Three-Way Valves; Mixing Valve; Diverting Valve - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

VALVE SELECTION AND SIZING

THREE-WAY VALVES

Three-way valves (Fig. 12) control the flow of liquids in
mixing or diverting valve applications (Fig. 13). The internal
design of a three-way globe valve enables it to seat against the
flow of liquid in the different applications. An arrow cast on
the valve body indicates the proper direction of liquid flow. It
is important to connect three-way valve piping correctly or
oscillations, noise, and excessive valve wear can result. Three-
way valves are typically have linear flow characteristics,
although, some are equal percentage for flow through the coil
with linear flow characteristics for flow through the coil bypass.
Ball valves are also available in a three-way configuration, while
two butterfly valves can be made to act as a three-way valve.
MIXING
VALVE
IN OUT
I
N
Fig. 12. Three-Way Valves.
LOAD
HOT
WATER
SUPPLY
BYPASS
A. LOAD BYPASS IN MIXING VALVE APPLICATION
THREE-WAY

DIVERTING VALVE

SUPPLY
BYPASS
B. LOAD BYPASS IN DIVERTING VALVE APPLICATION
Fig. 13. Three-Way Valve Applications.
DIVERTING
VALVE
IN OUT
O
U
T
C2334
THREE-WAY

MIXING VALVE

HOT
WATER
RETURN
RETURN
LOAD
C2335A
MIXING VALVE
A mixing valve provides two inlet ports and one common
outlet port. The valve receives liquids to be mixed from the
inlet ports and discharges the liquid through the outlet port
(Fig. 12). The position of the valve disc determines the mixing
proportions of the liquids from the inlet ports.
The close-off pressure in a mixing valve equals the maximum
value of the greater inlet pressure minus the minimum value of
the downstream pressure.
EXAMPLE:
A mixing valve application has a maximum pressure of
170 kPa on one inlet port, maximum pressure of 140 kPa
on the other inlet port, and minimum downstream pressure
of 70 kPa on the outlet port. The close-off pressure is
170 kPa – 70 kPa = 100 kPa. The application requires a
mixing valve with at least a 100 kPa close-off rating. The
actuator selected must have a high enough force to operate
satisfactorily.
In globe mixing valve applications, the force exerted on the
valve disc due to unbalanced pressure at the inlets usually
remains in the same direction. In cases where there is a reversal
of force, the force changes direction and holds the valve disc
off the seat, cushioning it as it closes. If the pressure difference
for the system is greater than the pressure ratings of available
globe mixing valves, use a ball mixing valve or two butterfly
valves in a tee configuration.
Globe mixing valves are not suitable for modulating diverting
valve applications. If a mixing valve is piped for modulating
diverting service, the inlet pressure slams the disc against the
seat when it nears the closed position. This results in loss of
control, oscillations, and excessive valve wear and noise.
Mixing valves are acceptable using about 80 percent of the
close-off rating, but not recommended, in two-position diverting
valve applications.
DIVERTING VALVE
A globe diverting valve provides one common inlet port and
two outlet ports. The diverting valve uses two V-port plugs
which seat in opposite directions and against the common inlet
flow. The valve receives a liquid from one inlet port and
discharges the liquids through the outlet ports (Fig. 12)
depending on the position of the valve disc. If the valve disc is
against the bottom seat (stem up), all the liquid discharges
through the side outlet port. If the valve disc is against the top
seat (stem down), all the liquid discharges through the bottom
outlet port.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
436