Inherent within the very nature of the energy industry, the hazards associated in refining oil and gas present a number of risks. One of the prominent and well documented risks is the potential for fire or explosions as a result of a break in containment. To combat this, facilities incorporate a combination of Flame and Gas Detectors to detect events before they become unmanageable.
With the increase in prevalence of flame and gas detection technologies in the 1980s, and with technological improvements improving to this day, deciding where to locate these detectors based on the hazard they are intended to mitigate became far more open to scrutiny.
History of F&G mapping
The question was, and still is, regularly posed: ‘How many detectors do I need, and where do I put them?’ A fair question in the context of a processing facility where traditional detection codes are simply not applicable. Placing heat detectors on a grid is intuitively not adequate for external processing facilities exposed to harsh environments like the North Sea. How, therefore, can we adequately position flame and flammable gas detectors, such that target fire sizes and gas clouds of concern can reliably, and verifiably, be detected?
As a result of this challenge, Micropack introduced the first F&G modelling assessment software in 1989. This technique was then developed by Micropack, BP and Shell, with Shell coining the term widely used today – ‘F&G Mapping’.
Just after the turn of the century, Shell and BP formalised their practices in to their own internal guidance documents, detailing methodologies against which F&G Detection systems should be designed where national/ international codes are not applicable. In 2005, Micropack introduced the concept of F&G Mapping to the ISA84 committee, which later rolled out ‘ISA TR84.00.07 Guidance on the Effectiveness of Fire and Gas Detection’. Since then, Micropack has continued to innovate in the field of F&G Detection Design through software development and refinement of best practices in relation to F&G Detection Design.
Risk based coverage analysis
There are countless potential ways in which a fire or gas release can impact on a facility. Certain processes present the potential for a gas jet/ liquid spray fire where pressures exist in the stream, flash fires/ fireballs are credible, in addition to Boiling Liquid Expanding Vapour Explosions (BLEVE) and hydrocarbon/ chemical pool fires. Gas releases can present an explosion hazard in congested areas and a hazard to adjacent areas through gas migration, as well as providing the potential for toxic gas release where toxic constituents exist within the stream. It is therefore critical that an appropriate methodology is applied to ensure the detector distribution will detect a fire or gas release at an acceptable stage along the event timeline to provide effective mitigation measures.
F&G Mapping allows the designer to position flame and gas detection devices within the facility and assess the coverage afforded by the system. This is automatically correlated against pre-specified targets that the system must achieve. An example of such targets applied for a flame detection system can be seen in Table 1.
These targets allow the designer to position flame detectors in such a way that small fires are detected early in high risk areas, whereas low risk areas allow the fire to grow to a more substantial fire size before activating any automatic actions (i.e. release of deluge/ shutdown of equipment). This meets the philosophy of risk/ goal based F&G detection design. One of the greatest features of such an approach is the ability to optimise the detection system and demonstrate this optimisation (providing the fewest number of detectors while meeting the predetermined detection targets).
The application of effective technology is equally critical to system optimisation, in addition to reliability of the detection system during operation. The strengths and limitations of each technology must be noted and accounted for within the design to ensure appropriate technologies are applied to the specific hazards of the facility. Each device will have its own unique ‘footprint’ which should be accurately represented in the mapping, as this can have a significant impact on the results of the assessment and the subsequent analysis of adequacy.
Figures 1 and 2 show the difference in detection footprint between an intelligent visual flame detector (iVFD) and a triple IR flame detector. Comparisons show that on facilities where visual detection is used, the detector count can be reduced by up to 30%. If we, therefore, are to optimise our layout, the detection technology selected will have a significant bearing on the results.
Change Management/ Operations
F&G Mapping does not only provide benefits during the design phase. The benefits of F&G Mapping can also be observed in a number of ways throughout the operational phase. The first relates to design in that as the process changes through redundancy/ replacement of old equipment (or through the introduction of new process or simply through alteration in the processing conditions), F&G Mapping can allow quick and easy revalidation of the design to ensure coverage is not degraded. This also shows if further optimisation can be achieved. Without F&G Mapping, designs carried out years before can be difficult to revalidate if the responsible engineer has moved on. Knowing the intentions behind detector placement can often be difficult to decipher. With F&G Mapping, however, the targets are detailed. These targets can be revalidated, with the coverage then recalculated to make sure the original design intent maintains its credibility.
Another benefit pertains to real time operations. One of the primary deliverables of an adequate F&G Mapping study is detailed detector contributions. An example of these contributions can be seen in figure 3. These contributions show the individual percentage contribution of the volume from each detector, but also, critically, the percentage lost should that detector be taken out of service.
Knowing the degradation of coverage when a specific detector is removed can be useful in design as it can help remove the detectors that do not contribute as effectively as possible. Crucially though, if a facility (which can have upwards of 200 flame detectors) has, for example, five detectors in fault at one time, this can provide a difficult decision for the maintenance team as to where to start. A quick look at the detector contributions, however, can direct them to the most important detectors that are out of service, and allow for that crucial maintenance optimisation function.
One of the most important factors in the design of safety critical systems is ensuring the design has a suitable number of sensors to achieve the required safety function, without significant over engineering. F&G Detection is no different, but a little more complex. F&G Mapping provides designers with the methodology and tools to review detection coverage against pre-determined targets, and ensure a suitable and optimised system is commissioned. There are countless other factors to be considered including implementation of an appropriate methodology based on the application, application of engineering judgment as to what adequate coverage looks like, and the application of appropriate detection technology. Pulling all these factors together has been proven to reduce costs of the system significantly and streamline the change management process throughout the operational phase of a facility.
F&G Mapping also provides the significant benefit of maintenance optimisation, which can save money and potentially lives. Where multiple detectors are in fault, maintenance engineers can quickly and easily verify the devices that are the more safety critical, and focus their efforts on bringing them back online.
For more information, go to www.micropackfireandgas.com