Process Control Lecture 9 (M2)

8/13/2019 Process Control Lecture 9 (M2)

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Process Control Elements

Process Engineering II (EI - 309)

Lalu Seban

Assistant Professor,

Dept. of Electronics & Instrumentation Engineering,

National Institute of Technology Silchar

Lecture 9 (Module - II)


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UNIT II (Lectures - 4)

PROCESS CONTROL ELEMENTS: Signalconversion - I/P, P/I Converters,Pneumatic and Electric actuators, ValvePositioner, Control Valve, Characteristics

of Control Valves, Types of controlvalves, Control valve sizing, cavitationand flashing.


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Once again to the basic


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In simple words Measure the PV

Evaluate (SP-PV)

Decision (by controller)

Action (by FCE)


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In microscopic view


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Language problem

Synchronization between sensor andcontroller, controller and FCE etc.

FCE should understand whatController speaks

We need a translator (I/P, I/V, P/I)

Better avoid mediators in real life


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Process Control Elements By Application

Sensors

A/D, D/A Converters

I/P, P/I, V/I, I/V ConvertersActuators/FCE


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Process Control Elements Depending on the energy source

Hydraulic(pressurized un-compressiblle fluid)

Pneumatic(compressed air)

Electric (current, voltage)


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Electric Vs Pneumatic Vs Hydraulic


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ELECTRIC PNEUMATIC HYDRAULIC

Accurate positionSuit to advance

control

No tubing

InexpensiveFast

No pollution

No return line

No stall damage

Large capacityLocking capability

Self lubricating

Easy to control

Smooth operation

Low speed

Expensive

Unsafe

Need breaker

Overheating

Low accuracy

Noise pollution

Difficult speed control

Need infrastructure

Expensive

Leakage problems

Difficult speed control

Need return line

Need infrastructure

Advantages

Disadvantages


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I to P convertor Controller speaks in I (4-20 mA)

FCE understands only P (3-15 Psi)

So we take the help of a mediatorcalled I/P converter

I/PAir supply20 psiCurrent

4 to 20 mA

Pneumatic 3 to 15 psisupplied to FCE


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I to P convertor


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P to I convertor Transmission over large distances

Input to an electronic logger orcomputer

Input P (3-15 Psi)

Output I (4-20 mA)

Make use of pressure capsule


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P to I convertor


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Problems. The electrical (4-20mA) temperature

transmitter is calibrated for a

temperature range of 500 to 750 Kelvin. Range and Span ?

Find the output of transmitter when

temperature is 400K, 500K, 600K, 700K, 750K, 1000K


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Is it clear 0 to 150 Psig, the range is 0 to 150 Psig, 0

for the 0%(4mA) and 150 for the 100 %

(20mA), the span is 150Psig 20 to 200 Psig, the range is 20 to 200 Psig,

20 for the 0%(4mA) and 200 for the100%(20mA), the span is 180Psig

40 to 350 F, the range is 40 to 350 F, 40for the 0%(4mA) and 350 for the 100 %(20mA), the span is 310F


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Problems. If the input to an I/P converter is 10mA,

what will be its output?

0mA, 4mA, 10mA, 12mA, 20mA

Represent every signal in percentageand then proceed


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Is it clear. Signal (x) to (%)

(%) to Signal (x

)

X 100min

max min

x - x(%)= x - x

X100

min max min

(%)x = x +(x - x )


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Process Control Elements

Process Engineering II (EI - 309)

Lalu Seban

Assistant Professor,

Dept. of Electronics & Instrumentation Engineering,

National Institute of Technology Silchar

Lecture 10 (Module - II)


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Final Control Element equipment to convert the control

signal (generated by controller)

into the action needed to correctlycontrol the process

The final control element adjust the

amount of energy/mass goes into orout from process as commanded bythe controller


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Hydraulic Actuators Using pressurised liquid for doing work

Pressure in whole liquid remain equal

Applications requiring large forces andprecise control

vehicle power steering and brakes,hydraulic jacks and heavy earthmoving machines


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Final Control Elements Hydraulic

Pneumatic

Electric


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Hydraulic Actuators

2

2

1 1 x 1

2 2 x 2

1 2

2 2 3

91 1 1

F P A

F P A

P P

F A

F A


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Electronic FCEs Heater

Blower

Motors

Solenoid valves


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Electronic FCEs Heater

Blower

Motors stepper, AC, DC

Solenoid valves


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Solenoid When the coil is energized the core will be

pulled in

SOLENOIDcorecoil

coil

core


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Solenoid

V


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Solenoid

V


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Control Valvesmechanism to regulate flow

by opening or closing

based on signal from controller


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Control ValvesLinear or Rotary motion valves

Electric or Pneumatic valves

Air to open or Air to close valves

Manual or Automatic valves

Linear or Quick opening or Equalpercentage valves


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Linear Motion valves closure member moves in a straightline to regulate flow

Gate valve

Globe valve

Diaphragm valve


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Rotary Motion valvesclosure member moves in an angularor circular path to regulate flow

Plug valve

Ball valve

Butterfly valve

Quarter Turn Valves: rotary valveswhich moves from fully closed to fullyopen in a a quarter turn


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RememberRotary valve :

turn Clock-wise to Close

Quarter turn valve :

perpendicular to pipe (line) toClose


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Based on actuationManually turning

using motors

Using pneumatic pressureair to open

air to close


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Valve Components


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Valve Bodyis the first boundary of a pressure

Valve

framework that holds all the partstogether

ends are designed to connect the

Valve to the piping


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Valve Disk part which regulate flow, dependingon its position.

With the Valve closed, full systempressure is applied across disc

valves named disc design

linear motion

gate, globe

rotatory motion butterfly, ball


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Valve Seat(s)ensures the seating surface for thedisk

fine surface finished seating for goodsealing

one seat globe valve

two seat gate valve


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Valve StemValve stem provides the necessarymovement to the disc, plug or the ball

one end connected to the actuator

second end to disc

linear or rotatory movement


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Valves

by construction type

The valving element rotates in the passagewayto stop flow.Eg.: Ball Valve, Butterfly Valve


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Valves


The valving element acts as a 'seal' or 'plug' inthe passageway to stop flow.Eg.: Globe Valve


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Valves


The valving element is 'inserted' into thepassageway to stop flow.Eg.: Gate Valve


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Valves


The valving element is 'pinched' against thepassageway from the outside to stop flow.Eg.: Diaphragm Valve


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Ball Valve


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Butterfly Valve


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Globe Valve


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Gate Valve


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Valve - Flow characteristicsLinear, Equal %, quick (fast) opening

% flow Vs % opening


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Valve - Flow characteristicsbased on trim (disc, seat)


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Valve Positioner

static friction

differential pressure

another control system within FCE !!

Process Control Lecture 9 (M2)

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