Inherent and Installed Valve Characteristics

An inherent flow characteristic is the relation between valve opening and flow under constant pressure conditions. The inherent characteristic of a valve is obtained when there is a constant pressure drop across the valve for all valve positions; the process fluid is not flashing, cavitating or approaching sonic velocity (choked flow); and the actuator is linear
(valve stem travel is proportional to the controller output).
The inherent flow characteristics do not reflect the actual performance of the valve as installed. The ideal condition of constant valve pressure drop (ΔP) is unlikely to be true and the ‘operating’ characteristics will have deviation from the inherent characteristics and is
termed the “Installed Flow Characteristics”.
The deviation in the characteristics depends on the pressure drop variation across the control valve, as the control valve operates from minimum flow at its initial travel position to its maximum flow at its fully opened position. The variations in pressure drop across the valve can be attributed to two basic causes:
1. The pump characteristic which results in an increase in pump head as the flow is reduced;
2. The reduction in line losses as the flow is reduced.
As a general rule, systems with a significant amount of pipe and fittings (the most common case) are best suited to equal percentage inherent characteristic valves. Systems with very little pipe (where the pressure drop available to the control valve remains nearly constant and as a result the inherent characteristic of the valve is also the installed characteristic) are better suited to linear inherent characteristic valves.
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