To ensure positive injection of foam concentrate, the discharge pressure rating of pumps at the design discharge capacity shall be in excess of what?

• A. The maximum water pressure available under any condition at the point of foam concentrate injection.
• B. The minimum water pressure available at the injection point.
• C. The ambient atmospheric pressure.
• D. The static pressure in the system.

Answer: (A)

In foam–water systems, getting foam concentrate into the water stream reliably depends on having enough driving pressure at the injection point. The pump’s discharge pressure rating at the design discharge capacity must be higher than the maximum water pressure that can exist at the injection point. This pressure differential ensures water will push through the injection point and draw the foam concentrate into the stream, achieving positive injection under all operating conditions. If the pump pressure could fall to or below the highest water pressure, foam concentrate might not be drawn in or could be forced back, leading to improper dosing. The other options don’t guarantee this constant, robust injection: minimum water pressure doesn’t cover peak conditions, ambient atmospheric pressure is not relevant to in-system injection pressures, and static pressure alone ignores dynamic pressure variations.