Loop drawings for smart instruments

March 12, 2013
Readers look to our experts for information on smart instruments

"Ask the Experts" is moderated by Béla Lipták, automation and safety consultant, who is also the editor of the Instrument and Automation Engineers' Handbook (IAEH). If you would like to become a contributing author of the 5th edition, or if you have an automation-related question for this column, write to [email protected]

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
[email protected]

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

P&ID documentation

Figure 1: Communication link symbols.

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
[email protected]

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.

Redundant FISCO segment diagram— components

Figure 2: An example of a segment diagram that serves the function of a loop diagram.

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
[email protected]

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
[email protected]

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)
[email protected]

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
[email protected]

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.
[email protected]

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
[email protected]

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
[email protected]

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
[email protected]

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

George Erk
[email protected]

See Also: What are Good Signal Filtering Tips?

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
[email protected]

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
[email protected]

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
[email protected]

See Also: If you can't read a P&ID diagram, you need this self-study course

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.

There must be some drawing that shows the location of the component field devices, interface devices, power supplies, terminators and junction blocks, even showing the routing if necessary. There must be some drawing that shows the function of each set of devices, perhaps as analog function blocks or discrete wiring diagrams, with some narrative that explains the use, operation and maintenance issues of the devices, as well as any associated hazards. There could be a common set of data sheets for devices by type.

Because a fieldbus is a communication system, there must be some sort of traffic diagram showing sources and destinations. Such a diagram shows interdependencies, such as who will be affected if a device is removed. This could be a problem for a mesh network, which can reroute messages through other devices.

Bill Hawkins
[email protected]

A: It wasn't that long ago those of us in the process industries—at least in the United States—had to be compelled to update P&I drawings under penalty of law . . . broadly speaking.

If there's no will to update P&IDs, then what's the likelihood loop diagrams or segment diagrams are updated? When the systems engineer decides she's using TI-101 to compensate FI-102, does she religiously make a note on the affected loop sheets, so the plant operator will be aware when he pulls the RTD?

See Also: Maximizing Control-Loop Performance

We evidently didn't have this religion when we were numerous (in relative terms) and now—at least in North America—we are few, perhaps too few. The "lifecycle cost" of useful and reliable loop sheets has already been priced out of the long-term budget (by austere staffing levels) . . . why should projects pay for them? And with the complexity and flexibility of microprocessor-based control systems, is there anyone knowledgeable and insightful enough to certify that every connection and interaction is clearly presented in a manner so that the fellow on the 3 a.m. call-out will notice and comprehend? I wonder how scarce such experts are, whether at clients or contractors, and how much time they have for such an exercise, should we convince our project managers the effort is worth it. My guess is they will be consumed with other complex and high-value-added chores, such as alarm rationalization, HMI optimization, LOPA and SIF analyses, and so on.

Our economies and our technology have perhaps outstripped the ability of paper to ever do the job again, except in the very simplest of process plants. I think the next handheld—maybe the 575?—will have a little wand like Mr. Spock's tricorder, and one will be able to wave it over an instrument, rendering a listing of all its interconnections in the system, as well as a diagnosis of its ailments. Or maybe it will be an app for your iPhone. Would someone invent it, please? And please make sure it's priced below my purchasing card's limit so I don't have to apply for capital funding to buy it . . .

In the real world, I have heard of plants creating "live" loop sheets or segment drawings on the DCS, populated with real-time data, and with the potential for right-click access to cross-references, diagnostics and manuals. If I were going to go to battle for some funding, I think that's what I'd be looking to create, in lieu of CAD drawings or paper . . . Hard disk space is cheap, and selected DCS workstations can be rendered read-only so even the most fat-fingered plant operator can drill down for relevant information.

John Rezabek
[email protected]

About the Author

Béla Lipták | Columnist and Control Consultant

Béla Lipták is an automation and safety consultant and editor of the Instrument and Automation Engineers’ Handbook (IAEH).

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