Loop drawings for smart instruments

Readers look to our experts for information on smart instruments

By Bela Liptak

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Another trend, perhaps a factor in the decline of loop drawings, has been reduced participation and influence by plant maintenance personnel on capital projects. I recall a time when E&I tech input carried a fair amount of weight during the design and engineering phase, though I'm sure that also varies considerably, depending on the corporate culture.

R. H. (Rick) Meeker, Jr., PE

A:  Loop drawings for smart instruments are no different than those for conventional instrumentation. The difference in the two systems is the digital component of the signal. The HART Communications protocol is used by most smart instrument vendors to impose a digital signal on the 4-20 mA signal. This allows additional information to be carried on the instrument wiring matrix. The addition of a digital component in the signal also allows other wiring methods, such as fieldbus and Ethernet, to be used instead of conventional wiring.

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For examples of loop drawings, I suggest you search Google Images for "instrument loop drawings" for several styles of loop diagrams.

John Dressel

A: In my experience, loop drawings are a must-have. They help technicians in troubleshooting. If you are using software, like InTools for design, they are easily done.

H S Gambhir

A: We need to get more folks thinking lifecycle cost rather than just their portion of the lifecycle.

George Erk

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A: As far as I am aware, there are no international standards yet for "smart" instrument loop drawings. You can either follow the ISA standard (S5.1) or the client's standard. However, a cover sheet is required to explain all the symbols used in the drawings.

Smart instruments connected to an asset management system require additional components, depending on what protocol is used (e.g., Foundation fieldbus, Profibus, Profinet, HART, Wireless HART, DeviceNET, ControlNET, ASi etc).

Raj Sreenevasan

A: There is a significant camp of people out there who believe it is NOT necessary to show the type of signal on a P&ID, since this information is not really important to the process people who normally use this drawing. The type of communication would of course be indicated in the instrument index from which the I/O list is prepared, as well as the appropriate data sheets and segment/network/loop diagram.

ISA5.4 is in need of an update to include verbiage and examples of loops using "fieldbus." (I use the quotation marks since there is Foundation fieldbus as well as other generic forms of this mode of communications.) However, the only real difference in showing "smart" instrumentation on loop drawings is the symbol for the communication link between the components of the loop, plus knowledge of the specific instrumentation vendor's terminal designations and interconnection requirements. And while ISA5.4 may be out of date, ANSI/ISA5.1 (2009) is current with the needed symbols to depict fieldbus communications.

Thomas C. McAvinew, PE

A: There are updates to show new transmission methods for wired digital connection and wireless, but a quick scan does not reveal any changes for the "smart-ness" on the symbol. I do not see any construction for CiF (control in the field) that one would use for Foundation fieldbus. There is no designation for the node (instrument) that holds the Foundation fieldbus link active scheduler (LAS) or the backup (bLAS). While there are indications in the symbols for showing on the "panel," there is no designation for indication where (which operator station or stations) the loop display should appear, as one might specify for a DCS to link the loop diagram to the DCS database.

Users are interested in the loop diagrams and symbols, but not enough to participate on the standards committees. Vendors don't care, since they cannot generate competitive advantage from these. Engineering contractors really care, but they have their own standards and probably don't want to share with ISA for fear that they may need to change their standards if ISA selects a different one.

I believe that ISA 5.4 is not ready for distribution of the update – long overdue.

Dick Caro

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A: Loop sheets evolved for wired 4-20 mA designs. A fieldbus reduces wiring, so something else is needed. The scope of a fieldbus is more appropriate to a single process unit or machine or to a group of equipment pieces linked by a common function. The information would be gathered in a maintenance manual rather than distributed over many loop sheets. The manual can be electronic, if suitable readers are available. A project manager might find it harder to say no to maintenance manuals.

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