Valve and Actuator Manual
Applications and Quick Tips Section
Application Note
Issue Date
977
VM-12
1292
Application Considerations:
Three-Way Valve Equal Percentage Flow Characteristic
The inherent equal percentage flow characteristic can be described by the following equation:
Inherent Flow Characteristics
If the pressure across a valve is held constant
regardless of the flowrate through the valve, the
resulting relationship between the valve stroke
and flow is called the inherent flow characteristic.
The shape of this characteristic is determined by
the shape of the valve plug. Different types of
plug shapes are used to optimize control
response for specific applications.
Q = QmR [(x/T)-1]
Where: Q = Flowrate
X = Valve Position
T = Maximum Valve Travel
Qm = Maximum Flowrate
R = Valve Rangeability
A plot of the characteristic for a two-way valve is
shown in Figure 1.
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
PERCENT VALVE STROKE
Figure 1: Inherent Equal Percentage Flow Characteristic
Two-Way Valve
© 1992 Johnson Controls, Inc.
Code No. LIT-977AN12
1
A three-way equal percentage valve has two
equal percentage plugs. A plot of the inherent
characteristic for this type of three-way valve is
shown in Figure 2. The total flowrate through this
valve is the sum of the Normally Open (N.O.)
and Normally Closed (N.C.) port flowrates at any
particular valve stroke.
control valve will see an increase in differential
pressure which is equal to the reduction in
pressure drop in the piping, coils, etc.
This pressure shift has a significant impact on
the actual installed valve flow characteristic. The
deviation from the inherent flow characteristic is
a function of a property called valve authority
(N). It is defined as the ratio of the full flow valve
pressure drop to the system pressure drop
(including the valve).
These inherent flow characteristics are valuable
for specifying a type of valve to be supplied by a
manufacturer, but they do not reflect the actual
performance of the valve once it is installed
within a system. Recall that implicit in the
definition of a inherent flow characteristic is the
stipulation that the pressure across the valve will
be constant regardless of the system flowrate. In
a real system, as the system flowrate decreases
the pressure drop across the valve will increase.
This occurs because the pressure losses for the
piping, coils, balancing valves, etc. will decease
exponentially with the flowrate. In turn, the
N = ∆P valve ÷ ∆P system
Where: N = Valve Authority
The installed flow characteristic can be
described by the following equation which is a
function of valve authority and the inherent valve
flow characteristic.
120
100
N .C . P O R T
N .O . P O R T
80
TOTAL FLO W
60
40
20
0
0
10
20
30
40
50
60
70
PERCENT VALVE STROKE
Figure 2: Inherent Equal Percentage Characteristic
Three-Way Valve
2 Application Considerations: Three-Way Valve Equal Percentage Flow Characteristic
80
90
100
Q in sta lle d =
0 .5
1
N
1
-1 +
N
Discussion
These two traits are not necessarily mutually
exclusive. As can be seen with the preceding
plot, a three-way valve with an authority of
ten percent can meet both of these performance
constraints. A good summary is provided in the
“Handbook of Control Valves” published by the
Instrument Society Of America.
1
k2
Where: Q installed = Actual Installed Flowrate
N
= Valve Authority, Decimal
Percentage
k
= Inherent Flowrate,
Decimal Percentage
Linear inherent flow characteristic provides a
change in flow which is linear with valve lift and,
thus, with the signal to the valve. A linear
characteristic would seem intuitively to be the
most desirable characteristic for control, since it
provides constant valve gain throughout the
stroke at a constant pressure drop. However,
inclusion of the valve into a system, with its
associated piping equipment, and control loop,
leads to considerations which generally make
“equal percentage” the most widely applied
characteristic. The equal percentage
characteristic produces a change in flow, with a
change in lift, that is a constant percentage of
the flow before the change was made.
The characteristics of a three-way valve with an
authority of ten percent is shown in Figure 3.
System Design Considerations
There are two items to be considered when
applying three-way way valves:
1. The valve should be selected so that its
installed characteristic, when combined with
the coil performance characteristic, will
allow a linear combined lift versus capacity
relationship.
2. The valve should provide a relatively constant system flowrate regardless of its stem
position.
120
100
80
60
N .C . P O R T
N .O . P O R T
40
TO TAL FLO W
20
0
0
10
20
30
40
50
60
70
80
90
100
PERCENT VALVE STROKE
Figure 3: Installed Characteristic Authority = 10%
Three-Way Valve
Application Considerations: Three-Way Valve Equal Percentage Flow Characteristic
3
Controls Group
507 E. Michigan Street
P.O. Box 423
Milwaukee, WI 53201
4 Application Considerations: Three-Way Valve Equal Percentage Flow Characteristic
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Valve and Actuator Manual
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