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

Processes including liquids, gases and bulk solids:

• Moving Bulk Material
• Heating, cooling, mixing and separating
• Measuring Properties

Outdoor facilities and high ambient temperatures, significant environmental protection requirements, Continuous process control, many analogue variables.

FACTORY AUTOMATION

Synchronized manufacturing processes:

• Moving, aligning, transporting parts.
• Mechanical operations
• Measuring Dimensions

Compact plants and limited ambient temperature range, Limited environmental protection requirements, Stat detection and motion control and Binary values dominate measurements.

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.

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.

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.

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.

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.

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.

CHARACTERISTICS OF INSTRUMENTS

Range: The upper and lower limits of the signal received by the instrument is specified as range. It is written as ”from (lower limit value) to (upper limit value).

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.

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.

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.

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.

Resolution
Resolution is defined as the smallest change that can be detected by a sensor.

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.

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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.

Interpreting Information

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

VALVE AND ACTUATOR SYMBOLS

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

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.

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.

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

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

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.

• 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.

• 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.

• 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.

• 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.

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.

• 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.

Electrical Junction Boxes
• In electronic loop diagrams, junction box
connections are represented by circles
• instead of squares as shown in Figure 1.38.

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.

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)

• 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)

• 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)