Process control 3 chapter

9AEI-406.31 & 32 1

Final Control Element

Basic Principle of Final Control Element

• Final control element operations involve the steps

necessary to convert the control signal, (generated by

a process controller) into proportional action on the

process itself.

9AEI-406.31 & 32 2

Final Control Element

• The process controller O/P signals are usually in one

of three forms.

• Electrical signal ( 4 -20 mA )

• Pneumatic signal (3 to 15 PSI)

• Digital signals usually TTL level voltages.

• By using the above signals the final control element

control the process.

9AEI-406.31 & 32 3

ELEMENTS OF FINAL CONTROL OPERATIONFig:1

ControlSignal

Signalconversions

Actuator

Final controlElement

Process

9AEI-406.31 & 32 4

Elements of a Final Control Operation

• Process-control application the conversion of a

process-controller signal to a control function can be

represented by the elements of a final control

operation as shown in fig 1.

9AEI-406.31 & 32 5

Description of each block

1. Control Signal:

The input control signal may take many forms such as

• An electric current

• Pneumatic pressure

• Digital signal etc.,

9AEI-406.31 & 32 6

Description…..

2. Signal conversions :

• This block refers to the modifications that must be made to the control signal to properly interface with the next stage of control, that is actuator.

• Thus, if a valve control element is to be operated by an electric motor actuator,

• Then a 4 - 20 mA DC control signal must be modified to operate the motor

9AEI-406.31 & 32 7

Description…..cont..

• If DC motor is used, modification might be current to

voltage conversion and amplification

• Signal conversions are often called transducers

because they convert control signals from one form to

other, such as

• Current to pressure

• Current to voltage

• Analog to digital

9AEI-406.31 & 32 8

Description….

3. Actuators:

• The actuator is a translation of the converted control

signal into action on the control element.

• Thus, if a valve is to operated then the actuator is a

device that converts the control signal into physical

action of opening or closing the valve

9AEI-406.31 & 32 9

Description….

4. Final control element:

• This device has direct influence on the process dynamic variable and is designed as an integral part of the process.

• Thus, if flow is to be controlled, then the control element, a valve, must be built, directly into the flow system.

• If temperature is to be controlled, then some control element that has a direct influence on temperature must be involved in the process.

9AEI-406.31 & 32 10

Description….

4. Final control element:

• This could be a heater / cooler combination, that is

electrically actuated by relays or a pneumatic valve to

control influx of reactants.

9AEI-406.31 & 32 11

Applications of final control element

• To regulate the flow rate in a fluid delivery system.

• To control the degree of backing of crackers in cracking

systems etc.,

• To maintain level, temperature etc., in process industries.

12 9AEI-406.33

Need for E/P and P/E converter

• Signal conversions are often called transducers because

they convert control signals from one form to other, such

as

• Electrical to pressure (E/P)

• Pressure to electrical (P/E)

• The electrical signal in the form of current signal.

13 9AEI-406.33

Need for Current-to-Pressure converter

The P/I converter is used wherever pneumatic signals must be converted to electronic signals for any one of the following reasons.

• Transmission over large distances.

• Input to an electronic data logger or computer.

• Input to Telemeter equipment.

• Instrument air not available at the receiver controller.

14 9AEI-406.33

Need for Current-to-Pressure converter

• When we want to use the low level electric current signal

to do work, it is much easier to let the work to be done by

a pneumatic signal.

15 9AEI-406.33

Pressure-to-Current converter

• The pressure-to-current converter, or simply P/I

converter, is complementary to I/P converter.

• In principle, any of the electronics pressure transmitters

could be used, but in practice, special devices are used

to improve accuracy.

16 9AEI-406.33

Pressure-to-Current converter

17 9AEI-406.33

• Because of this need for accuracy, most P/I converters

use a bellows input and a motion balance sensor.

• A typical high quality P/E converter is shown in Fig. 4.8.

The voltage output can be converted into current values

with number of standard circuits.

18 9AEI-406.33

• Bellow is convert the pneumatic control signal (3-15 psi)

in to displacement signal.

• Bellow elongation sensed by LVDT (Linear Variable

Differential Transformer.

19 9AEI-406.33

• The air signals are at low pressure levels (0.2 to 1

Kg/cm2, or 3 psi to 15 psi), and many of the pressure

detectors are not sensitive or not linear enough at these

pressures.

• A P/I converter should be at least 0.5% accurate and

preferably 0.25% to preserve the integrity of the initial

signal.

Current to Pressure Converter

20 9AEI-406.34

21 9AEI-406.34

• The current-to-pressure converter, or simply I/P

converter, is a very important element in process control.

• The I/P converter gives us a linear way of translating the

4-20 m A current into a 0.2 to 1 Kg/cm2 signal (3 to 15 psi

signal).

22 9AEI-406.34

Current-to-Pressure converter

• There are many designs for these converters, but the

basic principle almost always involves the use of a

flapper/nozzle system.

• Fig. 1 illustrates a simple way to construct such a

converter.

23 9AEI-406.34

Current-to-Pressure converter

Fig 1

24 9AEI-406.34

• The current through a coil produces a force that will tend

to pull the flapper down and close off the gap.

• A high current produces a high pressure so that the

device is direct acting.

25 9AEI-406.34

• Adjustment of the springs and perhaps the position

relative to the pivot to which they are attached allows

the unit to be calibrated so that 4 mA corresponds to 0.2

Kg/cm2 ( or 3 psi) and 20 mA corresponds to 1 kg/cm2

(or 15 psi).

• Fig. 2 illustrates the construction of one such converters

and also lists the various electric devices with which it is

commonly combined.

26 9AEI-406.34

Electro-pneumatic Transmitter

Fig 2

27 9AEI-406.34

• A permanent magnet creates a field that passes through

the steel body of the transmitter and across a small air

gap to the pole piece.

• A multi turn, flexure-mounted voice coil is suspended in

the air gap.

• The input current flows through the coil creating an

electromagnetic force that tend to repel the coil and thus

converts the current signal into a mechanical force.

28 9AEI-406.34

• Since the total force obtainable in a typical voice coil

motor with small current inputs is very less, a different

approach, namely, the use of a reaction nozzle, is

employed here to convert the force into a pneumatic

output pressure.

29 9AEI-406.34

• In this circuit, supply air flows through a restriction and

out the detector nozzle.

• The reaction of the air jet as it impinges against the

nozzle seat supplies the counterbalancing force to the

voice coil motor.

30 9AEI-406.34

• The nozzle back pressure is the transmitted output

pressure.

• In order to make the transmitter insensitive to vibration,

the voice coil is integrally mounted to a float submerged

in silicone oil.

• The float is sized so that its buoyant force equals the

weight of the assembly, leaving a zero net force.

31 9AEI-406.34

• Zero is adjusted by changing a leaf spring force.

• Span is adjusted by turning the range adjusting screw to

change the gap between the screw and the magnet thus

shunting some of the magnetic fielded away from the pole

piece.

• Such converter are called motion balance type converter.

• A force balanced type converter with signal feedback to

improve accuracy is shown in fig.3.

32 9AEI-406.34

Fig 3

9AEI-406.35 33

Actuators

• Actuators converts the control signal into action on the

control elements

Eg:

if a valve is to be operated then the actuator is a device

that converts the control signal into physical action of

opening or closing the valve

9AEI-406.35 34

Actuators…..

• The actuator must provide an accurate output position

proportional to the input signal in spite of various forces

acting on the output member.

• They often translates a control signals into a large force

or torque as required to manipulate some control

element

9AEI-406.35 35

Classification of actuators

Actuators are classified as

• Pneumatic actuators

• Hydraulic actuators

• Electro pneumatic actuators

• Electric motor actuator

9AEI-406.35 36

• Pneumatic actuators may operate directly from the

pneumatic output signal from a pneumatic controller or

they may employ a separate source of compressed air.

• The principle is based on the concept of pressure as

force per unit area.

• It net pressure difference (P2 – P1) is applied to a

diaphragm of surface area A; the net force (F) acting on

it is given as F = (P2 – P1) A.

Pneumatic actuator

9AEI-406.35 37

Pneumatic actuator in low pressure state

Fig.1

9AEI-406.35 38

Pneumatic actuator in high pressure state

Fig.2

9AEI-406.35 39

• Fig. 1 shows the condition in the low signal pressure

state.

• It consists of spring, control shaft and diaphragm.

• The pressure on the opposite side of the diaphragm is

maintained at atmospheric pressure by the open hole H.

• The input pressure applies a force on the diaphragm

9AEI-406.35 40

• It forces the diaphragm and connects the shaft to down

side against the spring force.

• The figure 2 shows the maximum control pressure and

maximum travel of shaft.

• The pressure and force are linearly related.

9AEI-406.35 41

• The shaft position is linearly related to the applied control pressure

Where

= shaft travel (m)

= applied gauge pressure (Pascals)

A = diaphragm area(m2)

K = spring constant (N/m2)

p∆

Ax p

k∆ = ∆

x∆

9AEI-406.35 42

Advantages

• Low cost

• Simple design

• Mechanical fail safe

• Suitable for high hazardous area

9AEI-406.35 43

Disadvantages

• Slow speed

• Lack of stiffness

• Instability

• Moderate thrust

44 9AEI-406.36

Electro Pneumatic Actuators

• When an electric control systems are employed, it is

often advantageous to use a pneumatic actuator.

• If a suitable air supply is available, a pneumatic actuator

can provide very large power output and may be

operated directly from an electric control system.

• This requires transducing the electrical output of the

controller into an input variable for the actuator.

45 9AEI-406.36

Electric Pneumatic Actuators

• The electro pneumatic pilot is shown in fig (1). It converts

an electrical signal input to a proportional air pressure

output.

• The input electric signal is usually a direct current.

• “Voice coil” motor accepts direct current as input signal.

46 9AEI-406.36

Fig: 1 Electro Pneumatic Pilot

47 9AEI-406.36


• The “Voice Coil” motor converts the direct current into a

mechanical force when placed in a magnetic field.

• The force causes a deflection of the balance beam,

covers the nozzle and results in an increase of output

pressure.

48 9AEI-406.36


• The output pressure acts on the feedback bellows to

cause a torque on the balance beam equal but

opposite to that of the voice coil.

• The output pressure therefore is proportional to the

input dc current.

• The electro - pneumatic actuator is shown in fig 2.

49 9AEI-406.36

ELECTRO PNEUMATIC ACTUATOR

Fig 2

50 9AEI-406.36

• The motion of the output of the actuator is related to the

balance beam through the feedback lever.

• The output position of the actuator is therefore proportional

to input direct current.

51 9AEI-406.36

Advantages

• Low cost

• Simple design

• Mechanical fail safe

• Suitable for high hazardous area

• Good control with control device

52 9AEI-406.36

Disadvantages

• Slow speed

• Lack of stiffness

• Instability

• Moderate thrust

• Quality air requirement

53 9AEI-406.37

Hydraulic Actuator

• Hydraulic actuator accepts a signal from a pneumatic

controller or an electric controller and employ hydraulic

pressure to drive an output member.

• The hydraulic actuator is used where high speed and

large forces are required.

• The basic principle is shown in figure 3

54 9AEI-406.37

Hydraulic Actuator

• The basic idea is the same as for pneumatic actuators

except that an incompressible fluid is used to provide

the pressure.

• The hydraulic pressure can be made very large by

adjustments of the area of the forcing piston A1.

55 9AEI-406.37

Hydraulic ActuatorFig 3

56 9AEI-406.37

• The hydraulic pressure is given by

PH = F1 / A1

• Where PH = hydraulic pressure (Pa)

F1 = applied piston force (N)

A1 = forcing piston area (m2)

579AEI-406.37

• The resulting force on the working piston is

FW = PHA2

Where FW = force of working piston (N)

A2 = working piston area (m2)

• Thus, the working force is given in terms of the applied

force by

2 1

W

1

A FF

A=

58 9AEI-406.37

59 9AEI-406.37

1. Find the working force resulting from 200N applied to a

1cm radius forcing piston

a) If the working piston has a radius of 6cm. Then

b) Find the hydraulic pressure

Example Problem

609AEI-406.37

Solution :

a) We can find the working force from,

2 2

21

1

6. .(200 )

1w

R CmF F N

R Cm

= = ÷ ÷

FW = 7200N

Example Problem Contd…

2 1

W

1

A FF

A=

61 9AEI-406.37

b) Thus, the 200N force provides 7200N of force. The hydraulic pressure is

WH -2 2

2

5H

F 7200NP = =

A (Π)x(6x10 m)

P =6.4x10 Pa

Example Problem Contd…

62 9AEI-406.37

Advantages

• High thrust

• Fast speed

• High stiffness coefficient

• Self lubrication

63 9AEI-406.37

Disadvantages

• Complex design

• Large heavy structure

• Hazardous

• Fluid viscosity sensitive

Electrical Actuator

9AEI-406.38 64

9AEI-406.38 65

Solenoid

• A solenoid is an elementary device that converts an

electrical signal into mechanical motion, usually in a

straight line.

9AEI-406.38 66

9AEI-406.38 67

Fig 1

9AEI-406.38 68

• A simple solenoid consists of a coil and plunger as

shown in Fig. 1.

• The plunger may be free standing or spring loaded.

• The coil may be operated by either dc or ac voltage.

9AEI-406.38 69

Specifications

• Solenoid specifications include the electrical rating and

the plunger pull or push force when excited by the

specified voltage.

• Some solenoids are rated only for intermittent duty

because of thermal constraints.

• In this case, the maximum duty cycle ( percentage on

total time) will be specified.

9AEI-406.38 70

Applications

• Solenoids are used when a large sudden force must be

applied to perform some job.

• These actuators are best suited for small, short-stroke

on-off valves, requiring high speeds of response.

• Solenoid-actuated valves can open or close in 8 to 12

milliseconds.

9AEI-406.38 71

• In Fig. 2 a solenoid is used to change the gears of a two-

position transmission.

• An SCR is used to activate the solenoid coil.

9AEI-406.38 72

Fig 2

9AEI-406.38 73

• In many process control pipe lines solenoid values

(where the plunger can act as value stem also) are used

for quick closing or shut-off operations.

• In large pipelines shut-off values are operated with the

help of pneumatic air or hydraulic oil which are in turn

controlled by solenoids valves fitted in the air line or oil

line.

9AEI-406.38 74

• Solenoids move in a straight line and therefore require a

cam or other mechanical converter to operate rotary

values.

9AEI-406.38 75

• A practical solenoid valve consists of the valve body, a

magnetic core attached to the stem and disc, and a

solenoid coil (Refer Fig. 4.11).

• The magnetic core moves in a tube that is closed at the

top and sealed at the bottom, allowing the valve to be

packless.

9AEI-406.38 76

9AEI-406.38 77

• A small spring assists the release and initial closing of

the valve. The valve is electrically energized to open.

• The figure shows both the reenergized and energized

conditions of the valve for better understanding.

9AEI-406.38 78

Disadvantages

• Although the force output of solenoids may not have

many electrical or mechanical limitations, their use as

valve actuators has economic and core (or stem) travel

limitations, and they are expensive too.

9AEI-406.38 79

• Designs are available with a separate positioned which

accepts 4-20 mA from controller and delivers a dc

output signal to the solenoid.

• Valve position feedback is obtained through the use of a

linear variable differential transformer (LVDT) mounted

directly on the valve.

Final Control Elements

9AEI-406.39 80

9AEI-406.39 81

Stepper Motor

• A stepping motor is a rotating machine that actually

completes a full rotation by sequencing through a series

of discrete rotational seeps.

• Each step position is an equilibrium position in that,

without further excitation, the rotor position will stay at

the latest step.

9AEI-406.39 82

Stepper Motor

• Thus, continuous rotation is achieved by the input of a train of pulses, each of which causes an advance of one step.

• It is not really Continuous rotation, but discrete, stepwise rotation.

• The rotational rate is determined by the number of steps per revolution and the rates at which the pulses are applied.

9AEI-406.39 83

9AEI-406.39 84

Stepper motor

Fig.1

9AEI-406.39 85

• The operation of a stepping motor can be understood

from the simple model shown in Figure1,

• Which has 90 per step.

• In this motor, the rotor is a PM that is driven by a

particular set of electromagnets.

• In the position shown, the system is in equilibrium and

no motion occurs.

9AEI-406.39 86

• The switches are typically solid-state devices, such as

transistors, SCRs, or TRICACs.

• The switch sequencer will direct the switches through a

sequence of positions as the pulses are received.

• The next pulse in Figure 2a will change S2 from C to D,

resulting in the poles of that electromagnet reversing

fields.

9AEI-406.39 87

Fig 2

9AEI-406.39 88

• Now, because the pole north/south orientation is

different, the rotor is repelled and attracted so that it

moves to the new position of equilibrium shown in Figure

2b.

• With the next pulse, S1 is changed to B, causing the

same kind of pole reversal and rotation of the PM to a

new position, as shown in Figure 2c.

9AEI-406.39 89

• Finally, the next pulse cause S2 to switch to C again, and

the PM rotor again steps to a new equilibrium position,

as in Figure 2d.

• The next pulse will send the system back to the original

state and the rotor to the original position.

• This sequence is them repeated as the pulse train

comes in, resulting in a stepwise continuous rotation of

the rotor PM.

9AEI-406.39 90

• The most common stepper motor does not use a PM,

but rather a rotor of magnetic material (not a magnet)

with a certain number of teeth.

• This rotor is driven by a phased arrangement of coils

with a different number of poles so that the rotor can

never be in perfect alignment with the stator.

9AEI-406.39 91

9AEI-406.39 92

• Figure 7.26 illustrates this for a rotor with 8 “teeth” and a

stator with 12 “poles.”

• One set of four teeth is aligned, but the other four are

not. If excitation is placed on the next set of poles (B)

and taken off the first set (A), then the rotor will step

once to come into alignment with the B set of poles.

• The direction of rotation of stepper motors can be

changed just by changing the order in which different

poles are activated and deactivated.

9AEI-406.39 93

FIG 7.26

9AEI-406.40 & 41 94

Control Valve

• A valve is a device that regulates the flow of a fluid

(gases, liquids, fluidized solids, or slurries) by opening,

closing, or partially obstructing various passageways.

• Valves are technically pipe fittings,. In an open valve,

fluid flows in a direction from higher pressure to lower

pressure.

9AEI-406.40 & 41 95

9AEI-406.40 & 41 96

• Control valves are valves used to control conditions

such as flow, pressure, temperature, and liquid level by

fully or partially opening or closing in response to signals

received from controllers that compare a "set point" to a

"process variable" whose value is provided by sensors

that monitor changes in such conditions.

9AEI-406.40 & 41 97

• The opening or closing of control valves is done by

means of electrical, hydraulic or pneumatic systems.

• Petitioners are used to control the opening or closing of

the actuator based on Electric, or Pneumatic Signals.

• These control signals, traditionally based on 3-15psi

(0.2-1.0bar), more common now are 4-20mA signals for

industry, 0-10V for HVAC systems.

9AEI-406.40 & 41 98

Construction of control valve

• The control valve is basically divided into three main

parts

• Valve body and Bonnet

• Stem

• plug

9AEI-406.40 & 41 99

1) Body 2) Head 3) Ball 4) Lever handle5) Stem

9AEI-406.40 & 41 100

Body and bonnet

• The main parts of a valve are the body and the bonnet.

• These two parts form the casing that holds the fluid

going through the valve.

• The bonnet is the casing through which the stem (see

below) passes and that forms a guide and seal for the

stem.

9AEI-406.40 & 41 101

• Valve bodies are usually metallic. Brass, bronze,

gunmetal, cast iron, steel, alloy steels and stainless

steels.

• Seawater applications, like desalination plants, often use

duplex valves, as well as super duplex valves, due to

their corrosion resistant properties, particularly against

warm seawater.

9AEI-406.40 & 41 102

Body materials for various applications

• Alloy 20 valves are typically used in sulphuric acid plants,

• Monel valves are used in hydrofluoric acid (HF Acid) plants.

• Hastelloy valves are often used in high temperature applications, such as nuclear plants,

• Inconel valves are often used in hydrogen applications.• Plastic bodies are used for relatively low pressures and

temperatures. PVC, PP (Polypropylene), PVDF (Polyvinylidene fluoride) and glass-reinforced nylon are common plastics used for valve bodies.

9AEI-406.40 & 41 103

Bonnet

• A bonnet acts as a cover on the valve body.

• It is commonly semi-permanently screwed into the valve

body.

• During manufacture of the valve, the internal parts are

put into the body and then the bonnet is attached to hold

everything together inside.

9AEI-406.40 & 41 104

Ports

• Ports are passages that allow fluid to pass through the valve.

• Ports are obstructed by the valve member or disc to control flow.

• Valves most commonly have 2 ports, but may have as many as 20.

• The valve is almost always connected at it's ports to pipes or other components.

• Connection methods include threadings, compression fittings, glue, cement, flanges, or welding

9AEI-406.40 & 41 105

Disc

• A disc or valve member is a movable obstruction inside

the stationary body that adjustably restricts flow through

the valve.

• Although traditionally disc-shaped, discs come in various

shapes.

• A ball is a round valve member with one or more paths

between ports passing through it.

9AEI-406.40 & 41 106

• By rotating the ball flow can be directed between

different ports.

• Ball valves use spherical rotors with a cylindrical hole

drilled as a fluid passage.

• Plug valves use cylindrical or conically tapered rotors

called plugs.

9AEI-406.40 & 41 107

Seat

• The seat is the interior surface of the body which contacts the disc to form a leak-tight seal.

• In discs that move linearly or swing on a hinge the disc comes into contact with the seat only when the valve is shut.

• In disks that rotate, the seat is always in contact with the disk, but the area of contact changes as the disc is turned. The seat always remains stationary relative to the body.

• Seats are classified by whether they are cut directly into the body, or if they are made of a different material:

9AEI-406.40 & 41 108

• Hard seats are integral to the valve body. Nearly all hard seated metal valves have a small amount of leakage.

• Examples: Gate, globe, and check valves are usually hard seated.

• Soft seats are fitted to the valve body and made of softer materials such as PTFE (Poly tetrafluoroethylene) or various elastomers such as NBR (Nitrile butadiene

rubber) , EPDM (ethylene propylene diene Monomer (M-class)

rubber), or FKM (Fakultas Kesehatan Masyarakat) depending on the maximum operating temperature.

• Examples: butterfly, ball, plug, and diaphragm valves are usually soft seated.

9AEI-406.40 & 41 109109

• Stem is nothing but a vertical metallic shaft or rod.

• The stem transmits motion from controlling device to the

disc.

• The stem typically passes through the bonnet when present.

• The motion transmitted by the stem may be a linear force, a

rotational torque, or some combination of these.

Stem

9AEI-406.40 & 41 110

Gaskets

• The seals or packings used to prevent the escape of a

gas or fluids from valves.

9AEI-406.40 & 41 111

9AEI-406.40 & 41 112

• The valve plug is screwed into stem and then pinned

• It can be a single seated plug or double seated plug

which is guided at the top and bottom

Plug

9AEI-406.40 & 41 113

• According to their basic shapes, plugs can be classified as

• Disc

• V – shape

• Contoured

Construction of control valve (contd..)

9AEI-406.40 & 41 114

9AEI-406.40 & 41 115

Various Types of valve plugs

9AEI-406.40 & 41 116116

Various plugsFig:5

9AEI-406.42 117

Types of control valves

• There are three basic types of control valves whose

relationship between stem position and flow rate

• These are :

• Quick opening control valve

• Linear control valve

• Equal percentage control valve

9AEI-406.42 118

Characteristics of control valves

Fig 1

9AEI-406.42 119

9AEI-406.42 120

Control Valve Flow Characteristics

• The most common characteristics are shown in the figure above.

• The percent of flow through the valve is plotted against valve stem position.

• The curves shown are typical of those available from valve manufacturers.

• These curves are based on constant pressure drop across the valve and are called inherent flow characteristics.

9AEI-406.42 121

9AEI-406.42 122

9AEI-406.42 123

Control Valve Flow Characteristics

• Linear - flow capacity increases linearly with valve travel.

• Equal percentage - flow capacity increases exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing Cv.

• A modified parabolic characteristic is approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.

9AEI-406.42 124

• Quick opening provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control. So it is limited to on-off service, such as sequential operation in either batch or semi-continuous processes.

• Hyperbolic

• Square Root

• The majority of control applications are valves with linear, equal-percentage, or modified-flow characteristics

9AEI-406.42 125

Quick Opening Control Valve

• This type of valve is used predominantly for full on /

full off control applications

• A relatively small motion of valve, for example ,

may allow 90% of maximum flow rate with only a

30% travel of the stem

9AEI-406.42 126

Linear valve

• This type of valve has a flow rate that varies linearly with

the stem position

• It represents the ideal situation where the valve alone

determines the pressure drop

9AEI-406.42 127

Linear valve

• The relationship is expressed as

max max

Q S=

Q S

Where Q = flow rate (m3/s)Qmax = maximum flow rate (m3/s)

S = stem position (m)Smax = maximum stem position (m)

9AEI-406.42 128

Equal percentage valve

• This is a very important type of valve employed in flow

control

• It has a characteristics such that a given percentage

change in stem position produces an equivalent change

in flow , i.e. an equal percentage

9AEI-406.42 129

• Generally, this type of valve does not shut-off the

flow completely in its limits of stem travel

• At the other extreme, the valve allows a maximum

flow Qmax.

Equal percentage valve (contd..)

9AEI-406.42 130

• For this type we define rangeability R as the ratio

R = Qmax / Qmin

• The curve in fig. 1 shows that increase in flow rate for a

given change in valve opening depends the extent to

which the valve is already open

• This curve typically exponential in form and is

represented by

Q = Qmin .RS/Smax

Equal percentage valve (contd..)

9AEI-406.43 & 44 131

Different Types Of Control Valves

• Globe control valve:

• Sliding Stem Control Valves:

• Lifting Gate Valves

• Rotating-Shaft Control Valves

9AEI-406.43 & 44 132

Sliding Stem Control Valves:

• Control valves in which the plug is operated by means

of reciprocating motion are termed sliding stem valves

and are of following types

1.Single seat plug valves

2.Double seat plug valves.

3.Lifting-gate valves.

9AEI-406.43 & 44 133

Single Seat Control Valve

Fig 1

9AEI-406.43 & 44 134


Fig:2, a single seat control valve and pneumatic actuator

9AEI-406.43 & 44 135


• Figure 2 is a diagrammatic representation of a single seat two-port globe valve.

• In this case the fluid flow is pushing against the valve plug and tending to keep the plug off the valve seat.

9AEI-406.43 & 44 136


• The vertical movement of the plug and stem of the

control valve changes the area of opening of the port.

• The flow rate of the fluid passing through the port is

therefore proportioned or throttled by positioning the

valve stem. The stem in turn positioned by the

actuator.

9AEI-406.43 & 44 137

Single seat valve plugs

Fig 3

9AEI-406.43 & 44 138

A few types of single seat valve plugs

• The V-port type plug is open on the inside and the

flow passes horizontally through the triangular

shaped area over the seat ring as in the fig. 3.

9AEI-406.43 & 44 139

Single Seat Plug Valves

• It has the following features:

a)It is simple in construction

b)It can be shut off to provide zero flow.

c)There is a large force acting on the valve stem because of the differential head acting across the port and seat area.

9AEI-406.43 & 44 140

Need for Double seat valve

• If a larger valve, having a larger orifice, is used to pass greater volumes of the medium, then the force that the actuator must develop in order to close the valve will also increase.

• Where very large capacities must be passed using large valves, or where very high differential pressures exist, the point will be reached where it becomes impractical to provide sufficient force to close a conventional single seat valve.

• In such circumstances, the traditional solution to this problem is the double seat two-port valve.

9AEI-406.43 & 44 141

Single Seat Plug Valves

Fig 4

9AEI-406.43 & 44 142

Double Seat Plug Valve

• The double seat valve has two port openings and

two seats and two plugs.

• The port openings are not usually identical in size

as in fig.4

9AEI-406.43 & 44 143

Single seat valve plugs

Fig 5

9AEI-406.43 & 44 144


This type has the following features.

• Net force acting on the valve stem is generally

small therefore pressure balanced.

• It cannot be shut off tightly because of differential

temperature expansion of value plug and valve

body

9AEI-406.43 & 44 145


• Few types of plugs for single seat and double seat

valves are shown in fig :5

• The piston type plug has one or more grooves along its

length and the flow passes vertically in the grooves

between the plug and seat ring.

9AEI-406.45 146

GATE VALVES

• Gate valves are used when a straight-line flow of fluid

and minimum restric-tion is desired.

• Gate valves are so named because the part that either

stops or allows flow through the valve acts somewhat

like the opening or closing of a gate and is called,

appropriately, the gate.

• The gate is usually wedge shaped.

9AEI-406.45 147

• When the valve is wide open, the gate is fully drawn up

into the valve, leaving an opening for flow through the

valve the same size as the pipe in which the valve is

installed.

• Therefore, there is little pressure drop or flow restriction

through the valve.

9AEI-406.45 148

Fig 1

9AEI-406.45 149

Disadvantages

• Gate valves are not suitable for throttling purposes since

the control of flow would be difficult due to valve design

and since the flow of fluid slapping against a partially

open gate can cause extensive damage to the valve.

9AEI-406.45 150

Lifting Gate Valves

• Lifting gate valves are shown in figure .3

• The gate valve in fig.1 is often used for fluids containing solid matter, because it presents an open area directly to the flow of fluid and does not involve a change of direction of flow stream.

• A gate valve can usually be shut off tightly by wedging into the seat.

• The chopping action at shut off is very useful for stringy materials such as paper pulp.

9AEI-406.45 151

9AEI-406.45 152

Fig 3

9AEI-406.45 153

Cross section of Gate valve

9AEI-406.45 154

Weir Valve

• It is also called Diaphragm valves (or) membrane

valves

• The weir valve of fig.5 particularly suited to certain

chemical fluids, because it has a smooth contour inside

the body with no “pockets” for solid matter and because

it has no packing gland around the stem.

9AEI-406.45 155

Weir Valve

• The flexible diaphragm of rubber or other non metallic

material is positioned by the plunger and stem.

• Fluid pressure inside the valve body holds the

diaphragm smoothly against the plunger.

9AEI-406.45 156

Fig 5

9AEI-406.45 157

9AEI-406.45 158

Applications of Diaphragm Valve

• Diaphragm valves can be manual or automated.

• Their application is generally as shut-off valves in

process systems within the food and beverage,

pharmaceutical and biotech industries.

9AEI-406.46 159

Rotating-Shaft Control Valves

• Control valves in which the restriction is accomplished

by the rotation of a plug or vane may be called

rotating-shaft type.

• Rotary type valves, often called quarter-turn valves,

• All require a rotary motion to open and close, and can

easily be fitted with actuators.

9AEI-406.46 160

Classification of Rotary shaft valves

1. Rotating-plug valves

2. Rotating ball valves

3. Butterfly valve

4. Louvers

9AEI-406.46 161

Eccentric plug valves

• Figure 1 shows a typical eccentric plug valve.

• These valves are normally installed with the plug spindle

horizontal as shown, and the attached actuator situated

alongside the valve.

• Plug valves may include linkages between the plug and

actuator.

9AEI-406.46 162

Fig 1

9AEI-406.46 163

Rotating Ball valves

• Figure 2 shows a ball valve consisting of a spherical ball located between two sealing rings in a simple body form.

• The ball has a hole allowing fluid to pass through. When aligned with the pipe ends, this gives either full bore or nearly full bore flow with very little pressure drop.

• Rotating the ball through 90° opens and closes the flow passage.

• Ball valves designed specifically for control purposes will have characterized balls or seats, to give a predictable flow pattern.

9AEI-406.46 164

Fig 2

9AEI-406.46 165

Advantages of ball valves

• Ball valves are an economic means of providing control with tight shut-off for many fluids including steam at temperatures up to 250°C (38 bar g, saturated steam).

• Above this temperature, special seat materials or metal-to-metal seating are necessary, which can be expensive.

• Ball valves are easily actuated and often used for remote

isolation and control.

• For critical control applications, segmented balls and balls with specially shaped holes are available to provide different flow characteristics.

9AEI-406.46166

Rotary Plug Valve

• The Rotating plug is shown in fig.3

• The plug is a cylindrical or conical element with a

transverse opening.

9AEI-406.46 167

Rotary Plug Valve

Fig 3

9AEI-406.46 168

Rotary Plug Valve

• It is rotated in the valve body by an external lever so

that the opening on one side of the plug is gradually

covered or uncovered.

• The shape of the opening or port may be circular, V-

shape, rectangular, or any form that is desired to

produce a given flow-angle characteristic.

9AEI-406.46 169

Rotary Plug Valve

• A rotating plug valve having a conical plug can generally

be closed tightly and has high range ability.

• This type of valve is often employed for throttling the flow

of oil to burner systems.

• The valve with a spherical plug that control the flow of

fluid through the valve body is called as ball valve.

9AEI-406.47 to 48 170

Butterfly valves

• The valve is similar in operation to a ball valve.

• Rotating the handle turns the plate either parallel or

perpendicular to the flow of water, shutting off the flow.

9AEI-406.47 to 48 171

9AEI-406.47 to 48 172

Butterfly valves

• Schematic diagram of a butterfly valve, which consists of a disc rotating in on bearings.

• In the open position the disc is parallel to the pipe wall, allowing full flow through the valve.

• In the closed position it is rotated against a seat, and perpendicular to the pipe wall.

9AEI-406.47 to 48 173

• Traditionally, butterfly valves were limited to low pressures and temperatures, due to the inherent limitations of the soft seats used.

• Currently, valves with higher temperature seats or high quality and specially machined metal-to-metal seats are available to overcome these drawbacks.

• Standard butterfly valves are now used in simple control applications.

• Particularly in larger sizes and where limited turndown is required.

9AEI-406.47 to 48 174

• A fluid flowing through a butterfly valve creates a low

pressure drop, in that the valve presents little resistance

to flow when open.

• In general however, their differential pressure limits are

lower than those for globe valves.

• Ball valves are similar except that, due to their different

sealing arrangements, they can operate against higher

differential pressures than equivalent butterfly valves.

9AEI-406.47 to 48 175

9AEI-406.47 to 48 176

Rotation of Disk in butterfly valve

9AEI-406.47 to 48 177

• Butterfly valves are valves with a circular body and a

rotary motion disk closure member which is pivotally

supported by its stem.

• A butterfly valve can appear in various styles, including

eccentric and high-performance valves.

• These are normally a type of valve that uses a flat plate

to control the flow of water.

9AEI-406.47 to 48 178

• As well as this, butterfly valves are used on firefighting

apparatus and typically are used on larger lines, such as

front and rear suction ports and tank to pump lines.

• A butterfly valve is also a type of flow control device,

used to make a fluid start or stop flowing through a

section of pipe.

9AEI-406.47 to 48 179

Butterfly Valve

• The total rotation of the vane is usually restricted to

about 60 degrees, because the addition of 30 deg

does not produce much further increase in flow.

• The V-port butterfly valve incorporates a V-slot in the

body so that rotation of the vane opens a portion of

the V-slot.

• The range ability may vary from 5 to 50 and tight shut

off may be obtained with special design.

9AEI-406.47 to 48 180

9AEI-406.47 to 48 181

9AEI-406.47 to 48 182

Application

• The butterfly valve is most often employed for the control of air and gas

9AEI-406.47 to 48 183

Louvers

• The louvers consists of two or more rectangular vane mounted on shafts one above the other and is inter connected so as to rotate together

• The vanes or operated by an external lever

• In the unirotational louver the vanes remain at all position

• In a counter rotational louver alternate vanes rotate in the opposite direction

9AEI-406.47 to 48 184

9AEI-406.47 to 48 185

Louvers

• Flow guide or some time installed between adjacent vanes in order to improve the effective ness of throttling

• A louver can not provide a tight shut-off because of the long length of seat surfaces

9AEI-406.47 to 48 186

Application

• Louvers are used exclusively for control of air flow (draft) at low pressure.

Engineering

Process control 3 chapter