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Will a Larger Pipe Size Help or Hurt?

 

Q.  I’m designing a pump system from a lake and have read and understand your calculation of FT HD needed for pump selection but it seems that the upstream (uphill) pipe diameter would be a factor in the calculation.  I was going to use larger pipe to reduce pipe resistance and valve pressure drop but it seems to me the weight of the additional water (back pressure) would be higher for a  larger diameter pipe than a smaller one.  It must be easier to push water up a 3/4″ column than a 1 1/2 inch column.  You mention nothing about this.  Excluding pipe resistance, does the pipe diameter play a roll in taxing the FT HD required?  Rephrased –  Does a larger diameter column of water have any effect on the static pressure or force required to move it?

A.  The short answer is that the larger pipe would be better because there would be less pressure loss in the pipe.  This is due to less “friction loss” as the water flows through the larger size pipe.  The larger amount of water in the bigger pipe has no impact on the water pressure.   A smaller pipe may create more friction loss however, so it can be worse than a larger pipe.    To find out, you need to calculate the friction loss in the different sizes of outlet pipe based on the flow and pipe size.  See the Friction Loss Calculators to calculate the friction loss in pipes.

More detailed answer:
One of the really hard to grasp principles of hydraulics is the relation of volume of flow, pressure, and the weight of water.  Odd as it seems a larger pipe will actually be easier for the pump.  It’s not the volume of water, but the height it is lifted that matters.  In a way this is a variation on the old saying “which weighs more, a pound of feathers, or a pound of lead?”  Obviously both weigh a pound!  This version could be phrased “which is easier for the pump, 5 GPM in a 1/2″ pipe or 5 GPM in a 2″ pipe?  Neither because 5 GPM is still 5 GPM regardless of the pipe size!  Yes, you would need more power if you were actually lifting more water, also we would need more power to lift the water higher, but neither is not what is happening.  The amount of water nor the height we are lifting hasn’t changed.

The other issue here is flow through a pipe.  This is the issue that actually makes the smaller pipe potentially worse than the larger.  Because the smaller pipe is smaller it is harder to force the water through it.  The resistance of the walls of the smaller pipe causes pressure loss as water flows through.  this is commonly called “friction loss”.  How much friction loss occurs depends on the flow rate and pipe size.  Both higher flows and smaller pipes sizes result in greater friction loss.  This is the only reason a smaller pipe would be worse than the bigger pipe.  How much worse is dependent upon the actual flow rate and pipe size.

As a general rule (ie: not always true, but is most of the time)  the pipe size of the pump outlet is almost always smaller than the size of pipe that will provide optimal flow from the pump.  In other words, if a pump has a 1″ threaded outlet, it is very likely that a 1 1/2″ pipe would be attached to the 1″ outlet for use as the outlet pipe.  Pump manufacturer’s tend to use smaller size inlets and outlets to save money.

More technical answer:
Think about feet of head.  As discussed in the Pump Tutorial, the number of feet of water depth determines the water pressure.  So 80 feet of water depth equals a pressure of 80 ft. hd.  This pressure will be the same regardless of the pipe size.  The water pressure at the bottom of an 80′ high 1/2″  pipe is exactly the same as the water pressure at the bottom of an 80′ high 6″ pipe, even though the 6″ pipe holds a lot more water.  A pump actually works by creating water pressure.  So for the pump there is no difference between pumping into either size pipe, the water pressure required to move the water into the bottom of both pipes is the same.  Now the pressure lost as water moves through the two pipes will be different.  Assuming a high rate of flow, a lot more pressure will be lost due to friction in the smaller pipe.    So for that reason using a larger pipe will be better.  Depending on the flow, however, it may be only very marginally better.  To find out you need to calculate the friction loss in the outlet pipe based on the flow and pipe size.  See the Friction Loss Calculators to calculate the friction loss in pipes or tubes of various types.



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