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I live on a small farm with my wife. We drink rain water from a rainwater tank. It's a small, simple system. However, we have water hammer occurring between the supply water tank (Supply Tank) and a piston pump (The Pump). The water-hammer seems to be damaging The Pump.

However, the water hammer only occurs when The Pump starts operating if the Supply Tank is full.

The full Supply Tank supplies water through a 32mm alkathene hose (The Line) to The Pump 80 meters away.

The Tank creates water pressure that seems to cause water hammer at The Pump and at 90 degree elbows in The Line before The Pump. I want to protect The Pump by creating a cushion that arrests the water-hammer.

I have not tried anything yet. We're on a frugal budget so we're looking for low-cost resourceful 'possible solutions' to try.

I have thought about creating an air-cushion using a T fitting that I could install just before The Pump. However, configuration requires additional 90 degree elbows and the flow does not seem to like the tight bends when the pump starts and stops.

Q1) Could installing an inline pressure reducing valve at The Tank end of The Line be helpful?

Q2) Can you suggest alternative options?

Q3) Do you have any thoughts about creating an air cushion just before the pump.

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  • $\begingroup$ How do you know there's water hammer? I mean what are the actual empirical observations that lead you to believe that? $\endgroup$
    – user28774
    Commented May 29 at 14:05
  • $\begingroup$ Water hammer usually (?always) results from a sudden change in the flow rate. Is there a sudden change in the flow rate, and if so, why? Can it be avoided? $\endgroup$
    – user28774
    Commented May 29 at 14:25
  • $\begingroup$ There is a complication. There is a branch at the pump. Water comes from the tank or water comes from the well. If I put an expansion pipe at the pump then the pump might not be able to draw the water up from the well which is 90 meters away and three meters down. So I would have to put the expansion pipe only on the line to the tank, just before the gate valve that is used to close it off when drawing water from the well which is line number two. $\endgroup$
    – Henk
    Commented May 30 at 1:22
  • $\begingroup$ To be clear about why I asked: the long, narrow pipes on the inlet side of the pump make me suspect cavitation, rather than water hammer, as the cause of any damage to the pump. $\endgroup$
    – user28774
    Commented Jun 3 at 11:08

2 Answers 2

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High pressure is a cause of water hammer in the pressurized side just because high pressure water moves faster. I don't think this is a solution in this low pressure side (I'm assuming you have correctly determined the location of the hammer). I think it will also be challenging to do pressure reducing in the inches of water range. I think you're just getting higher system flow rates when the tank is full.

The fix is almost always an expansion tank. The ones that go on hot water heaters have bladders to keep air in them. You could probably just put a vertical pipe in front of the pump, maybe even an open pipe if you don't draw air from it when the tank level is low. But a vertical pipe with a vent valve could easily be opened when the pump is off periodically to ensure you keep air in it.

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  • $\begingroup$ Would the vertical pipe be best with an airtight seal so that the compressed air inside could act as a cushion? Or with a small hole drilled at the top so insects could not get in but air could pass through that hole? $\endgroup$
    – Henk
    Commented May 30 at 1:15
  • $\begingroup$ Expansion/well tank should be used with a pressure sensing cutoff switch to complete the usual system that keeps your pump as healthy as possible. $\endgroup$
    – Abel
    Commented Jun 1 at 0:58
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The obvious solution is to use a purpose build water hammer arrestor. You should try that unless there's a reason you can't.

Water Hammer Arrestor Source

If you can't buy one, just build something functionally similar. They're not complicated.

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    $\begingroup$ A well tank or expansion tank also provide similar functionality, but via a bladder instead of piston. It's pretty standard to use a well tank with a pump setup like this. $\endgroup$
    – Abel
    Commented Jun 1 at 0:53

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