Modular barriers cut hazardous area maintenance

Author : Paul Gay

25 May 2010

A new generation of fieldbus barrier wiring hubs for connecting intrinsically safe field instruments to Foundation fieldbus networks has been launched into the hazardous area market by MTL Instruments. Networks using the 9370-FB Series Fieldbus Barrier carry sufficient energy to power the instruments in an intrinsically safe manner in the hazardous area.

Phil Saward, MTL’s Technical Manager for Industrial Networks, explained: “We are confident that the product’s innovative features will help users to achieve the improvements in efficiency and plant safety that are required in today’s demanding business environment. MTL responded to the demand for greater flexibility when using the High Energy Trunk technique and has exceeded customers’ expectations for easy installation and maintenance of field-mounted equipment.”

The system converts the energetic signals from the control system’s trunk line to multiple intrinsically safe spur lines, which connect to the network powered field instrumentation in the hazardous area. A single hub can accommodate up to 12 spurs which are intrinsically safe and therefore hot swappable permitting maintenance and modification of instrument circuits without the need to power down the network.

Previous generations of network barriers were DIN rail mounted but involved considerable wiring to connect the spurs. The complexity of wiring has been overcome with the introduction of the new 9370-FB modules which achieve their isolation characteristics by using a switching technology developed by MTL’s parent, Cooper Crouse Hinds.

The result is a product which minimises the time maintenance in the hazardous area; a procedure that took half an hour with traditional barriers now takes five minutes, claimed Saward. All parts of the system containing complex electronic circuits are housed in pluggable modules that can be removed and replaced while the equipment is live. This is in sharp contrast to conventional implementations, where serviceability is made difficult or impossible by the hard-wired nature of the field enclosure.

The High Energy Trunk technique removes the drawbacks associated with traditional Fieldbus Barrier implementations and according to MTL results in lower cost, safer operation and higher reliability throughout the life-cycle of the fieldbus network. The 9370-FB Series is 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.

Optional, integrated surge protection is also a key feature of the 9370-FB Series, and includes plug-in devices to protect the barrier modules from transients transmitted along the trunk and spur cables. In conjunction with additional protectors located at the fieldbus host, and at the fieldbus instruments, they allow the user to install and operate fully protected networks.

Unlike conventional fieldbus barrier enclosures, the plug-in surge protection devices can be included at the time of installation or retrofitted once the system is in service. This streamlines the selection process, avoids clumsy over-specification and allows protection to be added in response to site experience.

The barrier modules, terminator and optional surge protection devices are all certified for removal, and replacement, in the hazardous area without causing an ignition risk. This certification not only means safer maintenance, but also 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 easily 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.

MTL’s target markets for the barrier modules include the oil and gas and petrochemical sectors where distributed control systems are dominant in process automation. Pre-launch versions of the product have proved their reliability in the field and have been adopted by several leading companies.