Linear Valve; Equal Percentage Valve - Honeywell AUTOMATIC CONTROL Engineering Manual

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
100%
90%
20%
0% 20%
FLOW
GRAPH A
Fig. 10. Heat Output, Flow, and Stem Travel Characteristics of a Linear Valve.
— 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.
100%
90%
20%
90% 100% 0% 20%
STEM TRAVEL
GRAPH B
439
VALVE SELECTION AND SIZING
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:
100%
90%
20%
90% 100% 0% 20%
EXAMPLE:
When a valve with the stem at 30 percent of its total lift and
existing flow of 3.9 gpm (Table 2) opens an additional 10
percent of its full travel, the flow measures 6.2 gpm 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 9.9 gpm.
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 200F. entering
water and 50F 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.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
90% 100%
STEM TRAVEL
GRAPH C
C2332
Flow
Rate Change
3.9 gpm —
6.2 gpm 60% increase
9.9 gpm 60% increase