The simplest means of proportioning is accomplished by pre-mixing. With this method, pre-measured portions of water and foam concentrate are mixed in a container. In most cases, pre-mixed solutions are discharged from a pressure-rated tank using an inert gas such as carbon dioxide or nitrogen. An alternate method of discharge uses a pump and on-pressure-rated, atmospheric storage tank. The pump transfers the foam solution (under pressure) through piping or hose to the discharge devices.
Only AFFF concentrates can be used with the premix or dump-in methods. Protein base foams do not mix as readily as AFFF and will gradually settle out of the premixed solution. Specially diluted alcohol-resistant concentrates are used in specific pre-mix units. In dump-in applications, ANSULITE alcohol-resistant AFFF concentrate should only be used when the booster tank is equipped with a circulation pump and complete mixing can be accomplished through the recycle line.
A disadvantage with premix systems is that all the water is converted to foam solution. Other types of proportioning systems store the foam concentrate separately from the water supply so that either foam or water discharge is possible.
A Premix system is ideal for protection of small hazards where suitable continuous water supplies are not available. Typical risks include Pump Rooms, Dip Tanks and Engine Rooms.
Advantages
- Self Contained
- Does not need continuous water supply
- Does not need external power for operation
- Relatively inexpensive
Disadvantages
- Premix storage life is limited – needs to be replaced every 1 to 3 years
- Only suitable for small risks