108 CONTROL LOOP FOUNDATION: BATCH AND CONTINUOUS PROCESSES
General Symbol
Ball Valve
Globe Valve
Damper
Figure 7-12. Excerpt from ISA-5.1 Valve Body and Damper Symbols
When a positioner is used with a valve, the diaphragm representation may
be combined with a representation of the actuator and the valve body, as
is illustrated in Figures 7-16 to 7-19. The failure mode of the valve (i.e., fail
open or fail closed) is often indicated by the valve symbol in control sys-
tem documentation.
A special actuator symbol is defined by the standard for motorized actua-
tors. Motorized actuators are used in some industry segments because
upon loss of power, the last valve position is maintained. Also, better reso-
lution may be achieved using a motorized actuator for a specific applica-
tion, such as the basis weight valve that is used to regulate the thick stock
flow to a paper machine in paper manufacturing. [6] Solenoid actuators
that are used to automate valves used in on-off service are shown using a
special symbol.
Installation details are provided in the loop diagram documentation.
Some of details may also be provided in the P&ID. For example, the P&ID
and loop diagram may show the orifice that must be installed to measure
flow using a differential pressure transmitter. Hand-operated valves that
are used to block flow during startup or maintenance are shown since they
impact process operation if not properly set up. Also, the installation of
inline instrumentation, such as a magnetic flowmeter, is commonly shown
in a unique manner in the control system documentation. In addition,
measurement elements such as an RTD or a thermocouple are shown since
they may be physically installed some distance from the field transmitter
or the control system. To provide a consistent means of documenting the
physical installation, the ISA-5.1 standard includes symbols for many of
these common installation details and field devices. A sample of some of
these symbols is shown in Figure 7-14.
CHAPTER 7 – CONTROL AND FIELD INSTRUMENTATION DOCUMENTATION 109
Generic actuator,
Spring-diaphragm
Spring-diaphragm with
positioner
Linear piston actuator
with positioner
Rotary motor operated M
actuator
Solenoid actuator for S
on-off valve
Figure 7-13. Excerpt from ISA-5.1 Actuator Symbols
Restriction Orifice, With
Flow Transmitter FT
Hand Valve
FT
Inline Measurement
Measurement Element TE
Figure 7-14. Excerpt from ISA-5.1 Symbols for Other Devices
7.7 Equipment Representation
A representation of major pieces of process equipment is normally
included in control system documentation. This allows the field instru-
mentation installation to be shown in relationship to the process equip-
ment. Example process equipment representations are illustrated in
Figure 7-15.
A general vessel representation may be appropriate for vessels, agitators,
heat exchangers, and pumps that do not play an important role in the con-
trol system. For example, an agitator on a tank may not directly impact the
control associated with the tank level. A special representation is provided
110 CONTROL LOOP FOUNDATION: BATCH AND CONTINUOUS PROCESSES
Vessel, Jacketed Vessel,
Reactor
Atmospheric Tank, Storage
Heat Exchange
Agitator
Pump
Figure 7-15. Examples of Process Equipment Symbols
for a reactor. A jacketed vessel symbol may be used when a vessel is
heated or cooled by circulating liquid through an outside shell. Such a
design is commonly used in the batch industry, and permits the vessel’s
contents to be heated or cooled without coming in contact with the circu-
lating liquid. A symbol is also provided for flat-bottomed and cone-
shaped storage tanks that are open to the atmosphere. The examples
include a representation of a heat exchanger, which is used to heat or cool
a liquid stream. A symbol is defined for an agitator that may be used to
ensure good mixing of liquids in a vessel. Also, a pump symbol is shown
in these examples.
7.8 Documentation Examples
The four examples included in this section are designed to illustrate many
of the recommendations of the ISA-5.1 standard. In the first example,
questions and answers are used to highlight specific points that have been
covered in this chapter. A review of these questions will hopefully serve to
reinforce your understanding of concepts and terminology that will be
used in later chapters. By studying the control system and field instru-
mentation documentation, a lot can be learned about the process and the
way the control system is designed to work. Such background information
can be extremely helpful when you are working with or troubleshooting
the control system for a new process.
7.8.1 Example – Basic Neutralizer Control System
The neutralizer process may be used to adjust the pH of a feed stream. The
field devices and process controls that would typically be provided to this
CHAPTER 7 – CONTROL AND FIELD INSTRUMENTATION DOCUMENTATION 111
process are shown in this piping and instrumentation diagram illustrated
in Figure 7-16.
After reviewing this diagram, consider the following questions and
answers.
Feedforward AC FT FC
104 105 105
Reagent Signal
Stage 1 FT Characterizer
102 AC AY
103 103
FC Reagent
FC Stage 2
FV
FC AV 105
101 103
FV
101 AT
FT 103
101 Static Mixer
FC
Feed
Neutralizer
FV
AT 106
104
Discharge FC
Figure 7-16. Example – Basic Neutralizer Control System
Question: What tag number convention is shown in this
example?
Answer: The expanded tag number convention, defined by
the ISA-5.1 standard, is not used in this example since the
tag only contains function letters and a loop number.
Question: Flow measurements are made using FT101,
FT102, and FT105. What type of measurement technique is
used by these field devices?
Answer: The field device is shown as installed inline and
the liquid is conductive. Thus, a magnetic flowmeter is
installed.
Question: Do the valves have digital positioners?
Answer: Yes, an electric signal is shown going directly to a
box located on each valve. The pneumatic line from the
112 CONTROL LOOP FOUNDATION: BATCH AND CONTINUOUS PROCESSES
actuator is also tied to this box. Therefore, we can assume
that these valves are installed with a valve positioner.
Question: What type of valve is being used to regulate
flow?
Answer: The general valve body symbol is used and thus
the type of valve is not indicated in this drawing.
Question: Is the plant operator able to access the control
functions indicated by AC103, AC104, FC101, and FC105?
Answer: Yes, the circle indicating these control functions
contains a horizontal line that indicates the operator may
access this function.
Question: Can the operator access flow measurements
FT101, FT102, and FT105, as well as analytic (pH) measure-
ments AT103 and AT104?
Answer: No, the circle indicating these measurement func-
tions does not include a horizontal line. In this case, these
measurements are not directly accessed by the operator.
Question: Are any on-off (blocking) valves used in this
process?
Answer: Yes, two blocking valves are shown with electric
on-off actuators.
Question: What function is provided by AC103?
Answer: The function letters of the tag number indicate
that the function is analytic control.
Question: What measurement is used by AC103 in per-
forming its control function?
Answer: Since an electric signal line is shown from AT103
to AC103, then control must be based on the measurement
AT103 (pH).
Question: What is the purpose of the function shown
between AT103 and AC103, that is, AY103?
Answer: On the diagram, this function is noted as Signal
Characterization. When a complex calculation is done, it is
often represented by a single function with AY noted inside
the circle and a note provided to explain the purpose of this
function.
CHAPTER 7 – CONTROL AND FIELD INSTRUMENTATION DOCUMENTATION 113
Question: Does the vessel have an agitator?
Answer: Yes, the figure shows an agitator impeller in the
vessel, powered by a motor on top of the vessel.
Question: Is the pump fixed speed or variable speed?
Answer: Since control of the process is done using a regu-
lating valve, then it may be assumed that the pump is fixed
speed.
Question: How could the liquid in the vessel be heated?
Answer: A heat exchanger is shown in the recirculation line
and could be used to heat (or cool) the liquid in the vessel.
Question: How is the pH of the incoming stream adjusted?
Answer: The output of AC103 and AC104 go to a valve and
flow loop, respectively, that regulates the flow of reagent to
the inlet stream and recirculation line. Thus, it may be
assumed that reagent is used to adjust pH.
7.8.2 Example – Basic Column Control
The basic column control system shown in Figure 7-17 is somewhat more
complicated than the basic neutralizer control system. However, after
studying this example, it will become clear that many of the components
that make up the column control system are similar to those used in the
basic neutralizer control system. As we have seen, what is learned by
working with one process may be often applied to another process. Once
you become familiar with a few processes, it becomes easier to work with
other processes.
7.8.3 Example – Batch Reactor Control System
Batch processing plays an important role in some industries. In a batch
process, a vessel is charged with feed material that is processed in the ves-
sel through mechanical or chemical means (or both). At the end of the pro-
cess, the product—which may be a finished product or an intermediate
product for use in another process—is removed from the vessel. The batch
reactor example illustrated in Figure 7-18 is known as “continuous feed
batch,” in which feed material is continuously added to the vessel
throughout part or all of the batch processing. The reaction takes place,
and finally the product is pumped from the vessel. In comparing the basic
components to those seen in the previous two examples, the only new ele-
ment introduced by this example is an eductor (a device that produces
vacuum by means of the Venturi effect) that is used to remove gases cre-
ated by the batch reaction.
114 CONTROL LOOP FOUNDATION: BATCH AND CONTINUOUS PROCESSES
PV
PC 115
115
FO Vent
Feedforward LC
FT111 LT
Feed Tank 117 117
Distillate FC
FC 118
Receiver
114
FV FV
PT 114 118
115 FT FT
Reflux FO 114 118
FC
FC Overheads
111
FV FT111 Feedforward
111
FT
RTD
Column TT
111 Feed TC
FC
116 116
Storage Tank
FC LC
110 113
FV
110 FC
FT 112
LT
110 113 FV
FC Steam 112
FT
Bottoms 112
FC
Figure 7-17. Basic Column Control System
PC
128
TC FO
126 PV PT
127 PV
128
FC TT 128
122 126 Vent System
FO
Eductor
FV TV
122 126
FT
122 PC
FC FO
Anti-Foam Coolant 127 TC
124
FC
PT
121 TT
FV 127
125
121
FT TC
121 FC 125
Feed B
FC TT TV
120 124 125
FV
120 Coolant
FO
FT
120 FC Feed A LT
Batch Reactor LC
123 123
LV
123
Discharge FC
Figure 7-18
Figure 7-18. Example – Batch Reactor Control
CHAPTER 7 – CONTROL AND FIELD INSTRUMENTATION DOCUMENTATION 115
7.8.4 Example – Continuous Feed and Recycle Tank
In studying the example of a continuous feed and recycle tank shown in
Figure 7-19, you will note that the only new function that was not con-
tained in a previous example is FFC133. The function letters of the tag
number indicate that this new function is ratio control. The line connec-
tions indicate that the ratio control is implemented using two feed streams
to Reactor 1.
FC
135
FV
135
FT Recycle
135
FC
Makeup Recycle Tank
FC
LC LT 133
134 134 FV
133
FT
133 FC Reactor 1
FC
132
FFC FV
133 132
PC FT
130 132
Feed Tank FC Reactor 1
FO
FC
PV 131
130 PT FV
130 131
FT
131
FC Reactor 2
Figure 7-19. Example – Continuous Feed and Recycle Tank
Knowing the tag number and symbols on a P&ID is the first step in under-
standing which measurement and control functions have been installed on
the process. As the examples shown in Figure 7-16 to 7-19 illustrate, a
basic understanding of the control and process is possible if you are famil-
iar with the tag number and symbols on the P&ID. Later chapters on con-
trol implementation will address how the control functions shown on the
P&ID and loops sheet are implemented.
116 CONTROL LOOP FOUNDATION: BATCH AND CONTINUOUS PROCESSES
References
1. The International Society of Automation (ISA). http://
www.isa.org.
2. International Electrotechnical Commission (IEC). http://
www.iec.ch.
3. OSHA Standard 29 CFR 1910.119, Process safety management of
highly hazardous chemicals. http://www.osha.gov.
4. ISA-5.4-1991, Instrument Loop Diagrams, Research Triangle Park,
NC: ISA (ISBN 1-55617-227-3).
5. Skousen, Philip. Valve Handbook – Second Edition, Chapter 1.5.1,
McGraw-Hill Handbook (ISBN: 0071437738).
6. Lehtinen, Marjaana. More speed and accuracy for basis weight
control, pp. 22–24, Automation 1, 2004.
