P&ID’s

PIPING & INSTRUMENTATION DIAGRAMS

       

                              

                              

Format & Layout Title Block

Sheet Number Revision Number Date

       

                              

                              

Format & Layout Layout Rules of Thumb

No Scale Flow From Left to Right across page Important Process Streams Heavy Dark Less Important (utilities, instrumentation

lines lightweight) Horizontal Lines ‘break’ vertical Lines Process Lines Break Instrumentation

Lines Pumps along bottom Tanks above pumps

       

                              

                              

P&ID’s - Symbology Equipment Symbology Instrumentation Symbology

See the document “P&ID Symbols and Photos.DOC”

       

                              

                              

P&ID’s - Equipment Keep in mind the development of the P&ID’s

is really developing the process The way it will start-up / shut-down and

normally operate Procedure

Start with the Simulation and Equipment List

Draw Equipment (including spares) Draw the major process Lines, then Utilities

       

                              

                              

P&ID’s - Safety - ALWAYS!!!!! ALWAYS Consider Safety!!

Think, what happens if the control valve doesn’t operate as intended?

What happens if the pump stops working? How will the operators troubleshoot the problem? What if the operators ignore an alarm ...

Consider the need for redundant controls and equipment, but only if deemed necessary.

Redundant things are twice as likely to fail!

       

                              

                              

P&ID’s - Simplicity Rules The easiest process to operate and understand

is the simplest one K.I.S.S. Add “indicators” (flow, pressure,

temperature) for trouble shooting Don’t add “controls” unless they are

absolutely necessary Only resort to complicated controls when

you have to... Consider the consequences of operators not understanding them.

       

                              

                              

P&ID’s - Rookie Mistakes Don’t put level control on Storage Tanks

They’re there to take up variations in flow, therefore the level is supposed to ‘float’

Let Pressures in the process ‘float’ as much as possible, try not to over control operating pressures.

Don’t try to control liquid flow at two points in a pipe - remember conservation of mass

Don’t look at one P&ID only - Look at the big picture then consider the details

       

                              

                              

P&ID’s - Rookie Mistakes Draw the equipment the way it’s going

to be built - don’t oversimplify it’s representation more equipment/inst detail is better

Show Liquid levels / internals etc. It helps you design the controls It helps operators understand how it

works

FV

FT

FIC

FE

Set@1200 kg/hr

       

                              

                              

Simplicity Watch Out For Controls on Different

Sheets

AA

L F

       

                              

                              

P&ID’s - Costs Are Important Goal: Lowest Capital costs and Lowest

Operating Costs. Only build what’s required The Cheapest Process that meets the

need of all involved is the most profitable.

It’s got to work! After that, make it cheap as possible.

       

                              

                              

P&ID’s - Control The Flow that occurs in a pipe is a

function of the Pressure Drop.

Valves add or subtract from the natural pressure drop of the pipe.

Valves change the pressure drop to give us the flow rate we want

       

                              

                              

P&ID’s Control (Globe) Valves can have just

about any pressure drop and flow combination

On the other hand, pipes have very definite flow/pressure relationship

50 gizillion psi 5 psi

20 m3/hr

2” pipe, 20 m long, 20 m3/hr P = 5 kPa

       

                              

                              

P&ID’s Piping Pressure Drop

Resulting Flow Through Pipe

Pressure Differential

“System Curve”for 4” pipe, length 100 ft

       

                              

                              

P&ID’s Piping Pressure Drop

Resulting Flow Through Pipe

Pressure Differential

“System Curve”for 4” pipe, length 100 ft

       

                              

                              

P&ID’s Piping Pressure Drop

Resulting Flow Through Pipe

Pressure Differential

“System Curve”for 4” pipe, length 100 ftWith a valve that’s50% closed

       

                              

                              

P&ID’s In a Liquid System (non compressible

flow) two valves in series have the same flow rate through them

DP = 3 bar DP = 20 bar

Press7 bar(g)

Assume each segment of pipe has a DP of 1 bar

       

                              

                              

P&ID’s Backing Up

Press7 bar(g)

?

?

       

                              

                              

P&ID’s Pumps (Centrifugal)

Flow Through Pump

Pressure Increase That Pump Is Able To Produce Pump Curve

System Curve

       

                              

                              

P&ID’s - Control Systems First ask what controls the production

rate of the process? How will we control this production

rate

Flow control

       

                              

                              

P&ID’s - Control Systems Mentally take note of other critical

things that need to be controlled (flow, temperature, pressure, level) Feed - do we need flow control ? Multiple feeds - do we need to ratio

one flow rate to another?

Flow of B depends on A

A

B

       

                              

                              

Ratio Control

FV

FT

FIC

FE

FV

FT

FRIC

FE

PrimaryRate

RatioToPrimary

       

                              

                              

Pressure To Decrease the Pressure of a Tank

allow material to be removed (at a faster rate)

condense vapour faster

PICPT

PV

PIC

PV

PT

Control Downstream Pressure Control Upstream Pressure

       

                              

                              

Temperature Usually you increase the flow or

pressure of one stream. The effect of that change is to cause another streams temperature to change.

       

                              

                              

Level To Decrease the Level of a Tank

allow material to be removed (at a faster rate)

Tanks and Pumps Always Provide a way of preventing your

pumps from running dry Always Provide a way of preventing your

pumps from being “dead headed” Recirc the pump discharge to the tank with a

“minimum flow bypass”

       

                              

                              

Controls Failure Position of Valves

Most of the time it’s safer to bottle up the process and prevent failure from propagating downstream by making control valves “fail closed”

Occasionally it’s better to fail open ie. Cooling water