Fieldbus in hazardous areas
26 June 2015
Richard Barnes, a Senior Consultant at Emerson Process Management and Chairman of the FieldComm Group UK Marketing Committee, looks at some of the advantages of using FOUNDATION Fieldbus in hazardous areas.
Everyone wants to be successful. There is a saying though: be careful what you wish for - it may come true. FOUNDATION Fieldbus is successful. But its very success in establishing itself as a standard solution within process control and monitoring means its visibility is reduced!
Where FOUNDATION Fieldbus wins during deployment into hazardous areas is through two main areas.
The first area is through the reduced cable complexity of dropping multiple devices from the databus. Safe wiring can be made more secure, simple to deploy and supported through technology. The second area is the capability to automate the manner the fieldbus assets and their status is displayed to users via asset management software. This means users can evaluate the condition of the assets, analyse performance and plan for corrective action all whilst within a safe area. Figure 1 below shows one option for a high power trunk into the hazardous area. The spurs to devices from the fieldbus coupler use the Intrinsic Safety principle; the segment from host interface to fieldbus coupler is mechanically protected. The combination offers the users both complexity but in a simple to use packaged solution. When deployed into the hazardous area regulatory control function also follows and allows the users unparalleled flexibility.
The second area is through the availability of supplementary information relating to the status of the digital communications and the field based asset condition based upon self-test subroutines. This data allows the user to consider the asset in terms of its criticality to the business and the possibility to use this analysis information to plan maintenance and support activities.
If we return to the preliminary area the field network nodes require excitation from the bus and this power requirement of course increases per node. This by definition limits the number of devices that can be placed upon the bus when the spark ignition limitation is in place however careful planning and application of the monitoring and control strategy does not reduce the effectiveness of the bus technology or the savings that can be won.
When the hazardous protection philosophy is through mechanical containment, or the flameproofing of field devices and networking components, then the technology limitations are only limited by the drawbacks of maintaining the mixed protection philosophy. In itself the technology for nodes per segment is up to 32 however in practice the deployment of devices per master interface is limited to 16 devices to allow for the number of links and permitting a reasonable timing on the bus for real-time monitoring and control. Safety margins and future expansion often limit the number of devices per bus to between 8 and 12. Not to be forgotten is that many devices allow for multiple monitoring and control parameters and functions.
Process parameters are the primary reason for process automation however the technology to support this also permits overall business improvements and this leads us back to the secondary aspect to the win situation mentioned before. Provided the power is maintained to the segment under host fault condition, the function blocks embedded in the devices and on the segment will continue. Configuration of the function blocks will provide the parameters upon loss of communication to the host interface, to maintain process control relative to the last known set-point download or either ramp-up or ramp-down to a known good safe and controlled condition.
The process monitoring and control parameters in FOUNDATION Fieldbus come with status attached that can be used and reported to the control strategies and user interfaces. In addition to this the field device initiates it own self-test upon itself, the assembly it is part of and its environmental factors it has sensors in built to detect. The results of these can permit a skilfully designed device to provide not only failed asset status alarms but also predictive alarms that users can leverage towards planned maintenance strategies.
The incorporation of categorised alerts into device status as defined by the NAMUR NE107 recommendation and the corresponding section FF912 in the FOUNDATION Fieldbus technical specification allows a measured and timed response to problems and gradual degrading of the field based assets. The ability of a business to plan for key assets degrading means money savings for users. However this does mean that the users do need to adopt the technology to the fullest and not just rely on the process control aspect of the technology.
They also need to ensure their workforces are educated to both deploy and support their deployment of the technology. Blind faith in the technology to support itself is misplaced. In reality the support for FOUNDATION Fieldbus can be simplified and the basic steps to do so are clear and followed.
We are now at the section where I help you re-discover the basics of fieldbus.
FOUNDATION Fieldbus is:
* An all digital, serial, two-way communication system that interconnects intelligent measurement and control devices
* Essentially a Local Area Network (LAN) for field devices
* The name of the fieldbus technology developed by the Fieldbus Foundation
FOUNDATION Fieldbus allows PID control in the field and in various field devices; i.e., control anywhere. It also uses Device Description technology in a similar way as that of HART® technology.
Figure 2 shows a generic segment topology of the bus. For the non-technical people the terminators are to stop the signal falling off the bus, or even worse reflecting back and causing interference to the digital communications. The diagram shows the devices including the host interface at any position on the bus. Typically the host, bus power supply and a terminator are at one end (inside a cabinet located in the Local Equipment Room) and the other terminator is in a distribution junction box out on plant. The field devices are usually on short spurs and the main trunk takes the communication back to the host via a longer pair of wires. There are many variations on a theme however, and this also changes if you take into account the introduction of FOUNDATION for Remote Operations Management concept also being developed by the FieldComm Group. This places the host out in the field with the devices and uses a control bus over Ethernet/wi-fi for multi-type I/O and function block execution to a widely distributed architecture including asset management and Human Machine Interfaces.
When it comes to the practicality of the field wiring in hazardous areas though there is an obvious need to both fulfil a process monitoring and application as a primary role as well as deploy a safe electrical/electronic architecture through either limitation of energy (non-sparking) or containment of the ignition (flameproof/explosion-proof) or even both in combination.
For the business advantage, smart instrumentation in itself has provided the users with multiple benefits inherent to the use of digital technology. Design of devices has allowed manufacturers to provide accurate and reliable devices with exceptional turn-down and re-range capability without compromise to performance in the field. Deployment of the technology has also provided users (or rather the contractors representing the user during the design and commissioning phase) the advantages of the simplicity to install and implement (once they have properly planned the strategy and trained those responsible appropriately).
The key to success is how the ‘real’ end-users find advantage and acceptance when the field devices are digital. This aspect is often overlooked and leads to users rejecting the further deployment through the perceived ‘difficulty’ of dealing with the technology on a day-to-day basis.
Smart devices and their associated asset management invariably provide four inherent interfaces functions: Configuration, Diagnostics, Performance and Documentation – the benefits being:
a) the ability to optimise the function of the device to best fit the application to which it is being applied eg. a pressure transmitter may be pure pressure or d.p. or level or hydrostatic density, etc.
b) the device raises a flag upon the results of a fault being found during its continuous round of self-diagnosis or the device self-test can be initiated. For example, a performance check for deviation can be run as a method within a valve positioner/controller device during normal operation, and these alerts from the device can be incorporated into the Host System Operator Interface screens as part of an alarm mechanism or be run upon the asset management system for maintenance interface
c) the device performance will need to be established with (documented results) comparison to a reference standard. It would be prudent to ensure the calibration management for a tagged device is automatically tied to the device in-situ and its current configuration
d) the events of action upon and generated from the devices is automatically logged in an auditable database and that the device can be supported through the use of linked electronic documentation to the icon representing that tag in the field, thus avoiding the need to seek-out documentation that carries the details of periodic supportive or restorative actions to perform upon the devices.
It is insufficient for a system supplier to fail to plan for an effective field device asset management solution when providing smart devices. Often in practice it is found that the asset management is embedded within the DCS and does not allow for instrument engineers to access the devices. It is also common to forget that devices often need to be placed on bench testing and access to the system is not commonly available in the instrument workshop. These are areas where the ultimate end-user is often let down. Not that full access to the DCS is required – just asset management for support of devices and replacement where needed.
With intelligent devices and their status/event available ‘online’ this then allows the users to link this asset condition to automated (often semi-automated) reliability assessment and even corrective work order creation through the propagation of this information up the ISA 95 Model network topologies and back down to workers through the generation of CM work orders that are condition based rather than schedule based.
This allows the users to concentrate on the work needed to address real time degradation of assets and plan resources accordingly. A consideration of the asset criticality to align with the prioritisation of work according to the criticality of the alert (see FF912 /NE107) raised by the smart asset. So this is an illustration of how the ultimately the FOUNDATION Fieldbus technology fits into the work processes of users to streamline activity and meets the cost reduction needs through efficiency and leveraging on the technology.
When viewed in a hazardous context this allows users to plan how to maintain process plan based upon near-realtime condition feedback from the smart asset in the hazardous areas without exposure of workers to that environment. Dependent upon the richness of the information the manufacturer cares to embed into the device an investigative visit may be needed to confirm the reported data and this leads to proper selection of devices in the first instance by the user influence into the plant design.
About FieldComm Group:
The FieldComm Group is a global standards-based organisation consisting of leading process end users, manufacturers, universities and research organisations that work together to direct the development, incorporation and implementation of new and overlapping technologies and serves as the source for FDI technology. The FieldComm Group’s mission is to develop, manage and promote global standards for integrating digital devices into automation system architectures while protecting process-automation investments in HART and FOUNDATION Fieldbus communication technologies. Membership is open to anyone interested in the use of the technologies. For more information, visit their web site at www.fieldcommgroup.org.
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