Facts At Your Fingertips: Specification Considerations for Control Valves

Properly specifying process control valves for a plant project is critical to achieving efficient and effective processes. This one-page reference discusses key considerations for specifying control valves. Valve specification Process engineers should take the following aspects into consideration when specifying control valves to ensure that the valves are manufactured according to requirements. FIGURE 1. The diagram illustrates a control valve with fluid flowing through. The pressure drop is represented by ∆P[/caption] Flow coefficient and size. The size of a control valve is derived from the flow coefficient (Cv), which is a parameter defined as volumetric flowrate (in gal/min) of water through the valve at 60°F when pressure drop across the valve is 1 psi (Cv is calculated using the formula given in the standard ISA-75.01.01-2007). Valve controllability. The valve’s controllability must be sufficient over the full range of flowrates that the valve will experience. This can be ensured by estimating the maximum Cv and minimum Cv that correspond to maximum and minimum flowrate, respectively. In general, the controllability of a control valve is acceptable if its travel at maximum flowrate does not exceed 90% of the rated…