Which foam proportioning method uses an external pump to push foam concentrate into the fire stream?

• A. Venturi
• B. Premixing
• C. Batch Mixing
• D. Injection

Answer: (D)

Injection uses a dedicated pump to push foam concentrate from its storage into the water stream. This setup allows precise control of the foam-to-water ratio because the injection rate is driven by the external pump, which can be kept constant even when the main fire pump’s discharge pressure or flow changes. The concentrate is metered into the flowing water at the discharge side (or near the nozzle), so the foam proportioning is independent of the main pump’s performance.

In contrast, the venturi (ejector) method relies on the velocity of the water passing through a nozzle to entrain concentrate, so the mixture depends on flow and nozzle conditions rather than an independent pump. Premixing combines foam concentrate with water before discharge, giving a fixed mixture that isn’t adjusted on the fly. Batch mixing involves preparing a set amount of foam and water together in a tank or container, which is then discharged as a pre-m blended solution, again not responsive to real-time flow changes.