24 May 2010
In the early days of fieldbus, plants tested the technology with relatively small installations involving a few dozen instruments. But today, the process industry is making serious deployments of hundreds of instruments organised with hundreds of spur lines. To satisfy this need installers and maintenance people are calling for better ways of accessing the wiring hubs that are at the crucial point of the network.
These wiring hubs, or fieldbus barriers are located at the end of the high energy trunk line (sometimes called the ‘home run’). It is the point in the network where the signal comes from the control system, enters the hazardous area of the plant and has its signals converted to intrinsically safe levels and distributed to individual instruments or valves on spur lines.
One spur line could technically go to a dozen instruments, but in practice most installations put far fewer devices on one line, even as few as three or four if they are critical to the operation.
While the barrier itself is a fairly complex piece of electronic equipment, basically the installer only has to wire the trunk line in, and the spur lines out.
However, in one respect this simplicity is deceptive. Consider this combination—a high energy, non-intrinsically safe cable connected into the same box with intrinsically safe spur lines, all wired together inside an enclosure that is itself mounted in an explosive environment. Any work a maintenance engineer would do on this box would have to be performed very carefully.
Some boxes uses separate isolation switches to disconnect the trunk cable from the spurs. Other procedures call for de-energising the trunk cable completely before maintenance is performed, which of course has the disadvantage of shutting down the entire fieldbus segment. In general, fieldbus barriers in hazardous areas are not ‘live workable.’
Phil Saward, MTL’s Technical Manager for Industrial Networks, says it requires a bit of ‘electronic gymnastics’ working on barriers in hazardous areas.
‘Any maintenance activity in a hazardous area is excruciatingly expensive,’ he says. ‘Many end uses, quite frankly, don’t like fieldbus barriers. They don’t like working with them.’
9370-FB solution
Mr. Saward says MTL proposes a solution with its 9370-FB Series Fieldbus Barrier, designed specifically for connecting intrinsically safe field instruments to Foundation fieldbus networks in hazardous areas. It retains the use of the ‘High Energy Trunk’ technique.
All parts of the system containing complex electronic circuits are housed in pluggable modules that, unlike conventional fieldbus barriers, can be removed and replaced while the equipment is live.
This makes tracing and fixing faults easier, since rapid substitution can be used to pin-point the source of problems. Like it or not, today’s fault finding is often done by substitution and you can do this more quickly with the 9370 says Mr. Saward.
The layout
The trunk line enters the box on the lower left side and is connected into a special chamber that is isolated from the rest of the box. The one rule for maintenance personnel is: do not to open this chamber, under any circumstances. (Perhaps they will considering putting a padlock on it.)
Everything else inside the box is live workable, in hazardous environments.
This means the barrier modules, spur terminators and optional surge protection devices are all certified for removal, and replacement, in the hazardous area without causing an ignition risk. The barrier module—which accommodates up to six spur lines—can be released with only three screws.
This certification provides the ability for expansion in service. For example a 12-spur enclosure, fitted with one 6-spur module, can be optionally specified and installed; expansion to 12 active spurs is accomplished by plugging a second module into the enclosure. No other configuration is necessary, provided the fieldbus power supply is correctly sized and the full loading of the segment has been anticipated.
The use of an innovative flameproof connector from Crouse-Hinds makes the live working possible, explained Mr. Saward.
The 9370-FB Series are modular in construction, comprising of a fixed carrier assembly and pluggable components. These are assembled to form complete, self-contained enclosure systems in either stainless steel or carbon-loaded GRP, and sized according to the application requirements.
Surge protection
At the same time, MTL has introduced the FS32 Surge Protection Device (SPD) that can be installed in the 9370-FB or retrofitted to Foundation fieldbus systems.
The FS32 prevents surges and transient over-voltages conducted along the Trunk, or Spurs, of fieldbus systems from damaging the associated electronics such as terminators, spur blocks and the bus control equipment.
The FS32 uses a combination of solid state electronics and a gas filled discharge tube to provide surge protection up to 20kA. The surge protection circuit is designed to exhibit exceptionally low line resistance, and has negligible voltage drop to the spurs, says Mr. Saward.
In operation the FS32 does not adversely affect the performance, or operation, of the fieldbus or connected equipment. It allows signals to pass with little attenuation, while diverting surge currents safely to earth (ground) and clamping output voltages to safe levels.
MTL at the physical layer
Commenting on MTL’s position with fieldbus technology, Mr. Saward said, ‘With the introduction of the 9370-FB Series Fieldbus Barrier, MTL Instruments has strengthened its portfolio of solutions for taking Foundation fieldbus networks into hazardous areas.
‘MTL can now provide the complete range of physical layer products that satisfy both the FISCO (Fieldbus Intrinsically Safe Concept) and Fieldbus Barrier techniques for connecting to intrinsically safe fieldbus instruments in hazardous area networks.
‘MTL’s FISCO solution, launched in 2009, combines unique power supply redundancy with the benefits of the intrinsically safe technique to provide the highest levels of system availability and overall safety.
‘It is particularly suited to offshore applications, where it has been adopted by oil majors who require compact, lightweight field equipment and the low safety risk provided by intrinsically safe wiring.
‘The 9370-FB Fieldbus Barrier provides an alternative solution for applications that require heavily loaded fieldbus segments and long cable lengths, such as onshore refinery complexes.
‘While still supporting intrinsically safe fieldbus devices at the spur connections, the fieldbus trunk is implemented using the ‘increased safety’ technique that allows higher levels of power to be transmitted into the hazardous area. In contrast with existing Fieldbus Barrier implementations, the 9370-FB has ‘live pluggable’ electronic modules that remove the risks associated with maintenance of electronic apparatus in hazardous areas.
‘Plant operators can therefore choose between the FISCO and Fieldbus Barrier techniques from MTL to find fieldbus architectures that are best suited to their plant geographies and safety philosophies.’
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