Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
433
GLOBE VALVE
Globe valves are popular for HVAC applications. They are
available in pipe sizes from 12 mm to 300 mm and in a large
variety of capacities, flow characteristics, and temperature and
pressure capabilities. They provide wide rangeability and tight
shutoff for excellent control over a broad range of conditions.
Globe valves are made in two-way, straight or angle
configurations and three-way mixing and diverting designs.
Globe valves close against the flow and have arrows on the
body indicating correct flow direction. Incorrect piping can
result in stem oscillations, noise, and high wear.
A two-way globe valve has one inlet port and one outlet port
(Fig. 5) in either a straight through or angle pattern. The valve
can be either push-down-to-close or push-down-to-open.
Pneumatic and electric actuators with linear motion to operate
globe valves are available for operation with many control signals.
BUTTERFLY VALVE
Butterfly valves (Fig. 6) control the flow of hot, chilled, or
condenser water in two-position or proportional applications.
Butterfly valves are available in two-way or three-way
configurations. Tight cutoff may be achieved by proper selection
of actuator force and body lining. The three-way valve can be
used in mixing or diverting applications with the flow in any
direction. The three-way valve consists of two butterfly valves
that mount on a flanged cast iron tee and are linked to an actuator
which opens one valve as it closes the other. Minimum combined
capacity of both valves occurs at the half-open position.
Fig. 5. Two-Way Globe Valves.
BALL VALVE
Ball valves are available for two-position applications either
manual (hand) or power operated or for modulating applications
with direct coupled electric actuators. Ball valves are relatively
low cost and provide tight close off and available in two-way
and three-way configurations. As with all other valves, ball
valves must be properly sized to provide good flow control.
When used in modulating service, ball valves must be
specifically designed for modulating service as compared to
two-position service. Packing must provide leak-free sealing
through thousands of cycles to ensure trouble-free HVAC
service. The ball and stem should be made of stainless steel or
similar material that minimizes sticking to the seat.
Two-way ball valves have equal percentage flow control
characteristics and flow can be in either direction
Three-way ball valves can be used in either mixing or
diverting service. They have linear flow control characteristics
for constant total flow.
IN IN
PUSH-DOWN-TO-CLOSE PUSH-DOWN-TO-OPEN
C2328
M10403
Fig. 6. Butterfly Valve.
When butterfly valves are used for proportional control, they
must be applied using conservative pressure drop criteria. If
the pressure drop approaches the critical pressure drop,
unbalanced forces on the disc can cause oscillations, poor
control, and/or damage to the linkage and actuator, even though
the critical flow point is not reached.
Butterfly valves are usually found in larger pipe sizes. For
example, two butterfly valves could be piped in a mixing
application to control the temperature of the water going back
to the condenser. The valves proportion the amount of tower
water and condenser water return that is flowing in the
condenser water supply line.