PROFIBUS PA – Saving Money from the Start

  • Post category:PI NEWS
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Recently on a LinkedIn forum dealing with the process industry, someone asked the following question: “I am a new instrumentation engineer. How does 4 mA … 20 mA technology work and why not 0 mA … 20 mA?” What followed was an in-depth analysis of the origins and history of the technology, Ohm’s law, and compressed air. Lost in the heat of the discussion, two people made almost the same comment: “We should leave that behind and just switch to fieldbus!”

Journey back in time to the 1950s and 1960s: Three black and white television channels are a sought-after luxury. There are simple adjustment dials for both volume and brightness. The resolution is 30 lines—around 1280 pixels—with a refresh rate of 12.5 Hz. Electronics is finding its way into the process industry: Instead of compressed air, a thin two-wire cable is used for the power supply and instrument control. The resistance is high: Engineers must now carry a device with them all the time: the multimeter! The counterargument: “With compressed air, I can hear whether it is working if I pull out the hose.” The planner says: “Now I have to switch everything to the new technology and cannot just create a carbon copy of the previous system.” (The photocopier has already been invented, but has not yet been universally introduced.)

At this point, we will avoid repeating the advantages of digital communication using PROFIBUS PA or FOUNDATION fieldbus H1. As is also the case with conversion from compressed air to electrical signals, the benefits can be reaped when all of the tasks from planning and commissioning to operation and maintenance are also adapted.

The practical benefits come from the correct handling of the technology and how you take advantage of these benefits. The fieldbus already starts to pay for itself at commissioning time.

The fieldbus is essential for all those who are serious about the “Internet of Things” and Big Data. In this respect, the following instructions apply to all digital transmission systems, including HART and WirelessHART.

250 automation specialists attended the 2015 PI conference in Speyer. This conference is organized every two years by PROFIBUS and PROFINET International. Practical demonstrations were performed as part of workshops, showing how a fieldbus ensures effective process management. The workshop on planning and explosion protection showed, using the example of PROFIBUS PA, how planning needs to be adjusted, so that the first “profit” from a higher level of efficiency and shorter project duration can be seen at commissioning time:

Use typical variables, models, and templates! For PROFIBUS, the application profile PA 3.02 describes a standardized template example independent of manufacturer and software revision for each instrument type, which governs the devices and control technology. In practical terms, this means: configure the address, connect, and run. The PROFIBUS PA profile also handles device changes without any problems. The maintenance engineer no longer has to worry about compatibility between device and control technology. The new instrument is automatically compatible: configure the address, replace the device, connect, and run.

According to Pareto, approximately 80% of the loops can be efficiently planned and tested in advance and the parameters optimized. The purchasing department orders the devices with the optimized parameters, which are configured by the manufacturer in the factory. The majority of the measurements should be implemented in this way. This also minimizes issues of DCS integration.

Take Advantage of the Added Value of Devices! Measuring instruments with multiple variables eliminate the need for additional compensation calculations or supply values for energy, density, and mass in one telegram. The manufacturers will be happy to show how it works and how digital technology helps increase accuracy to the point where it can be utilized for billing purposes.

Plan the Installation in Advance! Trunk-and-spur topology is the industry standard for installation. This topology is simple to understand and corresponds conceptually to the old terminal board. Each field device is individually accessible. The limits for geographic dimensions or the device count are checked in advance taking cable length and voltage drop into consideration, e.g., with the free Segment Checker tool.

Segment Checker: 11 field devices, each with 100 m spur and 700 m trunk—plenty for most applications and still expandable

Verification of intrinsic safety in accordance with FISCO can be carried out at the same time using the documentation without additional costs. With the high-power trunk, cables over 1000 m long present no problems, even in hazardous areas up to Zone 0/Div. 1.

Check and define the shielding and grounding concept as part of planning activities. There are no standards for this. A specialist develops a suitable concept depending on conditions, such as the electrical installation, the geographic conditions, and so on. Don’t skip this step. It is crucial to establishing reliable communication.

Decide on the need for lightning protection at an early stage. It cannot be retrofitted in the control cabinet or in the field junction box. Modern lightning protection is self-monitoring and can automatically report to the plant asset management system when it ceases to function. This is real savings for the maintenance team, whose equipment can be affected by power surges or lightning strikes.

Special care should be taken with shielding. This is new and different! Therefore, the installation personnel should be informed about the small differences in a one-day training session. Otherwise, a fieldbus device is connected just as easily as a device with a 4 mA … 20 mA connection.

Time saved, satisfaction secured
Well-thought-out planning will be rewarded with a significant reduction in commissioning time—especially when compared with conventional 4 mA … 20 mA technology. A reduction of several weeks is possible for larger systems, for example, one with 1200 instruments. The production management will certainly be pleased about this. With automatically updating documentation in the control system, many of the corrections required in the documentation that would otherwise be performed manually are no longer necessary. A technical white paper from Pepperl+Fuchs discusses best operating practices for commissioning of fieldbus systems made efficient through diagnostics. It illustrates the potential efficiency and time saving in the context of a case study. The white paper is free to download.

One way or another: The next plant after this one should be planned from carbon copies or photocopies of projects planned using digital technology, right? No, of course not. Your best bet is to create a copy of your project in the Cloud and access the data from your smartphone. Just like you access the data in your field device—made possible by your fieldbus. You no longer take a compact transistor radio to work with you, do you?

Andreas Hennecke, Pepperl+Fuchs