System Integration & Control Systems Design


Fire prevention for mine vehicles

September 2012 System Integration & Control Systems Design

This article gives a basic introduction to the various actuation methods in use on mining vehicle fire suppression systems around the world.

The question is a good one because of the complexity associated with actuating these specialised fire extinguishing systems. Often, end users are faced with the difficult task of deciding what actuation method is best to use: should it be manual, should it be automatic, or should it be a combination. Then, should the activation be electrical or mechanical?

The system basics

Mining vehicle fire extinguishing systems generally contain an extinguishing agent that is dispersed from a storage container under pressure and distributed to the risk areas either by pipe work and nozzles, or by careful placement of the extinguishing modules. The two most popular choices for extinguishing agents on mining vehicles are dry power or water based (water containing surfactants and other chemicals). Other more specialised types also exist such as chemical aerosol or chemical impulse types.

However, what is common to all the types of vehicle extinguishing systems are the methods of detection and activation used to operate and deploy the system. These can be split into two categories: manual and automatic. Generally, it is considered bad practice to use an automatic system only, as will become evident later in this article.

The detection/activation categories can be expanded as follows:

* Manual: mechanical operation, electrical operation.

* Automatic: mechanical detection, electrical detection, chemical detection.

Principles of operation

Typically the extinguishing agent is stored in a pressure vessel that is operated by a release valve. The valve would require either a pressure pulse or a loss of pressure to activate it, although there are some types that operate by Bowden cables. Typically the pressure pulse (or loss) could be initiated using a solenoid; the control of the solenoid could be electrical or mechanical.

Note: electrical operation is usually preferred because an electrical system can be monitored, whereas the mechanical operation of a solenoid, by a pull cable for example, cannot. An electrical signal could also directly activate a specialised type of extinguishing agent by means of an electrical squib or detonator.

So we now we have two distinct types of mechanical operation of the fire extinguishing agent control valve, namely ‘loss of pressure’ (LOP) or ‘rise of pressure’ (ROP). Both of these methods can be provided either mechanically (pneumatically) or electrically via a solenoid valve.

Automatic LOP an ROP

Now it is easy to see that a mechanical (pneumatic) detection device could be constructed using a special polymer tube that melts at a very specific temperature. This tubing could then be connected to a loss of pressure (LOP) valve. When a fire occurs, the tube melts and releases the pressure in the tubing thus causing the release valve to operate. Such a system has a major advantage in that it fails safe. However, the drawback is any leakage from the detection tube, or damage to it, will result in unwanted operation of the system.

Chemical detection and activation is yet another method of automatic operation. The method of using a heat sensitive ‘cord’ that will ignite at a certain temperature is well known and is the preferred method of operation of fire extinguishing aerosol systems when used in small cabinets and enclosures (such as the thermo-cord system used in conjunction with Pyrogen units).

However, the heat sensitive cord is not robust enough for mining vehicle applications. Instead a similar chemical compound is packaged in a ridged stainless steel tube, when the temperature of the tube reaches a certain temperature (generally 180°C) the chemical combusts causing a pressure rise in the tube (ROP). By pneumatically connecting the chemical detector to the main valve in a similar fashion to the LOP system, the rise in pressure can be used to activate the valve.

Electrical detection

Electrical detection as applied to vehicle fire systems consists primarily of heat detection and flame detection (IR). The aggressive environment in the engine bay precludes the use of other types of detection devices. The electrical detection system will also include a control panel that may incorporate its own inbuilt electrical release button.

As stated earlier, all that is needed to operate the solenoid valve attached to the main valve (and discharge the fire extinguishing system) is an electrical signal, so an electrical detection system and purpose built control panel are ideally suited to vehicle fire protection applications. The electronic control panel should ideally monitor not only the integrity of the mechanical portion of the system through the use of pressure sensors, but also the integrity of the electrical detection system together with the integrity of the actuation circuit by monitoring the condition of the solenoid coil.

The electrical detection system uses electricity and this may be supplied by the vehicle’s batteries. However, consideration should be given to providing a dedicated rechargeable standby battery. A vehicle that enters the workshop for welding repairs may be at very high risk of fire damage if the vehicle’s electrical systems are isolated prior to repair work.

Electrical heat detection can be provided by point heat detectors (these are basically thermostats that operate on a bi-metal strip principle) or linear heat detection that uses digital linear heat detection cable. Digital linear heat detection cable has a very special construction of twin core springy wires that are coated with a special polymer that melts at a set temperature. The whole assembly is encased in a protective polymer sheath or woven steel braid. When a fire occurs the insulation melts and the wires spring together causing a short circuit which is detected by the control panel. An actuation mechanism using digital linear heat cable is very robust, can be routed around a vehicle engine bay to monitor specific areas and can be monitored electronically for integrity.

The final method of electrical detection is by detecting infrared radiation. There are many different types of infrared (IR) detectors on the market and all offer advantages and disadvantages. A mining vehicle is however very unique in terms of fire risk and it is not prudent to use IR detectors that have been optimised for long range, outdoor, small fire hydrocarbon fire detection. IR detectors need a clear field of view of the fire to operate correctly and should be placed in close proximity to the fire risk areas (short range). Fitting an IR detector to the cab roof to ‘look over’ the engine bay is asking for trouble. A short range dual frequency IR detector, situated inside the vehicle fire risk area is all that is required.

Conclusion

Hopefully this short article will give a little insight into mining vehicle fire system actuation methods. All of the methods of actuation have advantages and disadvantages and the only way to select the correct method or combination of methods is to apply some good engineering judgement and perform a competent risk assessment.



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