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Fire system 2,6 km underground

Moab Khotsong Gold Mine is one of AngloGold Ashanti’s Vaal River operations in South Africa. The mine is located around 180 km south-west of Johannesburg near the towns of Orkney and Klerksdorp. The mine has been in production since 2003 and currently ranks as one of the six deepest mines in the world, with its mining depth ranging between 2,6 km and 3,054 km below surface.

The mine exploits the Vaal Reef (VR) through three mining layers, namely the Top, Middle and Lower Mines. Moab Khotsong employs a scattered mining method with an integrated backfill support system.

Sperosens was recently tasked to design, supply, deliver, install and commission conveyor fire suppression systems at the following high-risk areas at AngloGold Ashanti – Moab Khotsong Mine:

• 77 Level – CV04 Reef, CV05 Waste, DP Waste and Rock Reef – Fire Suppression System.

• 101 Level – 101 Waste and 101 Reef – Fire Suppression System.

• 102 Level – Overlay and Underlay – Fire Suppression System.

Installation started in September 2016 and will end in March 2017. Once complete, this might well be one of the deepest operational fire detection and suppression systems in the world.

For the application it was proposed that a pre-engineered, standalone suppression system, designed in accordance with NFPA16, be installed at all the high-risk areas. This system is called a Compressed Air Foam System (CAFS) and is designed in accordance with NFPA16: Standard for Foam Spray Fixed Systems for Fire Protection.

The compressed air foam system

The compressed air (nitrogen) foam system is a fixed automatic fire prevention, detection and suppression system, consisting of a control unit, fire and heat detectors, a set of discharge spray nozzles, a pressurised water/suppression agent tank, a high-pressure nitrogen cylinder, a number of solenoid valves, and pressure and level switches. The unit can also be interfaced and displayed onto the SL2010 fire detection telemetry system or scada system.

The foam spray fire suppression system is a stand-alone, pre-engineered, modular fire suppression system that can be configured to supply various quantities of nozzles, from a minimum of two nozzles, up to a maximum of 21 nozzles per single cylinder system. The normal operating pressure of the conveyor belt tank is designed such that the discharge nozzles should operate at a constant adjustable discharge pressure of between 3 and 10 bar, thus allowing for a balanced and equal discharge from all the nozzles, depending on the specific design application and duration discharge required.

Principle of operation

A vessel is filled with a mixture of water and a foaming agent and is pressurised using a standard nitrogen cylinder. Two sensors are used to detect overheating:

• Plumber block temperature sensor – to detect the overheating of bearings.

• Infrared temperature sensor – to detect overheating of pulleys.

In addition, a Triple Spectrum (IR) Infrared Flame Detector is used to detect flames. The system will automatically discharge the vessel contents through a solenoid valve, piping system and correctly placed nozzles when a fire alarm is generated.

Installations of CAF solutions have been successfully implemented on surface conveyor belts since 2012.

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