Full title: Taking Ownership of the Fire and Gas (F&G) System While Reducing Cost and Increasing Safety: a Practical Approach to F&G Mapping

 By James McNay

Introduction

With the increase in prevalence of F&G detection technology in the petrochemical industry, deciding where to locate these detectors based on the hazard they are intended to mitigate is becoming more open to scrutiny. As a result, methodologies on how to ‘map’ detector layouts have emerged in the last decade. F&G mapping, however, has been applied for more than 30 years and is not as new an application as some would suggest.

The full version of this paper, presented at the 2017 ISA International Process Control and Safety Symposium [1], reviews options available for designing the F&G system, with emphasis on the credibility of design, optimisation of detection layout, and how 21st-century mapping tools can assist. This raises a philosophical and practice question: Does a completed F&G mapping model equate to a sufficient demonstration of competence and adequacy?

The paper evaluates the current methods of dealing with such a scenario, questions whether there are dangers associated with putting too much emphasis on the results of the software applied (and how it’s applied) during the mapping stage of the design, and highlights the dangers of not applying validation mapping tools at all.

Practical Application of Optical Flame Detection

In external environments where hydrocarbon hazards exist, the industry standard form of fire detection is typically optical-based flame detection. As one can imagine, the open-based designs of most oil and gas structures are exposed to extremely harsh, unpredictable conditions. For this very reason, it is entirely unacceptable to rely upon standard smoke/heat detection in these areas, even if the detection target is a fire of significant Radiant Heat Output (RHO).

Since the detection objective is to mitigate the hazard, we must ensure that our target fire size is small enough to allow either manual or automatic control actions to be undertaken in a safe and successful manner before the ‘potential fire size’ is realised.

This brings us to performance-based F&G mapping. Since the ‘grey area’ of F&G has been around for decades, some companies generated their own in-house methods of designing F&G, which often specify target fire sizes to be detected. In this case, the design team takes ownership in tailoring these fire sizes to the areas of the facility that are suited to the targets, placing strict targets in the high-risk areas and less-stringent targets elsewhere. This is the first route to increased safety and reduced costs through system optimisation.

The environment in which these optical devices are to be applied can be harsh, variable, and unpredictable, to say the least. This is evident in the following figures, which show a comparison of the Micropack (Engineering) Ltd. detection test ground in summer and winter in Aberdeen, Scotland.

Figure 1: Micropack (Engineering) Ltd. test ground in summer.

Figure 2: Micropack (Engineering) Ltd. test ground in winter.

Further information on flame detection technologies can be found in ‘Desensitisation of Optical Flame Detection in Harsh External Environments’^[2].

Flame Detector Mapping

For hydrocarbon risks, we can define a fire hazard by its RHO, specified in kW. RHO gives a good indication of the potential damage a fire can cause.

Table 1: Potential Offshore Hydrocarbon Risk Area Grades and Associated Fire Sizes.

Flame detection mapping software, including HazMap3D^[3], provides a percentage coverage of each analysed area and is a useful tool in providing an optimised layout based on multiple fire sizes. Only through analysing multiple fire sizes can we demonstrate a holistic approach to optimising the layout.

The following figures show a simple example flame detection assessment.

Flammable Gas Detection: Hazardous Areas

The main point regarding flammable gas detection is that it is virtually impossible to detect all leaks. It is also important to note that the fundamental principle of process-area gas detection is not to detect leaks, but to detect clouds. It is, therefore, imperative that only those clouds that would be of concern become the target. Historically, locating flammable gas detection adjacent to leak sources was commonplace, but it soon became apparent that even when the slightest increase in pressure is present, locating detectors close to those leak points becomes detrimental to detection reliability.

Gas Detector Mapping

The aim of a flammable gas detection system is to detect the presence of flammable gas accumulations of sufficient size that, if ideally ignited, could cause damaging explosive overpressures. One of the primary methodologies adopted for detecting gas release is application of a target gas cloud. This approach is essentially drawn from the UK HSE publication OTO 93-002,^[4] which presents data on the overpressures associated with various ignited gas accumulations.

The report concludes that a 6 m cloud of stoichiometrically mixed methane will not, if ignited efficiently in an area with a blockage ratio of 0.3 – 0.4, produce flame speeds greater than 100 m/sec or 125 m/sec respectively (speeds associated with overpressures of less than 150 mBar, a widely accepted minimum threshold for pressure-induced damage). As the blockage ratio changes, so does the gas cloud volume required to create an explosive overpressure of concern.

Gas detection assessment software typically provides a three-dimensional assessment of the volume under review and presents the coverage data in elevation ‘slices’. The gas hazard, as described in OTO 93 002, was represented in the initial programs by a 5 m diameter ‘hard-edged’ sphere of stoichiometric gas/air mix (to this day this is still commonly applied in the petrochemical industry).

It was recognised from the outset that such sharp transitions from gas to fresh air were clearly unrealistic (except in some special cases involving very low pressure, cold, and ‘heavy’ vapours). In the absence of any data that could realistically be classed as practical, there was no alternative, so this conservative approach has been used extensively to assess the adequacy of flammable gas detection arrangements.

A Joint Industry Project was conducted to establish the ‘true’ behaviour of flammable gas releases in confined process areas. Part of the data gathered during these tests included behaviour of the initial gas cloud, measured by a local three-dimensional array of gas detectors.

When reviewed, this showed (unsurprisingly) that the ‘core’ of flammable gas was surrounded by a diffuse layer, the concentration of which fell as the distance increased from the source concentration (nominally 200% LEL) to a final value of 0% gas in air.

Further study confirmed that it was reasonable (indeed, conservative) to assume that the idealized hard sphere was surrounded by a shell of gas of no less than 20% LFL at a distance of 5 m from the edge of the central dense cloud. This was then used to improve the performance of (particularly open path) gas detection systems. This approach was incorporated into some F&G mapping software.

Figure 6: Typical Simple 3D Gas Detection Assessment
(Beam Attenuation model inhibited for simplicity).

Conclusions

One of the most important factors in the review of F&G systems is to ensure that implementing an appropriate methodology based on the application is addressed. Applying a suitable methodology, combined with an adequate detection technology, has been proven to reduce costs significantly.

Many operators have their own guidance documents with respect to F&G mapping. Where these are specified, it is important to not only comply with them, but also to have an appreciation of the practical implications of the design, which may not be explicitly referenced in the guidance document. These philosophies should be accounted for with any mapping software applied.

Whichever methodology is applied, it is crucial that parties involved on both sides of a project (designers and implementers) are happy with the methodology at kick-off, are fully aware of the strengths and limitations of the selected methodology, and work together to ensure the resulting design is appropriate for the specific application. This way, ownership of the F&G design can be taken, while both parties can discuss the potential to optimise the layout at a very early stage, making significant cost savings further down the line.

James McNay BSc (Hons), MIFireE, CFSP, MIET, Chair of ISA Committee for Fire and Gas

References

¹‘Taking Ownership of the F&G System while Reducing Cost and Increasing Safety: A Practical Approach to F&G Mapping,’ J McNay, ISA International Process Control and Safety Symposium in 2016.

²‘Desensitisation of Optical Flame Detection in Harsh External Environments’, J McNay, July 2014, IFE Journal (also available on the members area of the ISA website: https://www.isa.org/division/safety/).

³HazMap3D, 3D Flame and Gas Mapping software, Micropack (Engineering) Ltd.

⁴Offshore Technology Report, OTO 93 002 UK HSE, April 1993.