control-valve 1
TRANSCRIPT
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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.
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. 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.
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SPECIFICATION OF VALVES-
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
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SPECIFICATION OF VALVES-
Operating temperature - Deg. C
Design Pressure / temp.
End connection
MOC desired
leak Tightness required Actuator : Pneumatic / electrical
Max. compressed air available : kg/cm2
Cv :
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CATEGORY OF VALVES
ON - OFF SERVICES :- Gate, Ball,
Diaphragm, Plug, Butterfly valves.
THROTTLING SERVICES :- Globe,
Butterfly, Diaphragm, Pinch valves.
NON - REVERSE FLOW :- Check valves.
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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 arenot primarilly used for slurries, viscous
fluid etc.
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GATE VALVES -
ADVANTAGES :
Low pressure drop
when fully open andtight sealing
DISADVANTAGES:
Causes Vibration, seat
disc wear in partialopen condition. Slow
response characteristic
and require large
actuating force.
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GLOBE VALVES -
In Globe Valves disc or plug is moved on
and off the seat.The seat opening is directlyproportional to the travel of the disc.It is
short stem travel, high seating capacity,
large pressure drop & high flowcontrollability.
Application :-It is used primarily for
throttling purposes. It may be considered ageneral purpose flow control valve.
Specifically very widely used for high temp.
application.
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GLOBE VALVES -
ADVANTAGES :
Faster to open or
close, most reliableform of seating,
throttling to control
the flow to any desired
degree, positive shut-off
DISADVANTAGES:
Seat disc wear in
partial open condition.
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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 purposeinvolving on off services.
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PLUG VALVES -
ADVANTAGES :
Normally small in
size, require lessheadroom and
available in wide
range of materials.
They provide tightshut off, quick
opening and low
pressure drop.
DISADVANTAGES:
Plug valve may be
subjected to galling.
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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 PTFEwhich 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
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BALL VALVES -
ADVANTAGES :
Low pressure drop,
tight shut off, quarterturn operation, easy to
maintain, low torque.
They are small in size
and low in weight.
DISADVANTAGES:
PTFE seats are
subjected to extrusionif the valve is used for
throttling. Fluid
trapped in the ball in
the closed positionmay cause problem of
build up of vapour
pressure & corrosion.
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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
degs. Fully opens or closes the valves.
Application :- Low pressure application
where leakage is unimportant.
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BUTTERFLY VALVES -
ADVANTAGES :
Simple , compact
form, quick opening,good controllability,
low pressure drop and
low weights and costs.
DISADVANTAGES:
No tight shut off.
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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 asmooth streamlined flow. When the valve is
closed the diaphragm is tightly seated
against the seta in the body known as theweir. The diaphragm is of PTFE.
Application :- Used for corrosive liquids at
low temp. and pressure.
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DIAPHRAGM VALVES -
ADVANTAGES :
The diaphragm
completely keeps theworking parts in
isolation from the
process fluids. Simple
in construction, easierto operate and
maintain.
DISADVANTAGES:
Not for high temp. and
pressure.
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CONTROL VALVE -
PRINCIPAL OF OPERATION It is based upon balanced forces between
pneumatic force from the diaphragm against
a mechanical force produced by the actuatorspring.The control valve works with a
pneumatic signal 3 to 15 psi. The motion
starts at 3 psi and ends at 15 psi.
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CONTROL VALVE- HART POSITIONER
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COMPONENT OF CONTROL
VALVE Actuator
Body
Trim
Diaphragm
Diaphragm plate
Actuator stem
Actuator spring
Seat
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Travel Indicator
Valve stem
Gaskests
Yoke
Handwheel
COMPONENT OF CONTROL
VALVE
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TERMINOLOGY
Flow Co-efficients (Cv) :-
The flow of water (G=1, T= 6 to 34
deg. C) through the valve at full lift inU.S gallon per minute with a pressure
drop across the valve of 1 psi.
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VALVE LIFT %
FLOW
%
EQUAL %
LINEAR
QUICK OPENING
CHARACTERISTIC CURVES
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Linear :-
Flow is directly proportional to lift.
Equal Percentage :-
Flow changes by a constant percentage of its
instantaneous values for each unit change in lift.
Quick Opening :-
Flow increases rapidly in a linear relationshipwith plug lift reaching a max. value at a low lift.