ARMACO STANDARD

Engineering Encyclopedia
Saudi Aramco DeskTop Standards

Drafting Instrument Loop Diagrams

Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s
employees. Any material contained in this document which is not
already in the public domain may not be copied, reproduced, sold, given,
or disclosed to third parties, or otherwise used in whole, or in part,
without the written permission of the Vice President, Engineering
Services, Saudi Aramco.

Chapter : Drafting For additional information on this subject, contact
File Reference: AGE10803 N. H. Alahaimer on 874-0876

Engineering Encyclopedia Drafting

CONTENTS PAGES

Information

Instrument Loop Diagram 1
Ild Symbols And Abbreviations 1
Interpreting Ilds 29
Interpret An Ild For A Pneumatic Instrument Control Loop 37
Interpret An Ild For An Electronic Instrument Control Loop 44
Tracing Current Flow In Control Loops 57
Computer Relays 59
Computer Relay Symbols 59
Instrument Systems 67
Foxboro Spec 200 67
The Honeywell Vutronik Control Loop 83
The Honeywell Vutronik Alarm Card 96
Examples Of Honeywell Cards 100
Honeywell Resistance To Current Converter Card 102

Work Aids 104

Glossary 119

Saudi Aramco DeskTop Standards


Instrument Loop Diagram

Every process control loop has an instrument loop diagram (ILD) drawn for it. ILDs give
more information about control loops than any other drawing. Although they are of interest
mainly to instrumentation engineers and tech-nicians they are one of the most common
drawings seen in Saudi Aramco.

ILD Symbols And Abbreviations

Handout No. 1 (Drawing No. 990-J-36492 Sheets 1 and 2) shows common ILD symbols and
abbreviations. Some of them will be described in detail in this module.

Orifice Plate. Figure 1 shows the symbol for a flow element orifice plate. Figure 2 shows an
orifice plate.

H L
LINE NUMBER

FLOW ELEMENT
ORIFICE PLATE
MARK NO.
BORE

CORRECT SIZE
NUMBER MUST MATCH
BORE SHOWN ON ILD

1.550
ORIFICE PLATE

OW
FL
CORRECT DIRECTION
NUMBER ON PLATE
MUST FACE UPSTREAM

FIGURE I. FLOW ELEMENT, ORIFICE PLATE

Saudi Aramco DeskTop Standards 1


ILD Symbols and Abbreviations(Cont'd)

Orifice Plate(Cont'd). An orifice plate is placed into a pipeline to cause a pressure differential
between its upstream and downstream flow. H and L stand for High and Low. They indicate
the high and low pressure sides of the plate. The difference in pressure is used to indicate
flow rate. Pressure differential varies as the square of the flow rate. Therefore, the square
root of the pressure differential reading is needed in order to obtain the linear value of the
flow rate.

Process variable measuring devices, such as orifice plates, are sometimes called elements.

The mark number, seen in Figure 1, is the identification, or tag, number given to the flow
element. Bore is the size of the hole, in inches, in the orifice plate. The line number is the
identification number of the pipeline.

Control Valve. Figure 3 shows the symbol for a control valve. The letter S above the small
triangle means there is an air supply to open or close the valve. The abbreviation INST means
that an instrument air signal is supplied to the valve positioner. The positioner is shown by
the square block.

The output air signal is shown going to the top of the valve. Therefore, the valve operates by
air pushing down onto the diaphragm.

The mark number for this valve would be PCV, TCV, LCV, or FCV (for pressure,
temperature, level or flow control valve) followed by the loop number.

Size rating is the size, in inches, of the valve inlet and outlet bore.

A. F. ACTION, sometimes shown only as ACTION, says what the valve will do if there is an
Air Failure (AF). The word OPEN or CLOSE will be shown after A.F. ACTION.

S OUTPUT

INST.
LINE NUMBER

MARK NO. DIAPHRAGM OPERATED GLOBE
SIZE RATING VALVE WITH POSITIONER
A. F. ACTION

FIGURE 3. CONTROL VALVE




Electrical Switches. Figures 4 and 5 show electrical switches. In Figure 4, NO means Normally
Open. NC means Normally Closed.

The letter C on its own means Common. By operating the Hand Switch, C can be connected
either to NO or to NC.

Mark No. is the identification of the switch.

In Figure 5, SET AT is the value of the process variable at which the switch will automatically
trip open or close. The value will be shown in psi, °F, or %, depending on the type of switch
used (that is, the type of process variable that is being controlled). The symbol % is often
used in level control. Level may be given not as a dimension but as a percentage of the vessel
capacity. For example the set point may be 75% to show that the vessel should be kept at
75% full.

NO
OR
NC

HAND SWITCH

C

MARK NO.

FIGURE 4
LINE OF EQUIPMENT
MARK NO.

SWITCH ( SINGLE )

MARK NO.
SET AT

FIGURE 5. LEVEL SWITCH ( SINGLE )




Lamps And Lights. Figure 6 shows the symbols used for lamps and lights.

LAMP

MARK NO.

RED

RUNNING LIGHTS

GREEN

MARK NO.

FIGURE 6

When a light is not identified by a color, the light will usually be white. The mark number
will give the number of the instrument loop to which the light is connected.

ILD Line Symbols. Figure 7 shows ILD line symbols. Lines may be broken to avoid drawing
over equipment or information. The line may then be continued on the other side of the
equipment or information.

PROCESS LINES

INSTRUMENT AIR LINES

INSTRUMENT ELECTRIC LINES LINE LINE
BREAK CONTINUES
INSTRUMENT CAPILLARY TUBES

FIGURE 7. ILD LINE SYMBOLS




Air Supply. Figure 8 shows more ILD abbreviations. Those on the left indicate air supply.
Those on the right are as stated.

D/P DIFFERENTIAL PRESSURE
+ POSITIVE TERMINAL
_ NEGATIVE TERMINAL
S AIR SUPPLY AO / AFS AIR OPEN / AIR FAILURE CLOSE
A/S AC / AFO AIR CLOSE / AIR FAILURE OPEN
H HIGH PRESSURE
L LOW PRESSURE
EITHER SYMBOL MAY BE USED. GND GROUND

FIGURE 8. ILD ABBREVIATIONS

Electrical Signal Lines. Figure 9 shows ILD Electrical Signal Lines.

RED

WHITE
WIRE
COLORS SHIELDED CABLE
BLACK

GREY THIS SYMBOL
INDICATES A SHIELD

FIGURE 9. ILD ELECTRICAL SIGNAL LINES

The wires are color coded to show which wires must be connected to terminal posts.

Instrument cables that carry low voltage signals are shielded to prevent outside electrical
energy from interfering with the signals.




Box and Cable numbering. Figure 10 shows box and cable numbering. The Junction Box (JB)
or Terminal Box (TB) number is located at the top of the box symbol shown in Figure 10.
Connections, called terminal posts, inside the block are numbered.

JB OR TB NUMBER TERMINAL NUMBERS
SHOWN HERE

TERMINAL BOX WITH
TERMINALS

CONDUIT OR
CABLE NUMBER

CONDUIT OR CABLE
NUMBER SHOWN HERE

FIGURE 10. BOX AND CABLE NUMBERING

The conduit or cable number will be written in the block near the electrical line symbol.
Cables are always identified in pairs, or groups of pairs, of wire.




Local Indicators. Figure 11 shows the symbols for Local Indicators. Range means the range of
the indicator scale.

The letters B and E in the Foxboro local indicator symbol give the polarity of the input signal
(+ve or -ve). (Foxboro is the name of one of the manufacturers of instruments used by Saudi
Aramco. Another manufacturer is named Honeywell.)

LOCAL INDICATOR

MARK NO.
RANGE

B E
+ _
FOXBORO LOCAL
INDICATOR CONNECTIONS

FIGURE 11. LOCAL INDICATORS




Temperature Sensing Elements. Figure 12 shows the symbols for Temperature Sensing
Elements.

The Range is usually from zero to the maximum process temperature the Resistance
Temperature Element (RTE) will measure in its loop, for example, 0 to 250°F.
Type on the thermocouple symbol identifies the metals in the thermocouple, for example,
IRON/CON would mean iron and constantan.
EQUIPMENT
NUMBER

RESISTANCE TEMPERATURE
ELEMENT

MARK NO.
RANGE
EQUIPMENT OR
LINE NUMBER

THERMOCOUPLE TEMPERATURE
ELEMENT

MARK NO.
TYPE

FIGURE 12. TEMPERATURE SENSING ELEMENTS

Transducer. Figure 13 shows the symbol used for a transducer. The figure shows that the
transducer is changing an electrical input signal to a pneumatic output signal. Other symbols
may show the transducer changing a pneumatic input to an electrical output.

+
_ TRANSDUCER

MARK NO.

FIGURE 13. TRANSDUCER




Level Transmitters. Figure 14 shows the ILD symbols for Level Transmitters. All four
symbols are very similar and all show the vessel in which the level is being controlled. Note
the symbol for an accumulator, which is shown with the dry leg transmitters. The
accumulator is used to remove liquid from the dry leg.

DRY LEG
VESSEL NO. L 1
OUT
H
N
MARK NO. S
RANGE
SUPPRESSION LEVEL TRANSMITTER WITH AIR SUPPLY
ELEVATION CONNECTION ( D / P CELL )

WET LEG
2
H RED
VESSEL NO. +
L _
GREY
MARK NO.
RANGE LEVEL TRANSMITTER
SUPPRESSION ( D / P CELL )
ELEVATION

DRY LEG

VESSEL NO. L RED 3
+
H _
GREY

MARK NO.
RANGE
SUPPRESSION
ELEVATION

WET LEG
OUT 4
VESSEL NO. H

L
S
MARK NO.
RANGE
LEVEL TRANSMITTER WITH AIR SUPPLY
SUPPRESSION
ELEVATION CONNECTION ( D / P CELL )

FIGURE 14. LEVEL TRANSMITTERS WITH D / P CELLS



All four transmitter types use differential pressure to measure level. Types 1 and 3 are the
same except that 1 is pneumatic and 3 is electronic. Both use dry legs.

Types 2 and 4 are the same except that 2 is electrical and 4 is pneumatic. Both use wet legs.
Pressure measurement is sometimes expressed as the height of a column of water. This is
because a column of water one foot high produces a known pressure of 0.433 psi.
Alternatively, a column of water 27.7 inches high produces a pressure of 1.0 psi.

We can use this information to convert liquid pressure measurements into liquid level
measurements.

DP transmitters can be fitted with a biasing spring kit. The spring can be used to adjust or
balance out certain differential pressure readings in order to give us the actual readings we
require. When the bias acts to oppose pressure on the high side, it is called suppression.
When it acts to assist pressure on the high side, it is called elevation. An example is shown
below.

100 '' WC
15 psig

SEAL HIGH LOW
LEG SIDE SIDE

P1 P2
BIAS

3 psig

0 '' WC
HL
P1 P2

The pressure of liquid in the seal (or wet) leg is not needed for determining the liquid level in
the tank. Therefore, bias can be applied to balance out this pressure. Because bias in this
case is assisting pressure on the high side, we have elevation.




Level Transmitters (Cont'd). Figure 15 shows how a differential pressure transmitter is used to
measure level in a vessel open to the atmosphere.

Atmospheric pressure acts on the top of the water and also on the low pressure side of the DP
cell. Therefore, the difference in pressure between the high and low sides of the cell is equal
only to the pressure exerted by the water level.

Example: If the DP cell senses a pressure differential of 10 psi it means that the level of water
is 10 x 27.7 inches.

OPEN TANK LEVEL MEASUREMENT

AIR PRESSURE

LOW PRESSURE
HEIGHT

WATER SIDE VENTED TO
ATMOSPHERE

H L

FIGURE 15. LEVEL MEASUREMENT USING A DP CELL




Level Transmitters (Cont'd). Figure 16 shows how a DP transmitter measures level in a closed
vessel.

200 '' TANK PRESSURE

200 ''
DRY LEG

WATER
100 ''

H L

DP CELL

FIGURE 16. LEVEL MEASUREMENT USING DP CELL

In order to obtain a differential pressure that depends only on the liquid level, the pressure of
the tank atmosphere must be cancelled out. This is done by connecting the low side of the DP
cell to the top of the tank. This connection is called a dry leg.




Level Transmitters (Cont'd). Figure 17 shows why wet legs are sometimes used.

AIR

200 ''

200 ''
WET LEG

WATER
100 ''

H L

DP CELL

FIGURE 17. LEVEL MEASUREMENT USING DP CELL

The atmosphere in a tank may carry vapor from the liquid. If a dry leg DP cell is being used,
some of the vapor will condense in the leg. After a time, liquid at varying levels could collect
in the leg. This would cause differential pressure readings that do not represent only the
height of liquid in the vessel.




To overcome this problem the wet legs are made to a known height, then filled with liquid.
Because the liquid level in the leg is constant, the pressure it exerts on the low side of the DP
cell is constant. This pressure can be taken into account when reading differential pressure.
Figure 17 shows that it is possible for the low side pressure to be greater than the high side
pressure. DP cells are always connected with their high side to the vessel.

Temperature Transmitters. Figure 18 shows the symbols for Temperature Transmitters. Range
gives the temperature range of the transmitter, for example 0 to 250°F.
EQUIPMENT OR
LINE NUMBER

RED
+
_

GREY

TEMPERATURE TRANSMITTER.
MARK NO. THERMOCOUPLE WITH INTEGRAL
RANGE ELECTRONIC mV / mA CONVERTER
EQUIPMENT OR
LINE NUMBER

RED
+
_

GREY

TEMPERATURE TRANSMITTER.
RESISTANCE TEMPERATURE DETECTOR
MARK NO.
WITH RTD / mA CONVERTER
RANGE

FIGURE 18. TEMPERATURE TRANSMITTERS




Pressure and Flow Transmitters. Figure 19 shows two kinds of transmitters, one for pressure and
one for flow. The difference is in the connection to the process. Pressure measurement
requires only one connection. Flow measurement requires two connections; one for the high
pressure side of the orifice plate, and one for the low side.
EQUIPMENT OR
LINE NUMBER

RED
+
_
GREY
PRESSURE TRANSMITTER

MARK NO.
RANGE

OUT

IN
S
FLOW TRANSMITTER WITH AIR
MARK NO. SUPPLY CONNECTION
RANGE

FIGURE 19

Note that the flow transmitter has two input lines (on the left). This is because the flow
transmitter is using differential pressure.

Range will show the calibrated range of each transmitter. Examples would be:

• 0 - 100 psi (for pressure transmitter)

• 0 - 100" W.C. (inches water column) - [for flow transmitter]




Controller. Figure 20 shows the ILD symbol for a controller.

IN
OUT
CONTROLLER
S MARK NO.
SET POINT
P. BAND
RESET
DERIVATIVE
ACTION

FIGURE 20. ILD CONTROLLER SYMBOL

The meaning of the terms shown on the controller are explained below.

Mark No. identifies the process variable or loop number which is being controlled.

Set Point is the process variable value to which the controller has been set. It is the value
needed for efficient and safe operation. The set point setting can be altered by the operator
when necessary.

P Band means proportional band. This is a setting which determines the amount the variable
measurement must change from the set point for the control valve to move through 100% of
its travel. For example, suppose the total travel of a control valve is 6" (that is from fully
closed to fully open is a travel of 6"). If a total deviation of the process variable from set
point is also 6" (that is 3" below set point to 3" above set point) then the P Band is 100%
(because a 6" movement of the variable causes a 6" movement of the valve).

Note that the controller has a constant pressure air supply. The output of this supply depends
on the input being received from the transmitter (which signal depends on the process variable
measurement).




Level Control. Figure 21 shows a level control system. The valve is fully closed when the
level is 3" above its set point. It is fully open when the level is 3" below its set point.
Therefore, the level must travel through its full range in order to move the valve through
100% of its travel (6"). Therefore, P (Proportional) Band is 100%.

1.5 FEET 1.5 FOOT

200 % 100 % 50 %
PB PB PB
6 '' VALVE 6 '' FLOAT
MOVEMENT MOVEMENT

VALVE A

SPAN
3 ''
SET POINT
3 ''
ZERO

VALVE B

FIGURE 21. LEVEL CONTROL SYSTEM




Level Control (Cont'd). Figure 22 shows the arrangement for a P Band of 50%. A total
deviation from the set point of 3" causes a 6" movement of the control valve. The P Band is,
therefore, 50%.

2 FEET 1 FOOT

200 % 100 % 50 %
PB PB PB
MOVEMENT MOVEMENT

VALVE A

1.5 ''
SET POINT
1.5 ''

VALVE B

FIGURE 22




Level Control (Cont'd). Figure 23 shows the arrangement for a P Band of 200%. A total set
point deviation of 12" causes a 6" movement of the control valve.

1 FOOT 2 FEET

200 % 100 % 50 %
PB PB PB
MOVEMENT MOVEMENT

VALVE A
SPAN

6 ''

SET POINT

6 ''

SPAN

VALVE B

FIGURE 23. PIVOT TO THE LEFT




Level Control (Cont'd). Reset may have a time value next to it. Reset is used with proportional
control to return a variable back to its set point. (Reset is also sometimes called Gain.)

For example, Figure 24 shows a stable process. The level is at set point and 50 gpm is
entering and leaving the tank.

WATER
IN

50 GPM
MAXIMUM LEVEL

SET POINT
MINIMUM LEVEL

WATER
OUT

50 GPM

FIGURE 24. STABLE PROCESS




Level Control (Cont'd). If for some reason the flow leaving the tank increases to 60 gpm the
level will fall. The float will then cause the control valve to open and input flow will increase.
However, the valve cannot adjust until after the level has deviated from set point. Hence, a
new stable condition may exist which is not at set point, as shown in Figure 25. The
difference between the new level and the set point is called offset.

WATER
IN

60 GPM

OFFSET
SET POINT

WATER
OUT

60 GPM

FIGURE 25. STABLE BUT OFFSET




Level Control (Cont'd). Reset is used to help the proportional control to bring the variable back
to set point. It does this by sending an extra signal to the control valve. The signal adjusts the
control valve until set point is reached. Then the reset signal stops.

The reset mechanism is part of the controller. It has a scale on which different times can be
set, for example from 0.1 to 50 minutes. A setting of 0.5 means that the control valve will be
adjusted every 0.5 minutes until set point is reached.

Derivative also may have a time value next to it. It is usually used only in Temperature
Control Loops. Derivative is sometimes called Rate Action or Integral.

Derivative is necessary because proportional plus reset control may take a long time to correct
temperature deviations from set point. Derivative action is concerned with how fast a
temperature is changing from set point.

If temperature is deviating only slowly from set point, the controller will make only small
adjustments to the control valve. Derivative action senses the speed of the change
immediately the change begins (unlike reset, which responds after the change has occurred
and caused offset).

If the rate of change is high, derivative immediately causes a large adjustment to be made to
the control valve to bring the temperature under control.

Derivative action stops when the temperature stops changing.

The derivation mechanism is also a part of the controller. It uses the same kind of time scale
as the reset unit.

Action will have Direct or Reverse next to it. Direct means that if the input signal to an
instrument is increased, the output signal from the instrument will also increase. Reverse
means that if the input signal increases, the output signal decreases.




Indicating Controller. Figure 26 shows the symbols used for an indicating controller. They are
the same as for a basic controller except that a scale range for the variable will be given.
Scales may be linear or square root.

Linear scales are used for those process variables which change in direct proportion to
changes in instrument output signals, e.g. level, temperature, pressure. Flow measurements,
however, are taken from differential pressure readings at an orifice plate. Differential
pressure changes in proportion to the square of the flow rate. Therefore, the square root of the
differential pressure must be found (or extracted) from a differential pressure signal in order
to find the flow rate. This is why some scales are square root.

IN
OUT

S
MARK NO.
SET POINT
INDICATING
P. BAND CONTROLLER
RESET
DERIVATIVE
ACTION
SCALE RANGE

IN
( IND. CONTROL )
OUT
( MANUAL CONTROL
SET UNIT )
MARK NO.
S INDICATING CONTROLLER
SET POINT
WITH MANUAL CONTROL UNIT
P. BAND
RESET
DERIVATIVE
ACTION
SCALE RANGE

FIGURE 26. INDICATING CONTROLLERS




Panel-Mounted Indicator. Figure 27 shows the symbol for a panel-mounted indicator. Range
gives the range for the indicator scale.

IN

INDICATOR
( 1 TO 3 POINTERS )
MARK NO.
RANGE

FIGURE 27. PANEL MOUNTED INDICATOR

Strip Chart Recorder. Figure 28 shows the symbol for a strip chart recorder. Mark numbers
and Range are given for each pen.

If more than one instrument loop is being recorded, additional input line symbols are added
for each loop. Notes may be given to explain more about the symbols.

GND, L1 and L2 mean Ground, Line 1 and Line 2, respectively.

IN
GND
L1
L2
MARK 1 ST. PEN

RANGE 1 ST. PEN RECORDER
( 1 TO 3 PENS )
MARK 2 Dn. PEN

RANGE 2 Dn. PEN

MARK 3 Dr. PEN

RANGE 3 Dr. PEN

FIGURE 28




Two-Purpose Instrument Devices. Figure 29 shows the ILD symbols for two components of a
loop combined into one.

The top output signal goes to a level transmitter or controller. The bottom output signal goes
to a final control element, such as a control valve.

OUT
VESSEL NO. IN
LEVEL TRANSMITTER /
OUT CONTROLLER
S
WITH AIR SUPPLY
CONNECTION ( DISPLACER )

TRANSMITTER CONTROLLER
MARK NO. MARK NO.
RANGE SET POINT
AND
P BAND
RESET

FIGURE 29. TWO - PURPOSE INTRUMENT DEVICES




Level Transmitter/Controller. Figure 30 shows the symbols used to denote a transmitter or a
controller. In each case, the appropriate information blocks would be filled in and the other
blocks left blank.
EQUIPMENT OR
LINE NUMBER

OUT PRESSURE TRANSMITTER
OR CONTROLLER WITH AIR
SUPPLY CONNECTION
IN S

MARK NO.
RANGE
SET POINT
TRANSMITTER P BAND
MARK NO. RESET
RANGE OR DERIVATIVE
ACTION
EQUIPMENT OR
LINE NUMBER

OUT TEMPERATURE TRANSMITTER
OR CONTROLLER WITH AIR
SUPPLY CONNECTION
IN
S
MARK NO.
RANGE
SET POINT
TRANSMITTER P BAND
MARK NO. RESET
RANGE OR DERIVATIVE
ACTION

FIGURE 30. LEVEL TRANSMITTER / CONTROLLER




Three-Way Solenoid Valve. Figure 31 gives the symbol for a three-way solenoid valve. This
symbol is usually connected to the symbol for the final control element. Most solenoid valves
are not very large. They are commonly used to shut off instrument air supply to control
valves.

SOLENOID OPERATED
THREE - WAY VALVE
ENERGIZED P-A
P A DEENERGIZED A - E

E
MARK NO.

FIGURE 31.




Three-Way Valve Operation. Figure 32 shows the normal operation of a three-way valve. When
the coil is energized, air flows to the control valve actuator without interruption.

When the solenoid coil is de-energized (which is what happens when the Emergency Shut
Down (ESD) button is pressed) the three-way valve closes. This blocks the flow of air to the
control valve. At the same time, the 3-way valve allows the air which is operating the control
valve to vent to the atmosphere. This causes the control valve to close.

AIR TO AIR FROM
SUPPLY ACTUATOR SUPPLY ACTUATOR

P A P A

E E
ENERGIZED DEENERGIZED

P - PRESSURE A - ACTUATOR E - EXHAUST

FIGURE 32



INTERPRETING ILDS

Handout No. 2 (Drawing 461-J-NA-942815) is a simplified ILD. The Title Block, shown in
Figure 33 below, identifies the loop that is on the drawing.

It is Flow Control Loop 101 (FC-101). The block says that FC-101 is part of a crude oil
pipeline at Berri-3 Plant, Ras Tanura, The Plant Number is 461.

The index letter, J, is the standard index letter for Instrument Loop Diagrams.

The Reference Drawing Block gives the drawing numbers of P&IDs and Instrument
Installation Schedules on which FC-101 can be found.

Reference is also made to the drawing numbers of Rack Power Distribution (Rack Pwr Dist.)
and ILD PC-301.



INTERPRETING ILDS (Cont'd)

Handout No. 2 shows that the ILD is divided into four parts: FIELD, FIELD JUNCTION
BOX, CONTROL ROOM PANEL REAR and CONTROL ROOM PANEL FRONT. (Large
junction boxes are sometimes called Marshalling Boxes.)

When reading an ILD, it is usual to start at the sensing element. In Handout No. 2, this is an
orifice plate, as shown in Figure 34.

Note: The Figures given inside the circles are for this module reference only. They do not
appear on an actual ILD.

7 4 NA - 942815 461 J 61845

REV. NO. SHT DRAWING NO. PLANT NO. INDEX JOB ORDER NO.

FIELD

8

E-9007
6
7

5

4-20 m ADC
4
9
MARK NO. FT - 101
RANGE 0-100''WC
2 H L
10'' - P - 145 - 1A1

3

MARK NO. FE - 101
1
BORE 6''

ILD SENSOR AND TRANSMITTER

FIGURE 34




The mark number (which is the same as a tag or identification number) of the
At 1 flow element is 101 (that is, FE-101). The BORE of the flow element is 6"
(that is, the hole through the orifice plate is 6" diameter).

2 H and L show on which side of the orifice plate high and low pressures are
sensed.

3 The pipeline is 10" pipe and the pipeline number is 10"-P-145-1A1.

4 The Mark Number for the Flow Transmitter is 101 (that is, FT-101). The
pressure measuring range of the transmitter is 0-100" water column (WC).

5 Auxiliary process lines take high and low pressure to the flow transmitter.

6 This is an electrically operated flow transmitter, as shown by the electric signal
lines .

7 The electrical signal lines are shielded all the way from the transmitter to the
next loop component.

8 E-3007 is the identification number of the electrical signal line cable.

Electronic loops use standard instrument signals of either 4 to 20 mA or 10 to
9
50 mA, direct current. The drawing shows that 4-20 mA DC is being used.




Figure 35 shows the JUNCTION BOX and CONTROL ROOM PANEL REAR instrument
signal wire line connections.

(1) Shows JUNCTION BOX-200 (J.B. 200). The left side cables come from the flow
transmitter and enter Terminals 1 and 2. Terminal 3 is used to ground the shielding on
the signal line.

(2) C-8101 identifies the signal line cable coming from JB 200.

(3) J. B. 320 is located behind the control room panel, that is, panel rear.

(4) CC-517 identifies the wire cable from JB 320 that goes to Flow Recorder (FR-101) on
the Control Room front panel.

(5) The wire line symbol shows a connection between Terminals 12 and 13. This is done
in order to complete a circuit.




Figure 36 gives information about control panel instruments.




(1) These are the incoming signals from JB 320.

(2) This is the ILD symbol for a three-pen recorder.

(3) Mark No. 1st Pen is for flow recorder FR-101. 0-10 Ã identifies the part of the strip
chart which is recording the flow in loop 101. The square root sign (Ã) shows that a
square root scale is being used.

(4) The 2nd Pen is recording the pressure in control loop PC-301. The range 0-100 refers
to the part of the strip chart that is recording pressure. The note symbol, 2 , refers to
the reference drawing in the Legend block.

(5) These are incoming signals from JB 320 to flow indicating controller, FIC-101.

(6) This is the basic ILD symbol for an indicating controller.

(7) These are the outgoing symbols from FIC-101.

(8) CC-518 identifies the cable between the FIC-101 and JB 320.




Figure 37 below shows again the wiring terminations In the junction box and rear panels.
(Reference should be made to the ILD as a whole.)

(1) The outgoing signals from FIC-101 go to the same JB 320 as do the incoming signals
to FIC-101. Different terminals in JB 320 are used for the incoming and outgoing
signal wires.

(2) C-8101 is the same cable that has the incoming signal lines.

(3) This is JB 200. It has the signal lines from the flow transmitter, FT-101. It also has
the outgoing signals wired to terminals 5 and 6.

(4) E-1115 identifies the signal cable wires from JB 200 to the field instruments.




Figure 38 below shows the field-mounted instruments which complete the control loop.

MARK NO. FTd - 101

1
3 S
2

4
S 6
10''- P - 145 - 1A1

MARK NO. FCV - 101
SIZE RATING 10'' GLOBE 5
A. F. ACTION CLOSE

FIGURE 38. ILD TRANSDUCER AND CONTROL VALVE

(1) These are the signal lines from JB 200.

(2) This is the symbol for a transducer. Mark No. identifies it as Ftd-101.

(3) The transducer changes the incoming electrical signal to an outgoing pneumatic signal.

(4) This is the basic ILD symbol for a control valve.

(5) The information block shows that the control valve is Flow Control Valve FCV-101.
It is a 10" globe valve. A.F. Action Close means it will close if there is an air failure.

(6) This is the pipeline number. It is 10" pipe, line number S-145.



INTERPRET AN ILD FOR A PNEUMATIC INSTRUMENT CONTROL LOOP

Figure 39 shows a simplified section of a P&ID. Control Loop number 113 is controlling the
level of tempered water in the surge drum 139-D-211.



(Cont'd)

The level in the drum is sensed by the level transmitter, LT-113. The transmitter sends
pneumatic signals to a level indicating controller, LIC-113, and to two level switches LS-
113A and
LS-113B.

In turn, LIC-113 sends pneumatic signals to a level control valve, LCV-113. If the level in
the drum goes low, the signals cause the control valve to open. This allows more make-up
water to flow into the drum. If the level goes high, the signals cause the valve to close. This
reduces the make-up water flow rate.

The level switches are connected to high and low alarms (XA-3-32 and XA-3-33). The
switches are set to operate if the drum level goes dangerously high or dangerously low. They
are operated by the pneumatic signals coming from the level transmitter. The 3 refers to the
row number on the control panel. The 32 and 33 respectively refer to the column numbers.
They give the locations on the control panel where the alarms can be found.

Figure 40 shows how the level control loop would look on an ILD.

The ILD is shown in sections in Figure 41 through 44



(Cont'd)



(Cont'd)

Figure 41 is the ILD symbol for the level indicating controller LIC-113. The range is from 0-
100. Because it is a level controller, the scale range is a percentage. Levels are usually
indicated as a percentage of the vessel capacity. 0 to 100, therefore, is the range from
completely empty to completely full. Note the triangle and letter S to indicate air supply.



(Cont'd)

Figure 42 is the symbol for a level transmitter. The symbol ÆP/P means that the differential,
pressure (ÆP) sensed by the transmitter is sent to the loop controller as a pressure (P). It will
be sent as a pneumatic pressure signal of 3 to 15 psi.

The figure shows that the transmitter senses the differential pressure at equipment number
139-D-211. This is the surge drum shown on the P & ID.



(Cont'd)

Figure 43 shows the level control valve. 3"-SC-160-IAIA identifies the make-up water
pipeline. This is the line the level control loop uses to control the level in the surge drum.



(Cont'd)

Figure 44 shows the level switches in the control loop. LS-113A is the high level alarm
switch. It is set to open when it receives a 12-psi signal from the level transmitter.

LS-113B is the low-level alarm switch. It will open when it receives a 6-psi signal from the
level transmitter.

Figure 40 shows that the switches are connected to alarms XA-3-32 and XA-3-33 on the front
panel of the control room. The alarms can be seen on windows 3-32 and 3-33



INTERPRET AN ILD FOR AN ELECTRONIC INSTRUMENT CONTROL LOOP

Handout No. 3 (Drawing Number J-415-NB-582636) shows an electronic instrument control
loop. Electronic loops are more complicated than pneumatic loops. There are two reasons for
this:

• Loop components are both field mounted and located in the control room. Also, the
instruments may be great distances away from each other. They must be connected
together by electric wires. The wires may pass through one or more junction boxes.

• The electric wiring connections between instruments must be done in such a way that
complete electric circuits are formed.



(Cont'd)

The top of Handout No. 3 shows a pressure control loop, shown again below in Figure 45.

The symbol for a control valve can be seen. It has Tag No. PCV-51. It is connected to
pipeline 4"-5-304-6A1 and has a 20-psig air supply.

Fig 45



(Cont'd)

Figure 46 shows the symbol for the transducer. You can tell it is a transducer because it has
two electrical connections, an air supply and a pneumatic output line. The two electric wires
are part of the control loop electric circuit. The current through the transducers varies with
changes in process variable values. Air at a constant pressure of 20 psig is supplied to the
transducer. The output value of the air pressure varies with changes in the transducer current.
Hence, electric signals are converted to pneumatic signals.

The symbol for a transducer is sometimes drawn as a square, but Foxboro, the company
which makes the instrument, draw it as a circle.

The letters E and B identify the terminal connections inside the transducer junction box. Note
again that the transducer needs a 20-psig air supply. The symbols shown in Figure 47 are for
locally-mounted air regulators.



(Cont'd)



(Cont'd)

Figure 48 shows the symbols for the field-mounted pressure indicator (PI-51) and the field-
mounted pressure transmitter (PT-51).

The transmitter is shown to be connected to a pipeline identified as 4"-S-305-3A1. The
circular symbol marked 'IND' shows that the transmitter has an indicator mounted on it.



(Cont'd)

Figure 49 shows the field junction box. All the instruments of Loop P-51 are wired into this
box. The box is identified as ETB3. The number that follows the ETB3 symbol is the
terminal number for the wire inside the terminal box.

The symbol marked 503 is a shield for the cable coming out of the junction box. It shields the
cable from outside electrical interference.



INTERPRET AN ILD FOR AN ELECTRICAL INSTRUMENT CONTROL LOOP
(Cont'd)

Figure 50 shows that the wiring goes from the junction box, through a marshalling box, and to
a panel interconnection junction box in the control room.

A marshalling box (MB) is simply a big junction box. It is usually located just inside the
control room building. It is a collection point for field wiring that comes into the control
room from many parts of the plant. From the marshalling box, the instrument loop wiring is
organized and routed to various display areas and panels in the control room.

The number of marshalling boxes in a plant depends on the size of the plant. Each box is
numbered. Figure 50 shows that on this ILD the marshalling box is MB7. The number that
follows each MB7 is the terminal number inside the box. There may be hundreds of wires in
each box.



INTERPRET AN ILD FOR AN ELECTRICAL INSTRUMENT CONTROL LOOP SET

Figure 51 shows, at the left, numbered blocks between the marshalling block and the panel
interconnection junction box. These are the individual wire numbers between the boxes.

The panel interconnection junction box is located behind the control room panel. It is usually
close to the loop controller. A short cable connects the controller to the junction box. The
cable carries a number of wires each insulated from the others. The wires are color coded.



(Cont'd)

The panel interconnection junction box symbols are shown in Figure 52.

The colors identify wires inside the connecting cable. Each of the lines below the colors
represents one wire inside the cable. The letter indicates the connection point on the cable
plug. For example, the violet wire in the cable is connected to point F in the cable plug. You
can also see from the figure that the violet wire is connected to the terminal strip at connection
number 5. Figure 53 shows the cable plug.

The letters BK at the top of the numbers column identify the terminal strip inside the panel
interconnection junction block. The ILD shows the identification of other terminal strips,
such as BH1, EA1 and so on.



(Cont'd)

The (+) and (-) signs indicate the polarity of each numbered terminal that is being used.



(Cont'd)

Figure 54 shows that there is a 100-ohm resistor connected across terminals 3 and 4 in the
junction box. Resistors are needed whenever an input signal is too high for other instruments
in the control loop.



(Cont'd)

Figure 55 shows the symbol for the pressure controller.

The controller needs a 118V 60Hz power supply. FOP No. F3-10 means that the instrument
is located on the Face Of Panel F3, in position 10. On other ILDs the abbreviation BOP
(Back of Panel) may sometimes be seen.



(Cont'd)

Figure 56 shows the recorder and its connections.



TRACING CURRENT FLOW IN CONTROL LOOPS

The symbol at the bottom right-hand corner of Figure 56 is for a three-pen recorder. Pen
number 2 records pressure values on PR-51. The recorder operates with a 118V, 60Hz
supply.

In order to record pressure values, the recorder must be connected to the pressure control
loop. It must receive signals that indicate the pressure values.

A study of the ILD, on Handout No. 3, shows that the power to operate the pressure
transmitter, PT-51, is supplied by the pressure controller, PC-51. The controller also operates
with a 118V, 60Hz supply. The ILD shows that a multi-wire cable connects the pressure
controller output to terminal 1 on terminal strip BK. A wire connects terminal 1 to the
positive side of the pressure transmitter, PT-51. The transmitter acts as a variable resistor. Its
resistance depends on the value of the process variable. Therefore, the current flowing
through the control loop changes as the transmitter resistance changes. And this change is a
measure of the process variable.

From the transmitter, the current flows through the pressure indicator, PI-51. From there it
goes to terminal BK-3. From BK-3 the current flows through a 100- ohm resistor to BK-4. A
wire connects BK-4 to BH-6. A wire from the multi-wire cable connects BH-6 to the plug.
The ILD shows this connection to be letter H on the plug (a violet colored wire). The current
goes to operate PR-51.

In order for the current to flow, there must be a complete circuit. Therefore, the current that
operates PR-51 must be returned to its source, PC-51. The ILD shows that this is done by
connecting a wire from the multi-wire cable (a brown wire) to terminal 7 on terminal strip
BH. This wire acts as a return wire. It takes the return current from PR-51 to BH-7. A wire
connects BH-7 to terminal 2 on terminal strip BK. A wire from the multi-cable wire is
connected to BK-2. The IDL shows this to be connection U on the plug (a grey color wire).
The connection completes the circuit.



(Cont'd)

Figure 57 shows the symbol for a panel-mounted alarm. The numbers 1 - 10 identify the
location of the alarm in the alarm display panel, i.e., Row 1, Column 10.

Tracing the wires from the alarm shows that it is connected to the multi-wire cable plug at
terminals 6 and 7 on terminal strip EO. Temperature switch TS-54 is connected by the multi-
wire cable (connections J and B) to these same terminals. Hence, the current passing through
TS-54 can also pass through alarm XA-1-10. If the supply fails, the switch will trip and set
off the alarm.



Computer Relays

Computer Relay Symbols

Symbols are used to show Computer Relays on ILDs. Details of other information related to
the relays may also be given. This section of the module covers the symbols and related
information.

Manufacturer's Symbols. Saudi Aramco uses instrumentation supplied by two manufacturers,
Foxboro and Honeywell. Relays supplied by these companies are drawn differently on ILDs.
An example is shown in Figure 58. The symbols are for adder/subtractor cards.



Computer Relays (Cont'd)

Manufacturer's Symbols(Cont'd). Foxboro instruments use the words "adder" or "summer" on
their cards (from "sum" meaning 'add').

Foxboro summer card output terminals are always the number 2 terminals. Honeywell
adder/subtracter card output terminals are always the number 6 terminals.

TP (terminal panel) followed by a mark number is used to identify terminals on Honeywell
Computer Relays.

Handout No. 4 (Drawing No. R84-A-NA-B44995 Sheet 1) is a P&ID for a deethanizer
system. Handout No. 5 (Drawing No. R84-J-NB46327 Sheet 1 A) is the ILD for Flow
Control Loop F-010 shown on the P&ID.




Manufacturer's Symbols(Cont'd). The ILD on Handout No. 5 shows that Honeywell relay
instruments are being used.

The ILD shows that the following instruments are to be found in the field (that is, out in the
plant area).

• Flow Transmitter, FT-010 NOTE: On this ILD the mark
• Flow Element, FE-010 numbers also include the Plant
• Flow Indicator, FI-010 number (R84).
• Flow Transducer, FTd-010
• Flow Control Valve, FCV-010

It also shows that the following instruments are found on the front of Control Panel CP-R84-
101.

• Flow Recorder, FR-010
• Flow Totalizer, FQI-010
• Flow Indicating Controller, FIC-010

Note that the flow indicator, FI-010, has a (non-linear) square root scale. This is because the
indicator is connected in series with the flow transmitter, FT-010, and the transmitter's
differential pressure signals have not yet passed through the square root extractor.

The auxiliary rack section shows that there are three Computer Relays being used. These are:

• FY-010A - a multiplier/divider card
• FY-010B - a square root extractor
• FQ-010 - a flow integrator card.



Computer Relays(Cont'd)

Computer Relay Symbols (Cont'd). Note that the function of the relays is shown at the top, as
shown in Figure 59.

The square root sign (Ã) indicates a square root extractor. The multiplication sign (X)
indicates a multiplier/divider card which is performing multiplication. (If a division sign ( )
were above the relay, the card would be performing a division function.) The integral sign (_)
indicates an integrator card.




Computer Relay Symbols(Cont'd). Figure 60 shows where the other information about the relays
was obtained from the ILD.




Computer Relay Symbols(Cont'd). A study of the ILD (Figure 60) shows the symbol

2
10-TT-016-12

The arrow enters the multiplier card at terminal number 8.

10-TT-016 tells us that a temperature transmitter, TT-016, is sending a signal to the multiplier
card. The 12 tells us that a wire from terminal 12 on the transmitter is connected to terminal 8
on the multiplier.

The number 2 in the box refers us to the Reference Drawings given on the right-hand side of
the ILD. 2 refers to ILD NB-B46327, sheet 35.

This kind of information is characteristic of ILDs. They show where an input signal comes
from and, if necessary, will make reference to another ILD to show the destination of the
signal.

Block number 7, just above TPAI-1, shows that the output from terminal 3 goes to 10TY-
010B. The reference drawing section refers to ILD NB-B46327 sheet 29. Sheet 29 is shown
in Handout No. 6. (Drawing No. R84-J-B46327 Sheet 29.) It shows that a TYPE E
thermocouple is used to sense the temperature in line 16"-P-1002-3A1. It also shows that a
3", globe type temperature control valve is fitted into line 3"-SC-1001-3A1C.

The symbols shown at the center of the auxiliary rack section of the ILD are for a computer
system. They are shown in Figure 61.




Computer Relay Symbols(Cont'd)

Note that the symbol MV/I represents the temperature transmitter TT-10. MV/I means it is
converting millivolts to current. Two input signals are shown entering the transmitter. One is
from TE-010; the other is from TE-015. Block 3 says that TE-015 is found on Sheet 34.

17/C cable, in the Rack Section, means 17 conductor cable. It is a cable containing 17
conductor wires. The cables are connected to the control instruments.

The ILD shows that lines 7 and 10 out of TPA-2-2 can be traced to the computing relays TY-
010A and TY-010B respectively.

TY-010A is the signal selector. The symbol above the card (<) is the mathematical symbol
for less than. In this case, the symbol means that the card is a low signal selector. If the
symbol was >, which means greater than, the card would be operating as a high signal
selector.

TY-010B is the adder/subtractor card. The Greek letter, capital sigma (_) above the card
means the sum of. It shows that the card is operating as an adder or subtractor, depending on
how the card is set. If a plus sign (+) is over the card, it means that the card is only adding.
The Greek capital letter delta (Æ) or a minus sign (-) is used to indicate a subtractor card.

Note that the input signal to terminal 5 on the adder/subtractor card comes from TPA1-1-3.
This shows again how ILDs are used to trace electric circuits from one drawing to another.

Other connections are shown going to sockets and pins for the computer control of the
temperature.



Instrument Systems

Saudi Aramco uses two control systems that are manufactured as complete units. The
systems are shown on ILDs. One of the systems is the Foxboro Spec 200 and the other is the
Honeywell Vutronic.

Foxboro Spec 200

'Spec' is an abbreviation for Simplified Package for Electronic Control. The basic
arrangement of the Spec 200 is shown in Figure 62.

PROCESS

TRANSMITTER
I
P
ALARMS
4 - 20 mA

4 - 20 mA
10 - 50 mA INPUT OUTPUT
RTD BUFFER AND CONTROL BUFFER AND
THERMOCOUPLE 0 - 10V FUNCTION 0 - 10V SIGNAL 10 - 50 mA
SIGNAL
mV CONVERSION CONVERSION
VOLTAGE

BASIC ARRANGEMENT OF A SPEC 200 LOOP

FIGURE 62



Instrument Systems(Cont'd)

Foxboro Spec 200 (Cont'd)

The system is a closed loop. The block symbol marked I/P (Figure 63) is used to show Spec
200 transducers. These transducers convert current energy (I) to pressure energy (P).

I
P

FIGURE 63

Input signals such as 4-20 mA, 10-50 mA, millivolts and ohms can be used by the system.
These signals are converted to 0 - 10 Volts DC signals by input signal converters. The 0 - 10
V signals are used by rack and panel mounted instruments, such as controllers, indicators,
recorders and alarms. Using small voltage signals makes the system safe to work on.

All Spec 200 instruments are connected in parallel. This allows components to be removed
from the loop without breaking up the system. It also means that the same voltage is applied
to all components.

Output signal converters are used to send 4-20 mA and 10-50 mA signals to field instruments.

The Spec 200 system consists of two areas: the display area and the nest area, as shown in
Figure 64.

The display area contains the recorders and indicators, and provides all the information
needed by operators.

The nest area contains the circuit cards for the control, computing, input and output
converters, alarm and conditioning units.

Nest units are fitted into sections called racks.





Figure 65 shows the operation of the Spec 200.

PROCESS

TRANSMITTER
5 VOLTS I
P
ALARMS
0 - 20 mA
5 VOLTS
4 - 20 mA INTPUT OUTPUT
BUFFER AND CONTROL BUFFER AND
0 - 10V FUNCTION 0 - 10V SIGNAL 0 - 50 mA
SIGNAL
CONVERSION CONVERSION

5 VOLTS 5 VOLTS

INPUT SIGNAL DISTRIBUTION

FIGURE 65





Suppose the following: A process control loop is for pressure control; the set point is 15 psi;
the transmitter has a range of 0-30 psi; the current range for the transmitter is 4-20 milliamps.

From the above it follows that a set point of 15 psi is equal to 50% of the transmitter's range.
This gives a signal of 12 mA (i.e. 50% of 4-20 mA range). As long as the process pressure
remains steady at 15 psi, the transmitter sends a 12 mA signal. When the 12 mA signal
reaches the input buffer and signal converter relay card in the nest unit, it is changed to a
voltage signal.

Spec 200 operates on 0-10 V. Since 12 mA is exactly half the transmitter range, the voltage
signal would also be exactly half its range, that is, 5 V. Therefore, the relay card in the
converter sends a 5 V signal to all other components in the control loop. For example, 5 volts
will be sent to the recorder and this will be seen as 15 psi on the recorder graph.

The transducer operates on a milliamp range. Therefore, the voltage signal must be converted
back to an amperage signal before it enters the transducer. This is done by the card in the
output buffer and signal converter in the nest unit.

The Spec 200 cards are used for specific functions. Some of these functions are described
below.




The Function of the 2AI-I2V Current to Voltage Converter Card. The Foxboro current converter
card Model No. 2AI-I2V is a solid state component located in the nest assembly. 2AI-I2V
stands for:

2 - A Spec 200 component
A - Analog signals in and out
I - Input instrument
I - Current signals in
2 - This is an isolated card
V - Voltage signals out

The 2AI-I2V card has only one function. It receives 4 to 20 mA signals from a field
transmitter and changes them to 0 to 10-volt signals. These are the signals needed by the
Spec 200 system. The voltage output is proportional to the current input.

The card can operate with two inputs and two outputs for dual operation. This means that the
card can receive and convert 4-20 mA signals from two transmitters.

The input (current signals) sides of the card circuit are isolated electrically from the output
sides (voltage signals). The two circuits are not connected by wires, but the input influences
the output because it passes through a transformer. This induces a proportional voltage in the
output side of the transformer coils.

Isolated cards are used because they give more protection to the cards. For example, a short
circuit in the transmitter circuit will not damage the card.

If the figure 3 were shown in place of the 2, it would mean that the card was not isolated.

The Function of the Controller Card 2AX+45. The Foxboro controller card, 2AX+45, has
electronic circuits that receive the input signals and modify them according to the control
settings. The card sends an electronic output signal to control a final control element, usually
a control valve.




The Function of 2AP+ALM-AR Alarm Card. Alarm cards cause alarms to sound in the plant
control room if operating conditions become abnormal.

The Foxboro 2AP+ALM-AR is a dual alarm card. That means that it can monitor two
different variables at the same time. 2AP+ALM-AR stands for:

2 - Spec 200 component
A - Analog signals in and out
P - Process component module
ALM - Alarm
AR - Dual absolute alarm - relay output

The card can be set to send output signals to two different alarm lights. For example, the
2AP+ALM-AR alarm card could monitor pressure for a low condition and temperature for a
high condition. The card can also be set to monitor both high and low conditions for the
same process variable. It could do this using only one input signal.

The alarm card is a solid state function card that slides into a module in the nest unit. The
card has two single alarm circuits with a common power supply. Each alarm has one input,
one set point, and one output. Alarm points are calibrated from zero to 100% of scale.

The alarm card receives voltage signals from other function cards, such as a square root
extractor, or a resistance-to-voltage temperature card. It has two relays built into it, one for
each alarm circuit. When the alarm is off, the relays on the alarm card are energized. The
relay contacts are normally open (NO) and this gives a no-alarm condition, as shown in
Figure 66.

TERMINAL NUMBERS
-4
LAMP
NC OFF
COM - +
-2 POWER SUPPLY

NO

+2
TERMINAL NUMBERS

RELAY CONTACTS OPEN
FIGURE 66




The Function of 2AP+ALM-AR Alarm Card (Cont'd. So long as the process variable that the alarm
card is monitoring stays within its set-point range, the relay will stay energized.

Imagine that the alarm circuit is monitoring a pressure control loop. The alarm is set to come
on if the pressure goes too high. At this condition, the voltage signal coming into the alarm
card will be at the value for which the alarm card has been set. This will cause the relay to be
deenergized. When this happens the NC contacts close, as shown in Figure 67, and the alarm
light comes on.

TERMINAL NUMBERS

-4

NC
COM - +
-2 POWER SUPPLY
LAMP
ON
NO

+2
TERMINAL NUMBERS

RELAY CONTACTS CLOSED

FIGURE 67


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