Page 49 - Valve Selection Handbook

36 Valve Selection Handbook

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.

Figure 2-9. Inherent Flow Characteristics of 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 installa-
tion. 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.
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 consid-
eration leads frequently to a valve size smaller than the adjoining pipe.