Calculated Cv versus Optimal CvJanuary 18, 2018 by Harry Woebkenberg
Most Application Engineers encourage the use of a manufacturer’s valve sizing program as a means to attain more accurate information for reliable operation than that which is achieved using alternate methods. Although these programs can be utilized to obtain several types of information, they are used most often for Cv calculation. Some sizing programs have a memo or note on the page marked “Sizing Report” that reads:
The Cv Calculated is the actual Cv required. When sizing for the following valve types, the figure should be divided by:
The Cv that is calculated by the sizing program is the actual Cv that is needed to pass the full amount of flow. However, when using the actual Cv, the valve will be required to operate completely open at 100% capacity in order to pass the flow. A valve that is running in a system at full capacity will have no reserve capacity as a control valve and could exhibit unacceptable droop when used in a regulator. The actual Cv must be modified so that the valve can pass the required amount of flow while at a lower percent of capacity, such as 70% (capacity percentages will vary according to type of valve).
The chart above provides guidelines to modify the actual Cv to create the optimal Cv, which allows the valve to operate within the desired capacity. For example, after the actual Cv for a pressure regulator application is calculated using the JVCV program, the chart directs to divide the resultant number by .7, as Pressure Regulators (PRV) will effectively operate at up to 70% of capacity. This prevents excessive droop during normal flow variations.
Back Pressure Regulators (BPRV) utilize a .5 factor to provide an extra margin of relief capacity. The .5 calculation factor for Back Pressure Regulators will also minimize gain and increase the accuracy of the valve.
You may notice that Pilot Operated Valves (PPRV/PBPRV), which are much less susceptible to droop or gain, require a smaller sizing factor. Two types of products that can be sized at the high end of the capacity range are temperature regulating valves and pneumatic or motor operated control valves. With a Temperature Regulator (TCV), as the process pressure is not moving the seat.
Like temperature regulators, Pneumatic and Motor Operated Control Valves can operate with high capacity percentages as neither droop nor gain factor into their performance. For these valves, a .9 sizing factor will provide sufficient capacity to ensure operation close to midrange.
To ensure maximum performance for Jordan Valve Regulators and Control Valves, click here to use the Jordan Valve Control Valve Sizing Program (JVCV).
For questions concerning Cv calculations, or any Jordan Valve product, please contact your Jordan Valve Client Consultant.
Jordan Valve Product Manager / VP Marketing