10.lecture p&id

P&IDsNotation, Construction, &Interpretation

By Peter WoolfUniversity of Michigan

Michigan Chemical ProcessDynamics and ControlsOpen Textbook

version 1.0Creative commons

Piping and InstrumentationDiagrams (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.

Piping and InstrumentationDiagrams (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 and

actuators

This P&ID does not imply: Supply and drain are atthe same elevation.The tank is 3x larger thanthe valve Pressure relief is on theupper 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

Example P&ID from design

Example P&ID from design with control relationships

Signal & Sensor Notation

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

Common line notation.. with lots of exceptions!


DT1

MA1

TC1Examples:

LI1density transmitter 1

Moisture alarm 1 Level indicator 1

Temperature control 1



TC1

Aside:It is not uncommon to seejust _C as an integratedalarm, controller,indicator and transmitter.

Thus TC1 often, butnot always implies italso senses andtransmits. TT1

TI1

TC1

TA1

Can mean..

More valve notation!



Flow sensorsFC1

Temperature SensorsTC1


Thermocouple schematic

Example Problem:The output of a chromel-alumel thermocouple is used to regulatethe temperature of a feed stream. When writing your controlprogram for this regulator, you refer directly to the EMF of thethermocouple instead of temperature. You know that the streamhas a temperature set point of 117C, so what is the EMF valueyou should set your controller set point?

We can extrapolateto a temperature of117 to get an EMF of4.79 mV.

Know Your Control Ranges


Composition SensorsUse composition sensors

sparingly, as they are(1) specialized: not every

composition can bemeasured easily

(2) Expensive(3) Often slow(4) High maintenance

Often you can infercomposition more easilyfrom physical properties(e.g. temperature in adistillation column orconductivity of asolution)

AC1


Composition SensorsUse composition sensors

sparingly, as they are(1) specialized: not every

composition can bemeasured easily

(2) Expensive(3) Often slow(4) High maintenance

Often you can infercomposition more easilyfrom physical properties(e.g. temperature in adistillation column orconductivity of asolution)

CC1


Polagraphic sensor

Photometer

Process Equipment


What is this? What is going on?


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

Notes:(1) Steam isgenerally controlledat the inlet, notoutlet (steam traps)(2) Cascading T tosteam pressureassumes steampressure variessignificantly


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..

P&ID Pitfalls


GOOD: Isolate equipment with valves to allow repair.

BAD: Surround equipment with control valves that will compete

P&ID Pitfalls


GOOD: Place control valves downstream of pumps toprevent starving the pump. (May also have a recycle torelieve pressure)

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


GOOD: Operate agitatorwhen the tank hassufficient liquid in it

BAD: Start agitator beforeblade is immersed in thefluid

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


Name that design flaw!

Safety valves

Valve beforepump

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?

Example P&ID from design

A much better example

Drawing P&IDs

Michigan P&ID templates can be used on: Visio (PC) OmniGraffle (Mac)(templates for both are on the wiki under

supplementary information for lecture10)

Example:Given a schematic of aprocess do the following:(1) Redraw the process asa formal P&ID using thetemplate(2) Add valves with properannotation(3) Add sensors with properannotation(4) Show valve/sensorconnections

1) Redrawn figure

2) Valves added and numbered

2) Valves added and numbered

Why not?

Redundant

Valvesafterpumps

CW afterexchanger

Valves afterpumps

Steam feedcontrolled,not output

3) Sensors added and numbered

3) Sensors added and numbered

Why not?

PC TC

PC

PCTC

FC

FC

LC

AC

LCPCTCFC

redundant

Slow, $$

Might have one, but might not care

Wrong

redundant

Dont care, cant changeredundant

Cant change

FC1: V1, V2, M1FC2: V1, V2, M2LC1: V1, V8, V2, M1 LC2: V1, V2, M2TC2: V7

4) Connect valves and sensors

FC1: V1, V2, M1FC2: V1, V2, M2LC1: V1, V8, V2, M1 LC2: V1, V2, M2TC2: V7

TC1: V5PC1: V6, V7, V8LC3: V1, V2, V3, SV1, M3, M4FC3: V3, M3FC4: SV1

4) Connect valves and sensors

Challenge:A, B, and C react to form aproduct D and a flammablegas byproduct E. Thereactor temperature isincreased with steam andcooled by a cold waterjacket. Mixing is achievedby an agitator andrecirculation.

For this system(1) Annotate valves and motors(2) Add and annotate sensors(3) Write out sensor valveconnections.

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

Take home messages

P&IDs provide a conceptual frameworkof your process and its controlarchitecture

Only measure the values that you canuse and need

Only control the things you have to

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