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Flow Diagrams and Instrumentation

Flow diagrams describe in a schematic drawing formatthe flow of fluids and gases through a unit or an entireplant. By using symbols to represent various pieces ofequipment, the flow diagram provides the piping designerwith an overall view of the operation of a facility.

The flow diagram used in this chapter is representativeof the types used by many companies in the piping indus-try. While actual symbols may vary slightly from onecompany to the next, the “look and feel” of flow dia-grams is the same throughout the piping industry.

Students must become familiar with the piping, equip-ment, instrumentation symbols and abbreviations used onflow diagrams, in order to be able to “read” and interpretthem.

One of the most difficult concepts for students to com-prehend is the absence of scale in the preparation of flowdiagrams. The flow diagram should be laid out in a verysimplistic and logical order and be read from left to right.It guides the drafter and designer in the same manner aroad map guides a traveler.

USES OF FLOW DIAGRAMS

The flow diagram is used by the piping group todevelop and lay out the plot plan. When developing theplot plan, the arrangement of the equipment in the facilityreflects, in part, the logical sequence of flow depicted onthe flow diagram. However, many other factors such ascode requirements, client standards and preferences,worker safety, and cost also influence the positioning ofequipment.

Once the plot plan is finalized, the piping designerroutes the pipe between two vessels as indicated by theflow diagram using piping specifications and accepteddesign practices. The flow diagram is usually “yellowedout” as each line is completed and incorporated into thedesign.

TYPE OF FLOW DIAGRAMS

Process engineers are responsible for developing flowdiagrams. In many large engineering firms, an entiredepartment is dedicated to the development of flow dia-grams. Today almost all flow diagrams are laid out withCAD, 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 diagramdeveloped by the flow diagram department. It includesthe following:

1. major equipment2. main piping3. direction of flow4. operating pressure and temperature5. major instrumentation

The process flow diagram will denote the following:

• Conditions to be used for the design of variouspieces of equipment (fractionation columns, pumps,heaters, etc.) required for facility operation.

• Operating and design conditions under which a par-ticular unit or piece of equipment will normallyoperate. Design conditions establish the limits thatequipment used in the facility can withstand. Designpressure is calculated to be at least 10% above themaximum operating pressure or 25# greater (which-ever is largest). The design temperature will be atleast the maximum operating temperature, butshould be at least 25 degrees above the normal oper-ating temperature.

• Composition of the commodities used in the processsequence as they enter and leave the unit.

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Figure 7-1 shows a sample process flow diagram.

Mechanical Flow Diagram

From the process flow diagram, the mechanical groupdevelops the mechanical flow diagram. The mechanical flowdiagram provides much more detailed data than the processflow diagram. Many companies refer to the mechanical flowdiagram as the “P & ID” (process and instrument diagram).Often referred to as the bible of the design process, thisdrawing provides the pipe drafter with the design criteria forthe unit. Mechanical flow diagrams include the following:

1. pipe line numbers and direction of flow2. pipe specifications and line sizes3. all equipment4. all valves5. all instrumentation with controlling devices

The mechanical flow diagram defines the exactsequence in which all equipment, valves, instrumentation,

connections, etc., are to be made on each pipe throughoutthe facility.

Figure 7-2 shows a sample of the mechanical flowdiagram.

The Utility Flow Diagram

The utility flow diagram shows the piping, valves, andinstrumentation for the basic plant utilities. Utilities areservices that are essential to the proper function of theplant. These utilities correspond to some of the same util-ities used in a typical house, such as water, gas, and sewerdrains.

Some of the common plant utilities are:

• steam • condensate

• fuel oil • utility air

• instrument air • cooling water

• drainage systems • flare system

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The flow diagram is a dynamic document. It may be revisedand updated during the project to reflect the client’s changes ormodifications 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 andmonitor these variables are:

Controller (C)Indicator (I)Gauge (G)Alarm (A)Recorder (R)

By learning these nine terms, students will be able tounderstand most of the instrument symbols found on amechanical flow diagram.

Figure 7-4 illustrates a combination of the symbols andabbreviations used to represent an instrument’s functionon flow diagrams. The first letter in the symbol indicatesthe instrument group, and the second and/or third lettersindicate the instrument type.

To indicate a change or to control the flow, level, pres-sure, or temperature, an instrument must first sense achange in the variable. Once a change has been detected,the instrument then transmits this information viamechanical, electronic, or pneumatic means to a controlpanel where it can be observed and recorded. At the sametime, the instrument may activate other devices to affectand change process conditions in the facility. Someinstruments are read in the plant at the instrument’s actuallocation. Others are displayed on a control panel locatedin an operator’s control room.

Instrument Types

Gauges. Gauges are instruments that measure the liq-uid level inside a vessel or the temperature and/or pres-sure in the piping system. Level, temperature, or pressuregauges are locally mounted to enable plant operators toobtain a visual reading.

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Controllers. Devices used to maintain a specified liq-uid level, temperature, pressure, or flow inside a vessel orpiping system. They activate the control valve that regu-lates the level, temperature, pressure, and flow in and outof the vessel.

Alarms. Signals via lights or horns that indicate theliquid level, temperature, or pressure inside a vessel is toohigh or too low or that there is no flow or reverse flow.

Indicators. Devices used to indicate the liquid level,temperature, pressure or flow rate inside a piping system.

Recorders. Devices used to record the liquid level,temperature, pressure, and flow rate inside a vessel orpiping system throughout a certain shift or period of time.

These same instruments may be found in combinationsuch as Level Recording Controller. Here the instrumentnot only records the liquid level but also sends a signal toa control valve to control the liquid level inside the vessel.

PIPING SYMBOLS

Figure 7-5 shows some flow diagram piping symbols.Notice all valves, no matter the pipe size and pound rat-ing, are drawn the same size. Generally, nozzles andreducers are not shown on the mechanical flow diagram.The flow diagram in Figure 7-2 shows reducers in orderto aid the students in visualizing and understanding theflow diagram and its relationship to the actual pipingarrangement drawing. Symbols used on flow diagramsrepresent the actual piece of equipment. Typically these

symbols have some resemblance to the actual pieces ofequipment in the field.

FLOW PLAN ARRANGEMENT

The flow plan should be arranged in a logical order offlow. The main flow through the unit should be obviousby even a brief examination of the flow plan. Use thefollowing checklist as an aid when developing a flowdiagram.

• Avoid crossing lines where possible.• Space equipment on the sketch to avoid overcrowding.• Use notes with symbols where necessary for clarity.• Use arrows to show flow direction.• Show equipment numbers when it is necessary to

identify equipment.• Show control systems on the sketch. The control

scheme is frequently the most important part of aflow plan sketch.

• Show important valves, orifice flanges, and controlvalves.

• Show flow directions through exchangers witharrows.

• Do not run lines diagonally across the drawing.• Label feed lines entering the unit from the field

where the line enters the unit. Label product linesleaving the unit by name.

• Do not draw lines any closer together than necessary.

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CHAPTER 7 REVIEW QUIZ

1. List five items shown on the process flow diagram.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

2. List five items shown on the mechanical flow diagram.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

3. List the four basic instrument groups.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

4. List the five instrument functions.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

5. What type of instrument is used to maintain a certain liquid level?________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

6. Identify the following instrument abbreviations:a. LG _______________________________________________________________________________________b. FA _______________________________________________________________________________________c. TI _______________________________________________________________________________________d. PC _______________________________________________________________________________________e. TRC ______________________________________________________________________________________f. LC _______________________________________________________________________________________g. PSV ______________________________________________________________________________________h. HCV _____________________________________________________________________________________i. LAH______________________________________________________________________________________j. LAL ______________________________________________________________________________________

7. Identify the following flow diagram abbreviations:a. DF _______________________________________________________________________________________b. SC _______________________________________________________________________________________c. CSO ______________________________________________________________________________________d. LC _______________________________________________________________________________________e. NO ______________________________________________________________________________________

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EXERCISE INFORMATION

Use the instructions accompanying Figure 7-6 to createthe flow diagram symbols with AutoCAD as shown. Usethe BASE command to place the base point as indicated.BLOCK each object using the exact symbol name pro-vided. OOPS the symbol to redisplay. SAVE the drawingas FLOSYMBL.

Exercises 1, 2, and 3.

Recreate the flow diagrams as shown. Using thesymbols in Figure 7-6 where applicable. Symbols repre-senting other pieces of equipment can be developed on anas-needed basis. Although equipment is not drawn toscale, it should be proportional to the other symbols usedin the drawing.

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CHAPTER 7 DRAWING EXERCISES

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