Chemical process plants consist of hundreds, or even thousands, of piping control loops all networked together. Control loops include control valves that are designed to keep important processes variable, such as pressure, flow, level and temperature, within a required operating range to ensure the quality of the end product. Sizing a control valve requires knowledge of the process conditions.
Determine the process flow condition to be controlled. An example is controlling the flow rate into a reactor. This would require an automatic control valve that opens up proportionately and accurately. This will determine the precision of the controls.
Determine the minimum and maximum flow rates the control valve must be designed to handle. If the reactor inlet piping is a 3-inch diameter pipe and there is a process feed pump, then the pump will have a maximum capacity. This will set the valve’s physical size that can be no larger than the diameter of the pipe.
Determine the maximum allowable pressure drop through the control valve. Control valves have an inherent restriction to flow that creates a pressure drop. If a certain pressure is required to feed the reactor, then the actual pressure drop through the valve must meet that requirement. This will determine the internals of the control valve. Once all of the process specifications have been determined, control valve manufacturers will match their products to the requirements.