Two-Way Valve; Quick-Opening Valve; Linear Valve - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

VALVE SELECTION AND SIZING

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.
TWO–WAY
VALVE
SUPPLY
Fig. 7. Two-Way Valve Application.
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.
NONLINEAR SYSTEM
RESPONSE
100%
0%
TEMPERATURE
Fig. 8. Linear vs Nonlinear System Control.
100%
90%
20%
0% 20%
FLOW
GRAPH A
Fig. 10. Heat Output, Flow, and Stem Travel Characteristics of a Linear Valve.
RETURN
LOAD
C2329
RESULTANT
LINEAR SYSTEM
CONTROL
EQUAL PERCENTAGE
CONTROL VALVE
100%
C2330
100%
90%
20%
90% 100% 0%

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.
100%
90%
0%
Fig. 9. Flow vs Stem Travel Characteristic
of a Quick-Opening Valve.

LINEAR VALVE

A linear valve may include a V-port plug or a contoured
plug. This type of valve is used for proportional control of
steam or chilled water, or in applications that do not have wide
load variations. Typically in steam or chilled water applications,
changes in flow through the load (e.g., heat exchanger, coil)
cause proportional changes in heat output. For example,
Figure 10 shows the relationships between heat output, flow,
and stem travel given a steam heat exchanger and a linear valve
as follows:
20% 90% 100%
STEM TRAVEL
GRAPH B
ENGINEERING MANUAL OF AUTOMATIC CONTROL
434
QUICK-OPENING
CONTROL VALVE
20% 100%
STEM TRAVEL
C2331
100%
90%
20%
0% 20% 90% 100%
STEM TRAVEL
GRAPH C
C2332