Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
438
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.
TWO-WAY VALVE
Two-way valves are available as globe, ball, or butterfly
valves. The combination of valve body and actuator (called
valve assembly) determines the valve stem position. Two-way
valves control steam or water in two-position or proportional
applications (Fig. 7). They provide tight shutoff and are available
with quick-opening, linear, or equal percentage flow
characteristics.
Fig. 7. Two-Way Valve Application.
SUPPLY
LOAD
RETURN
C2329
TWO–WAY
VALVE
Ideally, a control system has a linear response over its entire
operating range. The sensitivity of the control to a change in
temperature is then constant throughout the entire control range.
For example, a small increase in temperature provides a small
increase in cooling. A nonlinear system has varying sensitivity.
For example, a small increase in temperature can provide a
large increase in cooling in one part of the operating range and
a small increase in another part of the operating range. To
achieve linear control, the combined system performance of
the actuator, control valve, and load must be linear. If the system
is linear, a linear control valve is appropriate (Fig. 8). If the
system is not linear, a nonlinear control valve, such as an equal
percentage valve, is appropriate to balance the system so that
resultant performance is linear.
100%
0% 100%
NONLINEAR SYSTEM
RESPONSE
RESULTANT
LINEAR SYSTEM
CONTROL
EQUAL PERCENTAGE
CONTROL VALVE
TEMPERATURE
PERCENTAGE OF
FULL COOLING
C2330
100%
0% 100%
STEM TRAVEL
FLOW
C2331
90%
20%
QUICK-OPENING
CONTROL VALVE
Fig. 9. Flow vs Stem Travel Characteristic
of a Quick-Opening Valve.
Fig. 8. Linear vs Nonlinear System Control.
QUICK-OPENING VALVE
A quick-opening two-way valve includes only a disc guide
and a flat or quick-opening plug. This type of valve is used for
two-position control of steam. The pressure drop for a quick-
opening two-way valve should be 10 to 20 percent of the piping
system pressure differential, leaving the other 80 to 90 percent
for the load and piping connections. Figure 9 shows the
relationship of flow versus stem travel for a quick-opening
valve. To achieve 90 percent flow, the stem must open only 20
percent. Linear or equal percentage valves can be used in lieu
of quick-opening valves in two-position control applications
as the only significant positions are full open and full closed.