TABLE OF CONTENTS The measurement commonly applied to valves is the valve coefficient ( C v), which is also known as the flow coefficient. When selecting a valve for a particular application, the valve coefficient is used to determine the valve size that will best allow the valve to pass the required flow rate, while providing stable control of the process fluid. Valve manufacturers commonly publish C v data for various valve styles, which are approximate in nature and can vary usually up to 10 percent according to the piping configuration or trim manufacture.
If the C v is not calculated correctly for a valve, the valve usually experiences diminished performance in one of two ways: If the C v is too small for the required process, the valve itself or the trim inside the valve will be undersized, and the process system can be starved for fluid. In addition, because the restriction in the valve can cause a buildup in upstream pressure, higher back pressures created before the valve can lead to damage in upstream pumps or other upstream equipment. Undersized C v s can also create a higher pressure drop across the valve, which can lead to cavitation or flashing.
If the C v is calculated too high for the system requirements, a larger, oversized valve is usually selected. Obviously, the cost, size, and weight of a larger valve size are a major disadvantage. Besides that consideration, if...
