PROCESS INSTRUMENTATION BASICS

AND DIAGRAMS
 Process Instrumentation
• It is the technology of using instruments to
measure and control manufacturing,
conversion, or treating processes to create the
desired physical, electrical, and chemical
properties of materials.
 Automation
• the creation and application of technology to
monitor and control the process automatically
Process Automation
FACTORY AUTOMATION
 FUNCTIONS OF INSTRUMENTS
Instruments provide the following functions:

• 1. Collecting information about a measured variable.

• 2. Sending information to other instrument about measured variable.
• 3. Display information about measured variable.
• 4. Recording information about a measured variable.
• 5. Comparing what is happening to what should be happening.
This means comparing value of measured variable with the set point.
• 6. Making a decision about what action should be taken to adjust for
deviation from the set point.
• 7. Adjusting the manipulated variable.
• 8. Initiating an alarm when measured variable is either too high or too low.
 INSTRUMENTS IN A PROCESS
CONTROL LOOP
Sensor
• It is an instrument which is actually in direct
contact with the process.
• It first senses the value of a process variable
and converts it into a corresponding output.
• Examples are pressure to movement,
temperature to change in resistance, liquid
level to change in pressure. etc.
 INSTRUMENTS IN A PROCESS
CONTROL LOOP
Transmitter
• A transmitter carries a signal of the value of the
measured variable from the sensor to a controller.
Transmitters are necessary because the sensor and the
controller are often physically far apart. The
transmitter picks up the measurement provided by the
sensor, converts it to a standard signal, which can be
easily sent and read, and conveys the signal to the
controller. Sensors and transmitters are often
combined into one device. The two most common
types of transmission used in industry are Pneumatic
and Electronic.
 INSTRUMENTS IN A PROCESS
CONTROL LOOP
Controller
• It is a device which gets its input/measurement signal
from a transmitter and sends output control signal to a
final control element.
• It basically perform two functions. In first step it
compares the input measurement signal with the
desired value of variable known as set point. Set point
is prefixed according to process requirement. In the
second step, it generates a control signal depending on
the amount of error generated in first step and the
control action desired.
 INSTRUMENTS IN A PROCESS
CONTROL LOOP
Indicator
• It is an instrument which gives the
instantaneous value of the measured process
variable.
• Sometime it is a standalone operation such as
pressure gauges, thermometer, temperature
gauge. In other cases indicating function is an
integral part of either a transmitter or a
controller.
 INSTRUMENTS IN A PROCESS
CONTROL LOOP
Recorder
• Recorder is a device which keeps a permanent
record of the value of a process variable. It
gives instantaneous value of process variable
as well as its value in the past on either
continuous time bases or at fixed intervals of
time
 INSTRUMENTS IN A PROCESS
CONTROL LOOP
Final Control Element
• The final control element is that portion of the
loop which directly changes the value of
manipulated process variable. It receives its
input signal from the controller and act
accordingly to vary the manipulated process
variable. Final control elements include valves,
dampers, crane,, pumps and variable resistors.
INSTRUMENTS IN A PROCESS
CONTROL LOOP
 CHARACTERISTICS OF INSTRUMENTS

1. Every sensor is designed to work over a specified workable

range

2. Transducing elements must be used over the part of their

range in which they provide predictable performance and often
truer linearity.
CHARACTERISTICS OF INSTRUMENTS

Zero: It is the value of the measured variable at a datum. Zero,

therefore, is a value known to some defined point in the
measured range.
 CHARACTERISTICS OF INSTRUMENTS
Zero Drift
Drift is a gradual change in a variable over time when the process
conditions are constant.
1. One of the problems experienced with sensors occurs when the
value of the zero signal varies from its set value. This introduces
an error into the measurement equal to the amount of
variation, or drift, as it is usually termed.
2. Short-term drift is usually associated with changes in
temperature or electronics stabilizing.
3. Long-term drift is usually associated with aging of the transducer
or electronic components.
 CHARACTERISTICS OF INSTRUMENTS
Sensitivity
It is the ratio between the change in output of an instrument to
the corresponding change in measured variable (input).

The smallest change in input that can cause a control

element to change its output

Sensors may have constant or variable sensitivities, in which

cases they are described as having a linear or a nonlinear output,
respectively.

Clearly, the greater the output signal change for a given input
change, the greater the sensitivity of the measurement element.
 CHARACTERISTICS OF INSTRUMENTS
Resolution
Resolution is defined as the smallest change that can be
detected by a sensor.
 CHARACTERISTICS OF INSTRUMENTS
Time Response
The time taken by a sensor to approach its true output when
subjected to a step input is sometimes referred to as its response
time.

Fast sensors make it possible for controllers to function in a

timely manner.

Sensors with large time constants are slow and may degrade the
overall operation of the feedback loop.
 CHARACTERISTICS OF INSTRUMENTS
Linearity
A sensor described as a linear sensor when its output is directly
proportional to the input over its entire range. This relationship
appears as a straight line on a graph of output versus input.
In practice, exact linearity is never quite achieved, although
most transducers exhibit only small changes of slope over their
work range.
 CHARACTERISTICS OF INSTRUMENTS
Hysteresis: It is when the instrument shows different output
behavior while input increases (loading) and decreases
(unloading) over the same range. An RTD may exhibit hysteresis
while heating and cooling.
 CHARACTERISTICS OF INSTRUMENTS
Accuracy and Precision
Accuracy of a measurement is the term used to describe the
closeness with which the measurement approaches the true
value of the variable being measured.

Precision is the reproducibility with which repeated

measurements of the same variable can be made under identical
conditions.

In matters of process control, Precision is more important than

accuracy; it is normally more desirable to measure a variable
precisely than it is to have a high degree of absolute accuracy.
 CHARACTERISTICS OF INSTRUMENTS
Accuracy and Precision
Sensors are designed to be both accurate and precise.

A sensor that is accurate but imprecise may come very close to

measuring the actual value of the controlled variable, but it will
not be reliable in its measurements.

A sensor that is precise but inaccurate may not come as close to

measuring the actual value of the controlled variable, but its
measurements will differ from the actual value by nearly the
same amount every time. This consistency makes it
possible to compensate for the sensor error.
INTRODUCTION TO PROCESS (OR PIPING)
AND INSTRUMENTATION DIAGRAMS
• PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
• It is a standard set of symbols used to organize drawings of
Processes and demonstrate the relationship between
instruments and the various industrial processes they
monitor.
• These symbols may provide the following information:
1. The function the instrument perform
2. The measured variable in the instrument system
3. The connection between instrument and process
4. The location of instrument in the system
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)

INSTRUMENTS SYMBOLS
On an instrumentation diagram, each instrument in the system is
represented by a circle, called a balloon as shown in Figure 1

Letters, numbers, and lines drawn inside the balloon give specific
information about the type of instrument, its location, and the
function it performs.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)

1. INSTRUMENTS SYMBOLS
Lines or the absence of lines through balloons are used to indicate
where an instrument is mounted.

1. Solid Line
2. Broken Line
3. Without Line
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)

Solid Line
A solid line drawn horizontally through a balloon represents a
board-mounted Instrument .

Board-Mounted means that the instrument is mounted on a

structure with a group of instruments usually accessible to the
operator for normal use.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)

Broken Line
A broken line drawn horizontally through a balloon represents an
instrument that is mounted behind the board.

Behind the board is a term applied to a location that

(1) is within an area that contains the
instrument board,
and (2) is within or in back of the board, or is otherwise not
accessible to the operator for this normal use,
and (3) is not designated as local.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)

Without Line
A balloon drawn without a horizontal line represents an instrument
that is mounted locally, near the point of measurement or a final
control element. It is neither on nor behind a board. This is also
referred to as a field-mounted instrument.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)

1. TAG NUMBERS
The letters and numbers written inside an instrument balloon are
used to identify the instrument.

The functional identification is located in the top part of the

balloon. It is used to identify the measured variable and the
function of the individual instrument.

The loop identification is located in the bottom part of the balloon.

As its name implies, this code is used to identify the loop of which
the instrument is a part.

A loop is a combination of one or more interconnected instruments

arranged to
measure or control a process variable, or both.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
FUNCTIONAL IDENTIFICATION
The functional identification normally consists of two, three, or four
letters.

The first letter indicates the measured variable.

The remaining letters usually indicate the functions performed by

the individual instrument. For example, record (R), control (C) or
transmit (T).

The second letter in the functional identification may act as a

modifier of the first letter. For example, pressure (P) could be
modified to indicate differential pressure (PD). In such cases, a third
letter would be used to identify the function of the individual
instrument.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Loop Identification
A loop consists of one or more interconnected instruments.

These instruments are arranged to measure a process variable,

control a process variable, or both.

Each instrument in a loop must be identified with a loop

identification number as shown in the lower portion of the
instrument balloon.

The number shown on the instrumentation diagram is also shown

on the instrument itself to aid an identification.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Loop Identification
Each loop in a plant is assigned a specific number.

All of the instruments in a particular loop will have the same loop
identification number, regardless of the type of instrument
or the function it performs.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Loop Identification
Some loops may have two or more instruments performing the
same function. If this is the case, a consecutive letter suffix is
usually added to the loop identification number for each instrument
in order to differentiate two or more instruments with the same
functional identification in the same loop.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Interpreting Information
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
LINE SYMBOLS

Various types of line symbols are used on instrumentation

diagrams. These lines
may indicate:
1. Process piping,
2. Process connections, or
3. Signals
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Process Line Symbols

Process piping lines represent the piping that carries the working
fluid. These lines are dark and bold as shown below
Process connection lines are fine lines as shown in Figure 1 used to
represent different types of connections, such as

1. a mechanical link between two parts of an instrument system,

2. a connection between an instrument and the process, or
3. a connection indicating the supply line to an instrument.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Signal Lines
In industrial processes, it is often necessary to send a signal from
one instrument to another instrument.

Different types of lines are used on instrumentation diagrams to

represent the pathways for different types of signals.

The type of line that is used on the diagram will depend on the type
of signal transmitted.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Signal Lines
Pneumatic signal lines are fine lines, intersected by pairs of slashed
lines as shown below.
This symbol is used to show the path of a pneumatic signal in the
system.
In this example, the pneumatic signal line indicates that a
locally-mounted pressure transmitter is used to send a pneumatic
signal to a pressure recorder located on the control board.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Signal Lines
Electrical signal lines are used to represent the wiring or cables by
which electrical signals are conveyed. These signals are represented
by broken lines, or dashes. It can also be shown as fine line,
intersected by three slashed lines at regular intervals.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Signal Lines
Capillary tubing signal lines are fine lines with X drawn on its
interval along the line. Capillary tubing (filled system) is often used
to transmit temperature signals

Hydraulic signal lines are used to represent the piping used to transmit
signals by hydraulic pressure.

Electromagnetic or sonic signal lines represent signals that are carried

by heat, light or radio waves. These signal do not use piping, tubing, or
wiring for Transmission..
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
VALVE AND ACTUATOR SYMBOLS

Valves are used to control the flow of fluid. The most common types
of valves used in process systems are globe valves and gate valves.
These valves are both represented by a symbol consisting of two
triangles, connected at their points as shown in Figure 1.22.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
VALVE AND ACTUATOR SYMBOLS
Three-way control valves allow a process to be channeled in three
directions. A three-way control valve body has three intersecting
process piping lines.

Four-way control valves allow four options for the direction of flow.
Four process piping lines may intersect in the valve body.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
VALVE AND ACTUATOR SYMBOLS
Angle control valves allow the process to bend or turn. These valves
are usually installed if the process must be controlled at a point
where there is a bend in the process piping.
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
VALVE SYMBOLS
Hand actuators are used to control valves manually.

Valves are often controlled automatically. A common method of

automatic control is the use of a diaphragm
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
VALVE AND ACTUATOR SYMBOLS

solenoid actuator
 PROCESS & INSTRUMENTATION DIAGRAMS(P&IDs)
Valve Identification
Tag numbers may also be used to identify valves. The format,
letters, and numbers used in valve tag numbers are the same as
those used to identify instruments.

For example, the tag number in Figure 11 would be used to identify

a pressure control valve
in Loop 182
 READING A PROCESS AND INSTRUMENTATION DIAGRAM

Title Block
The title block of an instrumentation diagram is used to identify the
diagram. The title block is normally located in the lower right-hand
corner of the diagram.
A title block usually includes the following information:
1. Diagram number,
2. Diagram title , gives the name of the drawing and, in some cases,
a few words of description
3. Number of sheets ,specifies the number of sheets comprising the
diagram
4. Signatures section, this section may include the signatures of the
person who draw the diagram, the person who checked it, and the
person who approved it.
 READING A PROCESS AND INSTRUMENTATION DIAGRAM

Revisions Section
The revisions section of an instrumentation diagram provides
information about changes made to the original diagram.

The facility document control file contains information about

revisions to the instrumentation diagrams used at the facility. It is
important to check to make sure that the information shown in the
revisions section of the diagram reflects the changes
described in the document control file. If a revision described in the
document control file is not shown on the diagram, you will know
that you do not have an up-to-date copy of the diagram.
 READING A PROCESS AND INSTRUMENTATION DIAGRAM

List of Materials
The list of materials provides information about certain parts that
are used in the process system. The parts included in this list are
usually parts that are specific to a particular loop or part of the
instrument system. The information provided for each of the parts
specified in the list of materials may
include:
1. The tag number
2. The Manufacturer's Name.
3. The model number
4. The number used by the facility to identify the parts
 READING A PROCESS AND INSTRUMENTATION DIAGRAM

Notes
The notes provide supplementary information about the process or
instruments shown in the instrumentation diagram. There are two
types of notes used on instrumentation diagrams: general and local.
General notes usually apply to the entire diagram. References to
other diagrams may also be included in the general notes.

Local notes usually apply to a specific instrument or area of a loop.

Local notes are located near the instrument or area they concern.
Each note is connected to the instrument or area it refers to by a
leader line.
.
 INTRODUCTION TO LOOP DIAGRAMS

Complex industrial process systems are normally made up of many

process loops. Each loop performs a different function in the total
process system. As seen earlier a P & ID provides information about
the process system as a whole.
This type of diagram shows all of the process loops and their
relationship to the entire process system. By using a P & ID the
operation of a process can be traced from beginning to the end.

Loop diagrams contain more detailed information about each of the

loops shown on P & ID.
 INTRODUCTION TO LOOP DIAGRAMS
• The instruments are all concerned with the measurement and control of
temperature.
• All of the instruments are in the same loop (201).
• The temperature transmitter (TT-201) and the temperature valve (TV-201) are
locally-mounted and the sensor components of the transmitter and the valve are
connected directly to the process piping.
• The temperature recorder (TR-201) and the temperature controller (TC-201) are
board-mounted.
The three-way valve (TV-201) is actuated by a diaphragm actuator.
• Temperature signals are sent to the transmitter by way of capillary tubing.
• The temperature transmitter uses a pneumatic signal to transmit the
temperature measurement to the recorder and the controller.
• The controller sends a pneumatic signal to the valve actuator.
 INTRODUCTION TO LOOP DIAGRAMS
• Loop diagrams use many of the same symbols as
instrumentation diagrams as well as other symbols
which may not be used on instrumentation diagrams.

• In order to find the loop diagram for a particular loop,

the loop should first be located and identified on the
instrumentation diagram. The loop number, process
variable, and type of signal used in the loop can be
determined from the information provided on the
instrumentation diagram.
 INTRODUCTION TO LOOP DIAGRAMS
• Loop diagrams are usually divided into
sections as shown below.
• The sections can be used to help identify the
locations of the instruments in the loop.
• Field process area shows the locations of
field-mounted instruments.
• Field junction shows the locations of junction
boxes. This section usually does not contain
instruments.
• Panel rear shows the location of instruments
that are mounted behind the board.
• Panel front shows the location of
board-mounted instruments.
 LOOP DIAGRAMS SYMBOLS
Instrument Port Connections
• An instrument port is the connection point
between an instrument and a supply or signal line.
Knowing where these connections are located can
help you to trace problems that may develop in
the process loop.
• The symbol for an instrument port connection in a
pneumatic loop is a rectangle, drawn vertically
and attached to the instrument balloon.
 INTRODUCTION TO LOOP DIAGRAMS
• The rectangle is divided into squares. Each of
the squares represents a separate connection.
• Letters shown inside the squares are used to
indicate the type of connection.
• Typical identification letters include:
• S supply,
• I input, and
• O - output.
 INTRODUCTION TO LOOP DIAGRAMS
Junction Boxes
• Junction boxes are used to provide central connection points
for signal lines.
• Junction boxes are usually located in the field or at the rear of
the panel.
• The symbol for a junction box in a pneumatic loop is a
rectangle, divided into squares as shown in Figure 1.37. In this
case, however, the symbol is not attached to an instrument
balloon.
• Facility-designated numbers are usually used.
 INTRODUCTION TO LOOP DIAGRAMS
• The number of squares shown in the rectangle
will depend on the number of connections
available at the junction box.
• In some cases, a square will not contain a
number, indicating that particular junction box
connection has not been used.
• This symbol can also be used to represent
pneumatic manifold, bulkhead, or other types
of connections.
 INTRODUCTION TO LOOP DIAGRAMS
Electrical Junction Boxes
• In electronic loop diagrams, junction box
connections are represented by circles
• instead of squares as shown in Figure 1.38.
 INTRODUCTION TO LOOP DIAGRAMS
Connection points
• In electronic loops, the connection points are
terminals rather than ports.
• In some cases, plus and minus signs are used
to indicate the polarity of the terminals.
 INTRODUCTION TO LOOP DIAGRAMS
• Power Sources
• A loop diagram may provide information about
the power source for the loop.
• An air supply is represented on a loop diagram by
the letters AS, followed by the value of the air
supply pressure.
• The symbol for the power source is connected by
a leader line to the symbol for the appropriate
instrument supply port. (see Figure 1.40)
 INTRODUCTION TO LOOP DIAGRAMS
• The electrical power source is represented by
the letters ES, followed by the voltage
• of the supply. If the supply is AC, the
frequency is also shown. (see Figure 1.41)
 INTRODUCTION TO LOOP DIAGRAMS
• Shield , A circle surrounding the symbol for an
electronic signal line indicates that the line(s)
is shielded.
• Shielding reduces electrical interference with
the signal.
 CALIBRATION INFROMATION
• Loop diagrams often provide information
about the calibration values of instruments in
the loop.
Set Points
• The set point of an instrument is the value of
the input variable which sets the desired value
of the controlled variable.
• The symbol for a set point is a diamond. The
value of the set point is indicated inside the
diamond.
Controller Action

The controller action symbol indicates how controllers and other instruments
respond to changes in signals.
• The symbols for controller action is a vertical arrow located near the
instrument.
• The arrow may point up or down.

1. An arrow pointing up indicates that the value of the output signal of the
Instrument increases as the input increases. This is called(Direct Acting)

2. An arrow pointing down indicates that the value of the output signal of the
instrument decreases as the input increases. This is called (Reverse Acting)