process instru mentation n control

7/29/2019 Process Instru Mentation n Control

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University of Manchester Slide 1

Process Instrumentation and Control

J ohn Cuffe

Unit: Safety


Process Instrumentation & Control

It is necessary for the design engineer tounderstand the plant control philosophy

even at the Schematic/ Flowsheet/ PFDstage

This lecture is an overview of control fordesign purposes


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Basics of process control

Process instrumentation Control of unit operations

Process safety instrumentation


Basics of Process Control

Why is Process Control important?

Are we wasting time & money on it?


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Objectives of Process Control

Ensure stable process operation

h Particularly, keep the plant operating under safe conditions

h

Minimize damage to equipment due to variation in plantconditions

Ensure operation meets product specifications

Minimize impact of external disturbances

h Example: change in ambient temperature

Optimize process performance

h Maintain process throughput

h Minimize operating costs


Control Loop Components

The sensing instrument detects the measured variable and sends a signalto a controller, which signals the actuator to close or open a control valveand adjust the manipulated variable (usually a flow rate)

PTPAH

PAL

PICPVPT

PAH

PAL

PICPV

Process orutility stream

AlarmsInstrument line

Actuator

Final controlelement

Controller

Sensingelement


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Control Valve


Control Valves

The final controlelement is usually acontrol valveh Exceptions: electric

heaters, mixers,variable speed drives

The actuator is either amotor or a bellows thatopens or closes thevalve in response tothe signal

Actuator

Valve

Source: Valve Manufacturers Association, www.vma.org


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Types of Control Loop

Feedbackh Control system

measures changes in aprocess output and then

adjusts manipulatedvariable to return outputto set point

h Can be slow if processresponse time is long

Feed Forwardh Control system measures

disturbance and adjustsmanipulated variable to

compensate for it so thatcontrolled output is notaffected

h Requires greater knowledgeof system response

ProcessManipulated

variable

Controlled

output

Disturbance

Controller

ProcessManipulated

variable

Controlled

output

Disturbance

Controller

ProcessManipulated

variable

Controlled

output

Disturbance

Controller


Feedback Control

Control system measures changes in a process output and thenadjusts manipulated variable to return output to set point

Can be slow if process response time is long

ProcessManipulated

variable

Controlled

output

Disturbance

Controller

ProcessManipulated

variable

Controlled

output

Disturbance

Controller


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Feed Forward Control

Control system measures disturbance and adjusts manipulatedvariable to compensate for it so that controlled output is not affected

Requires greater knowledge of system response

ProcessManipulated

variable

Controlled

output

Disturbance

Controller


Feedback Control

Controller computes error between input and set point and adjustsoutput based on a control algorithm

Process

Sensing

element

Final control

element

Function

generatorSet

point

Output

Error

signal

Measured variable

Input

Manipulated variable

+

-

Controller


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Ratio Control

One stream is controlledin ratio to another

Often used for controllingfeed rates to try tomaintain stoichiometry

Also used in some typesof distillation columncontrol to set reflux ratios

FT

FFC

FFV

FT

FT

FFCFFC

FFV

FT


Cascade Control

One primary controller is used to adjust the set point of a secondsecondary controller

Used to minimize outside load variations and increase processstability

Example: reactortemperature (primarycontroller) cascades ontocoolant flow controller(secondary) to controlreactor temperature

FT

FIC

FV

Coolant

TIC

TE

TT

M

FT

FICFIC

FV

Coolant

TICTIC

TE

TT

M


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Process instrumentation Control of unit operations



What Can Be Measured? (& How Easily)

Easy

Temperature

Pressure Flow rate

V/L Level

Pressure difference

Conductivity

Difficult

L/L level

pH Certain components

h oxygen, sulphur, hydrogen,CO

Composition

Density

Voidage

Easy means cheap, reliable instrument with fast response timeand accurate measurement


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Temperature Measurement: Thermocouples

When a junction between dissimilar wires is heated, an EMF(voltage) is developed, which can be read by a millivolt transmitter

The junction is usually housed in a thermowell

THERMOCOUPLE HEADLEAD WIRE

A

B

A

B Cu

Cu+

-

COLD J UNCTION (T )2

MILLIVOLT

TRANSMITTER

ISATYPE A (+) B (-)

CONSTANTAN

CONSTANTAN

ALUMEL

CONSTANTAN

EJKT

CHROMEL

IRON

CHROMEL

COPPER

HOT J UNCTION (T )1

THERMOCOUPLE HEADLEAD WIRE

A

B

A

B Cu

Cu+

-

COLD J UNCTION (T )2

MILLIVOLT

TRANSMITTER

ISATYPE A (+) B (-)

CONSTANTAN

CONSTANTAN

ALUMEL

CONSTANTAN

EJKT

CHROMEL

IRON

CHROMEL

COPPER

HOT J UNCTION (T )1

LEAD WIRE

A

B

A

B Cu

Cu+

-

COLD J UNCTION (T )2COLD J UNCTION (T )2

MILLIVOLT

TRANSMITTER

ISATYPE A (+) B (-)

CONSTANTAN

CONSTANTAN

ALUMEL

CONSTANTAN

EJKT

CHROMEL

IRON

CHROMEL

COPPER

HOT J UNCTION (T )1HOT J UNCTION (T )HOT J UNCTION (T )1


Temperature Measurement: Thermocouples

Response depends on thermowell location and heattransfer

h Instrument error is usually 1 to 2 C

h There may be additional offsets if the thermowell is incorrectly

located

Response is fast if located in a flowing stream solocation is important

Sometimes thermocouples are also strapped to walls of

vesselsh For high temperature processes or processes with large

exotherms


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Pressure Measurement

Pressure instruments usually measuredifferential pressure

If one side is atmospheric pressure then thedifference is the process gauge pressure(usually written bar or psi), not absolutepressure (bara, psia)


Pressure Measurement

Several possible methods:

h Mechanical: measure displacement of a bellows or Bourdon tube

h Electrical: attach a strain gauge to a bellows

h Capacitance: diaphragm moves capacitor plate (most common

type)

h Piezoelectric: measures change in semiconductor conductivity

Pressure measurement devices respond quickly andaccurately

Differential pressure measurement is used as the basisfor flow and level measurement


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Flow Rate Measurement

Place a restriction in the flow path and measure the resultingpressure drop using a differential pressure (PD) cell

If fluid properties are known, results can be calibrated to flow rates

PD

Orifice Meter

PD

Venturi Meter


Orifice Plate


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Level Measurement

Displacement

Displacer moves up and down

with level due to bouyancy Displacer movement is

detected via mechanical ormagnetic linkage

Differential Pressure

Measures static head of liquid

using a differential pressurecell

Density of the liquid and vapormust be known and constant

Sensor element

PD




Process instrumentation

Control of unit operations Process safety instrumentation


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Flow Control Design Problem

The flow in a line containing a centrifugal

pump or compressor is required to becontrolled

How would you control this proposedarrangement?


Flow Control Solution

Most common arrangement is a control valve downstream of apump or compressor

Using a variable speed drive is a more efficient method, but highercapital cost

FT

FIC

FV

PI

M

FT

FIC

PIMFT

FIC

FV

PI

M

FT

FIC

PIM FT

FIC

PIM


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Level Control Design Problem

A process is to include a tank which is to becontinually filled but the level within it must

be maintained within certain limits

On the bottom outlet there is to be a pumprunning constantly during normal operation



Level Control Solution

Level control is neededwhenever there is a V/L orL/L interface

Level control setsinventories in processequipment

Many smaller vessels aresized based on levelcontrol response time

LV

LTLAH

LAL

LIC

M


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Pressure Control Design Problem

A gas is to be continually generated within avessel at above atmospheric pressure.

The pressure within the vessel must bemaintained at a certain level.

How would you control this proposed

arrangement?


Pressure Control

Pressure control is usually by ventinga gas or vapor

In hydrocarbon processes, off-gas is

often vented to fuel

In other processes, nitrogen may bebrought in to maintain pressure andvented via scrubbers

Most common arrangement is directventing (shown)

PV

PTPIC

PV

PTPIC


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Pressure Control Design Problem

The previous process changes and the gasnow contains a large amount of condensable

material

Again the pressure must be maintained at acertain level.

What changes would you make?


Pressure Control - Condensables Solution

If vapour has a highloading of

condensable material,then pressure controlis on the vent gasstream from thecondenser

PV

PTPIC

PV

PTPIC

PV

PTPIC


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Temperature Control Design Problem

A process stream is to be heated/cooledusing a heat exchanger.



Temperature Control: Single Stream

Heaters and coolersare usually controlled

by manipulating theflow rate of the hot orcold utility stream

Final control elementcan be on inlet oroutlet of utility side

TV

TETIC

Hot or cold

utility

Process

TT

TV

TETICTIC

Hot or cold

utility

Process

TT


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Process instrumentation Reading a P&ID

Control of unit operations



Role of Controls in Process Safety

Control system isinvolved in three levelsof process safetyh

Keeping plant operationsteady

h Sounding alarms to notifyoperator when variablesare out of limits

h Automatically shuttingthe plant down whennecessary

Automat ic Safety Shutdowns

PressureRelief System

Critical Alarms & Operator Intervention

Basic Process Control

Plant Design (Inherent Safety)

Emergency Response

in Community

Emergency Response

in Process Unit

Automat ic Safety Shutdowns

PressureRelief System

Critical Alarms & Operator Intervention

Basic Process Control

Plant Design (Inherent Safety)

Emergency Response

in Community

Emergency Response

in Process Unit


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Controlled parameters naturally fluctuate around set point

If the measured variable exceeds a preset limit an alarm should alertthe operator to take appropriate action

h Alarm limits should be set far enough from normal process variation toavoid nuisance alarms

If the measured variable exceeds a safe operating limit then an

automatic plant shutdown may be necessaryh Shutdown limit should be set far enough from alarm limit that the

operator has a chance to respond to the alarm

h But not so far that no time is left to safely shut the plant down

Process Control, Alarms and Shutdowns

time

Variable

AL

AH

Shutdown

Set point


Standards for Safety Instrumentation

ISA S84.01 Safety Instrumented Systems

h U.S. standard for emergency shutdown systems

h Primary goal is to protect people, not plant or profits

IEC 61508 & 61511

h IEC = International Electrotechnical Commission

h International standards for safety instrumented systems

Standards define requirements for sensors, solvers(logic), and final elements (valves, switches)

Consult most recent version of standards for current bestpractices

Other standards also recommend best practices foralarm levels, vessel sizing to allow adequate control, etc.


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Safety Integrity Levels

ISA S84.01 defines three levels of safety integritydepending on the availability of the Safety Systems

Availability = time the system is available / total time

Safety Integrity LevelsSIL Availability System redundancy

SIL 1 90 99% Non-redundant

SIL 2 99 99.9% Partially redundant

SIL 3 99.9 99.99% Totally redundant

Redundant system means instrumentation is duplicated


Safety Integrity Level

SIL should be determined during a processhazard analysis SIL required depends on risk of

operator exposure and injuryh Can be calculated using fault trees

SIL determines the type of instrumentation thatshould be used


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Process Alarms and Shutdown Trips

Software alarms can be set on instruments and controllers throughthe digital control system and show up on shared displays

Separate alarm and shutdown instrumentation can also be used, forhigher redundancy

LTLAL

LIC

TRIP

LTLIC

LSL LAL

LAL

UCA

UCA

S

LTLAL

LIC

TRIP

LTLAL

LIC

TRIP

LTLICLIC

LSL LALLAL

LAL

UCA

UCA

UCA

UCA

S


Caution on Software Alarms

There is a temptation to put lots of software alarms indigital control systems

If there are too many alarms then they can become a

distraction to the operatorsh Increasing the chance of human errorh Increasing the chance that the operator will ignore the alarm,

switch it off, or acknowledge it without taking action

Alarms should be carefully placed and calibrated tomake sure that they serve the purpose of the designer

Operators should be trained to understand theimportance of every alarm on the plant


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Valve Failure Positions

It is important to specify what happens to a control valveif the signal fails

The final valve position has an impact on process safetyand pressure relief scenarios and may affect otherinstrumentation

Fails open Fails locked incurrent position

Fails closed Failure modeindeterminate

Shared Display Devices

Most plant control rooms now useshared display devices that showthe outputs of multiple instrumentson a screen

Operator can see a flow diagram

that identifies where theinstrument is and can enter setpoints

Software also allows data to beplotted as trends

Data can be accessed remotely

Data is collected and logged forprocess records

S UOP

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