Chemical Engineering March 2017 - 84

Engineering Practice
Key Considerations in Specifying Control Valves
Follow this guidance to specify a control valve accurately during the design phase
Satyendra Kumar Singh
Simon India Limited
C
ontrol valves are one of
the most common and important
instruments used
in the chemical process
industries (CPI). They help to ensure
smooth and efficient operation
of process plants, by achieving the
desired operating parameters by
means of regulating the fluid flow in
connected pipes. The need to properly
specify control valves during the
design phase of a plant cannot be
overemphasized.
The size of a control valve is derived
from a parameter called the
flow coefficient (Cv), which is defined
as volumetric flowrate (in gal/min)
of water through the valve at 60°F
when pressure drop across the valve
is 1 psi (Cv is calculated using the
formula given in the standard ISA75.01.01-2007).
Process engineers
should take the following aspects
into consideration when specifying
control valves, to ensure that the
valves that are manufactured by the
vendors function according to the
requirements.
1. Controllability. While specifying
a control valve during the design
phase, the process engineer should
ensure that the valve's controllability
must be good over the entire range
between minimum and maximum
flowrates. This can be done by estimating
the maximum Cv and minimum
Cv
that correspond to maximum
flowrate and minimum flowrate,
respectively. In general, the controllability
of a control valve is deemed
acceptable if its travel at maximum
flowrate does not exceed 90% of the
rated travel, and if travel at minimum
flowrate is in the range of 10-20%
of the rated travel. This means the
ratio of estimated maximum Cv
to
estimated minimum Cv should preferably
not be more than 15. If the
ratio far exceeds this value, travel
at minimum flow may be less than
10% of the rated travel, or the travel
84
at maximum flow may be greater
than 90% of the rated travel - both
scenarios mean poor controllability
of the valve. In that case, pressure
drop across the control valve should
be increased so that the target ratio
can be lowered, as shown in Equation
(1). For incompressible fluids,
the ratio of maximum Cv to minimum
Cv is given by Equation (1):
(1)
(2)
Figure 1 shows a typical control
valve circuit. The following notes add
further explanation:
i. The segments represented by
P1A and BP2 represent items connected
to the valve (such as pipes,
fittings, heat exchangers, flow elements,
and more), whereas the
segment represented by AB represents
the control valve in the
complete circuit P1ABP2. The arrows
in this circuit represent flow
direction.
ii. ∆P denotes pressure drop.
iii. Indicated pressure drops are for
maximum flow.
Referring to Figure 1, if x is the
pressure drop across the control
valve circuit P1ABP2, and y is the
pressure drop across the control
valve AB for maximum flow, then the
pressure drop in the remaining part
of the circuit (consisting of the pipes,
fittings, heat exchangers, flow elements
and more; as represented by
the segments P1A and BP2) is x-y
for maximum flow.
If r is the ratio of maximum to
minimum flow, and z is the pressure
drop across the circuit P1ABP2 at
minimum flow, then - ignoring the
elevation difference between P1 and
P2 - pressure drop in the part of the
circuit other than the control valve
(that is, P1A and BP2) at minimum
flow is approximately = (x-y)/r2.
Pressure drop across the control
valve AB at minimum flow is
approximately:
From Equation (1) and Equation (2),
(Cv)max/(Cv)min is approximately:
P = y
A
P1
P = x
FIGURE 1. Shown here is a typical control valve
circuit, which is used illustratively in the descriptions
provided in the main text
B
P2
(3)
The following conclusions can be
drawn from Equation (3):
iv. As r>1 and z≥x, any increase in
y leads to decrease in the ratio
(Cv)max/(Cv)min. That is, better
controllability can be achieved by
increasing pressure drop across
the control valve at maximum flow.
(Note: z = x if P1 and P2 are fixed
pressure points, and in general z>x
if the control valve is located at the
discharge of a centrifugal pump
v. If r increases, y also increases
for the same ratio of (Cv)max to
(Cv)min. This means that pressure
drop across the control valve at
maximum flow should increase
with an increase in the ratio of
maximum to minimum flow, if
the same controllability has to
be achieved.
vi. In most common cases,
maximum flow is 110%, and minimum
flow is 50%, of normal flow.
In such cases, if pressure drop
across the circuit P1ABP2 remains
same for maximum and minimum
flows, then Equation (3) becomes
the following:
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