Everything you needed to know & more besides!
The original dry powder extinguishers appeared around the early 20th century & were simple tubes of bicarbonate of soda, chalk, etc that were dashed onto a fire by hand or a shaker.
These were not very effective and soon fell out of fashion, although they received attention during WW2 as possible devices for use on incendiary bombs, but the traditional media of water buckets & sand buckets were found as effective.
It was not until the end of the 1950's that the modern powder extinguishers first started to appear.
This time, the extinguishant, normally Sodium Bicarbonate plus water proofing agents (usually metal stearates) was contained in a cylinder and propelled by a CO2 charge onto a fire at a pressure of around 150 psi.
The design was similar to that of a cartridge water extinguisher, with some notable differences:-
- a gassing tube to fluidise compacted powder prior to discharge
- the need for a larger capacity CO2 cartridge masss for mass as powder is discharged in a stream of CO2 rather than be expelled by it.
Reseach in the US and by Nu-Swift in the UK lead to the development of the stored pressure extinguisher, with dry air, nitrogen and CO2 being used as pressurising gases in various makes, usually with a working pressure of 150-195 psi. Stored pressure has now overtaken catridge pressure as the main type of powder extinguisher
Early extinguishers had a fan shaped nozzle as a flat horizontal discharge was thought to be the most effective, but further research showed a conical discharge to be more effective and round nozzles soon became the norm.
Small models (1-7lb) were of total discharge, but larger models (7lb +) , other than National Coal Board approved models, had a shut off control to allow more effective use of the agent and reduce uneccesary powder clouds, which block vision.
The introduction of the modern powder extinguisher saw the birth of the "domestic/motoring" disposable aerosol extinguishers in the 60's, still used to this day
Design has changed little over the years, normally following general developments such as the advent of squeeze grip controlled discharge, although some manufacturers still produce strike knob type models.
It took many decades before the exact extinguishing nature of powder was known- original theories cited the force of discharge 'blowing out' the flames; or that the powder smothered by displacing oxygen; or that there was some cooling/smothering effect from the water & CO2 liberated by the contact of powder with flame; or that powder absorned all the heat.
The actual effect of powder requires knowledge of the fire tetrahedron as it does not cool, starve or smother to any appreciable extent, but in fact works on the chemical reaction that is fire, the powder particals 'mopping up' the free radical elements that are formed during and are important in propagating, the reaction of combustion.
Powders fall into three distinct groups:
BC Powders
ABC Powders
D Powders
BC Powders are the original type, based on Sodium Bicarbonate. They are alkaline in nature and are effective on Class B (flammable liquid/liquifiable solids) and Class C (flammable gas) fires. They are electrically non-conductive.
As they are alkaline they have some saponification effects and have a limited effect on Class F (cooking oils) fires, being installed in some kitchens for that purpose. Early cooker hood suppression systems used BC Powder before Wet Chemical extinguishants were developed, however BC Powder is not classified as a Class F agent and should not be installed for that purpose.
Very few extinguishers in general use contain BC Powder other than the aerosol type.
'High Performance' BC Powder. For the high risk petrochemical industry a BC Powder extinguisher with a more rapid knockdown than Sodium Bicarbonate was required and two specific types were invented:
- Potassium Bicarbonate BC Powder (Invented by the US Navy and often called Purple K after the pigmentation of the leading brand). More expensive than Sodium Bicarbonate the Potassium ion was found to have a greater affinity for free radicals and was twice as effective.
- Urea based Potassium Bicarbonate BC Powder (Monnex) As with other BC powders, Monnex interferes with the chemical reactions which occur within the combustion zone. However, the unique property of Monnex is that within the combustion zone, the high temperature causes the powder to explode and break into minute particles giving a very large surface area which effects the extinction of the flames. Monnex is the only powder to possess this unique property. Due to the low density of Monnex, it is not normally possible to charge an extinguisher to its normal capacity. A 75% charge by weight is considered average.
ABC Powders are mixtures of ammonium phosphate and ammonium sulphate, ground to selected particle sizes and treated with flow promoting and moisture repellent additives. They are acidic in nature and are effective on Class A (flammable solids), Class B (flammable liquid/liquifiable solids) and Class C (flammable gas) fires. They are electrically non-conductive.
In addition to the particle surface extinguishing effect of a Class B powder, Class ABC powders have low melting/decomposition points in the order of 150°C to 180°C. When these powders are applied to hot and smouldering surfaces, the particles fuse and swell to form a barrier which excludes oxygen and thereby completes the extinguishing process and prevents re-ignition.
Most powder extinguishers in service and almost all now marketed (except aerosols) are ABC Powder.
Different blends are available, the more ammonium phosphate, the more effective it is.
Class D powders are designed for fires in flammable metals and three main types are in use-
- Sodium Chloride for certain Class D fires, those involving alkali metals such as sodium and potassium, also zirconium, uranium and powdered aluminum, extinguishes a metal fire by fusing to form a crust. This excludes oxygen from the surface of the molten metal; a carbonaceous rafting agent prevents the powder from sinking into the surface of molten metal
- Copper extinguishing agent specially developed by the U.S. Navy for fighting lithium and lithium alloy fires. The copper compound smothers the fire and provides an excellent heat sink for dissipating heat. Copper powder has been found to be superior to all other known fire extinguishing agents for lithium.
- Ternary Eutectic Chloride, developed by UKAEA for uranium fires, which works similarly to Sodium Choride. It is extremely toxic.
Although modern powders are non-toxic, the discharge of a powder extinguisher in a confined space can cause a sudden reduction of visibility which may temporarily jeopardize escape, rescue or other emergency action. For this reason water-based extinguishers are to be preferred in hospitals, old people's homes and hotels. Powder extinguishers are prohibited to be used in PSV's and minibuses by statute law for the above reason.
Most fire engines carry an ABC Powder extinguisher aboard for small Class B, C & electrical fires and a common use of the powder extinguisher on an appliance is to tackle post box fires with minimal secondary damage to mail.
Other than the above scenerio powder extinguishers are considered to have high secondary damage potential and are not advised for protection of electrical hazards, especially sensitive electronics. Powder, specifically ABC powder is not permitted in or near aircraft as it can damage the metal superstructure.
When used on Class B fires, Powder must extinguish the whole fire area in an uninterupted application or flashback will occur as unlike foam there is no physical barrier - it's all or nothing. The lack of a securing blanket means there is a re-ignition risk. Also powder has no cooling properties, one of the reasons it is uneffective against class F fires as although it can extinguish the flame, the heat of the fat will cause immediate flashback.
Powder is used with Foam in motorsport, with powder being used for the initial rapid knockdown and Foam to cool and provide flashback protection.
Sodium Bicarbonate Powders, unless specially treated, are not compatible with Foams; Purple-K, Monnex and ABC Powders are generally less damaging, and often are used with AFFF & FFFP, but compatibility must be borne in mind where powder and foam are used together and a higher application rate of foam allowed for.
