Foundation Fieldbus Implementation
A fieldbus network consists of a power supply, power conditioner, optional repeater(s), cables, junction box(es), terminators and devices. The control loops configured on the network use function blocks that can be executed in the instrument, host device or a combination of both. The Foundation fieldbus specification covers the protocol, cabling, topology and other details. For more information, visit the Fieldbus Foundation website.
Like traditional 4-20 mA circuit designs, Foundation fieldbus can be implemented in hazardous locations using explosion-proof, intrinsically safe and nonincendive designs.
Explosion-proof designs are not practical for fieldbus implementations due to the cost, bulky enclosure size and inability to work on the network while energized without a hot-work permit.
Traditional Entity-based fieldbus designs use safety barriers and limit the available bus current to about 80 mA. A typical fieldbus device might draw 20 mA of current, meaning that, in theory, up to four devices on a segment could be powered up. Allowing only four devices does not realize the possible economic benefit of a network design when compared to traditional 4-20 mA, Entity-based implementations.
In an effort to reduce end-user engineering and increase the bus current available, two standards were created specifically for Foundation fieldbus implementation in hazardous areas.
The Fieldbus Intrinsically Safe COncept (FISCO) specification considers IS fieldbus as a system that allows the end user to specify FISCO-certified devices and integrate them without the engineering requirements of the Entity approach. The available bus current is increased to 120 mA in the IIC (A/B) gas group and 265 mA in the IIB (C/D) gas group. Using 20 mA devices, that would theoretically equate to six and 13 devices respectively. Engineering requirements to create safety documents are reduced, and the only safety documentation required is a listing of the devices utilized on the network. Reduced engineering and an increased number of devices are two obvious advantages of FISCO.
The Fieldbus NonIncendive COncept (FNICO), a derivative of FISCO, is a specification that considers nonincendive fieldbus as a system. It is intended for division 2 classifications and takes advantage of the less stringent requirements of a nonincendive design. End users specify FNICO-certified devices and integrate them without the engineering requirements of the Entity approach.
The available bus current is also increased to 180 mA in a IIC (A/B) gas group and 320 mA in IIB (C/D) gas group. Using the 20 mA device, that would theoretically equate to 9 devices and 16 devices respectively. For the same reasons, FNICO is an obvious advantage over Entity. FNICO has a bigger advantage over Entity in a division 2 area, but because of their reduced safety factor, they are only allowed in division 2 areas.
The High Power Trunk (HPT) is a hybrid approach, where the fieldbus trunk is installed nonincendive (non-sparking and not FNICO), and the individual device spurs are installed as intrinsically safe spurs. The trunk and safety barrier is installed in either a safe or division 2 area, and the spurs can be wired to devices located in either a division 1 or 2 area. The devices can be Entity, FISCO, FNICO or a combination. In the HPT design, the trunk cannot be worked on while energized without a hot-work permit; however, the spurs can be.
In summary, implementing Foundation fieldbus can seem like a daunting task, and when coupled with hazardous-area considerations, may approach information overload; however, it does not have to be an explosive concept. Believe it or not, with today’s technology and product offerings, fieldbus is simpler than anytime before to implement.
Determing the Right ISS Fieldbus
Key questions to ask when considering implementation:
- What’s the area classification? Nonincendive is allowed only in division 2 areas; intrinsically safe designs are allowed in division 1 and 2 areas.
- What are the size and scalability requirements? How many devices do you plan to implement? What are your future requirements for expansion? How long does the trunk need to be?
- What are the technology/product characteristics? Does the product provide short-circuit protection for the trunk and the spurs? If during maintenance you accidentally short a spur, will the trunk be protected? Are the system components hot-swappable?
- What about risk? What level of safety or risk are you willing to accept? Intrinsically safe designs take into account component failures and allow maintenance while energized without a hot-work permit. Nonincendive designs do not take into account component failures and may not allow maintenance while energized without a hot-work permit.
- What about maintenance and downtime? Can your process be down (fieldbus de-energized) in order to troubleshoot or expand the network? Does your facility have a hot-work permit philosophy allowing live work on energized equipment?
- Who’s doing the engineering? Will the engineering be done in-house? If you are using an engineering firm, interview the proposed staff. Try to use a firm that has experience in hazardous-area fieldbus design because this implementation is a hybrid approach for many A&E firms. The design requires knowledge of hazardous area classification, which is primarily an electrical engineering function, as well as control systems design experience, primarily a control-system engineering function. In many cases, these resources may exist in two separate departments (electrical and control systems).
- What about the control system? What type of control system will be used? Contact your control system vendor and inquire about technologies and products that have been tested and proven on your control system. Are there any software limitations or hidden costs that punish you when using fieldbus on your control system?
- What about technician expertise? What is the skill set of the maintenance technician(s) that will support this technology? Will training be required?
- How will the devices be calibrated? Until recently, calibrating fieldbus devices in-house was difficult. In the past couple of years a small number of fieldbus calibration devices have appeared on the market.
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