Handbook of air conditioning and refrigeration / Shan K

Valve Selection
ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.29
3. Quick-opening valve. This control valve gives the maximum possible flow rate when the valve
disk or plug is just lifted from its seat.
Rangeability is defined as the ratio of the maximum flow rate to the minimum flow rate under
control. An equal-percentage valve may have a very good rangeability of 50 :1. A linear valve may
have a rangeability of 30 :1.
The flow characteristics of equal-percentage, linear, and quick-open valves are shown in
Fig. 5.16.
The following control valve ratings should be considered during the selection and sizing of a
valve:
1. Body rating. The nominal body rating of the valve is the theoretical rating of the valve body
only, in psig. The actual body rating is the permissible safe water pressure for the valve body, in
psig (kPa�g), at a specific water temperature.
2. Close-off rating. That is the maximum pressure difference between the inlet and outlet ports that
a valve can withstand without leakage when the valve is fully closed, in psi (kPa).
3. Maximum pressure and temperature. These are the maximum pressure and temperature of water
that the whole valve, including body, disk, seat, packing, etc., can withstand.
Proper selection of water control valves depends on water system performance, load variations, pipe
size, control modes, etc. Today, the use of scaling factors on analog outputs in a DDC system permits
a nonlinear device to provide an output of linear response. However, select a control valve having a
linear relationship between a change in the controlled variable and the amount of travel of the valve
stem, or a linear system control characteristic over the operating range is still desirable when it is costeffective.
Hence, a linear valve is often used for the water system for which the controlled variable has
FIGURE 5.16 Typical flow characteristics of various types of control
valve.