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Onboard foam systems on single stage pump

Hello group,
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.

Replies to This Discussion

Permalink Reply by Drew Smith on July 17, 2008 at 7:41pm

Based on what I know, I have a different reason for why you got foam at 100 PSI versus 200 PSI.

First, is the 95 GPM nozzle that matched the 95 GPM eductor rated for a 100 PSI nozzle pressure or less? If it is 100 PSI and you got foam at 100 PSI what pressure was used to overcome the friction loss in the eductor and hose?

Second, is the gauge for the eductor plumbed at the entrance to the bypass eductor or somewhere else? I have see these plumbed AFTER the eductor (not much help) or on the pipe supplying the eductor by with a few elbows before the actual educator. Regarding the master pump discharge gauge, it usually comes off the impeller housing but that pressure will not accurately supply the eductor based on the diameter, length and number of bends in the plumbing between the impeller and the eductor.

Normal pump discharge pressures are the sum of nozzle pressure + hose friction loss +appliances +elevation (which is rarely a big issue). With an eductor, the reason for a 200 PSI inlet pressure is that the standard 100 feet X 1&1/2 plus nozzle requiring 100 PSI nozzle pressure needs 140-145 PSI. The friction loss in the eductor is around 60 PSI. This 200 PSI inlet is universal for all makes of eductors because it is based on engineering.

With 150 feet X 1&3/4 and 100 PSI nozzzle only about 115 PSI is needed but the eductor lets about 140 PSI out causing too much pressure in the line resulting in no venturi.
If you run this same scenario with a low pressure nozzle (150 ft X 1&3/4 + 75 PSI NP) you only need about 90 PSI but again at 200 PSI you are over pumping the line. When you do it at 100 PSI you appeared to get results.

Finally, to ensure it is properly proportioned to 95 GPM I would time hose long takes to drain a 5 gallon pail. If the eductor is working correctly you should empty the pail in about 100 seconds.

Permalink Reply by Bob Franklin on August 24, 2008 at 11:22pm

Drew,
Thanks for the reply, many good points given. The nozzle we were using was a 95 gpm @100 psi. The point I was trying to make was I know of department s who have never operated there on-board foam systems since changing over to the new apparatus that has a single stage pump. When they do, they attempt to pump at 200 psi which produces more gallons than the venturi can handle and the foam system fail to produce foam. I will try to drain the pail excersize.
Thanks again,
Bob

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