Equal Percentage Valve - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

— Graph A shows the linear relationship between heat output
and flow for the steam heat exchanger. Changes in heat
output vary directly with changes in the fluid flow.
— Graph B shows the linear relationship between flow and
stem travel for the linear control valve. Changes in stem
travel vary directly with changes in the fluid flow.
NOTE: As a linear valve just starts to open, a
minimum flow occurs due to clearances
required to prevent sticking of the valve. Some
valves have a modified linear characteristic to
reduce this minimum controllable flow. This
modified characteristic is similar to an equal
percentage valve characteristic for the first 5 to
10 percent of stem lift and then follows a linear
valve characteristic for the remainder of the
stem travel.
— Graph C shows the linear relationship between heat
output and stem travel for the combined heat exchanger
and linear valve. Changes in heat output are directly
proportional to changes in the stem travel.
Thus a linear valve is used in linear applications to provide
linear control.

EQUAL PERCENTAGE VALVE

An equal percentage valve includes a contoured plug or
contoured V-port shaped so that similar movements in stem
travel at any point in the flow range change the existing flow
an equal percentage, regardless of flow rate.
EXAMPLE:
When a valve with the stem at 30 percent of its total lift
and existing flow of 0.25 L/s (Table 2) opens an additional
10 percent of its full travel, the flow measures 0.40 L/s or
increases 60 percent. If the valve opens an additional 10
percent so the stem is at 50 percent of its full travel, the
flow increases another 60 percent and is 0.64 L/s.
100%
90%
50%
0% 10%
GRAPH A
Fig. 11. Heat Output, Flow, and Stem Travel Characteristics of an Equal Percentage Valve.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
100%
50%
10%
50% 100% 0%
FLOW
Table 2. Stem Position Vs Flow for
Equal Percentage Valve.
Stem
Change Position
— 30% open
10% increase 40% open
10% increase 50% open
An equal percentage valve is used for proportional control
in hot water applications and is useful in control applications
where wide load variations can occur. Typically in hot water
applications, large reductions in flow through the load (e.g.,
coil) cause small reductions in heat output. An equal percentage
valve is used in these applications to achieve linear control.
For example, Figure 11 shows the heat output, flow, and stem
travel relationships for a hot water coil, with 94 C entering
water and 10 C entering air and an equal percentage valve, as
follows:
— Graph A shows the nonlinear relationship between heat
output and flow for the hot water coil. A 50 percent
reduction in flow causes a 10 percent reduction in heat
output. To reduce the heat output by 50 percent, the flow
must decrease 90 percent.
— Graph B shows the nonlinear relationship between flow
and stem travel for the equal percentage control valve.
To reduce the flow 50 percent, the stem must close
10 percent. If the stem closes 50 percent, the flow reduces
90 percent.
— Graph C shows the relationship between heat output and
stem travel for the combined coil and equal percentage
valve. The combined relationship is close to linear. A 10
percent reduction in heat output requires the stem to close
10 percent, a 50 percent reduction in heat output requires
the stem to close 50 percent, and a 90 percent reduction
in heat output requires the stem to close 90 percent.
The equal percentage valve compensates for the
characteristics of a hot water application to provide a control
that is close to linear.
100%
90%
50%
10%
50% 90% 100% 0%
STEM TRAVEL
GRAPH B
435
VALVE SELECTION AND SIZING
Flow
Rate Change
0.25 L/s
0.40 L/s 60% increase
0.64 L/s 60% increase
10% 50% 90% 100%
STEM TRAVEL
GRAPH C
C2333
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