Loop Drawings for Smart Instruments

Readers Look to Our Experts for Information on Smart Instruments

By Bela Liptak

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Q: I am looking for information on loop drawings for smart instruments and sample drawings. If you could point me in the right direction it would be appreciated.

Paul Bulmer

A: Loop drawings for "smart instruments" are the same as for analog loops, except for the communication links between the components (Figure 1). The loop component symbols are given in ANSI/ISA5.1 (2009).

As to the need for loop diagrams, there is disagreement. In the view of American engineering design firms, loop drawings are not really necessary (are a luxury) because if the P&ID flow sheets are sufficiently detailed, then the information that the loop drawings provide can be obtained from the instrument index, specifications, I/O lists and wiring diagrams, piping diagrams, logics, cable schedules, etc. This view has evolved because the main goal of these firms is to minimize the number of man-hour-consuming documents, and thereby gain a competitive edge by tightening budgets through limiting engineering costs.

See Also: Going with the Flow

In other offices and in other parts of the world, such as Asia, the criterion is total cost, which includes not only design, but also operating, maintenance and insurance costs, and from that perspective, loop drawings are desirable because they make the loops easy to understand, as you do not need to look at several documents. Those firms consider the generating of loop diagrams valuable and use ISA5.4 as the basis, although it has not been updated for over a decade and does not give examples for fieldbus loops. For this group of engineers, it would be useful if ISA5.4 covered the protocols used; e.g., Foundation fieldbus, Profibus, Profinet, HART, Wireless HART, DeviceNET, ControlNET, ASi, etc.

So it seems that the main cause of the elimination of loop drawings is economic and can be short-sighted, because having them serves not only operational and maintenance convenience, but also can improve safety and thereby lower insurance costs.

Béla Lipták

A: Figure 1 shows the P&ID representation that I have been using based on latest version of ISA5.1, and Figure 2 is a sample of a segment diagram, which can be considered to be a replacement for the loop diagrams that I use when presenting certified training for the Fieldbus Foundation.

There is a trend toward using database-driven loop diagrams or their equivalent. This is well and good for the large facilities, however, there are far more smaller operations that continue to rely on the printed word or manually created CAD drawings for installation, commissioning and troubleshooting.

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.

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