Relationship Between Resistance Coefﬁcient and Valve Opening Position

The relationship between fractional valve opening position and relative ﬂow through the valve is denoted as ﬂow characteristic. When ﬂow at all valve opening positions is taken at constant inlet pressure ion, the ﬂow characteristic thus determined is referred to as inherent. Figure 2-9 shows such inherent ﬂow characteristics that are typical for ﬂow control valves.

In the most practical applications, however, the pressure loss through the valve varies with valve opening position. This is illustrated in Figure 2-10 for a ﬂow system incorporating a pump. The upper portion of the ﬁgure represents the pump characteristic, displaying ﬂow against pump pressure, and the system characteristic, displaying ﬂow against pipeline pressure loss. The lower portion of the ﬁgure shows the ﬂow rate against valve opening position. The latter characteristic is referred to as the installed valve ﬂow characteristic and is unique for each valve installation. When the valve has been opened further to increase the ﬂow rate, the pressure at the inlet of the valve decreases, as shown in Figure 2-10. The required rate of valve opening is, therefore, higher in this case than indicated by the inherent ﬂow characteristic.

Figure 2-9. Inherent Flow Characteristics of

Valves

Figure 2-9. Inherent Flow Characteristics of Valves.

Relationship Between Flow Rate, Valve Opening

Position, and Pressure Loss in a Pumping system.

Figure 2-10. Relationship Between Flow Rate, Valve Opening Position, and
Pressure Loss in a Pumping system.

If the pump and system characteristic shows that the valve has to absorb a high- pressure drop, the valve should be sized so that the required pressure drop does not occur near the closed position, since this will promote damage to the seatings from the ﬂowing ﬂuid. This consideration leads frequently to a valve size smaller than the adjoining pipe.