CONTROL VALVE - THEORY
& SIZING
• The control valve, commonly named the
final control element of control contains a
pneumatic device that converts the control
signal from the controller in action,
regulating the process fluid flow.
. Valves make up approximately
15% of the total expenditure for
the materials and equipments for
the refinery or chemical
processing industry.
. Valves are used to handle
variety of liquids , gases ,
steam, air etc.
SPECIFICATION OF VALVES•
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Flow medium
Flow : kg/hr or Nm3/hr Max/Min/Normal
Inlet pressure : kg/cm2 Max/Min/Normal
Outlet pressure:kg/cm2 Max/Min/Normal
Max. allowable diff. Pressure : kg/cm2
Density of medium : kg/m3
Viscosity
Cv : Valve Flow Coefficient
SPECIFICATION OF VALVES•
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Operating temperature - Deg. C
Design Pressure / temp.
End connection
MOC desired
leak Tightness required
Actuator : Pneumatic / electrical
Max. compressed air available : kg/cm2
Fail Safe Action
CATEGORY OF VALVES
• ON - OFF SERVICES :- Gate, Ball,
Diaphragm, Plug, Butterfly valves.
• THROTTLING SERVICES :- Globe,
Butterfly, Diaphragm, Pinch valves.
• NON - REVERSE FLOW :- Check valves.
GATE VALVES • The Gate Valves are characterized by a
sliding gate which is moved by the stem
perpendicular to the direction of flow.
• Application :- It is used for on-off
application. Suited for high temperature and
pressure use with variety of fluids. They are
not primarily used for slurries, viscous fluid
etc.
GATE VALVES • ADVANTAGES :
Low pressure drop
when fully open and
tight sealing
• DISADVANTAGES:
Causes Vibration, seat
disc wear in partial
open condition. Slow
response characteristic
and require large
actuating force.
GLOBE VALVES • In Globe Valves disc or plug is moved on
and off the seat.The seat opening is directly
proportional to the travel of the plug.It is
short stem travel, high seating capacity,
large pressure drop & high flow
controllability.
• Application :- It is used primarily for
throttling purposes. It may be considered a
general purpose flow control valve.
Specifically very widely used for high temp.
application.
GLOBE VALVES • ADVANTAGES :
Faster to open or
close, most reliable
form of seating,
throttling to control
the flow to any
desired degree,
positive shut-off
• DISADVANTAGES:
Seat disc wear in
partial open
condition.
PLUG VALVES • It consist of body, plug and cover. The plug
is tapered or cylindrical. In the open
position, the bore in the plug connects the
inlet and outlet ends of the valve providing
straight line flow.
• Application :- The plug valves are
extensively used in refinery, petrochem and
chemical industries and for general purpose
involving on off services.
PLUG VALVES • ADVANTAGES :
Normally small in
size, require less
headroom and
available in wide
range of materials.
They provide tight
shut off, quick
opening and low
pressure drop.
• DISADVANTAGES:
Plug valve may be
subjected to galling.
BALL VALVES • It is improvisation of the plug valve. It is
basically a ported sphere in a housing.The
seat matching the ball is circulars so that the
seating stress is circumferentially uniform.
The seats are usually made up of PTFE
which is inert to all chemicals, has low
coefficient of friction and resiliency.
• Application :- It is used for flow &
pressure control and shut off and for
corrosive fluids, slurries, normal liquid &
gases. For high temp. & pressure
BALL VALVES • ADVANTAGES :
Low pressure drop,
tight shut off, quarter
turn operation, easy to
maintain, low torque.
They are small in size
and low in weight.
• DISADVANTAGES:
PTFE seats are
subjected to extrusion
if the valve is used for
throttling. Fluid
trapped in the ball in
the closed position
may cause problem of
build up of vapor
pressure & corrosion.
BUTTERFLY VALVES • They are rotary valves with a disc rotating
at right angles within a pipe section body.
• The stem passes through this disc and is
supported by both ends of the body.
• The shaft is ensured to the disc either by
bolts or pins. Rotating the stem through 90
degrees. Fully opens or closes the valves.
• Application :-Large flow control, Low
pressure application where leakage is
unimportant.
BUTTERFLY VALVES • ADVANTAGES :
Simple , compact
form, quick opening,
good control,large
flow, low pressure
drop and low weights
and costs.
• DISADVANTAGES:
No tight shut off.
DIAPHRAGM VALVES • The flexible diaphragm is connected to a
support member known as compressor,
which represent the closure member. When
the valve is opened the diaphragm is lifted
out of the flow path and the fluid has a
smooth streamlined flow. When the valve is
closed the diaphragm is tightly seated
against the seat in the body known as the
weir. The diaphragm is of NEOPRENE
• Application :- Used for corrosive liquids at
low temp. and pressure .
DIAPHRAGM VALVES • ADVANTAGES :
The diaphragm
completely keeps the
working parts in
isolation from the
process fluids. Simple
in construction, easier
to operate and
maintain.
• DISADVANTAGES:
Not for high temp. and
pressure.
CONTROL VALVE PRINCIPAL OF OPERATION
• It is based upon balanced forces between
pneumatic force from the diaphragm against
a mechanical force produced by the actuator
spring.The control valve works with a
pneumatic signal 3 to 15 psi. The motion
starts at 3 psi and ends at 15 psi.
CONTROL VALVE
OPERATIONAL FORMS
• AIR TO CLOSE:- 3 PSI – FULL OPEN
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15 PSI – FULL CLOSE
• AIR TO OPEN :– 3 PSI – FULL CLOSE
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15PSI – FULL OPEN
• At pressure higher than3 psi applied on
diaphragm,it will start overcoming the spring
force and move diaphragm plate which in turn
moves stem connected and operates the plug.
SAFETY CONSIDERATION
• In addition to control function the control valve serve as a
safety device in process.There are two basic considerations
relative to the safety aspect
• 1)The position of the valve when equipment fails
• 2) The position of the valve when itself fails
VENT PORT
ACTUATOR
POSITIONER
YOKE
VALVE BODY
CONTROL VALVE- HART POSITIONER
COMPONENT OF CONTROL
VALVE
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Actuator
Body
Trim
Diaphragm
Diaphragm plate
Actuator stem
Actuator spring
Seat
COMPONENT OF CONTROL
VALVE
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Travel Indicator
Valve stem
Gaskets
Yoke
Hand wheel
DOME
SPRING
DIAPHRAGM
AIR SIGNAL
STEM
COUPLER
INDICATOR
SCALE
GLAND
BONET
PLUG
OUTLET
VALVE SEAT
INLET
VALVE BODY
THREE WAY
VALVE
TERMINOLOGY
• Flow Co-efficient (Cv) :The flow of water (G=1, T= 6 to 34
deg. C) through the valve at full lift in
U.S gallon per minute with a pressure
drop across the valve of 1 psi.
CHARACTERISTIC CURVES
FLOW %
QUICK OPENING
LINEAR
EQUAL %
VALVE LIFT %
• Linear :Flow is directly proportional to lift at constant
pressure drop
• Equal Percentage :Flow changes by a constant percentage of its
instantaneous values for each unit change in lift.
• Modified flow:- Combination of linear and equal
percentage
• Quick Opening :Flow increases rapidly in a linear relationship
with plug lift reaching a max. value at a low lift.
On-Off service
Guidelines For Control Valve Specification
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Valve trim is of stainless steel
Harden SS or Stellite is used for
Pressure drop of 150 psi or more
Steam
Slurry fluids
More temp
Teflon packing used for less than 200 deg. Temp
Asbestos used above 200 deg temp
VALVE POSITONER
• Valve requiring extra force due to process
condition such as Large pressure drop,Temp
control,Large size , Split range control,
BOOSTER RELAYS
• Booster relays are used to reduce time lags resulting from
long transmission lines or controller outputs are
insufficient to meet capacity of actuator
• Application
• 1)Pneumatic line more than 500 feet
• 2)Large volume valve actuators
• 3)Fast process control system
HAND WHEELS
• Hand wheels are mounted on control valve for manual
operation in emergency
• APPLICATIONS
• Critical process loops services such as reaction or venting
systems where control is maintained in case of air fail or
diaphragm failure
• Control loops where bypass is not used