"So take a good look at my face...
If you look closer, it's easy to trace..."

("The Tracks of My Tears," by Smokey Robinson et.al, 1965)

ANALYZING A P&ID

Reading and analyzing a P&ID is like doing forensic CSI work: There's a lot of little hints all over the place that collectively reveal what's going on.

The nearby P&ID excerpt features carbon filter vessels V200A and V200B.  Arrows have been drawn on the P&ID to show the six TIs (TI 3A through TI 3F) that indicate the temperature of the carbon beds that are packed in the two vessels.

This P&ID excerpt has been enlarged to show TI 2 on a process stream line. On the P&ID, TI 2 is below and to the right of V200B. A hand-drawn arrow shows where TI-2 is located on the P&ID.

Just like TI 3A through TI 3F, TI 2 was first drawn by making a circle.

Then the tag name (TI 2) was added into the circle.

Then a light connecting line was drawn from the circle to the process stream line to let the P&ID reader know where the real hardware could be found in the real process.

 

PTOA Readers and Students recently learned in PTOA Segment #6 the real hardware for an ISA TI symbol would look something like this picture on the left. The temperature sensing hardware is protected from the process fluid by the thermowell. The thermowell extends into the process piping.

Process Flow Lines are Dark
Connecting Lines are Drawn Fainter

PTOA Readers and Students should also notice that the process stream line drawn in the P&ID is darker than the lighter line that connects the TI circle to the process stream.

The darker lines on the P&ID represent process flow. The darker lines on the P&ID represent pipes that have the process fluid flowing through them.

Lines that connect instruments to the process or other hardware (like vessels) are drawn with lighter, more faint, lines.

Arrowheads Point Out Which Way
The Process Stream is Flowing

PTOA Readers and Students can determine which way the process flow is going by looking at the arrowheads on the P&ID.

The nearby P&ID excerpt shows a piece of hardware labelled P-3 Vacuum Pump is situated to the left of TI-2 (there is a hand-drawn arrow pointing at TI-2).

The arrowhead on the process stream line coming out of P-3 Vacuum Pump indicates that the flow out of this pump is sensed by TI-2 and then flows into a piece of hardware labelled V-220. The "V" means "Vessel."

PTOA Readers and Students can conclude that the purpose of TI 2 is to indicate the process temperature of the process stream before the flow goes into V-220.

There must be a good reason to know the process temperature before the process stream flows into V-220.

 

 

DIFFERENT P&ID,
DIFFERENT BUT SIMILAR SYMBOLISM

Recall that the ISA nomenclature used in P&IDs may differ slightly between processing facilities.

The nearby P&ID excerpt is from a different P&ID.  This P&ID features two pieces of hardware that are labelled E-101A and E-101B (sneak preview: the "E" stands for "Exchanger").

Analyzing the flow through E-101A and E-101B will be covered in a later PTOA segment.

For now, PTOA Readers and Students should notice all the TIs around E-101A and E-101B  … hand-drawn arrows indicate where the TIs are located.

The four TIs that appear on the P&ID around E-101A have the tag names TI 384C, TI 384D, TI 385A, and TI 385B.

The four TIs that appear on the P&ID around E-101B have the tag names TI 384A, TI 384B, and TI 385 C.

Several TIs clustered around a piece of equipment can mean only one thing and lead up to another P&ID analysis conclusion:

These things called E-101A and E101B must have some important role related to process temperature at that point in the process.

But what are the other circles with tag names that begin with "TW" that touch the TI ISA symbols?

And why is the connecting line drawn from the TW circle instead of the TI circle?

"TW" means "Thermowell."

The Exchanger P&ID is more detailed than above Carbon Filter P&ID.

In the Exchanger P&ID each TI is paired with a TW. This P&ID goes through the detail of accurately showing that the TW is the actual piece of hardware that has an interface with the process fluid.

The two excerpts from real P&IDs illustrate how there can be slightly different symbolism used in process design schematics. In both cases, the real TI hardware looks like the TI and thermowell shown nearby.

Take Home  Messages: The dark lines on P&IDs illustrate the flow of the process fluid through hardware.  The dark lines represent the pipes through which the process fluids flow in a real plant.

Arrowheads on the P&ID illustrate which way the process fluid/stream is flowing. 

Lighter lines are used to connect ISA instrument symbols...like TIs and TWs...to the process lines and pieces of equipment that are needed to convert raw materials into final products. 

The placement of hardware like TIs is intentional and gives the P&ID reader a hint as to what the purpose of the nearby equipment might be. 

Symbols used in P&IDs can slightly differ.

©2015 PTOA Segment 00007
Process Industry Schematics

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