1.1.
WHAT IS A P&ID?
1.2.
P&ID PIPING SYMBOLOGY LEGEND
1.3.
INSTRUMENTS AND EQUIPMENTS IN P&ID
1.3.1.
Local indicators in P&ID
1.3.2.
Transmitters in P&ID
1.3.3.
Switches in P&ID
1.3.4.
Valves in P&ID
1.3.5.
Other equipments in P&ID
1.4.
P&ID TYPICAL INSTRUMENT DRAWINGS
1.5.
P&ID TYPICAL VALVE DRAWINGS
1.6.
P&ID CODES
1.6.1.
P&ID instrument codes
1.6.2.
P&ID pipe codes
1.7.
P&ID CONTROL LOGICS
1.8.
P&ID SCOPE INDICATIONS
1.9.
P&ID TITLE BLOCK
1.1.
WHAT IS A P&ID?
My first instrumentation lesson and the first lesson that
I usually teach to new mates is "How
to read a P&ID".
Furthermore, explain a P&ID is a very common test in
a technical interviews related to industrial process control.
What is a P&ID?
"Pipe
And Instrumentation Diagram"
As its name suggests "P&ID" is a diagram
where It is represented pipes, instruments, valves, pumps and other
devices; It is indicated also nominal
diameters, rating, identification names, materials, fluids, control
logic...etc.
P&ID example:
The P&IDs are drawn by the process department and
supported by the other departments.
There is always an oficial copy, It is called "P&ID Master". The location of
this copy is usually with process department.
Sometimes instrument department has its official copy
where is indicated the instrument
department comments.
Periodically, each department will indicate theirs
changes and relevant annotations to the "P&ID Master".
Coordinate this action with an standard way and report to
every affected people on time, is one of the most important point that should
be managed in an engineering.
1.2.
P&ID PIPING SYMBOLOGY LEGEND
The first P&ID in a project is the symbology legend
sheet.
The symbology legend drawings are similar in each project
and according to ISA (See ISA-5.1).
If It were your first day working as instrumentation
engineer, I recommend you to study the project symbology legend sheet. It will
help you to interpret the P&ID drawings and understand the scope of the
project.
P&ID symbology legends examples:
1.3.
INSTRUMENTS AND EQUIPMENTS IN P&ID
As I said at the beginning, the instruments and
equipments are implemented in the P&ID, the following pictures are some
instrument representation examples:
1.3.1.
Local indicators in P&ID
The local indicators show us the measured values in the
equipments and process lines. The local indicators will be very useful for the
maintenance and the production staff.
Examples:
1.3.2.
Transmitters in P&ID
The transmitters send the measured values to the control
cabinets, allowing us to control and monitor the process. These devices can be
specified with a local indicatios.
Examples:
1.3.3.
Switches in P&ID
The switches send a digital signals when the measured
variable reaches a certain value.
The "switches" may trip for high or low value.
Examples:
1.3.4.
Valves in P&ID
The control valves and manual valves will be drawn in the
P&ID diagrams. These equipments are used to control the flow in the process
lines.
Examples:
1.3.5.
Other equipments in P&ID
P&ID example:
1.4.
P&ID TYPICAL INSTRUMENT DRAWINGS
The instruments usually have: the sensor (transducer) and
the transmitter.
Temperature transmitter example:
The sensor (transducer) interprets the measured variable
and converts it to other measurable energy.
For example a resistance temperature detector transducer
(PT100) converts the temperature in a resistance value. This value is not
usually sent directly to the control cabinet, instead it is converted in a
head-mounting transmitter. The transmitter sends the final signal (4-2mA HART
usually) to the control cabinet.
The head-mounting transmitter can be represented like the
following drawings:
Also, It can be indicated with the transducer or the
thermowell.
If the temperature transmitter isn’t head-mounting, the
transmitter can be installed in the cabinet or in a junction box.
The "flow transmitters" can be represented with
different drawings.
It can be indicated the transmitter or the transmitter
and the primary flow element.
Some examples:
1.5.
P&ID TYPICAL VALVE DRAWINGS
We are going to divide the valve types in five groups:
Valves and actuators can be represented with different drawings
like the instruments, for example:
With or without Positioned & Actuator:
With or without handwheel:
With the fail position:
With the status during normal operation:
1.6.
P&ID CODES
Every projects usually follow more or less, the same
coding guidelines thaht are supported by the ISA or other standard as the KKS.
If we define a correct coding criteria, we could save a
lot of time.
The codes apply to equipment, signals, alarms, valves,
trays, control and electrical cabinets, junction boxes, instruments, pipes,
cables ...etc.
1.6.1.
P&ID instrument codes
In the example, the “TAG”
(name) is "20-FT-1982-A".
This code will not be repeated for other plant devices.
-First part: “the
Unit”. It indicates two digits "20".
The instrument is installed in the unit “20”.
To split the project in different units is very common in Oil & Gas big
projects, however, it is less common in power plants.
Note:
(An instrument department manager told me one day) " The secret to solve a
complex problem is to know how to split the problem in smaller parts"
-Second part: "the
instrument type”. It indicates two
letters "FT". It is a flow
transmitter.
Other examples:
AT - Analyzer / FT - Flow transmitter / TT - Temperature
transmitter / LT - Level Transmitter
-Third part: “the
Loop Number”. It indicates "1982".
Each control loop is named with a sequential number. A “Loop” is a set of devices
with a control logic link.
Control Loop example:
-Fourth part: “Suffix”.
“A” indicates that there is a
redundant device at least, as the "A"
and "B" in the previous drawing.
Redundant equipments are defined in critical loops. If a
device is not working, the loop control will use the redundant one.
1.6.2.
P&ID pipe codes
-First part: “Size”.
It indicates "2" (nominal
diameter two inches).
-Second part: “Fluid
Type” It indicates "GI" (nitrogen).
-Third part: “Sequential
Number”. It indicates "45455".
-Fourth part: “Pipe
specification” It indicates "1S5".
We can find the code meanings in the Project document: "pipe
specification".
In these case: "1" -> Rating 150# / "S5" -> Pipe material
stainless steel.
1.7.
P&ID CONTROL LOGICS
In the following drawings is indicated a part of P&ID
control logics.
All control logics can not be detailed in a P&ID. There
are other specific documents, as functional descriptions or control strategy.
In general, the control logics are easier to draw in
Oil&Gas P&ID (where the control loops are more simples) than electrical power plant (where every control
loops are interconnected)
In the picture below: "There is a flow transmitter
that sends a hardwired signal to the control system and the control system
monitors the measured value"
In the picture below: "There is a flow transmitter
that sends a hardwired signal to the control system and the control system
monitors the value and it will generate two alarms, if there are low flow value
or high flow value "(The alarms could be represented by a hexagon too)
In the picture below: "There is a flow transmitter
and a control valve whose signals are wired to the control system. The control
system monitors the flow rate and use
the process value in a internal PID that controls the valve position.
In the picture
below: "There is an off valve. It is wired to the control system. The
valve can be opened or closed by a plant operator through a "HS"
(Hand Switch).
A "HS" is a configurated button in the SCADA
screen.
In the picture below: "There is a pressure gauge, a
motor-pump and an on-off valve. If the measured pressure is below a fixed
value, an alarm is generated by the control system and it will active an "Interlock
signal". The interlock will stop the pump, and it will close the valve.
1.8.
P&ID SCOPE INDICATIONS
The scope of “black-box suppliers” should be represented
in the P&ID.
“Black-Box” is a devices group installed and supplied by
the same vendor.
If you are designing a plant you should coordinate that “black-boxes”
limits fit with the rest of the devices.
1.9.
P&ID TITLE BLOCK
P&IDs must have a standard title-box as other project
oficial drawings.
Note: It has been
tried to collect into this report some basic concepts to help anyone who begins
to walk on the instrumentation way. It is always an honor to teach someone who
want to learn.
In this article, I would like to thank those
professionals who have repeatedly explained us and will continue to explain you
all over again what the instrumentation and control is. As conclusión, I would
like to give you two messages:
1º- For those who have carried on working as engineers
for several years, I honestly belive that part of your duty is to maintain the
excitement to learn in the young people (even prioritizing this vs. a project).
2º- For the beginners, don’t let anyone to take your
hunger to learn and improve.
Julio.C.F.L 28/04/2015
