Fire Safety Science Digital Archive

When charging chemical foam extinguishers it is unusual to use warm water for the outer sodium bicarbonate charge and hot water for the inner aluminium sulphate charge to ensure easy and complete solution of these salts. There is same danger in using warm water for the sodium bicarbonate charge as too high a temperature can cause conversion of the bicarbonate to carbonate with a consequent reduction in the volume of carbon dioxide that can be produced by the reaction of the two solutions. In many cases, particularly with the Armed Services, warm or hot water may not be available or temperature control may be difficult under the prevailing conditions. If cold water were found to be satisfactory the charging process would be much easier and the need for careful temperature control would be avoided. The sodium bicarbonate charge provided for an extinguisher gives a 10 per cent solution and should be completely dissolved at 230C and above. The quantity of sodium bicarbonate is always more than is necessary to complete the chemical reaction with the aluminium sulphate so that sufficient bicarbonate would be taken into solution at temperatures lower than this. In the case of the aluminium sulphate, it has been shown that practically all the salt can be dissolved at 14C so that there seems no need to use hot water as advised except as a means of making the mixing process easier. This note describes a limited number of experiments carried out to study the performances of 2 gallon foam extinguishers which were charged using water at tap temperature.