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Flow
Diagrams and
instrumentation
Flow diagrams describe in a schematic drawing format
the flow of fluids and gases through a unit or an entire
plant. By using symbols to represent various pieces of
equipment, the flow diagram provides the piping designer
with an overall view of the operation of a facility.
The flow diagram used in this chapter is representative
of the types used by many companies in the piping indus-
try. While actual symbols may vary slightly from one
company to the next, the "look and feel" of flow dia-
grams is the same throughout the piping industry.
Students must becom e fam iliar with the piping, equip-
ment, instrumentat ion symbols and abbreviat ions used on
flow diagrams, in order to be able to "read" and interpret
them.
One of the most difficult concepts for students to com-
prehend is the absence of scale in the preparation of flow
diagrams. The flow diagram should be laid out in a very
simplistic and logical order and be read from left to right.
It guides the drafter and designer in the same manner a
road map guides a traveler.
USES OF FLOW DIAGRAMS
The flow diagram is used by the piping group to
develop and lay out the plot plan. When developing the
plot plan, the arrangement of the equipment in the facility
TYPE OF FLOW DIAGRAMS
Process engineers are responsible for developing flow
diagrams. In many large engineering firms, an entire
department is dedicated to the development of flow dia-
grams. Today almost all flow diagrams are laid out with
CAD, using third-party piping packages such as Pro-
Flow or individually developed company packages.
Process Flow Diagram
The process flow diagram is the first flow diagram
developed by the flow diagram department. It includes
the following:
1. major eq uipment
2. main piping
3. direction of flow
4. operating pressure and temperature
5. major instrumentat ion
The process flow diagram will denote the following:
9 Cond itions to be used for the design of various
pieces of equipment (fract ionation columns, pumps,
heaters, etc.) required for facility operation.
9 Operating and design condit ions under which a par-
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2
Pipe Draftingand Design
Figure 7-1 shows a sample process f low diagram.
Mechan ical Flow Diagram
From the process f low diagram, the mechanical group
develops the mechanical f low diagram. The mechanical f low
diagram provides much more detailed data than the process
flow diagram. Man y co mpanies refer to the mechanical f low
diagram as the "P & ID" (process and instrument diagram).
Often referred to as the bible of the design process, this
drawing provides the pipe drafter with the design criteria for
the unit. Mechanical flow diagrams include the following:
1. pipe l ine numbers and direction of f low
2. pipe specif ications and l ine sizes
3. all equipment
4. all valves
5. all instrumentation with controll ing devices
The mechanica l f low d iagram def ines the exact
sequence in which a l l equ ipmen t , va lves , ins t rumenta t ion ,
connect ions , e tc . , a re to be made on each p ipe th roughout
the facility.
Figure 7 -2 shows a sample o f the mechanica l f low
diagram.
The Uti l i ty Flow Diagram
The uti l i ty f low diagram shows the piping, valves, and
instrumentation for the basic plant uti l i t ies. Util i t ies are
services that are essential to the proper function of the
plant. These uti l i t ies correspond to some of the same uti l-
i t ies used in a typical house, such as water , gas, and sewer
drains.
Some of the common plant uti l i t ies are:
9 s team
9 fuel oil
9 instrument air
9 drainage systems
9 condensa te
9 utility air
9 coo l ing water
9 f lare syste m
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114
Pipe Draftingand Design
The flow diagram is a dynamic docum ent. It may be revised
and updated during the project to reflect the client's changes o r
modifications imposed by governmental regulations.
Figure 7-3 shows a sample utility flow diagram.
FLOW DIAGRAM INSTRUMENTS
Instruments function by sensing changes in the vari-
ables they monitor. The four basic instrument groups are"
Flow (F)
Level (L)
Pressure (P)
Temperature (T)
The types of instruments used to sense, control and
monitor these variables are:
Controller (C)
Indicator (I)
Gauge (G)
Alarm (A)
Recorder (R)
By learning these nine terms, students will be able to
understand most of the instrument symbols found on a
mechanical flow diagram.
Figure 7-4 i l lustrates a combination of the symbols and
abbreviat ions used to represent an instrument 's function
on flow diagrams. The first letter in the symbol indicates
the instrument group, and the second and/or third letters
indicate the instrument type.
To indicate a change or to control the flow, level, pres-
sure, or temperature, an instrument must first sense a
change in the variable. Once a change has been detected,
the instrument then transmits this information via
mechanical , electronic, or pneumatic means to a control
panel where i t can be observed and recorded. At the sam e
time, the instrument may activate other devices to affect
and change process conditions in the facility. Some
instruments are read in the plant at the instrument's actual
location. Others are displayed on a control panel located
in an operator 's control room.
Instrument Types
Gauges. Gauges are instruments that measure the l iq-
uid level inside a vessel or the temperature and/or pres-
sure in the piping system. Level, temperature, or pressure
gauges are locally mounted to enable plant operators to
obtain a visual reading.
HOT OIL
TO & FROM
E-101
COOUNC 1
ATER
TO & FROM
E-102
,,
PSV-2
TO FLARE
FROM V-102
-r
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Flow Diagramsand Instrumentation
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L O C A LL Y M O U N T E D IN S T R U M E N T
BO A R D M O U N T E D IN S T R U M E N T
FLO W I N S T R U M E N T S
F ~ ) F L O W , ~ ~ ,
FL O W E L E ME NT
- )
F L O W N D IC A TO R
FR F L O W R EC O R D ER
FRq F L O W R E C O R D I N G C O N T R O L L E R
L E V E L I N S T R U M E N T S
@
_C
J
, G )
L E V E L A L A RM
L E V E L A L A R M H I G H
L E V E L A L A RM L O W
L E V E L C O N T R O L L ER
LEVEL GLASS
T E M P E R A T U R E
T ~ )
TI
T F
C
I N S T R U M E N T S
T E M P E R A TU R E A L A R M
TE MP E RA TURE I ND I CA TO R
T E M P E R A T U R E R E C O R D E R
T E M P E R A T U R E R E C O R D I N G
CO NTRO L L E R
T E M P E R A T U R E W E L L
P R E S S U R E
~C
@
(~)
Pl(
@
~V RELIEF VALVE
I N S T R U M E N T S
P R E S S U R E C O N T R O L L E R
P R E S S U R E I N D I C A T O R
P R E S S U R E R E C O R D E R
P R E S S U R E I N D I C A T I N G
CO NTRO L L E R
P R E S S U R E R E C O R D I N G
CO NTRO L L E R
P R E S S U R E S A F E T Y V A L V E
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Contro l le rs . Devices used to main ta in a spec i f ied l iq -
uid level, temperature, pressure, or f low inside a vessel or
piping system. They activate the control valve that regu-
lates the level, temperature, pressure, and f low in and out
of the vessel .
Alarms. Signals v ia l igh ts o r horns tha t ind ica te the
liquid level, temperature, or pressure inside a vessel is too
high or too low or that there is no f low or reverse f low.
Ind ica tors . Devices used to ind ica te the l iqu id leve l ,
temperature, pressure or f low rate inside a piping system.
Recorders . Devices used to record the l iqu id leve l ,
temperature, pressure, and f low rate inside a vessel or
piping system throughout a certain shif t or period of t ime.
These same inst ruments may be found in combinat ion
such as Level Recording Controller . Here the instrument
not only records the l iquid level but also sends a signal to
a control valve to control the l iquid level inside the vessel .
PIPING SYMBOLS
Figure 7 -5 shows some f low d iagram p ip ing symbols .
Notice all valves, no matter the pipe size and pound rat-
ing, are drawn the same size. Generally, nozzles and
reducers a re no t shown on the mechanica l f low d iagram.
The f low d iagram in Figure 7 -2 shows reducers in o rder
to aid the students in visualizing and understanding the
flow diagram and its relationship to the actual piping
ar rangement d rawing . Symbols used on f low d iagrams
represen t the ac tua l p iece o f equ ipment . Typica l ly these
Flow Diagramsand Instrumentation 117
symbols have some resemblance to the ac tua l p ieces o f
equipment in the f ield.
FLOW PLAN ARRANGEMENT
The f low plan should be arranged in a logical order of
f low. The main f low through the unit should be obvious
by even a brief examination of the f low plan. Use the
fo l lowing check l is t as an a id when develop ing a f low
diagram.
9 Avoid crossing l ines where possible.
9 Space equip ment on the sketch to avoid overcrowding.
9 Use no tes w i th symbols w here necessary fo r c la r ity .
9 Use arrows to show flow direction.
9 Show equipment numbers when i t i s necessary to
iden t i fy equ ipment .
9 Show control sy stem s on the sketch. The control
scheme is f requent ly the most impor tan t par t o f a
f low plan sketch.
9 Show important valves, or if ice f langes, and control
valves.
9 Show f low d i rec tions th rough exchangers wi th
a r r ows .
9 Do not run l ines diagonally across the drawing.
9 Lab el feed l ines e ntering the u nit from the f ield
where the l ine enters the unit . Label product l ines
leaving the unit by name.
9 Do not draw lines any closer together than necessary.
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8 Pipe Drafting and Design
CHAPTER 7 REVIEW QUIZ
1. List f ive i tems shown on the process f low diagram.
2. List f ive i tems shown on the mechanical f low diagram.
3. List the four basic instrument groups.
4. List the f ive instrument functions.
5. What type of instrument is used to maintain a certain l iquid level?
6. Identify the following instrument abbreviations"
a . L G
b. FA
c. TI
d. PC
e. TRC
f. LC
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EXERCISE NFORMATION
Use the instructions accom panying Figure 7-6 to create
the flow diagram symbols with AutoCAD as shown. Use
the BASE command to place the base point as indicated.
BLOCK each objec t us ing the exact symbol name pro-
vided. OOPS the symbol to redisplay. SAVE the drawing
a s F L O S Y M B L .
Flow Diagrams and Instrumentation 119
Exercises 1 2 and 3
Recreate the flow diagrams as shown. Using the
symbols in Figure 7-6 where applicable. Symbols repre-
senting other pieces of equipm ent can be developed on an
as-needed basis. Although equipment is not drawn to
scale, i t should be proport ional to the other symbols used
in the drawing.
F LO W S H E E T S Y M BO L S
TO D RAW FLOW SHEET SY MBOLS SET GRID TO .125 AND SNAP TO .0625
THE .125 GRI D SH OW N BEL OW HAS BEE N ENL ARGE D FOR BETTER VISUALIZATIION.
DRAW AND BLOCK EACH SYM BOL. BASE POINT ( } i ~ ) &
%
SYMBOL NAME L AME] HAVE BE EN INC LUD ED FOR PRO PER IDENTIFICATION.
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