10.Lecture

7/28/2019 10.Lecture

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P&IDs

Notation, Construction, &

Interpretation

By Peter Woolf University of Michigan

Michigan Chemical Process

Dynamics and ControlsOpen Textbook

version 1.0

Creative commons

7/28/2019 10.Lecture


Piping and Instrumentation

Diagrams (P&IDs)What it is not:

• Not an architectural diagram of a process.

Positions in a P&ID do not correspond to a3D position, but more a connectivity.

• Not to scale

• Not a diagram of the reaction kinetics

• Not a control diagram (block diagram),influence graph, incidence graph, Bayesiannetwork, or correlation network.

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Piping and Instrumentation

Diagrams (P&IDs)What it is:

• Shows relative location of process equipment,

sensors, actuators in a process

• Conceptual outline of a chemical plant

• Provide common language for discussing a plant

• Show control connections between sensors andactuators

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This P&ID does not imply:

• Supply and drain are at

the same elevation.

•The tank is 3x larger thanthe valve

• Pressure relief is on the

upper left side of the tank.

• V1 is within sight of S001

• Does not imply that all tanks are of the same size

• Does not imply impeller type or location in CSTR

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Example P&ID from design

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Example P&ID from design with control relationships

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Signal & Sensor Notation

Figures from http://controls.engin.umich.edu/

Common line notation.. with lots of exceptions!

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DT1

MA1

TC1

Examples:

LI1density transmitter 1

Moisture alarm 1 Level indicator 1

Temperature control 1


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TC1

Aside:It is not uncommon to see just _C as an integrated alarm, controller,

indicator and transmitter.Thus TC1 often, but not always implies it also senses and transmits. TT1

TI1

TC1

TA1

Can mean..

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More valve notation!


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More valve notation!


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Flow sensorsFC1

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Temperature SensorsTC1


Thermocouple schematic

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Example Problem:

The output of a chromel-alumel thermocouple is used to regulate

the temperature of a feed stream. When writing your control

program for this regulator, you refer directly to the EMF of thethermocouple instead of temperature. You know that the stream

has a temperature set point of 117°C, so what is the EMF value

you should set your controller set point?

We can extrapolate

to a temperature of

117 to get an EMF of

4.79 mV.

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Know Your Control Ranges


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Composition Sensors

Use composition sensorssparingly, as they are(1) specialized: not every

composition can bemeasured easily

(2) Expensive

(3) Often slow (4) High maintenance

Often you can infer composition more easily

from physical properties(e.g. temperature in adistillation column or conductivity of asolution)

AC1


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Composition Sensors

Use composition sensorssparingly, as they are(1) specialized: not every

composition can bemeasured easily

(2) Expensive

(3) Often slow (4) High maintenance

Often you can infer composition more easily

from physical properties(e.g. temperature in adistillation column or conductivity of asolution)

CC1


Polagraphic sensor

Photometer

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Process Equipment


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What is this? What is going on?


Reactor or heat exchanger Temperature controls pressure, controls valve(example of cascade control)

Notes:(1) Steam isgenerally controlled at the inlet, not outlet (steam traps)

(2) Cascading T tosteam pressureassumes steam pressure variessignificantly

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What is this? What is going on?

CSTR

Questions:(1) What do the flow controllers do?(2) How does the exit flow influence the temperature? Answer: This is a batch process.Moral: A P&ID alone only tells part of the story..

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P&ID Pitfalls


GOOD: Isolate equipment with valves to allow repair.

BAD: Surround equipment with control valves that will compete

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P&ID Pitfalls


GOOD: Place control valves downstream of pumps to prevent starving the pump. (May also have a recycle to

relieve pressure)

BAD: Place control valve upstream of pumps. Will starvethe pump, causing damage to pump and wear on parts.

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GOOD: Operate agitator when the tank hassufficient liquid in it

BAD: Start agitator beforeblade is immersed in thefluid

Note: This may not be apparent from the P&ID, but does affect how you operate your system. Fill tank THEN turn on agitator, not the other way around!

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Name that design flaw!

Safety valves

Valve before pump

Where dothese go?

Other possible issues:(1) Is pressure if E-1 the best metric, or might you also

need temp?(2) How can you drain E-1 if liquid remains?(3) Should V7 be a control valve to control the pressure?

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Drawing P&IDsMichigan P&ID templates can be used on:

• Visio (PC)

• OmniGraffle (Mac)

(templates for both are on the wiki under

supplementary information for lecture

10)

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Example:Given a schematic of a

process do the following:(1) Redraw the process asa formal P&ID using thetemplate(2) Add valves with proper

annotation(3) Add sensors with proper annotation(4) Show valve/sensor connections

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1) Redrawn figure

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2) Valves added and numbered

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2) Valves added and numbered

Why not?

Redundant

Valvesafter pumps

CW after exchanger

Valves after pumps

Steam feed controlled,not output

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3) Sensors added and numbered

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3) Sensors added and numbered

Why not?

PC TC

PC

PC

TC FC

FC

LC

AC

LC PC TC FC

redundant

Slow, $$

Might have one,

but might not care

Wrong

redundant

Don’t care, can’t changeredundant

Can’t change

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FC1: V1, V2, M1FC2 : V1, V2, M2 LC1: V1, V8, V2, M1

LC2: V1, V2, M2

TC2: V7

4) Connect valves and sensors

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LC2: V1, V2, M2

TC2: V7

TC1: V5 PC1: V6, V7, V8 LC3: V1, V2, V3, SV1, M3, M4FC3: V3, M3

FC4: SV1


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LC2: V1, V2, M2

TC2: V7

TC1: V5 PC1: V6, V7, V8 LC3: V1, V2, V3, SV1, M3, M4FC3: V3, M3

FC4: SV1


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Challenge: A, B, and C react to form a product D and a flammablegas byproduct E. Thereactor temperature isincreased with steam and cooled by a cold water jacket. Mixing is achieved by an agitator and recirculation.

For this system(1) Annotate valves and motors

(2) Add and annotate sensors(3) Write out sensor valveconnections.

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Solution: (see figure)Note: may need to zoomin to the figure to read the annotation.

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Take home messages• P&IDs provide a conceptual framework

of your process and its control

architecture

• Only measure the values that you can

use and need

• Only control the things you have to

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