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Old 31-01-2009, 08:51   #1
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Pipe Head Resistance

Does any body have a handy dandy chart that will allow for the calculation of head pressure and resistance for pipe/tube, bends and fittings. My need is to calculate the actual GPH for my bilge pump system at the discharge end after it has run through a stated size pipe with so much head through so many feet of hose and fittings.
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Old 31-01-2009, 09:13   #2
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Check with the Mfg. Most pump Mfgs supply that chart or will send you one. The head is the real issue depending on the type of pump.(Diaphram or impeller type) I got a very nice booklet from my local plumbing/AC/Heating store for free. (Not Home Depot)
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Old 31-01-2009, 09:54   #3
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Can you put the pump in a bucket with a measured amount of water an time it.
This would be more accurate as it would also include any power deviations.
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Old 31-01-2009, 11:03   #4
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There are tables and calculations for this. My brother, a third engineer from Cal Maritime, learned this stuff and he says that theory and reality are rarely the same. Your dealing with a lot of variables here. Run, diameter, wall and joint smoothness, pressure, viscosity, pressure head and turn radius are the major variables. As you know, as with any calculations, the more variables you are dealing with, the less accurate will be your estimate.

Be wary that the ratings you get for pumps are for (unrealistic) ideal conditions, meaning full voltage, no run and no pressure head. Less than ideal conditions can slow down a pumps rate considerably.

I think the most accurate thing you could do is to build a physical model and measure that. Doing it with tables and formulas will probably be frustrating and probably not yield a very accurate estimate.
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Old 31-01-2009, 11:49   #5
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Friction Head (hfs) or (hfd) refers to the resistance in the pipe and fittings. It varies with the size, condition and type of pipe, the number and type of fittings, the flow rate, and type of liquid.

Friction Loss Tables (and more):
http://www.eljay.com/pdf/friction%20loss.pdf
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Old 31-01-2009, 14:05   #6
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Rule (ITT) has head / flow info in the installation instructions for 1m & 2m of head (Lift)

What brand are you planning on using?
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Old 31-01-2009, 16:18   #7
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If you haven't done the calculation before, it's a steep learning curve and it's even harder if you want to calculate the flow rate through the system given the pump's discharge pressure. Perry's Chemical Engineers Handbook will tell you how to do it (and it can be done with a high degree of accuracy, comments of others on this thread notwithstanding). You can find the book in most university libraries. There are equivalent Mechanical Engineers Handbooks that will provide the same information. But if you haven't done this type of calculation, you'll be miles ahead having someone (an engineer) with experience do it for you.

Most pumps of any size come with a pump curve that gives you the flow rate and power for a given head - the flow rate tends to remain fairly level against head for awhile, and then drop off pretty quickly as the head (against which the pump is pimping) is increased. When the discharge is completely choked off (by a closed valve, for example) the pump curve twlls you the maximum head the pump will develop (which it does at zero flow). All of this pertains to centrifugal type pumps. Positive displacement pumps such as diaphragm pumps, behave a little differently. Hope some of this helps.
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Old 31-01-2009, 19:53   #8
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Thanks for the info guys. It turns out the Nigel Calder has some charts in his book Boatowners Mechanical and Electrical Manual. He states that he made some gross simplifications as it is harder as Gord states to measure resistance because of the difference pipe and fitting variables. I was making a decision between a Rule 3700 and Rule 4000 bilge pump. The 4000 has a 2" barb for 2" hose the 3700 is 1 1/2" using Calders chart it seems that the large hose will give me about 5' less head. This makes a substantial difference with possible out puts from my calculations using Rules data the 4000 will give me about 2200 gph at the stated optimal 13.6 voltage, the 3700 is about 1300 gph. As the 4000 and the 3700 have the same foot print, I opted for the 4000. This pump is being located higher in the bilge. I am locating a smaller pump lower down.
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