Primary function of the pressure control valves is to match the flow of fluid through the regulator to the demand for fluid placed upon the system. If the load flow decreases, then the regulator flow must decrease also. If the load flow increases, then the regulator flow must increase in order to keep the controlled pressure from decreasing due to a shortage of fluid in the pressure system.
In the regulating valve shown in image, a diaphragm is used with a poppet valve to regulate pressure. As pressure in the upper chamber increases, the diaphragm is pushed upward, causing the poppet to reduce flow, bringing the pressure back down. By adjusting the top screw, the downward pressure on the diaphragm can be increased. requiring more pressure in the upper chamber to maintain equilibrium. In this way, the outlet pressure of the regulator is controlled.
Pressure Limiting Valves : Image (a) and (b) show the construction of pressure sequence valve and the symbol of the valve respectively. The pressure relief valve is mounted at the pressure side of the hydraulic system. Its function is to limit the pressure in the system on an acceptable value. In fact a pressure relief valve has the same construction as a spring operated directional or non-return check valve. When the system gets overloaded the pressure relief valve will open and the pump flow will be leaded directly into the hydraulic reservoir. The pressure in the system remains on the value determined by the spring on the pressure relief valve and hence the system is safeguarded against excessive pressure.
Pressure sequence valves : Image (a) and (b) show the arrangement of pressure sequence valves in pneumatic system and the symbol of the valves respectively. Pressure sequence valves control the sequence of operation between two branches in a circuit. They enable one unit to automatically set another unit into motion. This valve is similar to a relief valve except that, after the set pressure has been reached, the sequence valve diverts the fluid to a second actuator or motor to do work in another part of the system. Fluid is free to flow into cylinder A. The first sequence valve 1 blocks the passage of fluid until the piston in cylinder A moves to the end of its stroke. At this time, sequence valve 1 opens, allowing fluid to enter cylinder L. This action continues until all three pistons complete their strokes.
Process Control Valves
The process control valves are used to control the rate of fluid flow of a process control system, for example, controlling the rate of fluid flow into a tank. The operation is based on a variable resistance to the flow of fluid by altering the cross- section of the pipe through which the fluid flows. A process control valve consists of an actuator and valve. The actuator regulates the pressure of the system where as the valve controls the flow of fluid into or out of the system. The most common form of
process control valve actuator is pneumatic diaphragm actuator.
Pneumatic diaphragm actuator : A simple construction of pneumatic diaphragm actuator is shown in image It consists of a diaphragm, made of rubber, with the input pressure supply acting on one side and the atmospheric pressure acting on the other side. image (a) shows the condition when the input pressure from the controller is low. In this state, the spring maintains the diaphragm and the connected stem in the upper position. When the input pressure from the controller is increased the diaphragm moves down against the spring force and hence the stem as well, as shown in image (b). The pressure and spring force are directly proportional and the compression of a spring is linearly related to the force acting on the diaphragm.
Construction and Working principle of Process control valve
Diagram shows a simple construction of typical pressure control valves using a pneumatic diaphragm actuator attached to drive the stem which is used to control the flow. The operation of pneumatic diaphragm actuator is explained in the previous section. The operation (up and down movement) of diaphragm actuator moves the inner-valve plug attached to the stem within the valve body. The size of the opening between the plug and seat varies when the stem and plugs moves up and down. The shape of the plug determines the amount of actual opening of the valve.
Types process control valves : Classification of process control valves are based on the valve body and plug.
Valve bodies : The valve may be single seated (contains one plug with one seating surface) or double seated. Image shows different forms of valve bodies. Single seated valves, as shown in image (a), have only one plug and the fluid flows through one path in the valve. These types of valves are recommended if the valve is required to close tightly. Double seated valves as shown in image (b) have two plugs and hence the fluid entering into the valve splits into two streams. These types of valves have an advantage of less force required to operate the valve with an accurate positioning.
Plugs : The relationship between the stem movement and the effect on the flow rate is determined by the shape of the plug. There are 3 basic types of control valve plugs as shown in image.
Quick opening plug valves : These types of valves are used for large change in flow rate with a small movement of the valve stem and act as an on/off control of flow rate.
Linear contoured plug valves : In this type, the flow rate varies linearly with stem position. Here, the valve alone determines the pressure drop. Therefore the change in flowrate = K x (change in stem displacement) where K is the constant. The relationship between flow rate and stem position may be expressed.
Equal percentage plug valves : In this type of plug, for a given percentage of changes in stem position an equivalent change in flow rate occurs. In normal situations, this type of valve does not shut-off the flow completely and the minimum flow which occurs when the stem travels in its limit.
