Loop drawings for smart instruments

Readers look to our experts for information on smart instruments

By Bela Liptak

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It may save a few pennies now, but if you need to reconstruct a loop during or after an incident or at the proverbial 3 a.m. on Saturday morning, the lost production revenues quickly exceed the cost of preparing the minimum level of documentation. We need to get more folks thinking total lifecycle cost rather than just their portion of the lifecycle.

Ian Verhappen

A: I have not seen a "true" loop drawing in over 15 years. They are now merely a wiring drawing in another form and are more geared to construction/commissioning rather than operations/maintenance.

A true loop drawing would convey some sense of process functionality. There should be some indication of the related components. Often there would also be a simplified process sketch. Furthermore, many loop drawings do not even show the power supply details pertaining to the instrument; e.g., how to isolate the 24VDC from an instrument; what are the fuse ratings? It makes me wonder if anyone actually uses loop drawings anymore.

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.

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