With the exception of sprinklers, most fixed fire extinguishing systems rely on total flooding with gas. Here Tim Nichols looks at some ingenious examples of equipment using gas or foam for specific, local risks.
Most fixed fire extinguishing systems fall into two categories – total flood of an extinguishing gas into the area to be protected, or local application into a defined area. Of these applications, a large majority are protected by halocarbon or inert gas systems for total flood, and carbon dioxide for both total flood and local application. Variations of these applications include the use of water mist, and wet chemical (for example, for kitchen hood protection). Foam is applied widely as a protection system in the petrochemical and transport industries and can be use in specialist local applications, as can fixed dry powder systems.
Dry powder and foam tank
Fundamentally, the standalone hardware for the extinguishing agents of dry powder or foam is the same. An example is a 15-bar system comprising a pilot cylinder pressurised with dry nitrogen, and a vessel which contains the extinguishing agent. At the pre-design stage, it is possible to choose the volume and dimensions of the equipment, incorporating directional valves as applicable to the protection of the risk. The vessel that contains the extinguishing agent is made in carbon or stainless steel, depending on requirements.
This particular system differs from some others, in that it incorporates an automatic device which prevents incomplete mixing of pressurising nitrogen that can occur in the first stages of discharge. Extinguishing vessels are available in sizes from 60 litres through to 1000 litres.
The unit is activated by a nitrogen pilot cylinder that can be initiated by an electric signal, or via manual release. The pilot cylinder is fitted with a pressure gauge and low-pressure monitoring. At the exit of the pilot cylinder, a high-flow pressure reducer is installed that reduces the nitrogen pilot cylinder pressure from 150 bars to the working pressure of 15 bars. The extinguishing agent then enters the vessel, where the discharge is withheld by a pneumatic valve. Once the pressure in the vessel is sufficient to ensure that complete mixing of the extinguishing agent and nitrogen has occurred, the pneumatic valve automatically opens allowing system discharge.
The pneumatic valve also serves as a sealing device to prevent the contamination of the extinguishing agent inside. The vessel contains openings for drainage or removal of contents, and various pressure connections for the different actuation arrangements that make up the system.
The extinguishing agent can be a foam premix, depending on the type of foam required, or dry powder. In the case of dry powder, two types are normally chosen, one of which is suitable for Class A, B or C fires; and the other for Class D fires.
Systems in action
Petrol service stations and fuel storage and transfer points present a complex fire hazard for extinguishing systems. Flammable liquids (Class B fuels) may be spilt when vehicles are filling at the pumps, leading to fire or explosion risk from vapour emitted. Traditional fire fighting methods involve the use of portable extinguishers or sand, which is also used for mopping up spilt diesel. The extinguishing agent is a pre-mix foam solution that forms a coating over the liquid which has caught fire to exclude oxygen, provide cooling and to prevent fuel spread and re-ignition.
In a petrol station forecourt in Spain, for example, a fixed automatic system – based on that described above – covers an area of approximately 12 square metres of vehicle lane, or the area adjacent to the pumps. The system consists of detection, control and a fixed extinguishing component.
Fire detection is achieved through the use of a heat sensitive tube that in quiescent condition is pressurised. On detecting heat – between 80 degrees C and 100 degrees C – the tube ruptures, lowering the pressure. This pressure drop is detected by a micro-switch, which activates the discharge of the system annunciates at the alarm panel. Outputs from the panel include the standard local alarm and remote activation, plus options such as for local shutdown of pumps.
The extinguishing system is configured by means of two ramps per lane, built into the kerb of the pump isle. The discharge of extinguishing agent is through two nozzles, one of which is housed in each of the two ramps that cover the pump area affected by the fire. The ramps are separated by 1.2m and are centred above the pump shaft. An electric/manual release point is located on an outside wall in case of manual activation being required.
The detection system has an accumulator that is pressurised at 15 bars, and which ensures maintenance of the release system during an unspecified period of time. Fitted at the end of the heat-sensitive tube is a valve for testing integrity against leakage, and a pressure gauge which displays if the pipe is in service.
LEDs indicate system status including: normal service pressure; low detection tube pressure; and nil pressure that would indicate actual discharge.
Combustible metals
Class D fires are those involving combustible metals, such as magnesium, titanium, zirconium, sodium, lithium and potassium. The problem in using extinguishing agents suitable for Class A, B and E fires is that these metals are highly reactive to the agents themselves. They continue to burn even in the presence of nitrogen (precluding the use of IG-55, IG-541 or IG-100 systems) and react with carbon dioxide. Halocarbons can act as accelerants and water can create hydrogen, the reaction to which is almost explosive.
The only suitable extinguishing medium is non-standard dry powder, as standard bicarbonate-based dry powder produces carbon during the reaction that fuels the fire. Suitable powders may vary, depending on the metal fire tackled, but include those based on sodium chloride, copper and graphite. The heat of the fire causes the dry powder to cake and form an exterior crust, which excludes air, prevents re-ignition and attenuates heat, so putting out the fire.
Class D powders are applied in portable fire extinguishers, but for some applications this is neither convenient nor safe – you would not want to tackle by hand a fiercely burning metal fire. Other systems include trolley units and small systems of similar size and capacity. Larger systems use equipment designed above. The amount of dry powder needed will be a 1:15 powder-to-metal ratio, depending on the application.
Some applications require a combination of two different agents, notably foam and dry powder. The equipment described above is equally suitable for this task after modifications to the storage. An example of this approach would be in a metal processing plant where oil is the fire risk, but if the machine ran dry then a metal fire could result. The foam would be effective on the oil and a Class D dry powder on the metal fire. An alternative use for twin agent systems is to use the extinguishing effectiveness of both foam and dry powder. The dry powder provides very fast knockdown and the foam a lasting seal to prevent re-ignition.
LPG Fire Ltd is a manufacturer of a full range of approved fixed extinguishing systems and products for almost every application. Tim Nichols may be contacted at [email protected]
