This case describes an investigation into the fire safety requirements of a plant that processed methane-rich waste gas from nearby mines. It demonstrates the value of tailoring the fire safety advice for the circumstances. In this case, instead of a detailed quantitative assessment of all possible fire scenarios, a comprehensive qualitative review of the fire scenarios and the general principles for their management was undertaken.
A gas drainage plant (GDP) processed methane-rich gases from nearby mine sites. To accommodate increased demand from expanded mining activities, it was being expanded from two gas-processing trains to three.
Mine gases were processed through the GDP and sent to:
- An adjacent electricity generation plant (EGP)
- Flare stacks for flaring to atmosphere
- Dedicated vents for emergency (unburnt) gas release to atmosphere.
A site layout is provided in Figure 1.
The flammable gases posed a risk of fire to the plant. With the expanded GDP, it was decided that an upgrade of the fire protection system at the site might be required.
Fire safety review requirements
Fire safety requirements for any facility are, by necessity, particular to that facility. Fire safety reviews require a good understanding of:
- The materials that are present, their quantities and how they are stored
- The processes involved at the site
- Existing and proposed safety features
- Existing and proposed fire safety features.
A site visit is normally required when a review is undertaken.
As there was already a GDP in operation, fire detection, prevention and protection measures already existed and a fire water supply was in place, although there was uncertainty about whether it would be sufficient for additional fire duties associated with the plant expansion. Fire hydrants were sited around the perimeter, but the rationale for their location was not recorded. It was clear that a more process-specific design basis for fire prevention and protection was needed.
At the time of the study, the requirement was to understand the broad fire safety requirements so that design could begin on these aspects of the expanded GDP. Detailed quantitative modelling of the consequences of fire scenarios was not required at this stage and detailed hydraulic calculations for the fire water supply was to be carried out further down the track.
This was not a formal fire safety study. Rather than a full-scale study, a simplified version was requested instead, to help the site managers to understand the scale of the fire safety challenge. The main steps are shown in Figure 1.
For the GDP upgrade, the preliminary information gathered included:
- Site layout drawings
- Process flow diagrams
- Piping and instrument diagrams (P&IDs)
- Process safety descriptions
- General site safety information
- Photos of equipment as well as aerial site photos
- Existing fire water system diagrams.
The preliminary information also included site layout drawings and hazardous area zones for the EGP.
There were gaps in the information provided, particularly for existing plant, materials and equipment. It was anticipated that the site visit and interviews with employees would fill those gaps.
The storages of all the materials, whether flammable or not, were summarised along with commentary on any fire safety measures in place (or planned). An example is in Table 1, which is extracted from the full list.
A good understanding of the process plant was required to produce fire scenarios. The site visit provided an understanding of the setting of the plant and its proximity to its neighbours. It also provided an overview of the scale of the plant and the standard of maintenance and housekeeping for the existing process. A walk-through of the process and the storage of all existing materials on-site was also provided.
Fire scenarios and management options
The materials list and the process description were used to prepare a list of hazardous scenarios that may lead to fires or explosions on site. These necessarily involve the materials on site that are a fire hazard, including:
- Methane-rich mine gas
- Natural gas piped into the site to supplement mine gas for electricity generation
- Liquid petroleum gas (LPG) in bottles for forklifts, instruments and flare pilot flame ignition
- Diesel, oil and lubricants
- Transformer oil
- Small quantities of flammable chemicals.
All of the materials may be hazardous if incorrectly managed. The materials that have the ability to generate a significant hazard were examined in particular detail in the scenarios that were considered, some of which are described below.
Kinds of fires
A number of kinds of fires and other events to which the fire safety system might have to respond were identified. Some of the scenarios are listed below with definitions of the terms used in the following paragraphs:
- Mine gas jet fire or flash fire
- Natural gas jet fire or flash fire
- Methane gas or other flammable gas jet fire, flash fire or BLEVE
- Pool fire at oil storage
- Transformer explosion near generators
- Transformer explosion at the switchyard.
Jet fire. A jet fire is a high intensity fire that can only be extinguished by isolating its fuel source. Its heat radiation can impact personnel and equipment in surrounding areas. Fire water is needed to cool surrounding equipment to limit further damage. To determine the firewater requirements, the surface area of equipment to be cooled must be calculated in conjunction with quantitative modelling of the radiation impacts of the fire.
Flash fire. A flash fire occurs when a leak of flammable gas forms a vapour cloud that is not ignited initially. Eventually, when the vapour cloud reaches an ignition source, the flammable part of the cloud generates a flash fire or, if in a confined area, a vapour cloud explosion. Quantitative modelling of the extent of the vapour cloud before ignition determines the flame envelope. The overpressure effects of the vapour cloud can be calculated according to the degree of confinement of the gas.
BLEVE. A boiling liquid expanding vapour explosion or BLEVE is an explosion caused by the sudden rupture of a vessel containing liquefied flammable gas (or pressurised flammable liquid) due to fire impingement. The vessel becomes overheated, the internal pressure rises and the vessel ruptures. The contents instantaneously flash into vapour and ignite, producing a turbulent ball of flame that rises into the air and burns out. Quantitative modelling is required to determine the radiation effects of a BLEVE.
Pool fire. A pool fire is a fire associated with a flammable liquid storage. Its radiation effects can be modelled quantitatively by understanding the size of the pool and the rate of burn-down of the flammable liquid.
The gas extraction, GDP and electricity generation areas and their associated processes were examined to identify where flammable material might be released, with further cross-checking from similar hazardous facilities and materials.
Eighteen specific scenarios were identified. For example, 5 scenarios were associated with gas extraction:
- Mine gas flash fire or vapour cloud explosion inside the riser
- Mine gas jet fire or flash fire during shutdown conditions
- Methane gas bottle flash fire, vapour cloud explosion or BLEVE
- Mine gas overland pipeline flash fire or vapour cloud explosion inside the pipeline along the route
- Mine gas overland pipeline jet fire or flash fire along the route during shutdown conditions.
Consequences for each scenario were not calculated. However, a coarse estimate of the kind and extent of damage was prepared as a guide for setting priorities.
Table 2 shows an example of a scenario analysis template. It was assumed that best practice mechanical design, integrity checks, procedural reviews, training and testing would underpin the management points listed.
A substantial amount of useful information can be generated from a site visit by personnel with relevant expertise and experience, including perspectives about the scale of the facility, its setting, its proximity to neighbouring structures and environmental conditions. Not only does it allow for confirmation of documented information, but features that might not be recorded formally can be identified; for example, the proximity of a particularly sensitive neighbouring structure or environmental feature might be so familiar to site personnel that they overlook it when preparing information for the review.
To gain the greatest possible benefit from the (often) short period of time available for a visit, preparation is essential. It is beneficial to have prior knowledge of the process (even in outline), the kinds of materials expected to be on site and in what form, and the kinds of fire prevention and protection measures that would be anticipated. A list of questions and information requirements should be prepared for the site visit.
Someone who is knowledgeable about the site, the process, materials and their storage, and the fire protection and prevention measures should guide the visit. They might also, through anecdotes, reveal information about the facility that has not been provided in advance, such as the culture of the organisation or its maintenance philosophy. This can give rise to further questions to probe aspects of fire safety that were not considered prior to the visit.
Following the visit, connections with the site personnel are a valuable means for dealing with further queries. Involvement of site personnel may also assist in acceptance of the review outcomes, particularly if they have provided significant assistance.
Simplified qualitative approach
For a plant upgrade involving hazardous materials, it might be that a formal fire safety study is not required, although fire safety always needs to be considered and some quantitative analysis will be required for fire safety design.
There are several circumstances in which quantitative analysis is required:
- Where a facility has large volumes of hazardous materials and would be considered a fire safety risk to neighbouring properties
- As part of a formal land use safety planning assessment for regulators
- For sizing equipment, such as hydrants and sprinkler or foam suppression systems, to provide assurance that fire scenarios will be prevented or their effects will be reduced as far as possible, to limit damage to structures and equipment as well as injury and loss of life.
In this case, a qualitative approach was cost-effective and had several useful purposes:
- To inform the facility’s management about the scale of the fire challenge for the site
- To provide guidance about the kinds of fire equipment to be considered
- To justify further expenditure on the project and ensure it was targeted at the most important aspects of the design.
With the benefit of previous experience of similar sites and their potential fire scenarios, a qualitative analysis of the kinds of requirements for fire safety can be generated in a very short time, while still ensuring that it is relevant and tailored to that site. This is useful as a starting point for scoping further investigations into fire safety on site. It limits the cost of the review and reduces demands on the time of site personnel.