One of the main areas of debate when specifying control valves is determining the size of the valve required. Often process engineers will know the pipe diameter used in an application and it is tempting to take that as the control valve’s defining characteristic. Of greater importance are the flow conditions within the system as these will dictate the size of the orifice within the control valve. The pressure either side of the valve and the expected flow rate are essential pieces of information when deciding on the valve design.
Inside the valve body, the actuator design is often either a piston or a diaphragm design. The piston design typically offers a smaller, more compact valve which is also lighter and easier to handle than the diaphragm designs. Actuators are usually made from stainless steel or polyphenolsulpide (PPS), which is a chemically-resistant plastic. The actuator is topped off by the control head or positioner.
Older, pneumatically operated positioners had a flapper/nozzle arrangement and operated on 3-15psi, so no matter what the state of the valve, open closed or somewhere in between, the system was always expelling some compressed air to the atmosphere.
Compressed air is an expensive commodity, requiring considerable energy to generate and when a manufacturing line is equipped with multiple pressure relief valve all venting to the atmosphere, this can equate to a considerable waste of energy. It is important to not only establish the most appropriate valve design, but also a cost-effective solution that takes account of annual running costs.
Improving reliability
Many process control environments offer less than ideal conditions for long-term reliability. Moisture-laden atmospheres, corrosive chemicals and regular wash-downs all have the capacity to shorten the service life of common rail control valve F00vc01334. One of the potential weaknesses of the actuator is the spring chamber where atmospheric air is drawn in each time the valve operates.
Actuator
The actuator is a component attached to the injector control valve F00rj02130, which controls the valve movement. Without the actuator, the internal mechanism of the valve cannot be moved.
The feedback pneumatic signal comes to the actuator, directly connected to the main valve body. The main control valve moves relative to the actuator. When the feedback signal becomes high—indicating a more increased flow—the actuator closes, closing the main valve.
When the feedback signal decreases—indicating low flow—the actuator opens, simultaneously opening the valve to increase the flow.
