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  • From: Bill Jones <billj AT harborside.com>
  • To: homestead AT lists.ibiblio.org
  • Subject: Re: [Homestead] Ram pump rules of thumb
  • Date: Fri, 01 Sep 2006 17:08:23 -0700

Don Bowen wrote:

I know Gene is looking into Ram pumps and I think several others may be interested. Today I looked at some home made ram pumps, made from plumbing parts. These would be good to experiment with. The name is:
Erosion Free Water Rams
Don Price
563-519-0248

He said that a ram needs a minimum of three feet of fall. Lift water is proportional to fall. For 10 feet of fall, if lift is 50 feet, the output will be 1/5 the input water, for 100 feet the output water will be 1/10 the input water. Five feet of fall will be half that and three feet will be one third, 20 feet of fall will be double and so on.



Our little seasonal creek dried up early and totally after the logging on the hill. So I've given up on a ram pump for watering the summer garden. But after thinking a while, I decided that it's still worthwhile to have one just for winter use. If we had to use much of the meager winter output from solar panels on raising water from our deep well, there would be none left for lighting. And why, with so much water on the surface (in winter)?

It's easy to show that this rule is true from the law of conservation of energy. Since power is proportional to both height and the rate of water flow, and the power out will always be less than or equal to the power in, if the height doubles the the flow rate must be at least halved, etc. But as I recall they were using an efficiency factor of about 60% at the webpages I visited, so expect the actual flow to be much less.

I found a source for PVC ram pumps ready-made for $150. They have a clear window chamber and a diaphragm to prevent waterlogging (i. e., the cushiony air bubble in the chamber disappears and all the pressure from the ram is placed upon your plumbing system, possibly rupturing it).

By the way, another little engineering problem I've encountered is that I did not know how to predict the rate of flow through a tube exiting a holding tank (in practice this must be less than the flow rate at the source, otherwise the holding tank would eventually empty and air would be sucked down). The key is to realize that the water will accelerate until the force of gravity on it equals the frictional force of the water line. In other words, it accelerates until head loss equals the amount of head your have. So if you look up the vertical distance the water drops in the _body_ of the head loss table, in the column that corresponds to your pipe diameter, then the flow rate off to the side will be the resulting flow! You may have to estimate a value in between two numbers.

The reason for worrying about this is that the holding tank has to be deep enough to provide enough pressure so that even after the friction with the downpipe is overcome, there is still the minimum water flow to operate the pump (2 GPM in the case of the $150 model I found). If the downpipe is fairly short, this is not a problem, but downpipes longer than 100' may require a fairly deep (more than a foot) holding tank to keep up the necessary pressure to sustain the minimum flow rate. It's still not a problem, but for instance, a holding tank only 6" deep will not feed a terribly long downpipe properly, and it would be a nuisance to find this out after the fact, especially if the holding tank is placed inconveniently on a cliff side.

Bill
S. Oregon coast






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