What process date is required to size a Control Valve?
Medium – What is passing through the valve? – if it is a special liquid give specific gravity (at flowing temperate), critical pressure, vapour pressure and viscosity.
Pressures – What is the maximum pressure that the valve needs to be rated for? What are the upstream and downstream pressures for each of the maximum, normal and minimum flow rates.
Flow rates – Maximum, normal and minimum. The maximum is used to select the valve size, the minimum to check the turndown requirement and the normal to see where the valve will control.
Temperature – Maximum temperature for design plus temperatures at maximum, normal and minimum flow conditions.
Please see the relevant enquiry sheets for additional information that may assist in the sizing and selection of the control valve required.
Cv is the capacity of the valve to deliver flow with an available differential pressure (∆P) across the valve. The Engineer of Record typically specifies the differential pressure that should be used when selecting the full flow Cv of the valve.
Cv by definition is the number of gallons per minute (GPM) a valve will flow with a 1 psi pressure drop across the valve. For example a valve with a Cv of 10 will flow 10 GPM with a 1 psi pressure drop. The formula used to select the valve Cv with the specified differential pressure is: Cv=GPM/((SQ RT(∆P)). If the specified ∆P is 4 psid and full flow is 10 GPM the calculated CV is 10/SQ RT of 4 = 5.
Factory available Cv’s are limited and sometimes the valve Cv will be selected to deliver full flow with a ∆P that is less than specified and the valve could be considered oversized. Other times the valve Cv may be selected to deliver full flow with a ∆P that is great than specified and the valve could be considered undersized. An oversized valve will cause hunting, poor control and premature actuator wear due to excessive cycling. An undersized valve may not be able to provide enough flow to meet design specification if the actual available ∆P is inadequate.
To learn more about valve sizing and selection click here
The valve coefficient, Cv, is a number which represents the capability of a valve (or any flow component) to flow a fluid. The larger the Cv, the larger the flow at a given pressure differential. By definition, a Cv value of one is the Cv required to flow one gallon per minute (gpm) of water at 60’ F with a pressure differential of one psi. Flow is proportional to the value of Cv.
For example, a Cv of 150 would then equate to 150 gpm of water at 60’ F with a differential pressure of one psi. The formula below allows for calculation of flow rate and pressure drop through a valve with a specified Cv value.
During the process of valve size selection, the same formula is used determine the valve Cv necessary for a required flow rate and desired pressure drop. Butterfly valve Cv values vary with the disc angle and valve size. In the chart below, we show the approximate Cv value of butterfly valves ranging from 2" to 24" and at various disc angles.
