Turndown Ratio refers to a valve’s
installed characteristic; it is the ratio between the normal maximum system flow and the minimum controllable flow. For example, when installed with low valve authority
(the ratio between the pressure drop across the valve to that of the entire system), a control valve with ‘linear’ flow characteristic may exhibit quick opening properties. Therefore, the valve’s installed Turndown Ratio of 3:1 would be lower than its theoretical Rangeability of 30:1. In the same application, a control valve with ‘equal percentage’ characteristic would have a higher Turndown Ratio (e.g. 20:1).
It is not possible to state a valve’s Turndown Ratio without understanding the system parameters. However, if the operating conditions are known and a specific Turndown Ratio is desired, SAF can design a
custom trim geometry to suit the application. Having a control valve that provides fine pressure control at a low percent open, as well as capacity for high flow rates, can be very advantageous.
