Loop Drawings for Smart Instruments

Readers Look to Our Experts for Information on Smart Instruments

By Bela Liptak

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A lot of this arises from the constraints imposed by drawing automation tools/instrument databases.

Simon Lucchini

A: Control system deliverables are moving away from the realms of ISA-based conventional drafting as they are more and more driven by the self-documenting software tools used. Engineers do not need to do as-built drawings as a separate exercise.

Loop drawings' content is driven by the client maintenance person who is using it. In most cases, it could be too simple as the diagnostics tool, and one does not really need reference to loops as there are not too many wires to trace in the field or in the system cabinets.

See Also: Control Loop Improvement

Clients are moving away from individual loop drawings, and are content with segment drawings irrespective of the fieldbus technology used.

Clients are demanding more value for the money they spend, and want to limit engineering costs as the hardware has been already made commercially off the shelf (COTS).

L. Rajagopalan (Raj)

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: The insurance cost would be high after a fire or an accident that occurs to any plant that has no loop drawings. The responsibility is the owner's if he or she takes the risk. ISA needs a universal language and interpretation on loop drawings.

Gerald Liu, P. Eng.

A: I have witnessed a real move away from loop drawings in the last 10 to 15 years. This has paralleled the overall tightening of budgets, both for capital projects, which has translated to shedding any "optional" design documentation, such as loop drawings, and for plant maintenance and engineering support, which has translated to reduced E&I technician and controls engineering staffing levels and higher turnover. 

See Also: Cascade, Scan Time, PID Tuning

In a lean plant environment, loop drawings are more of a luxury, mostly duplicating information found elsewhere, even though elsewhere means at least four other documents (P&ID, I/O wiring diagrams, piping diagrams, instrument spec sheets, and possibly SAMAs, logics, cable schedules and instrument lists). At the same time, in some plants, there has been a greater adoption of SAMA diagrams, sometimes better referred to as "control functional diagrams," loosely based on the old SAMA standard, but updated and adopted to control of industrial processes.

I agree as well that loop drawings are particularly useful, especially to plant personnel, because one must not refer to a number of other documents to get the whole picture of a loop as historically provided on a single loop drawing. Nonetheless, loop drawings have become viewed as a luxury by those seeking places to cut design and engineering costs. It's more difficult to assign a value to the maintenance and reliability costs attributable to inadequate or less effective/efficient forms of documentation, while it's relatively easy on a capital project to decide it's not needed for installation and saves X design dollars.

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.

See Also: What is the Best PID Execution Time?

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.

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