MEP201 Mechanical Engineering Drawing

1st semester 2005-2006

S R Kale

Lecture

Control Diagrams

Controls (2005) 2

Control Drawings - Background

Pre-requisites• Schematic, P&I drawings

Control philosophy• About the plant• From equipment/package manufacturers

Team: All designers (EE, ME, ChE, I&C, …)

Output• Wiring of control cables• Governing logic for control software

development

Interdisciplinary: Chem, EE, Aero, BioMed, ..

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Control Drawings - Types

Sequence ControlSequence Control drawingsInterlock DrawingsBlock Interlock Diagrams

ON/OFF : If … then …

Modulating Control Control Logic Diagram (feedback, modulating type)

Sense feedback signal, Set point Controller (PID) Output signal Device Feedback signal

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Sequence Control Drawings /

Block Interlock Diagram /Interlock Diagrams

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Sequence Control Drawings(Block) Interlock Diagram

Schematic

Control philosophy

Incorporate instruments in schematic to generate P & I drawing

Interlock diagram

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Sequence Control Diagram – Basics

• Logic from process designer & eqpt. manufacturer:

What are the safe operating limits?

What action to take if safe limits are exceeded?Logic or its execution – not unique

• Within the limits, automatic control system operates.

• Sequence control supercedes automatic control.

• ON-OFF control• Signal processing – via relays and/or software

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Sequence Control Diagram – Objectives

Operate equipment and system within safe limits.

Auto start / stop of equipment

WHY?

• Avoid damage to equipment (expensive repairs)

• Initiate automatic protective action in case of malfunction.

• Avoid plant shut down by switching to stand-by device.

• Safely shut-down system even if all safety back-ups don’t work.

• Safe start-up and shut-down of system.

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Interlock diagram - ElementsCONDITION LOGIC ACTION

PS1 L.O. SUPPY PR. L L1 LAMP ON

Parameter

Value

I.D. I.D.

Device

Action

Connect with lines

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Logic GatesAND

OR

NOT

TIME DELAYOn delay/Off delay Period ‘T”

&

OR

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Bearing Lube Oil Schematic

LUBE OILTANK

BEARING

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Bearing Lube Oil SystemRequirements (order by importance): Oil pressure at inlet

• Indication on gauge• Generate alarm if low• Generate signal to trip turbine if low low

Oil temperature at inlet• Indication on gauge• generate alarm if high

Oil flow rate• Measure (?) expensive – not really needed• Visual indication of flow

Oil level in tank• Indication• Alarm if low

Power supply is available for pump• Indication• Voltage and current drawn

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Bearing Lube Oil I & C Diagram

LUBE OILTANK

BEARING

LI1

SG

L1

PS1L

L3

TS1H

PI1

TI1

LS1

L4

L

L2

PS2LL

RLY1

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Interlock: Bearing L.O. systemCONDITION LOGIC ACTION

PS1 L.O. SUPPY PR. L L1 L.O. PR. L -LAMP ON

PS2 L.O. SUPPY PR. LL RLY1 FRANCIS TURBINE TRIP

L2 L.O. PR. LL -LAMP ON

TS1 L.O. SUPPLY TEMP. H L3 L.O. TEMP. H -LAMP ON

LS1 L.O. TANK LEVEL L L4 L.O. T. LVL L -LAMP ON

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Domestic water booster systemBooster pump, ground tank, overhead tank

Pump to start ifOverhead tank is empty AND Ground tank has enough water

Pump to stop ifOverhead tank is full OR Ground tank is empty

Indications to operator (panel lights or PLC display):Overhead tank - Full, Empty.Ground tank – Full, Empty.No power supply.Pump status – Running, Stopped.

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Domestic Booster Schematic

LS1H

LS2L

LS3L

LS4H

GROUNDLEVELTANKGROUND

LEVELTANK

GROUNDLEVELTANKOVERHEADTANK

BOOSTER PUMP

4 Signals –One motor.

RELAYS

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Interlock: Domestic water system

CONDITION LOGIC ACTION

LS1 G/L TANK LEVEL H

LS3 O/H TANK LEVEL L

SW1 BOOSTER PUMP ON

SW1 BOOSTER PUMP OFF

LS2 G/L TANK LEVEL L

LS4 O/H TANK LEVEL H

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Domestic water system - Alternate

Domestic water system

LS4 OVERHEAD TANK LEVEL HIGH H

LS2 GROUND TANK LEVEL HIGH H

LS3 OVERHEAD TANK LEVEL LOW L

LS1 GROUND TANK LEVEL LOW L

START PUMP

STOP PUMP

&

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Diesel engine of DG set/Loco

Diesel engine

On engine start signal (key) energize starter motor ifWater temperature is OK AND Fuel tank is not empty

After starting ifLube oil pressure is low after 3 seconds, then SHUT-OFF(TIME DELAY)

Engine is running, then SHUT-OFF ifLube oil pressure falls to 3 bar(g) ORJacket water temperature rises to 105 oC ORSpeed increases to 1545 RPM ORSpeed decreases to 1425 RPM.

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Air conditioner (window/split)

After power switch is turned ONWait 2 minutes then start compressor (TIME DELAY)

Switch-off running compressor ifCool space temperature falls to oC

Switch-on compressor ifCool space temperature rises to oC AND2 minutes have elapsed since compressor was shut-off (TIME DELAY)SET POINT OFFSET for cut out/cut in

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Airplanes

Airplanes(ref.: aeroPeru603/Air Crash Investigation)

Auto-pilot gets engaged if2 out of 3 readings of altitude/speed are same.(TWO OUT OF THREE LOGIC)IF NOT, fly the plane manually

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Francis turbine

Francis Turbine (Badagrah Nala)

2 x 100 % Lube oil pumps, give START signal to stand-by pump if

• Turbine is running AND• L.O. level in tank in NOT low AND• Standby pump is available AND• (Other pump has been switched OFF OR• Lube oil header pressure is LOW)

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Set point valuesGeyser thermostat* switch is in the power line itself.Set-point is say 55 oC At water temp. 55 oC, power is switched off. At what temp. is it switched on?

If at 55 oC then ??? Confusion!

Therefore: one switch for one sensing only,i.e. one TS for HIGH and another TS for LOW.And, offset cut-out and cut-inHIGH set at 55 oC + T, and LOW set at 55 oC -

T.

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Sequence Control Diagram – Examples

• BFP of CCPP : start and trip permissive• Standby BFP of CCPP : start permissive• Representation on P&I diagram• Typical interlock diagram - CCPP

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Sequence Control – Haywire

• Equipment damage• Equipment/device does not start or shut-off

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Sequence control - Summary

Outcome:

• Details of signal sources• Signal connections• Control logic for programming and hardware• Merge with layouts to generate control cable

routing, its length and size (BoM)• Modify piping drawings to accommodate

instruments/switches

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Modulating Control

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Automatic Control System

Automatic modulating control• Dynamic real-time control• Sense signals (parameters)

Generate control signal Adjust control device

• Feedback control loops• Every machine has its control system

P-based, PLC-based, DAS• Expensive, need maintenance, critical.

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Automatic controls- Example

• Control loop #1 (CCPP steam system) :Flow control• Control loop #2 (CCPP steam system) : Pressure & t

emperature control

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Automatic controls - Elements

Name and symbolController output not taken as input for

sensing resulting action, e.g. signal produced for adjusting valve stem. As a result of this action what is the position of the valve stem?

Sense valve stem position with a position sensor.

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Automatic controls

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