E. Osakue

1

BASIC INSTRUMENT SYMBOLS Introduction Instruments are critical components in piping systems because they are needed to ensure smooth flow of fluid and maintain desired production levels. They are used to monitor and control parameters like flow rate, pressure and temperature. They sense, transmit, indicate, record or control these parameters. An instrument must detect a change in a parameter like temperature, pressure or flow to be useful. This change is converted into a signal like current or voltage that is calibrated for interpretation. Often there is the need to change a signal from one energy form to another. For example, a current signal may have to be converted to a pressure signal or verse versa. A transducer is used for this application. Fig. 1 shows some instruments in a pipe run. Several instruments may be mounted on a pipe branch. Instruments shown include flow valves, pressure valves and gauges, thermometers, level indicators, etc.

Fig. 1 Pipe run instrumentation In instrumentation diagrams standard symbols are used. Complete industry standard for instruments symbols and lettering abbreviations are in the controlled document ANSI/ISA-5.1. Common Process Variables The common process variables are:

• Analysis (A): Analyzer function

• Flow (F): Volume flow rate measure

• Level (L): Capacity measure

• Pressure (P): Operating pressure

• Temperature (T): Operating temperature Instrument Types and Symbols The common types of instruments in piping processes are:

• Gauge (G): Measures level or value of a variable

• Alarm (A): Signals (light or sound) when variable value is out of range

• Indicator (I): Indicates level or value of a variable

• Recorder (R): Records level or value of a variable

• Controller (C): Uses variable signal level to control instrument

E. Osakue

2

Perhaps valves are the most common type of instruments in piping systems. They are used to isolate equipment and piping devices, regulate flow, prevent backflow, regulate and relieve pressure. Popular valves are gate, globe and control valves. Knife valves are similar to gate valves but occupy smaller space, weigh less and cost less. The five process parameters of flow, level, pressure, temperature, and analysis have special instruments that are designed to sense and monitor them. The sensing instruments or sensors are called primary elements such as flow element (FE), pressure element (PE), temperature element (TE), level element (LE), and analyzer or analysis element (AE). Gauges measure the pressure, temperature or liquid level in equipment. Indicators display the level of process parameters. Recorders record the level of process parameters while controllers are used to activate other devices so as to maintain a specific level of a process parameter. Alarms produce light or sound signals when operating levels of process parameters are too high or too low. They are also activated when there is no flow or a back flow occurs. Also instrument indicators, such as pressure (PI), temperature (TI), volume flow (FI), and volume level (LI) are very vital for operating piping systems. They may be mounted directly on the pipe branch or on a central panel.

Fig. 2a Instrument symbols

Fig. 2b Instrument symbols

E. Osakue

3

Fig. 2c Instrument symbols

Process flow diagrams and piping drawings show instruments using symbols. Instrument symbols are combinations of balloons, lines, letters, and numbers. Balloons are basic shapes such as circles, pentagons, etc. used to enclose the letters and numbers of instrument symbols. The circle is the most popular balloon. A single circle indicates a stand-alone instrument. Touching balloons mean the instruments share a common housing. Other shapes often modify the circle. Balloons indicate a) location, b) accessibility, c) general characteristics such as shared or discrete mounting, or computerized. The American National Standards Institute (ANSI) and The Instrument Society of America publish the complete industry standard for instrument symbols and lettering abbreviations in the controlled document ANSi/ISA-S5.1 and ISO 14617-6. Fig. 3 shows common balloons associated with instrument symbols.

Fig. 3 Instrument and mounting

E. Osakue

4

Referring to Fig. 3, a symbol of a circle inside a square describes an instrument that performs more than one function such as shared display or shared control. A computer function is represented by a hexagon and a programmable logic controller is represented by a diamond inside a square. Instruments may be mounted locally on the pipes and equipments in the field or on panels in control rooms. Locally mounted instruments are connected most often with small-diameter fittings on instrument lines. They must be placed where they can be seen and read easily. In the control rooms, panels, dashboards and consoles provide locations for access to the instruments. The console, panel, or board mounted instruments receive signals from the field instruments comprising mainly of sensors and transmitters. Panel or board mounted instruments have a horizontal line at the middle of the balloon symbol. If the horizontal line is of the broken or dashed type, the instrument is mounted behind at the panel or board. The letters in a symbol appear at the top of the balloon and identify the type and function(s) of the instrument. The letters must be interpreted in order, beginning with the first. The first letter identifies the process variable such as flow, pressure, analysis, etc. The second letter identifies the primary function of the instrument while subsequent letters can indicate other functions or are primary function qualifiers. Table 1 below gives some instrument function abbreviations.

Letter First Order Use Succeeding Order Use

A Analysis Alarm

B Burner, Flame

C User choice Controller

D User choice Differential

E Voltage Element

F Flow

G User Choice Gauge

H Hand High

I Current Indicator

J Power

K Time Control station

L Level Low

P Pressure Undefined

Q Quantity Totalize, Quality

R Radiation Recorder

S Speed, Frequency Safety, Switch

T Temperature Transmitter

U Multivariable Multifunction

V Vibration Valve, Viscosity

W Weight Well

Y Event, State Solenoid, Coil, Compute

Z Position

Table 1: Instrument function abbreviation The numbers inside a balloon are located at the bottom and identify the loop or tag number for the instrument. A loop number groups all instruments monitoring and controlling a process variable into one control unit. Equipment may have several loop numbers associated with it if more than one process variables must be controlled for proper functioning. Loop numbers are normally assigned by designers or engineers. Fig. 4 shows an example of an instrument symbol.

Fig. 4: Instrument symbol

Referring to Fig. 4; the letters are FE and the numbers are 150. FE……. Flow Element (Function abbreviation) 150…… Loop reference number

E. Osakue

5

The instrument symbol is linked to a device by a connection line of the appropriate signal type. The line represents a physical signal link that could be air (pneumatic), liquid (hydraulic), current, or voltage. Instrument pipes are normally smaller in size than the process lines. Instrument pipes may be connected with a coupling to the process lines. Common instrument pipe sizes are 0.5”, 0.75”, and 1.0” nominal diameters. The control of process variable is done after a comparison is made between actual values and desired values. A difference in these values necessitates the need to effect a change and a control signal is transmitted to a final control device, usually a valve, in the control loop. The control signal is used to activate the final control devices that effect changes in the process conditions. In manual control, the final control device is physically adjusted by somebody. In automatic control, the final control device is activated by means of electrical, pneumatic, hydraulic, or some other mechanisms without human intervention. The figures below show how instruments are represented in flow diagrams. Control loops have two or more instruments connected to ensure the monitoring and control of a process variable. Control loops are numbered for documentation and reference purposes.

Fig. 5 Temperature indicator Fig. 6 Flow indicator (Orifice meter)

Fig. 7 Pressure indicator Fig. 8 Pressure recorder

Fig. 9 Instrument mounting symbols