’’ innovave’ - WordPress.com · ’’ innovave’ ’entrepreneurial’ global’ 15 Emergency shutdown system (ESS), Interlocks and SIS are automatic actions usually stop

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The plant operation is managed from the control room, supported by the field, laboratory and maintenance personnel. The heart of the system is the Distributed Control System (DCS)


Process Transmitters and Actuators

Data Highway(Shared Communication Facilities)

......

DataStorage Unit

HostComputer

System Consoles PLC

4-20 mA

LocalConsole

LocalControl

Unit

4-20 mA

LocalControl

Unit

LocalConsole

Honeywell TDC 3000, Yokogawa centum XL, Rosemount RS3, Fischer Provox, Foxboro IA series, ABB Bailey Infi90, etc…


Plant Optimization

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

Smart Sensors

Smart ControlValves and Controllers

LocalArea

Network

Smart Sensors

Smart ControlValves and Controllers

LocalArea

Network

H1 Fieldbus Network H1 Fieldbus Network

H1 Fieldbus H1 Fieldbus

Data Storage

PLCs

High Speed Ethernet

CEAG I/O

P5001 0 ... 150 bar

•  First introduced 1988. Based upon smart valves, smart sensors and controllers •  Uses data highway to replace wires from sensor to DCS and to the control valves. •  Foundation Fieldbus (FF), Profibus (Process Field Bus), ControlNet, P-Net, SwiftNet ,

WorldFIP, Interbus, EtherCAT, SERCODS etc


ProcessorPower Supply

Data Highway

PLC Cabinet

Programming Interface

I/O Modules

Input Devices

Output Devices

PLC is a robust device that can perform various functions: -  Digital I/O -  Analog I/O -  Timer -  Display -  Etc

Often used for (but not limited to) sequence operation such as sequence control, start-up and emergency shutdown


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


ALARMS

SIS

RELIEF

CONTAINMENT

EMERGENCY RESPONSE

BPCS

Strength in Reserve

•  BPCS - Basic process control

•  Alarm - draw attention

•  SIS - Safety interlock system to stop/start equipment

•  Relief - Prevent excessive pressure

•  Containment - Prevent materials from reaching, workers, community or environment

•  Emergency Response - evacuation, fire fighting, health care, etc.

A U T O M A T I O N


Redundancy – Key Concept in Process Safety

Seriousness of event

Four independent protection layers (IPL)

Relief System

Safety Interlock System

Alarm System

Basic Process Control System

Process


•  Technology - Multiple PIDs, cascade, feedforward, etc. •  Always control unstable variables (Examples?) •  Always control “quick” safety related variables

- Stable variables that tend to change quickly (Examples?) •  Monitor variables that change very slowly

- Corrosion, erosion, build up of materials •  Provide safe response to critical instrumentation failures

- But, we use instrumentation in the BPCS?

LC

LT

PRV

FCV

LI

FI


LC

LT

PRV

FCV

LI

FI

Control Variable: Liquid Level (to be maintained at selected set point) Manipulated variable: Outlet Flowrate

Gd

GP Gv GC

GS

d

y

ym

ySP e c +

+ +

-

u

Liquid level is measured, compared to setpoint. Controller determines control action, which is translated into the value of the manipulated variable


TC

v1

v2

Heated Tank Example Steam is used to heat up water in a tank. Set Point: Desired Temperature Mechanism 1.  Temperature of water is measured 2.  Error is computed. Error = Set point – measured temperature 3.  Control action is computed based on error using control algorithm (PID) 4.  Control action = flowrate of steam

Temperature valve Controller

GC

GM

GF GP +_

Temperature sensor

T m TSP c e

TM


Feed

Steam

Product

LT

LC

Process

Final Control Element Controller

GC

GM

GF GP +_

Measurement

y m ySP c e

yM

PC

PT

Cooling water

TT TC

LT LC


●  Developed in the 1940’s, remains workhorse of practice ●  ONE controlled variable (CV) and ONE manipulated variable

(MV). Many PID’s used in a plant.

GC = KC (1+ 1τ I S

+τ DS )

Δu = KC e+ 1τ I

e+∑ τ D

ΔeΔt

⎛⎝⎜

⎞⎠⎟

Controller performance adjusted by fixing Three TUNING PARAMETERS: 1.  Proportional Gain KC : Move the controlled variables to desired value

2.  Integral time : Makes the response oscillatory 3.  Derivative time : Dampen the oscillations.

τ I

τ D


●  Alarm has an anounciator and visual indication –  No action is automated! A plant operator must decide.

●  Common error is to design too many alarm –  Easy to include; simple (perhaps, incorrect) fix to prevent repeat of safety incident –  Example: One plant had 17 alarms/h - operator acted on only 8%

●  Establish and observe clear priority ranking

- HIGH = Hazard to people or equip., action required - MEDIUM = Loss of RM, close monitoring required - LOW = investigate when time available

LC

LT

PRV

FCV

LI

FI

LAH LAL

Alarms from operating conditions going out of normal range, operator action (closure of FCV by operator in example) required to force operating condition (level in example) back to normal


●  Emergency shutdown system (ESS), Interlocks and SIS are automatic actions usually stop part of plant operation to achieve safe conditions –  Can divert flow to containment or disposal

–  Can stop potentially hazardous process, e.g., combustion

●  Capacity of the alternative process must be for “worst case” to prevent “unusual” situations –  We must be able to start up and shut down

–  Very fast “blips” might not be significant

Sensor Logic

Solver Final

element

Sensing of process parameter

(transmitter or switch)

Final control element (for example actuated valve)

Comparison with limit


LCV

LC

LT

PRV

FCV

LI

FI

LAH LAL

LSHH

ESD valve

LAHH

I1

SIS limits

Sensor Logic solver

Final element


The automation strategy is usually simple, for example,

If L123 < L123min; then, reduce fuel to zero

steam

water

LC

PC

fuel

How do we automate this SIS

when PC is adjusting the valve?


The automation strategy may involve several variables, any one of which could activate the SIS

If L123 < L123min; or If T105 > T105max ……. then, reduce fuel to zero

SIS 100

L123 T105 …..

s

Shown as “box” in drawing with

details elsewhere


The SIS saves us from hazards, but can shutdown the plant for false reasons, e.g., instrument failure.

1 out of 1 must indicate

failure

T100 s

2 out of 3 must indicate

failure

T100 T101 T102

Same variable, multiple sensors!

s

False shutdown

Failure on demand

5 x 10-3 5 x 10-3

2.5 x 10-6 2.5 x 10-6

Better performance, more expensive


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 ………….

BPCS and Alarms SIS and Alarms associated with SIS


● Overpressure –  Increase in pressure can lead to rupture of vessel

or pipe and release of toxic or flammable material

● Underpressure –  Also, we must protect against unexpected

vacuum!

● Relief systems provide an exit path for fluid –  Benefits: safety, environmental protection,

equipment protection, reduced insurance, compliance with governmental code


•  Entirely self-contained, no external power required

•  The action is automatic - does not require a person

•  Usually, goal is to achieve reasonable pressure

- Prevent high (over-) pressure - Prevent low (under-) pressure

•  The capacity should be for the “worst case” scenario

LC

LT

PRV

FCV

LI

FI

LAH LAL


•  No external power required -

•  self actuating - pressure of process provides needed force!

•  Valve closes when pressure returns to acceptable value

•  Relief Valve - liquid systems

•  Safety Valve - gas and vapor systems including steam

•  Safety Relief Valve - liquid and/or vapor systems

Process

To effluent handling


Rupture Disk/Burst Diaphragm

•  No external power required

•  self acting •  Rupture disk / burst

diaphragm must be replaced after opening

.

Process

To effluent handling


•  In some cases, relief Valve and Burst Diaphragm are used in series, why?

•  What is the advantage of the two in series?

•  Why not have two relief valves ( or diaphragms) in series?


•  Why is the pressure indicator provided?

•  If the pressure increases, the disk has a leak and should be replaced.

•  Is it local or remotely displayed? Why?

•  The display is local to reduce cost, because we do not have to respond immediately to a failed disk - the situation is not hazardous.

•  What is the advantage of two in series?

•  The disc protects the valve from corrosive or sticky material. The valve closes when the pressure returns below the set value.

In some cases, relief Valve and Burst Diaphragm are used in series, why?



●  You are to examine the Flash drum system shown in the next page. The purpose of the plant is to separate multiple mixture of mostly A and B plus some other heavy components to produce main product leaving the top at 95 mol % of A. Steam at 5 bar is used to bring the temperature at the desired saturation temperature of 130 oC.

●  Nominal operating conditions are as follows: §  T1= 70 oC §  T2= 120 oC §  T3= 100 oC §  T4= 110 oC §  P9 = 3 bar

●  Propose the layer of protection needed to ensure safe operation of the plant


FCV 7

FCV 2

FCV 5

TI 4

TI 3

FI 1

TI 1

FI 6

FI 2

TI 3

TI 2

TI 8

FI 8

LC 8

FI 9

FCV 9

FCV 8

PC 9

Process Fluid Steam

V-40 E-30

E-20

XAD= 95 mol %

XAF= 50 mol %


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’’ innovave’ - WordPress.com · ’’ innovave’ ’entrepreneurial’ global’ 15 Emergency shutdown system (ESS), Interlocks and SIS are automatic actions usually stop