Balancing of a Water and Air System (PDF

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All automatic control valves must be installed with the direction of flow opposing the
closing action of the valve. In other words, the direction of flow must be such that the
flow and pressure tend to hold the valve open. If the valve is installed the opposite
way, it will produce chattering. Double-seated or balanced valves (Figure 11.23) are
generally used when high differential pressures are encountered and tight shutoff is
not required. The flow direction through these valves tends to close one port while
opening the other port. This design creates a balanced thrust condition that lets the
valve close smoothly, without water hammer, regardless of the differential pressure.
Three-way Valves
Three-way valves are generally classified as mixing or diverting. This refers to the
internal construction of the valve, not the application. The internal differences are
necessary so that the valve seats against flow. Mixing valves have two inlets and one
outlet (Figure 11.24). Diverting valves have one inlet and two outlets (Figure 11.25).
Either valve may perform a temperature-control action (mixing application) or a flowcontrol
action (bypassing application) depending on its location in the system (Figure
11.26). Diverting valves, however, should not be substituted for mixing valves and
vice-versa. Using either design for the wrong application would tend to cause chatter.
Another type of three-way valve is used in the supply line to coils in a three-pipe
system (Figure 11.27). This valve has two inlets and one outlet. One inlet is supplied
with heating water and the other is supplied with chilled water. Depending on the
thermostatic controls in the
occupied space, the valve opens to allow either heating water or chilled water into the
coil.
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