A control valve is the single most important element in any fluid handling s ystem. The basic plan in sizing a valve is to determine the flow coefficient (Cv) of a valve.
Rangeability Factor: This describes the ability of a valve to stay on its theoretical characteristic at the bottom end near the closed position. This is the minimum value that should be considered if good control on light load is to be achieved. The ratio between the full flow and the minimum controllable flow is the rangeability factor.
Rangeability factor (RF) = Maximum flow / minimum controllable flow
The rangeability factor is measured under laboratory conditions, with a constant differential pressure applied across the valve. Rangeability is a characteristic of the valve itself and it depends on its design and tolerances.
Turndown Ratio: This is the ratio between the maximum normal flow and the minimum controllable flow. It is substantially less than the range ability if the valve is oversized, either by error or deliberately to allow for an occasional peak load.
Turndown Ratio (TR) = Maximum flow (installed) / minimum controllable flow
The higher the turndown ratio is, the better the controllability will be.
Valve Authority: As a control valve closes, the pressure drop across the valve increases so thatwhen the valve is completely closed, the differential pressure drop across the valve matches the pressure drop from the supply to the return line. This pressure drop is known as P-max. When the valve is completely open, the pressure drop across the valve is at its lowest point and is referred to P-min. The ratio (N) P min ÷ P max is the valve authority.
The increase in pres sure drop across the valve as it closes is important to note. Valves are rated based on a constant pressure drop. As the pressure drop shifts, the performance of the valve changes. The method to minimize the change in valve performance is to maintain the Valve Authority (N) above 0.5. Authority is related to turndown.
Turndown ratio = Rangeability X (Authority)0.5
