You ain't gonna rock and roll no more
Yakety yak
(Don't talk back!)

("Yakety Yak," by Lieber and Stoller of the Coasters, 1958)

 

HOW ISA TRANSMISSION SIGNALS APPEAR
ON A MODERN P&ID

The previous PTOA segment covered the various types of signal transmission used in automated process control. The ISA P&ID symbol for each transmission type was matched with real-world applications.

This PTOA segment illustrates how some of the signals are used on a P&ID.

PTOA Readers and Students will eventually learn how to interpret the above P&ID so do not fret about that now.

PTOA Readers and Students will eventually recognize that this graphic depicts the Level and Temperature control plan for the bottom of a random tower. The tower is too tall to be completely drawn on the P&ID excerpt; the rest of it is implied to exist above the zig-zaggy lines.

For now just observe how the components of the control system are connected.

 

Data Link Signals on P&IDs

The left side of P&ID graphic shows data linked signals as lines with evenly spaced clear circles.

The ISA symbols on either side of the data link lines represent process control components in a DCS system.

The two components are "talking to each other" via data links; the components are yakety yakking with a whole bunch of 0s and 1s.

The arrows on the data link lines help the P&ID reader know how the control loop works. Without the help of the arrows on data linked signal lines, it would be more difficult to figure out the control loop plan.

PTOA Readers and Student will learn more about the fundamentals of automatic process control in later PTOA segments.

 

Electronic Signals on P&IDs. 

All the dashed lines that appear between components in the graphics represent electrical signals.

The electrical signals are between the range of 4-20 milliAmps for reasons that will be explained in future PTOA segments.

In the real world, the electrical signals between the control loop components in the drawing would be connected via wires that are in conduit.

 

 

A First Exposure to Level Control

The right side of the graphic shows the level control plan for the tower.

Like a bathtub or a kitchen sink, the tower in the picture is a container that holds liquids.

All containers that hold liquids have the chance of overflowing.  For this reason, all containers that hold liquids in a processing facility have a level control plan to make certain that they do not overflow.

Level Control Loop 900

No crime has been committed but it is time for PTOA Readers and Students to do some forensic work by collecting hints from the P&ID that reveal the level control scheme for the tower in the graphic.

At the right side of the graphic below, find LT 900 (Level Transmitter 900), a local instrument out in the processing area that appears to be physically attached to the bottom of the Tower by an upper "leg" and a lower "leg."

LT 900 is transmitting an electrical signal to LIC 900 (Level Indicating Controller 900).

How do PTOA Readers and Students know that the signal type transmitted from LT 900 to LIC 900 is electrical?

PTOA Readers and Students that are confused by the inquiry need to review the previous PTOA Segment.

The symbol for LIC 900 has a line drawn through the horizontal diagonal. Is LIC 900 out in the processing area or in the control room?

PTOA Readers and Students that are confused by the inquiry need to review how ISA Symbols are Tell-Tale Signs (PTOA Segment #11).

Returning attention to the P&ID exerpt, the output signal from LIC 900 is an electrical signal sent to what looks like a "mushroom cap." The mushroom cap is part of Level Valve 900 (LV 900, which is unfortunately not tagged in the graphic).

The ISA valve icon (in particular, that mushroom cap looking thing) indicates that LV 900 is a pneumatically operated valve; in the Real World LV 900 would look like one of the blue pneumatic valves shown to the right.

How can an electrical signal of 4-20 mA be sent to a pneumatically operated valve that uses air pressure signals between 3 and 15 psi.

The answer is: not possible! No can do!

As shown in the graphic, there's no way for yakety yakking to take place between the output of LIC 900 and the input signal to (untagged) LV 900.

The ISA symbol representing Level Transducer 900 (LY 900) is missing from the graphic. The function of missing LY 900 would be to  convert the electrical signal from LIC 900 into a pneumatic signal that (untagged) LV 900 would understand.

ISA decided to use the letter "Y" for hardware components that transduce one signal type to another because the letter "T" was already spoken for by hardware components that were "transmitters."

Without the control loop component LY 900, the entire control plan for controlling the liquid level at the bottom of the tower could not work.

Such careless P&ID symbol omissions are common but nevertheless frustrating.

It's a similar frustration to finally, unequivocably understand the offsides rule in futbol... and then be aware that the sideline Assistant Ref doesn't know the rules well enough to call it.

 

Take Home Messages: After reading this PTOA segment, PTOA Readers and Students now have a visual concept of how data link and electrical signals appear on a P&ID. 

ISA Data Link signals are 0s and 1s in the real world and are represented on P&IDs as clear circles at spaced intervals on a line.  The arrows on data link signal lines help the P&ID reader interpret the control loop plan.

ISA Electrical Signals are 4-20 mA in the real world and are represented on P&IDs as dashed lines. 

Conversion between signal types electrical and pneumatic signals requires control loop components called Transducers. The ISA letter for a Converter or Transducer is a "Y."

PTOA Readers and Students will learn when to infer process automation equipment exists in the real world even when not shown on P&IDs by using all the other hints on the graphic. 

 

©2015 PTOA Segment 00015
Process Industry Schematics

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