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Fieldbus payback from the commissioning stage

14 July 2015

Andreas Hennecke, product marketing manager Fieldbus Technology at Pepperl+Fuchs, looks at why using a fieldbus as the infrastructure to power instrumentation requires customised working processes He also looks at the benefits it can offer to both planners and plant engineers.

Recently on a LinkedIn forum dealing with the process industry, a new instrumentation engineer asked how 4 -20mA technology works and asked why not 0-20mA.  Ann in-depth discussion followed looking at the 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!"

If we could journey back in time to the 1950s and 1960s we would find that three black and white television channels were considered to be a sought-after luxury, with simple adjustment dials for both volume and brightness. The resolution would be 30 lines – around 1280 pixels – with a refresh rate of 12.5 Hz. 

At the same time we would see electronics finding its way into the process industry. Instead of compressed air, a thin two-wire cable stared to be used for the power supply and instrument control. Resistance to this technology change was high among engineers who were required to carry a device with them all the time – the multimeter. Their argument against change was that with compressed air it was possible to hear whether a system was working if you just pulled out the hose. The planner’s argument was they had switch everything to the new technology and could not just create a carbon copy of the previous system. 

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
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 (IoT) and Big Data. In this respect, the following instructions apply to all digital transmission systems, including HART and WirelessHART. 

Around 250 automation specialists recently attended the 2015 PI conference in Speyer. This conference is organised every two years by PROFIBUS and PROFINET International (www.profibus.com). 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.

The advice was to use typical variables, models, and templates. For PROFIBUS, the application profile PA 3.02 describes a standardised template example, independent of manufacturer and software revision, for each instrument type, which governs the devices and control technology. In practical terms, this means you should be able to 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. 

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

Take advantage of added value 
Do take advantage of the added value of devices. Measuring instruments with multiple variables eliminates the need for additional compensation calculations or supply values for energy, density, and mass in one telegram. Manufacturers should be happy to show how it works and how digital technology can help increase accuracy to the point where it can be utilised for billing purposes. 

Do also 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, for example with the free Segment Checker tool (www.segmentchecker.com). 

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

Shielding and grounding
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. Never skip this step because 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 can offer real savings for the maintenance team, whose equipment can be affected by power surges or lightning strikes. 

Special care should also be taken with shielding. This is new and different so 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.

Well-thought-out planning will be rewarded with a significant reduction in commissioning time – especially when compared with conventional 4-20mA 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 from www.pepperl-fuchs.de/fdh1.


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