Sign, sign...Everywhere a sign.
Blockin' out the scenery, breakin' my mind.
Do This! Don't Do That! Can't you read the sign?

("Signs," by Les Emmerson
sung by the Five Man Electrical Band,  1970-71)

 

ISA NOMENCLATURE REVEALS
WHERE THE INSTRUMENT HARDWARE CAN BE FOUND

The ISA realized that P&ID symbols needed to evolve so that they could alert P&ID readers which instruments were in the processing area and which were located in a control panel of a control room.

The ISA had already established that hardware located in the processing area would be drawn with tag names written into clear circles. These instruments are also sometimes referred to as "field mounted" because they are located "out in the field" somewhere in the processing plant area.

In the above graphic, all of the ISA symbols with clear circles  indicate hardware that would be located in the processing area.

On a P&ID the tag names would be completed; the unique tag number assigned to the instrument would be written near or perhaps below the letters of the tag name.

Simple Heat Exchanger P&ID
shown with Local Temperature Controller

How do P&ID readers know that TC 1 (Temperature Controller #1) is located in the processing area?

The graphic to the left is a simplified P&ID that shows flow through a piece of temperature-changing equipment called a shell-and-tube heat exchanger.

PTOA Readers and Students will eventually understand the graphic completely so do not fret about that now. Shell-and-tube heat exchangers are the subject of future PTOA segments.

PTOA Readers and Students should recognize that the simplified P&ID indicates that TC1 (Temperature Controller 1) is  located "in the field" near the processing plant.

How do PTOA Readers and Students know that TC1 is in the processing area?

ISA Nomenclature
for Control Room Instruments

The ISA decided that when hardware (like a controller or a recorder or an annunciator panel) was located away from the processing area...for example, in a control panel of a control room... the ISA symbol would be slightly altered as follows:

• A circle is drawn with a definite line across the horizontal diameter.
• The letters of the tag name are written above the diameter line.  Just like the local instrument tag names, the letters of control room instruments would indicate which process variable the instrument is dedicated to (T=Temperature, P=Pressure, F=Flowrate, or L=Level) followed by letters that defined the function and type of instrument.
• The tag number of the instrument is written below the diameter line.
• Just like with local instruments, a faint line would be drawn from the circle to the place where the control room instrument was located...like on a process flow line or on a piece of equipment.

The graphic at the right illustrates that Flow Indicating Controller 55 (FIC 55) is located in a control room.

PTOA Readers and Students understand that FIC 55 is in the control room because a dark line is drawn across the horizontal diameter of the circle.

Would the controller hardware be just randomly hanging around and hard to find in the control room? Heck no!

The symbol infers that FIC 55 would logically be found in a control panel like the ones shown in the previous PTOA segment and like the control room in the photo below. Another way of describing the whereabouts of the instrument is to say it is "board-mounted" because it will be found on a control board panel.

 

What other Instruments on the List
Could Be Found in a Control Room?

The graphic at the very top of this page is repeated below for easy reference.

As previously mentioned, the ISA nomenclature reveals that FIC 105 is board-mounted in a control panel of a control room.

The ISA symbols also reveal that Pressure Indicating Controller 105 (PIC 105) and Level Transmitter 65 (LT 65) are also board mounted in the control panel of a control room.

The ISA nomenclature reveals that seven other instruments on the list below are located in the control room. Which instruments are they?

Simple Heat Exchanger P&ID
shown with Controllers Mounted
in a Control Board Panel

The below graphic is a simple P&ID of another heat exchanger. PTOA Readers and Students will eventually know how to interpret everything on the drawing so don't fret about that now.

For now PTOA Readers and Students just need to recognize that the ISA symbols reveal that FC 1 and FC 2 (Flow Controller 1 and Flow Controller 2) are located in the control room like the one in the above graphic.

How do PTOA Readers and Students know the FC 1  and FC 2 are located in the control room?

What other instrument represented as an ISA symbol in the drawing can be found in a control room? What process variable is this instrument concerned with and what is the function of the instrument?

Hint: T is for Temperature!

Take Home Messages: The ISA created P&ID symbols that informed the P&ID reader where the hardware instruments could be found.  The tag names of local instruments are drawn in clear circles but instruments that are located in a control board panel of a control room have a line drawn at the horizontal diameter. For instruments located in the control room, the letters of the tag name are drawn above the horizontal line and the number of the instrument is drawn below the horizontal line. 

Both local and control room P&ID symbols use the same conventions for the letters of the tag name: the first letter represents the process variable (T for Temperature, P for Pressure, F for Flowrate or L for Level) and the second and third letters indicate the function of the instrument and what kind of instrument it is.  For example, the ISA tag name FIC 55 is translated as "Flow Indicating Controller #55."

©2015 PTOA Segment 00011
Process Industry Schematics

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