Decode your Digital Future

Understand where your plant can achieve benefits from a digital fieldbus

By Ian Verhappen, MTL Instruments

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Justifying an expenditure can be challenging — especially when you want to introduce a new technology to your operation such as one of the fieldbuses that form the basis for the digital plants of the future. So, here, we’ll look at the advantages and savings that you realistically can expect with such a fieldbus.

Justification differs depending on whether you are in the capital phase (engineering, design, construction and commissioning) or operations phase, which typically accounts for the 80% of the total lifecycle cost of the project. It can be tough in either phase. After all, during the capital phase, the project manager wants to have the work completed yesterday for as little money as possible. During operations, the plant manager wants to have the plant run continuously (that is, with no maintenance or downtime) for as little money as possible. So, you must make a strong case — and this demands a realistic appraisal of benefits.

Each facility, of course, is unique and will have its own absolute values for determining the actual return on investment. However, there’re some useful general guides on where to look for justification opportunities for digital fieldbuses.

Capital phase
Don’t expect any significant savings in the engineering and design cost at present because all the systems and procedures in place at most companies are designed for analog loops. As engineering companies become more familiar with digital technologies and develop the associated tools and procedures, potential savings may turn up in the following areas:

Documentation/drawings. To achieve potential savings in documentation, the end user must review the purpose of each of the drawings to be generated. For example, a loop diagram is used to identify all the components — field devices (sometimes including the input transmitters and control devices), intermediate terminals and junction boxes, and the control system terminations — for a single signal or control loop. For an analog system, this normally is a single wire pair for each input or output signal.

With fieldbus, there’re multiple devices on a single wire pair; so to continue to identify all the components that could be affected by a change to one device on the wire loop, you’ll have all the devices on that network represented on a single drawing. This potentially can reduce the number of drawings by an order of magnitude.

Also other drawings, such as those for the marshalling cabinet, could be eliminated altogether because with fieldbus communications there’s only one type of signal wire in the system and this wire can be brought straight through to the control system cabinet.

Construction. This is the most intensive part of the project and also where most of the “problems” associated with digital systems are created. Just as with analyzer sample systems where more than 80% of the problems result from the sample system, with fieldbus systems more than 80% of the issues come from the installation of the network. Fortunately, field staff are becoming more familiar with some of the fieldbus technologies and vendors now are developing better diagnostic tools for spotting potential problems prior to full system commissioning.

One difficult-to-quantify benefit of using digital systems is the ease with which they can be expanded for minimal change in capital cost. This is especially important today with the increasing use of modular systems and skids. Fieldbus systems make it possible to pre-wire the module or skid at the fabrication facility and land a single cable in a junction box on the edge of the module to complete the tie-in to the control system. This same junction box also can be used to connect a laptop version of the control system to completely test the functionality of the unit before it leaves the fabricator’s yard.

More functionality from devices. One potential digital system advantage often overlooked is the ability to use a single field device for multiple signals on the single pair of wires. An obvious example of this is a Coriolis meter, where flow, mass, volume and temperature as a minimum can be obtained. Other opportunities include using differential pressure meters also to record bulk pressure and vortex flow meters also to measure fluid temperature. These latter two can provide as a third calculated signal flow measurements compensated for fluid expansion as a function of temperature (larger effect on liquids) and pressure (larger effect on gases).

For control valves, limit switches to detect the end limits of the valve stroke either can be virtual in the device or, in the case of some manufacturers, simply a short wire from the switch to a terminal block in the positioner (effectively a jumper) that reads and transmits the signal to the control system.

Besides this feature, digital positioners can include customized characterisation curves that allow the process engineer to truly match the behavior of the valve to the process. For example, the process may require quick opening for the first 10% of the signal to get the valve on or off the seat to avoid chattering, linear behavior from 10% to 75% (the range of the valve where most control happens) and then equal percentage for the final 25% of the stroke to provide better linearity at the upper end of the valve opening position.

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