Chemical Engineering March 2017 - 87

FL
FL
2()PV
P1
P1
FF
without causing choked flow, if
pressure drop is less than critical
pressure drop but greater than
∆Pincipient cavitation [2],where:
2()PV
FF
∆Pincipient cavitation =
= KC P1()PV
P
FF
Where:
Kc = The coefficient of incipient cavitation,
which is less than FL [2].
3. Multiple operating cases.
Whereas a control valve is generally
specified
for
three
cases
- minimum, normal and maximum
flowrates, with corresponding
pressure drop - there may be
more than three operating cases.
In such situations, normal flowrate
and corresponding pressure drop
should be specified in accordance
with the normal operating case,
whereas other operating cases (if
there are more than two) should
be narrowed down to two cases.
When narrowing down the operating
cases, Cv should be estimated
for each case. Then, minimum and
maximum flowrates (and corresponding
pressure drops) should
be specified in such a way that
they correspond to the minimum
and maximum Cv
of the control
valve, and the Cv corresponding to
all other cases should lie between
minimum Cv and maximum Cv. As
actual Cv
is not available when a
control valve is specified, the estimated
Cv should be used.
4. Selection of type of valve. Butterfly
valves, which are compact and
generally have a relatively low cost,
are often the first choice. However,
constraints may dictate otherwise.
For instance, if high pressure drop
across the valve is required, a globe
valve may be the better choice.
Because the resistance of a globe
valve is higher than that of a butterfly
valve, higher pressure drop can be
obtained across a globe valve with
reasonable size.
In liquid applications, high pressure
drop could lead to cavitation.
As the pressure-recovery factor of
globe valves tends to be higher than
that of other valve styles, cavitation
can often be avoided with the use of
globe valves.
In the case of gases, high pressure
drop could lead to choking
flow conditions, which can generate
excessive noise. Noise can be minimized
with a globe valve with the
use of cage-guided trim. However,
if the available pressure drop across
the valve is low, then a butterfly
valve may be the preferred choice.
Meanwhile, V-notch ball valves
can be preferred where high rangeability
is required. Standard, roundported
ball valves are generally used
for on-off applications. Table 1 provides
valve-selection guidelines.
5. Leakage class. The allowable
control valve seat leakage is specified
in terms of ANSI/FCI 70-022006
leakage class. This standard
recognizes six classes of allowable
seat leakage (Class I, II, III, IV,
V and VI). Class I means highest
allowable leakage; Class VI means
least allowable leakage [3]. Generally,
control valves for CPI applications
are specified with leakage
Class IV. However, in situations
where tight shutoff is required, at
least Class V should be specified.
If a control valve is discharging to
a flare, or is controlling (on-off) fuel
flow to the burner of a fired heater
or furnace, it should be specified
with Class VI leakage.
6. Flow characteristics. The most
common types of inherent flow characteristics
are the following [1]:
* Linear - A valve with an ideal
linear inherent flow characteristic
produces a flowrate that is directly
proportional to the amount
of valve plug travel, throughout the
travel range.
* Equal percentage - Ideally, for
equal increments of valve plug
travel, the change in flowrate regarding
travel may be expressed
as a constant percent of the flowrate
at the time of the change.
* Quick opening - A valve with
quick-opening flow characteristic
provides a maximum change
in flowrate at low travel rates. A
quick-opening characteristic is
basically linear through the first
40% of valve plug travel (corresponding
to 70% of maximum
flowrate), and there is little increase
in flowrate with further increase
in plug travel.
The flow characteristic of a valve depends
on its trim design. While ball
valves and butterfly valves have fixed
characteristics, globe valves can
have any of the three characteristics,
depending on plug or cage design.
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The type of flow characteristic
should be specified in the process
datasheet, considering the parameter
to be controlled, or the pressure
drop scenario in the system.
For flow or level control, linear characteristic
should generally be specified.
In general, linear characteristic
should also be specified if most of
the pressure drop (as a proportion of
total pressure drop in the system) is
across the valve itself so that pressure
drop across the valve remains
nearly
constant
for
varying
flowrates.
Equal-percentage characteristic
should be specified for pressure
control, or where a high proportion
of the total pressure drop occurs
in the system other than the valve
(that is, in pipes, fittings, equipment
and so on). It should also be specified
where pressure drop across
the valve varies with varying flowrate.
A quick-opening characteristic
should be specified for on-off applications.
As in most of the common
systems, pressure drop across the
control valve varies significantly with
flowrate, so equal-percentage flow
characteristics are most commonly
specified [1].
n
Edited by Suzanne Shelley
References
1. Emerson Process Management, " Control Valve Handbook, "
4th Ed., p. 18, pp. 33-36, p. 46, pp. 59-60, pp.
109-110, p. 136.
2. " Masoneilan Handbook for Control Valve Sizing, " 7th Ed.,
pp. 7-8, p. 10, 1987.
3. American National Standard, Control Valve Seat Leakage,
ANSI/FCI 70-2-2006, Cleveland, Fluid Controls Institute,
Inc., p. 2, 2006.
Author
Satyendra Kumar Singh is the
general manager (Head of Department)
- Process for Simon India
Limited (Mehtab House, A-36,
Ground Floor, Mohan Co-operative
Industrial Estate,
New Delhi110044,
India; Email: sty_singh@
yahoo.com; satyendra.singh@
adventz.simonindia.com). He has
more than 24 years of experience
in engineering consultancy and engineering procurement
construction in the fields of petroleum refining,
petrochemicals, chemicals, oil-and-gas, and ammonia.
Singh holds a Bachelor of Technology (Honors) degree
in chemical technology from Harcourt Butler Technological
Institute (Kanpur, India), and an Masters of Business
Administration from Indira Gandhi National Open
University (New Delhi, India). He is a Chartered Engineer
(India), and a Fellow of The Institution of Engineers
(India); and has published numerous papers on management
and engineering subjects.
87
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