Overview of Loop Diagrams in Process Control Systems
Loop diagrams are essential documents in process control systems, providing schematic representations of hydraulic, electric, magnetic, or pneumatic circuits. They detail instrumentation arrangements, signal connections, power connections, and termination information. Guidelines and standards for creating loop diagrams are outlined, emphasizing the importance of accuracy and completeness in development. The diagrams include symbols for instrument connections, energy supplies, and instrument actions, ensuring clear communication for maintenance and installation purposes.
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06/06/2017 LoopDiagrams Chapter 07 Loop Diagrams EMEC125 Overview &Definitions Loop diagrams are the most popular of all the documents generated for a process controlsystem. Definitions: Loop Diagram A schematic representation of a complete hydraulic, electric, magnetic, or pneumatic circuit. (International Society of Automation, 2003, p.299) Loop Instrumentation arranged as a combination of two or more instruments or functions arranged so that signals pass from one to another for the purpose of measurement and indication or control of a process variable. a) A self-contained device that measures and controls a process variable. (International Society of Automation, 2009, p.3.1.40) Sample LoopDiagram Loop_Diagram_Sample.pdf (CADtech Consultants, n.d.) 1
06/06/2017 Loop Diagrams What s On Them? Everything a controls technician needs to maintain and/or install a system of instrumentation Includedis: Electrical and pneumatic signalconnections Power connections to instruments andcontrols Indication and control devices, along with the connections to the process, to the piping, the ductwork, vessels, tanks,etc. Controllers whether computer or pneumatic Source and service utilities i.e. compressed air, water, electrical power, etc. Termination information that includes the identification of junction box(es), terminal strip numbers, terminal block numbers and possibly even pneumatic portnumbers Guidelines ISA-5.4-1991 Instrument Loop Diagrams spell out a few guidelines for creating loop diagrams. The standardstates: Drawings should be American Society of Mechanical Engineers (ASME) B size (11 x 17 )drawings. Only one loop should be depicted on a loop diagram. Paper can be portrait or landscape. Landscape is the preferred and is the most commonorientation. Should be divided into sections that relate to component locations, such as and according toISA-5.4: Field or Field ProcessArea Cable SpreadingRoom Computer I/OCabinet/Cabinet PanelFront ControlPanel Console Developing a LoopDiagram Development can startwhen: A standard has been established as to the content. The P&IDs are completed andissued. Specification forms have been completed. All devices and instruments have been specified and preferably ordered. Junction boxes, if used, have been defined, a.k.a. marshalling panel. Termination diagrams for control computers are ready for loading. 2
06/06/2017 Instrument Connections Symbols referenced in ISA-5.1 and ISA-5.4 apply for instrument loop diagrams. However, expansion of those symbols to include connection points, energy sources (electrical, air, hydraulic), and instrument action is necessary to provide the information required on instrument loop diagrams. (The Instrumentation, Systems, and Automation Society, 1991,p.12) Identify junction box or terminalassembly XXXX 1 2 3 4 Identify each Instrumentterminalsorports with identifying letters, numbers or both. (It is suggested that identifying numbers or lettersbethe manufacturer s designations) 7 8 15 16 FIC 103 1 2 5 6 connection used Instrument terminals orports General terminal or bulkheadsymbol Loop Diagram EnergySupply Electrical PowerSupply Identify electrical power supply followed by the appropriate supply levelidentification and circuit number ordisconnect identification L1 L2 G ES 115V,60Hz Panel A,Circuit12 TR 105 AS 20PSIG (IA or PA are optional alternatives forairsupply) AirSupply Identify airsupply followed by air supply pressure TT S 104 Hydraulic fluidSupply Identify hydraulic fluid followedby the fluid supply pressure WT S HS 50PSIG 103 InstrumentAction Identifies the response of the device to applied signals. The signals can be electronic orpneumatic. A transmitter and/or controller can provide two actions, direct or reverse: Direct Acting The output signal increases as the process variable (PV) increases. Reverse Acting The output signal decreases as the process variable (PV) increases. Loop diagram instruments should reference the action as direct or reverse. DIR can be used for direct and REV can be used for reverse. 3
06/06/2017 Instrument Action Instruments REV LT LSH 104 TC 104 104 REV DIR These instrument symbols show the control action by placing an acronym for the action near the instrument bubble. DIR identifies the instrument as Direct Acting and REV identifies the instrument as Reverse Acting. Because most transmitters are direct acting the DIR designation is optional and if not present the instrument can be assumed as directacting. P&ID Loop100 PIC 100 PIT 100 PV 100 1" 10" 150 CS004 10" 10" 6" 300 6 7 6" Loop Diagram P-100 Field ProcessArea FROM 01-D-001 TRANSMITTER: MODELNO: SPEC: NO: 3 AS 20 PSIG CONTROLLER: MODELNO: SPEC: NO: 0-10 PSIG PIT S 100 O I PIC O 3-15 PSIG PIT-100-(B) AS 20 PSIG S 100REV 10"150CS 004 ELEV 114'6" ELEV 119'0" 3-15 PSIG PV-100-(B) AS 100 PSIG I S O FO ELEV 120'0" PV 100 VALVE: MODELNO: SPEC: NO: TO FLARE REFERENCE DRAWINGS P&ID XXX INSTALLATION DETAILS NOTES: 1. PV-100 FULLYOPEN AT 3 PSIG FULLYCLOSEDAT 15PSIG 4
06/06/2017 8 7 6 5 4 3 2 1 PLC 01 H H PLC 01 TT 01 TT 01 G G F F TE 01 TE 01 E E D D REVISIONS DESCRIPTION ZONE REV DATE APPROVED C C F4 02 Changed transducer from Air Products to Automation Direct Universal Mar. 3, 2012 F7 01 Modified wiring for Type K T/C Feb. 24, 2003 B B NCC PLC Training Panel Input Temperature Transducer/Transmitter Loop 01 DWG NO REV A 02 A SIZE FSCMNO NCC145-04 B SHEET 1 OF 1 SCALE None 1 8 7 6 5 4 3 2 5
