Plant Operation Systems

DR. AA

1

Typical Objectives of Plant Operation

1. Protect people

2. Protect Environment

3. Protect Equipment

4. Maintain Smooth operation

5. Achieve Product rates and quality

6. Profit = optimising first five

7. Monitoring & diagnosis

These are Achieved Through Process Control…

2

Distributed Control System (DCS)

DCS (Distributed Control System) is a computerized control system used to control the production line in the industry

DCS was introduced in 1975 by Honeywell (TDC2000) and Yokogawa (CENTUM)

Other vendors: ABB (Bailey etc), Foxboro, Emerson (Fischer, Rosemount etc)

3

DCS Architecture

Process Transmitters and Actuators

Data Highway

(Shared Communication Facilities)

......

Data

Storage Unit

Host

Computer

System

ConsolesPLC

4-20 mA

Local

Console

Local

Control

Unit

4-20 mA

Local

Control

Unit

Local

Console

4

Fieldbus Technology

Introduced 1988 but underwent many

development

Standard IEC61158 introduced in

1999

Based upon smart devices installed

in the field.

Uses data highway to replace wires

for signal transfer.

Can mix sensors, transmitters, and

control valves from different vendors

CEAG

I/O

P5001 0

... 150 bar

•Foundation Fieldbus (FF)

•Profibus (Process Field Bus)

•Others: ControlNet, P-Net, SwiftNet ,

WorldFIP, Interbus, EtherCAT, SERCODS etc

5

Fieldbus Architecture

Plant Optimization

.................

Smart

Sensors

Smart Control

Valves and

Controllers

Local

Area

Network

Smart

Sensors

Smart Control

Valves and

Controllers

Local

Area

Network

H1 Fieldbus Network H1 Fieldbus Network

H1 Fieldbus H1 Fieldbus

Data Storage

PLCs

High Speed Ethernet

6

PLC Architecture

ProcessorPower

Supply

Data Highway

PLC Cabinet

Programming

Interface

I/O Modules

Input

Devices

Output

Devices

7

We desire independent protection layers, without common-

cause failures - Separate systems

sensors

SIS system

i/o i/o ………….

sensors

Digital control system

i/o i/o ………….

DCS handles controls

and alarms functions. PLC handles SIS and

Alarms associated with SIS

Redundancy

8

Control Diagram of a Typical Control Loop

Controller

F1

T1

T

F

F2

T2

TC

Actuator

System

TT

Sensor

System

9

Sensor System

Sensors

– Temperature, Flow, Liquid Level, Pressure, pH,

Transmitter

10

Temperature Measurement

Expansion Thermometer – Liquid in Glass

– Mercury in Steel

– Bimetallic Elements

Thermoelectric Thermometers (Thermocouple) – Type J, K, T, R, S, ...

Resistance Thermometers – Thermister

– Resistance Thermal Detector (RTD), e.g. Pt100

Radiation Thermometers (Optical Pyrometers)

11

Pressure Measurement

Direct pressure Measurement

– manometer

Indirect Pressure Measurement

– Bourdon Tubes, Bellows, Diaphragms

Electrical Pressure Transducers

– Capacitive, Resistive, Inductive

Other Pressure Transducers

– Force Balance, DP Cell, Piezoelectric Transducer

12

Flow Measurement

Point Velocity Measurement – Pitot Tube, Hot Wire Anemometer, Transit Time

Velocimeters

Gross Volume Flow Measurement – Venturi Tube, Orifice Plate, Nozzle, Dall Tube,

Rotameter, Turbine Meter, Positive Displacement Meters (piston, gear etc)

Gross Mass Flow measurement – Direct Method - Momentum Type, Thermal Type

– Indirect Method - calculate density & pressure etc.

13

Liquid Level

Dipstick, Sight glass, Float

Diaphragm

Load Cell

Manometer, Direct Pressure, Differential Pressure

Capacitive Probe

Ultrasonic

14

Chemical Composition

Refractive Index

Spectroscopy

– IR/UV/Visible Spectrophotometer, Mass Spectrophotometer, Atomic Absorption Spectrophotometer

Chromatography

– Gas Chromatography, Liquid Chromatography

15

Actuators System

On/Off

– On-Off Valves

– Pumps (motor)

Variable position

– Control Valves

– Variable speed pump • DC motor

• 3-phase motor

• Turbine drive

16

Control valve

17

Butterfly valve

18

Actuator

19

Valve with actuator & positioner

20

Why are some valves Fail-Opened, While Others Fail-Closed

21

Typical response of Control Valves

A. Quick-Opening B. Linear C. Square-root D. Equal Percentage

Percentage of

Maximum flow

Percentage of Stem Travel

A

B

D

C

Why do we need different shapes of plugs in the

control valves ?

22

Motor Speed Control

DC Motor Speed Control

– Adjust the power of motor by varying current or voltage

AC Motor Speed Control (3 Phase Motor)

– Adjust the power of motor by varying the frequency of the AC cycles

Steam Driven Turbine

– Adjust the turbine speed to adjust the pumping rates

23

Safety Features

Alarms & Enunciators

Interlocks

– To isolate the impact of process failures from one section from another

Rupture Disks, Pressure Relieve Valves, etc

24

Controller System

Stand Alone Controller

Supervisory Control

DCS

25

How Reliable are Those Instruments?

Reliability can be estimated using the following equation

teR

Here R is reliability, is the annual failure frequency (failure/year) and t is time (year)

teRp 11

The failure probability can then be estimated:

Here p is the annual probability of failure

26

Safety Integrity Level (SIL)

A SIL is a measure of safety system performance, in terms of probability of failure on demand (PFD). The higher the SIL is, the more reliable or effective the system is.

Every Safety Instrumented Function (SIF) has a SIL classification guided by the IEC 61508 standard

ANSI/ISA S84.01 and IEC 61508 require that companies assign a target SIL for any new or retrofitted SIS.

Three sector specific standards have been released using the IEC 61508 framework, IEC 61511 (process), IEC 61513 (nuclear) and IEC 62061 (manufacturing).

27

SIL and PFD

Safety Integrity

Level (SIL)

PFD

(Low Demand Mode)

PFD

(High Demand Mode)

1 > 10-2 to < 10-1 > 10-6 to < 10-5

2 > 10-3 to < 10-2 > 10-7 to < 10-6

3 > 10-4 to < 10-3 > 10-8 to < 10-7

4 > 10-5 to < 10-4 > 10-9 to < 10-8

PFD - Probability of Failure on Demand per year

• Low Demand Mode – intermittent operation (less than once a

year)

• High Demand Mode – Continuous operation or systems that

operates more than once a year

28

Examples

Component Failure Rate

(faults/year)

Reliability R=e(-t)

Failure Probability

P=1-R

DP Cell 1.41 0.24 0.76

Control Valve 0.6 0.55 0.45

Standalone Controller

0.29 0.75 0.25

29

How to Assign SIL Level?

PFD requirement typically determine by the PHA Team. Based on this, required SIL is identified.

There are various methodology available e.g. HSE Research report no 216 as well as others

30

Typical Product SIL (General Motors)

Product SIL Suitability

Level

FL4000 Flame Detector (Multi-

Spectral IR)

3

FL3111 Flame Detector (UV) 2

S100C Combustible gas detector 3

Field Mounted Display 2

TA102A Controller 2

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End of Topic 1.3

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