Ram pump -- is water compressibility really relevant?

[Swamp Thing]

Is it really necessary to consider the compressibility of water in order to understand a ram pump?

Steve usually does an excellent job of explaining this kind of thing, but this time I'm not so sure...

Note: Despite the thumbnail, he is not proposing a perpetual motion device.

 

[Ken Fabian]

I built and used one for a time - a variant that included a pressure vessel packed with closed cell rubber rather than just an air chamber - without that there was too much backflow from the non-return valve. It was a design intended for low water 'head' (or drop) and it did work but my site had low water levels and was near the lower limit for the design - only about 750mm - and sand was a problem but a screen filter just slowed the flow too much.

Lack of water compressibility was critical to a strong 'water hammer' - initially I used a polyurethane washer in the 'slam valve' (not sure about correct terminology) and just that bit of compressibility prevented it working. I had to use different materials than the initial design to get it to work.

Even a bit of expansion each 'hammer' from using less rigid materials - a different kind of 'compressibility' - was enough to prevent it working properly.

High density polypropylene worked for the valve washer but the valve seat in the main body - cast iron in place of the original PVC because it was not rigid enough and the hammer impulse was reduced - rapidly rusted and pitted, no doubt accelerated by the sand, sigh. (If I had enough money at the time to use stainless steel I probably could have bought a commercially available variant ram pump by Glockemann. That sort uses a diaphram that pushes a piston.)

It was not very reliable and didn't last beyond a year and a half. In deeper clear water with plenty of drop it would have worked much better and I could have used the more durable PVC instead of cast iron.

 

[Baluncore]

Is it really necessary to consider the compressibility of water in order to understand a ram pump?

Yes. Water is effectively not compressible, when compared to the elasticity of a steel pipe. If the reverse was true, the ram pump could not function.

Ram pumps are easier to understand and work with, if you make the wise assumption that water is incompressible. What is unfortunately elastic, are the pipes, which explains why the twelve or so meters of fixed diameter induction pipe, closest to the pump must be made from a metal such as steel, not from plastic. The remaining upstream part of the induction pipe can be made from cheap and flexible "polypipe" without problems. The steel pipe ensures that a significant pressure pulse is generated and maintained, which will only very slightly expand the elastic steel pipe.

The steel pipe must withstand the water hammer pulse when the waste valve slams shut. That pressure wave travels away from the waste valve, pushing a small amount of water through the check valve, into the bottom of the air reservoir and outlet chamber, as it passes, on its way back up the induction pipe.

The pressure wave generated by the waste valve closing, moves upstream through the induction pipe to the inlet, where it is inverted by the reflection from the open-ended induction pipe, to become a depression wave that runs back down to reopen the waste valve for the next cycle. There is a convenient complication here, in that the pressure inversion due to reflection, may be from the junction between the steel and the plastic induction pipes, since the significantly greater elasticity of the plastic can appear to be an open-end. The same inversion may be obtained from a step increase in pipe diameter at that junction.

The check valve below the air reservoir prevents the pumped head from affecting the lower pressure at the waste valve due to the lower head of water in the induction pipe. The water outlet line from the bottom of the air chamber can be small in diameter, and several kilometres long. That is because the flow through the outlet line is steady, driven continuously by air pressure, and so the flow does not pulse.

There must be air in the chamber to continuously oppose the outlet head, but that air also allows the check valve to operate. It is sometimes necessary to induce a few small bubbles of air into the pump body, to maintain sufficient air in the outlet chamber. Excess air will be vented, through the outlet line as compressed bubbles in the water. The air chamber pressure and volume is therefor self-regulating.

 

[Swamp Thing]

I may not have worded my question in the best way.

Steve Mould's explanation is based on the idea that you can't make complete sense of the device unless you assume that water does compress a little during the pressure spike, sending a shock wave upstream all the way to the reservoir.

I was skeptical about the need to invoke such ideas, and the two replies above seem to confirm my suspicion.

Also, since posting my question it occurred to me that the presence of that air chamber almost guarantees that the water isn't going to compress or swell the pipes to any meaningful extent. I mean, why would the water "bother" to compress when there is a compliant cushion of air to take the shock?

Thanks for the detailed and informative answers.

 

[Baluncore]

If Steve Mould was a perfectionist, he would never commit a video to YouTube. Like every publication, it can only be third-rate. If a few mistakes were fixed it could be imagined as being second-rate, until it was published, when it would naturally revert to being third-rate. First-rate is perfection, and is unobtainable. A publication must be designed for a particular educational level. Maybe we need seven different videos about the hydraulic ram pump, each targeted to a particular audience. The fundamentals need to be correct, but the hand waving will be different.

Steve Mould's explanation is based on the idea that you can't make complete sense of the device unless you assume that water does compress a little during the pressure spike, sending a shock wave upstream all the way to the reservoir.

That is strictly not a shock wave, it is a sound wave in a liquid, the sound of a step function.
A shock wave is a self-maintaining pressure and temperature step in a gas.

The presence of sound waves tells us that water is compressible, but that is where it ends. The "pressure spike" can also be seen as a transient wave travelling along a transmission line, or as a bulge in the pipe that travels with the sound pressure inside.

Also, since posting my question it occurred to me that the presence of that air chamber almost guarantees that the water isn't going to compress or swell the pipes to any meaningful extent. I mean, why would the water "bother" to compress when there is a compliant cushion of air to take the shock?

The system is dynamic, not static. Unlike hydrostatic pressure, the step function is so short that it is not everywhere the same at once.

As the waste valve closes with a snap, a pressure wave travels away from the valve. That forms a wavefront across the induction pipe. As that front passes the check valve, the peak pressure is clamped by the exit of water to the air chamber on one side, but only part of the energy pushes water through the valve, the rest of the wavefront continues to swell the pipe as it travels on towards the induction pipe inlet, where it is reflected. The peak pressure of that travelling step pulse, was limited to some extent, while passing the small aperture of the check valve, to the pressure in the air chamber.

The reflected wave is inverted and of similar but negative amplitude to what remained after passing the air chamber check valve earlier. Note also, there is a negative pressure limit due to cavitation at the reflector. That reflected depression pulse has no problem overcoming the static pressure head in the pump chamber, so the waste valve is opened by atmospheric pressure without any problem.

The forward step-up pressure wave, will be partly cancelled by its inverse on reflection. The pressure pulse width will be determined to some extent by the return transit time of the induction pipe. It will also be limited by the harmonic content of the waste valve closure.