What are the fundamental passive fire protection solutions offered for refuge chambers?
Fire dangers exist in underground mining and tunnelling operations. The heat transfer properties of these fires into refuge chambers underground depends greatly on the position of both the fire and the chamber. It is difficult to give a fire rating to a chamber due to the changing environment of its position and the mobile nature of the fire risk. Refuge chambers are therefore currently designed to protect against breathing contaminants and lack of oxygen, but not excessive heat load over a long duration.
The current MineARC refuge chambers are designed for a minimum duration, including the uninterruptible power supply (UPS) and breathing contaminant scrubbing, of 36 hours. This figure is based on the burn time of the rubber tyres and the fuel load of a large mining vehicle. These types of fires are accepted examples of mining operation risks.
The positioning of a chamber is very important. A standard refuge chamber (Standard Design or Compact Design) will not be able to sustain life within the chamber if ambient conditions exceed 47°C. Calculations on how the chambers are position depend almost solely on the ambient conditions. Upgrades for higher ambient conditions are available.
MineARC is unable to produce a guideline for positioning distance of chambers for fire protection. The heat loads on it would be incredibly difficult to establish. Current recommendations from the Department of Mines and Petroleum are that chambers must be within 750m any underground personnel (1500m apart). Distances of this nature would likely produce conditions favourable for refuge chamber positioning.
Fire Rated Refuge Chamber Construction
MineARC has constructed a number of passive fire rated refuge chambers, where insulation and protection against fires of short duration and high intensity were used.
MineARC can provide a passive insulated chamber that is:
- Constructed of the same outer steel materials.
- Lined with insulation boards (DuraSystems, ASKIN or Bondor) to cover all external surfaces. These boards are specifically installed to limit heat transfer occurring through the steel structure into the occupied volume.
- All gaps in the insulation are filled with a fire rated sealant and caulk for protection. This insulation also limits the heat transfer into the occupied space.
- The insulation lined with steel cladding before it’s painted as per normal refuge chamber construction and materials.
- The fire rated caulk is also provided as a material within the chamber to protect against any unforeseen gaps.
- Fire rated glands are used for all cable passes into the chamber.
- Plugs are available to block possible heat transfer. In extreme cases, heat may transfer through the check valves at the front of the chamber.
Insulation to Protect Refuge Chamber Components from Fire
A key consideration is heat removal from the chamber’s occupied space. A refuge chamber can become uninhabitable without the use of an air conditioner to protect against latent heat and humidity. It is essential to add insulation to protect the unit. If the exterior condenser of the air conditioner is exposed to extreme heat, causing it to fail.
MineARC’s current passive fire protection for refuge chambers includes insulated shielding at the rear of the chamber. The rear of the chamber houses the UPS cabinet; batteries are required to power the refuge chamber’s life support systems (scrubbing units, air conditioning and lighting), in the event of a loss of mains power. The battery cabinet doors also include intumescent grill blocks that allow ventilation of the cabinet. These blocks expand to shut when exposed to elevated temperatures and a fire’s naked flame, forming an incombustible barrier to the spread of fire and hot smoke.
To comply with all the current refuge chamber guidelines, the chamber also must include a portal window as well as a secondary means of egress (an escape hatch). The passive protection solutions for these include a portal window cover and an external opening “kick out” escape hatch that is able to be caulk-filled and insulated. The internal occupied space of a passive fire rated chamber is almost identical to a standard MineARC refuge chamber.
Time/Temperature Curves for Tunnelling
Fire Protection International Time/Temperature curves for Tunnelling Operations
MineARC has not yet been asked to provide a European tunnelling fire rated chamber compliant to any time/temperature curve. These curves map the temperature required to be protected against over the duration the chamber is in use.
The tunnelling regulations, for passive fire protection, are generally based on hydrocarbon fuels which burn more intensely but for shorter periods of time, however, if a large mobile truck were to ignite, the time period would increase. From the reviews done on the tunnelling standard for passive fire protection, the following time/temperature curve is used. Reviewing the curve, it is clear that while passive protection can be developed for the occupied space, the current air conditioning system, which is rated to an external temperature of 50 degrees, will need to be suspended for much of the fire duration. As such alternative cooling would be required to meet the curve above.
Heat Suppression – Misting and Fire Extinguishing
MineARC also has misting systems that emit water vapour into the surrounding atmosphere. This effectively reduces the ambient conditions and increases the ambient operating temperatures to as high as 60°C. Fire suppression devices have also been fitted to chambers in areas that are likely to experience some heat loading that can be extinguished from the chamber.
Alternative Cooling Options in Tunnelling
Secondary cooling methods are a critical component to meeting any tunnelling time/temperature curve. This ensures there is an ability to continue to cool the occupied space when the primary air conditioning is suspended.
Currently, MineARC is able to provide a MARCiS liquid carbon dioxide (CO2) cooling as a secondary cooling method. However, the CO2 bottles are required to be kept below 40 degrees within an insulated section of the chamber. This part of the chamber is cooled by the air conditioner during its operation, and once the MARCiS is activated the bottles cool their own insulated room. The exhaust gas, CO2, is then plumbed to the battery cabinet and air conditioner bay to eliminate the oxygen and reduce the risk of fire spread. This cooling method, however, is large and at times impractical. Thermal storage methods and continued research and development into secondary intrinsically safe cooling continue at MineARC.
Did you know? The exact level of protection from a potential threat such as a fire, the positioning of the refuge chambers, and the possible methods that could be utilised for operating in higher ambient conditions are all factors being continually developed at MineARC.
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