Taking fieldbus into hazardous areas

According to industry surveys by the ARC Advisory Group, the process fieldbus market is now primarily split between Foundation Fieldbus and Profibus.

Each is available in two versions - H1 and HSE for Foundation, and DP and PA for Profibus - although Foundation H1 leads the way in terms of both actual installations and future plans to install. The survey highlights, however, that fieldbus networks are more likely to be found on smaller systems than on the very large plants requiring over 2000 I/O points.

To some extent, this is only to be expected. Operators of large installations like oil refineries or petrochemical plants are inherently conservative by nature, and unlikely to take the lead in adopting relatively unproven technologies. More significantly perhaps is the added factor that, by their very nature, large process plants of this kind involve considerable numbers of hazardous areas that generally can only be operated under intrinsically safe conditions.

Although both of the popular fieldbuses were designed from the start to support intrinsically safe (IS) operation, initial designs at the physical (or wiring and connection) layer of a network limited the number of instruments or devices that could be powered on an IS bus segment. Put simply, the more instruments needing power from the bus, the larger is the current required. And as IS barriers effectively work by limiting the current flowing into the hazardous area network, there had to be a limit on the number of instruments per segment.

According to Roger Highton of MTL Instruments, this restriction meant that many installations were being designed with, say, at most only two control loops per segment, with the number of devices on each segment limited to single figures.

That is probably acceptable on smaller plants but on plants with I/O in the thousands, the result could be scores if not hundreds of individual fieldbus networks - all needing to be wired into the distributed control system. This removes at a stroke one of the main advantages of fieldbus, its huge reduction in field wiring and installation costs.

As well as being senior product manager for MTL's hazardous area business unit, Highton also acts as the director for the Fieldbus Foundation's European End User Councils. In both capacities, he has been actively involved in the development of both the FISCO and FNICO concepts in the IEC standards on the use of electrical equipment in explosive gas atmospheres.

FISCO (Fieldbus Intrinsically Safe Concept) is a relatively new standard for IS field wiring that allows more field devices to be incorporated on to longer cable lengths in Zone 1 hazardous areas. According to Highton, it removes the severe current limitations (around 80mA) of earlier hazardous area fieldbus specifications, allowing the same simple design rules to be used as in general purpose fieldbus systems. It also reduces the safety documentation required to just a list of devices on the segment.

In collaboration with Relcom, the Oregon, US-based manufacturer of fieldbus wiring components, MTL has developed two FISCO power supplies that have recently been passed by Emerson Process Management for integration into its DeltaV control system, arguably the most widely installed Foundation Fieldbus-based system of its kind. The Model 9121-IS can provide 110mA of output current, which would typically support more than five IS devices operating in Group IIC, while the 9122-IS model provides up to 250mA to support more than 12 devices in Group IIB. Each power supply repeats the fieldbus signal, so preventing any degradation of signal between the IS network and the non-hazardous parts of the control network.

The FISCO concept comes out of work by the German PTB standards organisation and has also been incorporated into Profibus PA, the IS version of Profibus DP. The FNICO (Fieldbus Non-Incendive Concept) concept, on the other hand, largely derives from the US and is applicable only to Zone 2 and Division 2 hazardous areas.

Despite this less onerous duty, MTL maintains that a FNICO-based fieldbus brings the benefits of FISCO - fewer practical constraints on device numbers, ease of documentation, and so on - to Zone 2 installations, with the added advantage of allowing live working on the hazardous area network without need for gas clearance. The IEC is currently working on a draft that will combine both FISCO and FNICO into the one standard.

Meanwhile, MTL and Relcom offer power supplies for FNICO applications, in addition to a wide range of MegaBlock wiring hubs for both systems.

Another company offering power supplies and connection devices for IS fieldbus systems is Hawke International. Based in Ashton-under-Lyne, Lancashire, Hawke takes neither the FISCO nor FNICO approach, opting instead for a conventional 'entity concept' technology. Rather than concentrating on the IS barrier, Hawke says it approaches the problem from the device coupler point of view. According to the company, its resulting system can drive the most devices and/or longest segment cable of any IS fieldbus solution - up to 16 devices in Group IIC applications with 500m of cable.

Hawke's Route-Master Series 100 (RM100) system features dual redundant power supplies and can support up to eight trunk isolator modules, each of which can provide up to 350mA for the fieldbus devices. The trunk isolator modules can also power FISCO-certified devices, since these have entity parameters (the maximum voltage, current and power allowed for each device) consistent with IS fieldbus requirements.

Hawke argues that, while the FISCO design increases the allowable current from 80mA to around 110mA for IIC applications, it does so at the expense of 'significant operation restrictions', reducing the allowable segment length from 1900m to 1000m, and allowable spur lengths from 120m to 30m.

With MTL claiming that 70 per cent of all Foundation Fieldbus installations worldwide have been implemented using its power supplies and conditioners, Hawke clearly has a battle on its hands for market share. But it has already made a start with the winning of a contract in September to provide Foundation Fieldbus IS power supplies for a $61million Amersham Health diagnostics plant in Lindesnes, Norway. Scheduled for completion in January 2005, this plant will run under the control of an Emerson DeltaV system - perhaps demonstrating yet again the pragmatism that now drives the whole fieldbus debate.