Control-Valve 1

5/20/2018 Control-Valve 1

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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.

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Control-Valve 1