Today’s process control valves offer an ever wider range of features and benefits for industries that require precise control over fluids, steam and other gases. With so many control valves to choose from it is important to establish the features that will deliver the most cost-effective design for a particular application.
Control valves are used to manage the flow rate of a liquid or a gas and in-turn control the temperature, pressure or liquid level within a process. As such, they are defined by the way in which they operate to control flow and include globe valves, angle seat, diaphragm, quarter-turn, knife and needle valves, to name a few. In most cases the valve bodies are made from metal; either brass, forged steel or in hygienic applications 316 stainless steel.
Actuators will use an on-board system to measure the position of the valve with varying degrees of accuracy, depending on the application. A contactless, digital encoder can place the valve in any of a thousand positions, making it very accurate, while more rudimentary measurements can be applied to less sensitive designs.
One of the main areas of debate when specifying globe control valve 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.
