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illustration

My dad has a tub by a lake. He has a water pump in the lake to pump up water into a tub.

He claims that if he use a flexible hose, the pressure in the hose at the top will be lower than if he use a rigid hose. The pump has to keep a flexible hose stretched, causing a pressure drop, he says.

I say that if the diameter of the hose doesn’t change while the pump runs, there will be no pressure drop, so once the pump is fully running, it doesn’t matter if the hose is flexible or not.

Who is right?

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    $\begingroup$ +1 for the drawing $\endgroup$ – OpticalResonator Jul 12 '18 at 12:30
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There is no difference once the flow is stabilized.

In a dynamic situation, the hose may yield under increase of pressure; the hose stretches, more water is accumulated in it, outflow is lower than inflow, and outflow pressure is lower. But the hose can't stretch indefinitely - at certain point its tension offsets the pressure, amounts of flow stabilize, pressure and flow rate at outlet rise, and the situation becomes equivalent to rigid hose in all respects. Whether the hose is rigid or not, the forces acting on its walls don't move them, so no work is done on the hose - the pressure is not affected.

And of course, the moment you switch the pump off, the flow may continue for a while in the soft hose, as the pressure drops and the hose contracts against it; that wouldn't happen with a rigid one.

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  • $\begingroup$ I disagree: Elastic is only one force which causes pressure loss. Internal friction, which depends on wall material and shape, matters too. Further, a "rigid" hose will arguably have fewer bends, and curves cause pressure loss too. $\endgroup$ – Carl Witthoft Jul 12 '18 at 18:01
  • $\begingroup$ If we assume internal friction and amount of bending is identical, SF is correct though, isn’t he? $\endgroup$ – Enselic Jul 13 '18 at 11:25
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    $\begingroup$ ...and in common scenarios like pump in lake - hose - tub, the difference would be completely unnoticeable - unless the soft hose crinkles obstructing most of the flow. Neither water flow speed is significant enough to generate considerable obstruction to laminar flow, nor will the shape deviate from round in a significant fashion. Never mind, from commonly accessible hoses flexible enough to make this matter (folding under own weight) the only kind would be fire hoses, and these have enough of diameter it really doesn't matter. $\endgroup$ – SF. Jul 13 '18 at 11:52
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Going around corners does induce a pressure drop, so if the rigid hose has fewer tight bends, it will perform (slightly) better in this application. Realistically, however, the gain is marginal, and you’re better off making the decision on financial/availability criteria.

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Yes hoses or pipes always cause some pressure drop when flowing, it may be small. For example ; part of a test given to industrial painters is " how much pressure drop is there for each of different lengths of hose when using an airless paint sprayer? " . That is, they must be able to adjust for the pressure drop between the pump and the nozzle for various flow rates and hose lengths

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  • $\begingroup$ for those who do not understand ; look up Reynolds Numbers in Wikipedia . A basic tool used by chemical engineers to determine pressure drop of a fluid/liquid flowing through a hose/pipe/tube. $\endgroup$ – blacksmith37 Jul 13 '18 at 18:16
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Listen to your Dad.

A flexible hose requires a minimum water pressure to maintain the characteristics required for all of the above factors. A 100' hose will see a significant pressure drop at the outlet, and a flexible hose will have some level of contraction if the source pressure is not high enough, which invalidates all of the assumptions.

I have filled up a lot of inflatable pools at distance from the spigot. The difference for rigid vs flexible in terms of GPH can be very dramatic.

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For Same size and same length of pipe lines, pressure drop will depend only on number of bends and internal smoothness of the pipe line. So depending upon above factors you may find out out in which case pressure drop will be higher.

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