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Chapter 7: Piping and Instrumentation

Diagrams (P&IDs)

A. General DescriptionPiping and instrumentation diagrams (P&IDs) are the foundation of the I&C engineering package of deliverables. The P&ID is a multidiscipline drawing that presents information from the piping, pro-cess, I&C, systems integration, and electrical departments. The P&ID is the “daughter” of the pro-cess flow diagram (PFD), a mechanical department product, and the heat and material balance (HMB) sheet, a process engineering department product. The PFD is simply a mechanical drawing that shows the major equipment items and their associated piping interconnections. The HMB takes the PFD and layers on the “physics” of the system. The HMB provides detail on the amount of heat-ing or cooling that is needed, the flow rates, the dwell times, and the throughput. From these the P&ID is developed.

The “owner” of the P&ID is usually the process engineer or whoever is serving in that capacity. This individual, or group, coordinates the information that is being placed on the drawing to make sure it is consistent with its intent. A coordinator is needed since information flows to this document from many separate sources. Following is a listing of some of the different engineering disciplines that provide input to the P&ID production effort and the type of information provided:

• Process engineering: drawing content, process data, vendor equipment depiction• Mechanical: pipe ID numbers, equipment numbers and labels, graphical depiction of equip-

ment and piping• Instrumentation: instrument numbers, graphical presentation of instruments and instru-

ment wiring/tubing• Controls/systems: control logic depiction• Electrical: motor ratings, motor wiring depiction

Using the PFD and HMB as a guide, the instrumentation P&ID designer places flow transmit-ters at places in the process that need flow information, level transmitters on tanks, and so on. Each item shown on the P&ID is given a unique identification (ID) number called a P&I tag, which allows the item to be tracked throughout the design.

Size and distance have only a minimal effect on the process, so the P&ID has only a minimal relationship to the physical area. Some detail, such as floor number, can be helpful information.

B. Purpose PFDs and HMBs are purely conceptual documents. They have only a minimum of information relat-ing to the specifics of the piping or instrumentation of a system. The P&ID provides the link between the conceptual and the actual. It is a graphical index of all the process equipment, all major sections of pipe, and all instruments. Pneumatic tubing is shown where the information is important to communicate controls, and electrical wiring information is presented in the form of dashed lines to communicate functional intent to the electrical and instrumentation departments. Pipe specifica-tions, tag numbers, and sizes are shown. Heat tracing requirements are indicated, as are all safety

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142 SUCCESSFUL INSTRUMENTATION AND CONTROL SYSTEMS DESIGN

devices. Computer logic is shown to a degree, as are alarms and process-control elements. In short, the P&ID is the road map for the engineering design team.

C. ContentThe P&ID communicates the means by which the raw material is manipulated as described in the HMB. It answers the question, “How will it be done?” To accomplish this, a unique set of symbols and a means for identifying, or tagging, them have evolved, both of which are well documented by ISA - The Instrumentation, Systems, and Automation Society in its standard ISA-5.1-1984 (R1992)–Instrumentation Symbols and Identification.1 Another standard, ISA-5.3-1983–Graphic Symbols for Distributed Control/Shared Display Instrumentation, Logic, and Computer Systems,2 applies to the depiction of items for computer display.

1. Symbology (ISA-5.1-1984 and ISA-5.3-1983)Figure 3 shows a typical flow control loop. Instrument “bubbles” are placed at their functional loca-tions in the process pipe. While the physical distance between the items could be great, they are depicted in close functional proximity.

In this example, flow element FE-10 is shown upstream of flow-control valve FV-10. The flow element is an orifice plate that forces the flow of the liquid through a beveled hole that is specially sized to achieve a certain pressure drop across it for a given flow rate. A flow transmitter (FT-10) samples the pressure drop and generates an electrical output proportional to flow (dashed lines indicate the electrical signal). The signal is fed to a panel-mounted meter (FI-10a) and then on to an analog input point in the computer (FI-10b), which is presented as a display to the operator. After the signal is in the computer, it is processed and a control signal is generated to force a flow-rate cor-rection. This correction signal is routed to a flow-control analog output point (FC-10), where the sig-nal is converted back to an electrical signal and fed to a current-to-pneumatic (I/P) transducer that converts the 4–20 mA electrical analog signal to a 3–15 psi pneumatic analog signal. This signal is then fed to the control valve to force it to open or close as needed to achieve the desired correction.

In practice, the two computer symbols are usually combined into one that represents both com-puter functions, as shown in Figure 4.

Figure 3. Typical P&ID symbology

FIELD

INSTRUMENT

FI

10a

PANEL

MOUNTED

INSTRUMENT

FI

10b

COMPUTER

FUNCTION

ELECTRICAL

SIGNAL

INTERNAL

SYSTEM LINK

FC

10

COMPUTER

FUNCTION

FIELD

INSTRUMENT

ELECTRICAL

SIGNAL

PNEUMATIC

SIGNAL

I/P

PROCESS

PIPE

FT

10

FV

10

FE

10CONTROL

VALVE

2" - CS-140-S135

PIPE ID

FY

10AS

FLOW

Whitt2003.book Page 142 Thursday, July 10, 2003 4:05 PM

PART III – CHAPTER 7: PIPING AND INSTRUMENTATION DIAGRAMS (P&IDS) 143

Signal lines are differentiated to indicate the medium of the signal. For example, the dashed lines indicate an electrical signal, while the solid line with the two cross-hatches indicates a pneu-matic signal.

2. Symbol IdentificationThe tagging process is also well documented by ISA, which presents a two-element numbering scheme of the format:

XXXX-YYYY,

where XXXX is the tag prefix that provides indication of function and YYYY is a sequential identifier to make the tag unique.

Sometimes a middle element is inserted to indicate a process area, such as a building number or a process material designator.

a. PrefixThe prefix is the important part of the identifier. In the ISA tagging method, the tag prefix letters are position dependent. The first letter indicates the physical property being mea-sured or controlled (e.g., pressure, flow, temperature). The second or third letters are mod-ifiers. In Figure 4, F in the first position indicates a flow-control item. FT in the leftmost bubble indicates that the item is a flow transmitter. FI is a flow indicator, FC is a flow con-troller, FY is an I/P transducer,* and FV is a flow-control valve.

Note: An “S” as a second letter can be a modifier to the first letter, or it can be classified as a “succeeding” letter. This can be a bit confusing. If “S” is used as a succeeding letter, it applies to emergency protective primary elements. In this case, a device normally labeled PCV could also be labeled PSV if it is used as a safety device. The term xCV implies a self-actuated control valve, such as a pressure regulator. The succeeding letter combination CV should not be used in cases where the valve is not self-actuating. Thus, FCV would not be appropriate in the case of the valve shown in Figure 4. If the letter “S” is used as a “suc-ceeding” letter, such as “LSH”, it designates a switch. The way to tell the difference is if the device in question is generating a discrete (on/off) signal, then the “S” in the second posi-

Figure 4. Typical P&ID symbology showing combined computer functions

* A 4–20 mA DC current-to-3–15 psi pressure converter.

FIELD

INSTRUMENT

FI

10a

PANEL

MOUNTED

INSTRUMENT

COMPUTER

FUNCTION

FIC

10

ELECTRICAL

SIGNAL FIELD

INSTRUMENT

ELECTRICAL

SIGNAL

PNEUMATIC

SIGNAL

I/P

PROCESS

PIPE

FT

10

FV

10

FE

10CONTROL

VALVE

2" - CS-140-S135

PIPE ID

FY

10AS

FLOW



tion indicates the device is a switch. If the device is reacting to a variable process condition, then the “S” signifies a safety function.

Following are a few of the more common prefix arrangements:

• LSHH: level switch high-high• LSH: level switch high• LSL: level switch low• LSLL: level switch low-low• LAL: level alarm low• PT: pressure transmitter• PDT: pressure differential transmitter• AT: analyzer transmitter• TE: temperature element• TT: temperature transmitter• PDSH: pressure differential switch high• KQL: time quantity light (i.e., time is expired)• PY: pressure transducer• ZSO: position switch (open)• HV: hand valve• HS: hand switch

b. SuffixThe suffix portion of the tag is merely a sequence number unique to all the items in that control loop. In the example shown in Figure 4, all the items are “sequence ten” items, which indicate that all of the items depicted are linked to the same control point in the pro-cess.

D. Practical ApplicationFigure 5 is the P&ID depiction of the product tank example discussed in the Introduction (see Figure 2). This is the control problem that will be described throughout this part of the book.

Note that only one support leg is shown on the vessel. Detail that is not directly related to the control philosophy should be minimized. Refer to ISA standards ISA-5.1-1984 and ISA-5.3-1983 for more information on P&ID symbols.

The following narrative explains the TK-10 liquid level control.

1. Tank Level: LT-10, LSH-10, LSLL-47The tank level is reported to the computer by level transmitter LT-10. The computer then reports the tank level to the operator as represented by the LI-10 bubble. LT-10 has an alarm output that has been calibrated as a high level alarm. Note that the bubbles are touching to indicate that they are contained in a single device. This alarm output is hardwired into the pump circuitry such that a high level forces the fill valve closed. If level switch low-low LSLL-47 trips, then the pump will be stopped through a hardwired interlock.

2. Tank Fill: HV-13, ZSC-13The tank is filled by a “wild” flow rate that is not controlled upstream but is allowed to run wild into the vessel as long as valve HV-13 is open. A computer command will open the valve provided the high level alarm generated by a switch on the level transmitter does not activate. Valve HV-13


PART III – CHAPTER 7: PIPING AND INSTRUMENTATION DIAGRAMS (P&IDS) 145

will “fail” closed (FC) upon loss of air supply by virtue of the pneumatics surrounding the three-way solenoid HY-13 and the position of the spring inside the valve actuator.

Closed-position limit switch ZSC-13 closes if the valve closes. A computerized limit closed sta-tus bit, labeled ZLC-13, informs the operator of this event. Note: The author prefers to use “L” in the second position (ZLC) for discrete signals, as opposed to “I” (ZIC), which is reserved for analog inputs. The “L” indicates “lamp”, while the “I” indicates “indicator”. Also, an “A” in the second position implies an alarm type signal, which is also discrete.

3. Tank Discharge: PP-10By the use of hand valves, the contents of the tank may be emptied into a trench in the floor, or it may be directed to discharge pump PP-10. This pump runs based on the position of hand switch HS-15B. This switch is a hand-off-auto (HOA) type of switch. In the hand position, the pump runs provided the interlocks are set. In the off position, the pump stops. In “auto,” the pump runs based on a computer command (HS-15A), again, provided the interlocks are set.

Interlocks for this pump are

• stop upon a low level (LSLL-47), and• stop upon a low or high pressure (PAH-48).

If the tank fill valve closes with the pump running, there will be a risk of pulling a vacuum on the tank. Therefore, a vacuum relief valve PSV-58 has been added. This valve has an integral pilot and will open to relieve the vacuum.

Figure 5. TK-10 controls P&ID presentation



4. Pump Discharge Pressure: PIC-48Pump discharge pressure is managed by pressure control loop 48. The computerized pressure-indi-cating controller (PIC-48) maintains a constant pressure at the output of the pump by sampling pressure, comparing the actual pressure to a desired set point, and then modulating valve PV-48 to reduce the error. Pressure transmitter PT-48 samples the pressure of the pump discharge as it pushes product into a downstream product tank. If the pressure in the line increases, indicating reduced demand downstream, then recirculation throttling valve PV-48 is opened slightly to recir-culate more product back into TK-10. If downstream pressure decreases, indicating increased demand, then the recirculation valve moves toward the closed position. This allows the constant-speed motor driving the pump to run safely. When product isn’t needed downstream, then the con-stant recirculation eliminates the need for an agitator.

If the computer detects a high or low pressure and the signal remains for a certain time period, then the operator is notified and the pump is stopped.

This scheme, or portions of it, will be discussed throughout the remainder of this book.

E. P&ID SummaryThe P&ID is perhaps the single most important document in terms of communication of ideas. It becomes the true starting point for the design package. It is a multidiscipline document that pro-vides a means for coordinating the efforts of the entire design team. A very basic P&ID is shown in Figure 6.

Before this P&ID can be released, the mechanical information has to be layered on. This is infor-mation such as pipe number and equipment specifications. In some cases, detailed interlock notes appear here. However, this minimized drawing is sufficient for the controls discussions in the fol-lowing sections.

Figure 6. Basic P&ID drawing

SCALE SHEET

SIZE CAGE CODE DWG NO. REV

A

B

C

D

A

B

C

D

8 7 6 5 4 3 2 1

8 7 6 5 4 3 2 1

INSTRUMENTATION & DESIGN PRIMERSIGNATURES DATE

DRAWN

CHECKEDAPPROVED

PROJ ENGRPROJ MGR

OFPROJECT NO.

BUSE CLIENT DWG NO

PIPING & INSTRUMENTATION DIAGRAMPRODUCT DISTRIBUTION SYSTEM

TK-13 PRODUCT TANK

P&ID - 001NONE IDP 1 1

DWG

XXXXXX

XXXXXX

XXX

X/X/XXXXX

XXX

REVISIONSREV DESCRIPTION DATE APPR

DATE CADENGREVISIONREVDRAWN

Published by ISA PressPut the address here...

HS15A

LSL47

LAL47

TK-10

HS15B

PY48

ASI/P

PV48

M

LI10

3-WAYLT10

HY13

AS

HV13

FC

IPT48

HOA

LSH10

ZSC13

ZLC13

I

PIC48

PAH/L

PAH-48

PP-10

FO

PRODUCT TO TK-11

FROM PRODUCT HEADER

TRENCH

DRAIN

2nd Floor

3rd FLOOR

HS13

I

VAC

PSV58

PAL-48 I

I

I INTERLOCKYL15B

AUTO YL15A

RUNNING

LAH-10LAM-10LAL-10

SYMBOLS

COMPUTER FUNCTION

FIELD DEVICE

XXXYYY

XXXXXX

AS

I/P

PAH/L

PLC-GENERATED ALARMPressure Alarm HighPressure Alarm Low

AIR SUPPLY

CURRENT TO VOLTAGE TRANSDUCER

ELECTRICAL SIGNAL

PROCESS PIPING

I ELECTRICAL INTERLOCK


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