You sending me signals, I send them back.
Don't take a rocket scientist to see that you
Sending me signals.
I send them back to you.
I send them back.

("The Signal," by Madcon, 2013)

 

ISA SYMBOLS FOR PIPING
AND HARDWARE CONNECTIONS

The most recent PTOA Segments kept reiterating that automatic process control requires the transmission of signals between the processing area and the Control Room and/or the various cabinets and components of a DCS system.

What are these mysterious signals?

And how does the P&ID reader differentiate these signals when reading a P&ID?

The nearby graphic entitled "Piping and Connection Symbols" illustrates the ISA symbols that reveal what type of signal is sent between the components of an Automatic Control Loop.

Working from the top of the schematic downward, PTOA Readers and Students already know that:

A dark line represents piping through which the process streams flow. The process stream could be gas, liquid, or a mixture of phases.

A more lightly drawn line is:

• a minor process line that supports the process or
• a line that connects a piece of hardware to the process line or piece of processing equipment.

 

ISA SYMBOLS FOR SIGNAL TRANSMISSION 

The mid-through-bottom part of the graphic illustrates how ISA decided to differentiate each type of signal transmission on a P&ID.

In order of appearance on the graphic, PTOA Readers and Students will learn how each ISA signal symbol translates into the real world of process industry.

Electrical lines are drawn as dashed lines.

Dashed lines are used to represent current (4-20 milliamps) or volts (1 to 5 volts usually).

Unless otherwise stated, PTOA Readers and Students can assume the dashed lines are transmitting an electrical signal in the range of 4-20 mA (milliamps).

Some PTOA Readers and Students may wonder why ISA uses the term "electrical signal" when "electronic signal" may seem more appropriate for a 4-20 mA signal.

Rest assured that the term "electrical signal" is a more accurate signal description that keeps Industrial Process Instrumentation Technicians, like the one in the nearby photo, happy.

In the processing facility, electrical signals are delivered via wires that are enclosed in a conduit (or nowadays by armored cable where applicable).

The following definition of "conduit" comes from the PTOA's Department of Redundancy Department:

A conduit is what pipes that contain electrical wires are called.

Each wire is wrapped in a certain way to protect against electro-magnetic interference from other wires that are enclosed within the same conduit and other sources. High voltage conduit is separated from low voltage conduit.

But where does the electricity power source originate?

Electricity can be purchased from the local utility provider or generated on site. Electrical generation and distribution are featured in future PTOA Segments.

PTOA Readers and Students just need to be aware that automatic control systems that use electricity assume that no interruption in electricity utility service will occur. Therefore, most processing facilities have backup electricity generators.

 

Pneumatic lines (aka, typically air pressure lines) are drawn as a line with slanting double hash marks drawn over it at spaced intervals.

A pneumatic air pressure signal is between 3 and 15 psi (psi means "pounds of pressure per square inch area).

In the actual processing plant, pneumatic signals are delivered through air supply lines called Headers. "Header" is just the name given to a pipe that is involved with the distribution of a utility throughout several areas of a processing facility.

Where does the air for the pneumatic instruments come from?

An Instrument Air Compressor is located in the utility area of plant and discharges Instrument Air into the Instrument Air Header.

An Instrument Air Compressor is a little upgrading unit all by itself. "Ambient Air" ... the same air that PTOA Readers and Students breathe... is the feedstock into the Air Compressor. Clean and dry pressurized air is discharged into the Instrument Air Header at 80-150 psi.

The most prevalent pneumatic instruments are pneumatically operated control valves that receive a signal to open or close more...or maybe just hold the same position.

The nearby photo features a row of pneumatic control valves. PTOA Readers and Students will learn how pneumatic valves work in later PTOA segments.

The black piece of hardware at the leftmost of each of the big blue control valves is an air regulator that reduces the air pressure from the header to approximately 30 psi/206 kPa.

The silver boxes with 3 dials are Valve Positioners that manage the air used to make the valve adjust its movement as commanded.  The silver tube delivers air to the part of the valve that actuates the valve movement.

 

Data link lines are represented as small clear circles spaced at regular intervals in a line.

Data link means digitized data (0 and 1s) is the communication media.

In digitized systems, PTOA Readers and Students are likely to hear Process Operators and Control Room Operators describing that the various DCS components "talk to each other."

Electricity utility is required to support digitized data.

The electrical wires needed to link between the various DCS field modules and the Control Room will be enclosed in a conduit.

Alternatively, DCS systems that have been upgraded into their own Process Control Networks (PCNs) link the various control components via ethernet cables.

 

Capillaries are small tubes filled with liquid.

The ISA symbol for a capillary system is a line drawn with Xs over it at spaced intervals.

The data link symbol discussed above is one of the newest transmission signals and capillary signal transmission is one of the oldest.

The most predominant use of capillary sensors PTOA Readers and Students will run into is their application in the old trustworthy pneumatic Foxboro 43 AP local controller.

PTOA Readers and Students will recall that the Foxboro 43 AP is a local indicating controller that can be modified to be a Temperature Indicating Controller (TIC) by incorporating a liquid filled capillary probe as a temperature sensor.

The connection to the capillary probe is dangling from this Foxboro 43 AP on the bottom left side of the instrument.

On a P&ID, this connection line would be shown as a line with Xs.

 

The ISA symbol for a hydraulic signal are drawn as "L"s spaced on a line.

Hydraulic pressure is pressure created by the height of a column of liquid.

Hydraulic pressure is a subject covered in future PTOA Segments.

Hydraulic-actuated valves are used in control designs when a lot of force is needed to move valves.

The below schematic shows a typical well head valve (which has the nickname of "Christmas Tree Valve"). A well head is the valve that interfaces with oil and gas extraction above and below the Earth's surface.

Surface Safety Valves on well head systems use hydraulically-actuated valves.

If the below schematic were drawn as a P&ID, all those lines shown with black arrows would be replaced with ISA symbols of spaced "Ls."

 

The ISA symbol for electromagnetic and sonic signal transmission is a line with sinusoidal waves drawn around it at spaced intervals.

Electromagnetic Signal transmission is based upon use of Faraday's "Law of Electromagnetic Induction." Electromagnetic Induction is more thoroughly featured in the upcoming PTOA PV Pressure Drivers/Prime Movers Focus Study.

 

 

Take Home Messages: PTOA Readers and Students now have a visual concept of what the ISA symbols for signal transmission that appear on P&IDs mean in the real world of process technology. 

ISA Pneumatic Air Signals are 3-15 psi in real life and represented on a P&ID as double slanted hash marks marked on a line at spaced intervals.

Instrument Air is distributed throughout the plant via the Instrument Air Header.

The ISA electrical signal on a P&ID is dashed lines. PTOA Readers and Students can assume the ISA electrical signal is 4-20 mA (milliamp) unless otherwise instructed the signal is a voltage. 

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.

Electricity utility is needed to support both electric and data signal transmission. Conduits (or sometimes cables) are used to connect electricity-based control components to each other. 

 

©2015 PTOA Segment 00014
Process Industry Schematics

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