (self-operated for steam) should be higher than the actuator of the regulator and lower than the receiver before and after the valve. The condenser should be filled with cold water before use and once every three months after that.