Ram pump -- is water compressibility really relevant?

[Baluncore]

Baluncore - I disagree - the amount may be small but there will be some backflow, a physical volume of water returning to the drive pipe before the delivery valve closes.

If there is backflow through the check valve, then it is minuscule, and irrelevant. When you are filling a bucket from a tap or faucet, and you cut off the water, does backflow occur?

Are you insisting that physical backflow, rather than an acoustic depression wave, must take place for the ram pump to repeatedly cycle?

However, in the traditional design above any air bubbles before the delivery valve would compress and rebound, making backflow (including some out the air valve), even if the delivery valve did not move - making it possible to cycle regardless. In the case of delivery valve not opening I would expect air to accumulate below it and reinforce that effect.

You seem to be saying that the pressure in the pump chamber could pulse and cycle, and that pulse pressure might be insufficient to open the check valve, due to the presence of air in the chamber.

The peak pressure in the chamber is the ram pressure, independent of an air bubble below the check valve.

The pressure head across the air vent or restricted pinhole, is pushing some water or excess air, to flow out of the pump chamber. The relative flow rates are dictated by viscosity and inertia.

Not only does air moving through the check valve keep the air reservoir full, but the excess air, expanding as it rises, reduces the average density of the column, and so can push water to a greater height, a bit like a trompe in reverse.

 

[Ken Fabian]

@Baluncore - If I am wrong in this it is wrong with conviction. I don't think I am wrong in this.

There is a rebound of water hammer hitting a compressible body of air, be it air in the air chamber in the time before the delivery valve shuts or bubbles before a stuck delivery valve - which imparts a reversal of direction of the water column. The momentum of the upward flow before gravity has its way makes a negative pressure, which is what is needed for the impulse/waste valve to re-open.

No matter that the volume of up-flow is minute, a definable period of negative pressure at the impulse valve results from it. That re-opens it.

The air valve eats some of that rebound, as does productive water delivery past the delivery valve but what is left of it makes negative pressure from the reversal of direction of the water column.

Sorry, I just don't buy the acoustic wave hypothesis, no matter if that is the citable wisdom on the matter.

 

[Baluncore]

No matter that the volume of up-flow is minute, a definable period of negative pressure at the impulse valve results from it. That re-opens it.

When high induction pipe flow drags the waste valve up, and closes it, the pump chamber pressure peaks and remains at high ram pressure until the reflection returns. Then the pressure in the pump chamber falls to the head pressure of the induction pipe. At that point, the waste valve sinks into the still water below, because the waste valve is weighted to be greater than the head pressure of the induction pipe, acting on the waste valve area. Water then begins to flow around the waste valve again, but with insufficient velocity to close it immediately. The flow then builds up until the cycle repeats.

I see no need for backflow to open the waste valve, as it sinks into the still water below.