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Pump ratings and why you never seem to get what's promised


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This is Eric at FlexPVC.com® again.

This time I'm going to write about pump ratings.

Most of the time people buy a pump based on it's flow. GPH (gallons per hour) or GPM (Gallons per minute) or LPM, etc for the metrically inclined.

But what most people don't know is the conditions these pumps are rated at.

I'm going to explain how they rate them and how you can get closest to the full rating.

#1. Most all pumps are rated with a flooded suction and with at least 10 times the diameter of straight pipe going into the suction side of the pipe.

That means a pump with a 1.5" entrance will have at least 15" of straight pipe going into it. The pumps are engineered to pull from that STRAIGHT column of water. So if you have a 90 or any other direction change on the input of your pump, you will never see the full flow rating.

#2. Most pumps are rated with a draw from a tank, a very large tank. This provides almost a perfect smooth source of water to draw from. Not a tank with a bunch of pumps swirling the water around. Back in the day, Jacuzzi pumps were tested with a 8' in diameter, 30" deep cistern.

Now, how do you get around these problem?

#1, Obviously try to keep your intake straight and pull from a pool of water that does not have any swirls in it.

#2. For large pumps (1/2hp and larger) you can build a flow straightener. Here is one I built for a 5hp 25K gph pump.


Here is what it looks like on the inside before the straighteners are inserted: (notice the nice radiused intake. Very important!)


Here is what it looks like after they are inserted:


I used SDR21 purple pipe because it's the cheapest pipe you can buy and it's pretty thin wall. You just "stuff" the pipes in till they are tight. They will bend and flex around. That's what you want. After they are all tight, you just dribble primer in all around them. Each contact point of each pipe will weld together. You then add glue to bond them in case of vibration.

Then you put the other end on and you have a flow straightener.

Just this one mod alone increased flow from 21K gph to 23K gph. Initially the people who did the plumbing had the Tee right up against the pump. Moving it 20" away we went from 18K gph to 21k gph. The pump is still a little away from it's 25K gph rating, but that is because it's not in a true flooded suction situation. The water reservoir is about 30" deep and the pump is about 12" deep so it has to draw up about 18". That is why it does not get to it's rated 25K gph. But when I got there the pump was only at 72% of it's rating and when I left it was at 92% of it's rating and the flow over the waterfall was greatly improved.

Anyway, I hope you found this informative.


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That's really interesting. Reminds me of the throttle body spacers people put on their cars which look really pointless but actually can measurably increase HP and torque by creating a more laminar airflow pattern.

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Semi OT, but the throttle body spacers don't really increase the hp/tq, but what they do is move the peaks because you are changing the vol. of air in the intake system as well as the length from the TB to the valve. What I've seen is that they tend to "rock" the curves about the peak point based on the length and the peak rpm point is based on the total vol in conjunction with the cam/ports combination, so it can move it up or down the rpm range.

Back on semi-topic there is a gizmo the hucksters sell called a vortex generator [not to be confused with aerodynamic vortex generators which aircraft and race cars use with good results] which actually has vanes in it to "swirl" the incoming air. They claim more mileage and power.. but when they are tested, just the opposite is shown, less power, worse fuel mileage. So "straightening" the air is probably the best approach, just like in the example given above.

Ie, it works! :-)

Thanks for the reply.

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