Just had an interesting problem with a customers apparatus. They have a single stage 1250 mid-ship pump with an Akron 95 GPM by-pass foam system. The complaint was that the department could not get the system to operate properly. They would go step by step according to the directions and would only have foam for just a brief second and then nothing but water.
The hose lay was 150 ft. 1-3/4 with a constant gallon nozzle (95 gpm). The instruction plate gave a step by step instruction and pump pressure stated 200. After trying the sytem, they were correct in that at 200 psi there was a brief application of foam and then only water. I removed the line from the metering valve to the eductor to inspect the ball in the eductor. When the line was charged, the ball would drop to open for the foam concentrate, but would immediately close as pressure was increased. Normally you can push the ball open, but this had a lot of pressure behind it.
After checking the system it dawned on me that the 95 gpm @ 200 psi were designed for two-stage pumping systems! With a two-stage pump, the first stage supplies 95 gpm at 100 psi, routes the 95 @ 100 to the second stage which only adds and additional 100 psi. The total is 95 gpm at 200 psi as the instructions indicate.
A single stage pump is a volume pump, meaning that for every pound of pressure there is a double increse in gallons when compared to a two stage pump. I cut the pressure down to 100 psi instead of 200 and we covered the apron with foam. The single stage pump at 100 psi was flowing the 95 gpm required by the inductor. At 200 psi we were producing 190 gallons which was causing a back pressure and closing the ball on the eductor.
Remember: In the PRESSURE POSITION you get HALF the gallons at TWICE the pressure.
In the VOLUME POSITION you get TWICE the gallons at HALF the pressure.
I thought I would share this information in case anyone was having a simular problem with your on board foam system.