Ram Pumps

People are always fascinated by ram pumps. I think partly because they achieve the seemingly impossible task of pumping water to a higher height than the water supplying the pump, and they do it for no added energy input. This is often misunderstood as needing no energy, but even a casual understanding of the laws of thermodynamics tells us this is impossible. You will notice I said no added energy input because fundamentally what a ram pump does is harvest the energy of a lot of water flowing through the pump from a low head source to pump a much smaller volume of water to a higher head. Head being the height of the water relative to the pump.

Imagine if you will a pipe with an internal diameter of 100mm with water flowing through it. If the pipe is around 25 m long then the weight of the water in the pipe is close to 200kg, remembering that one liter of water weighs one kg. This is basically one of those large oil drums full of water.

Water is basically incompressible — meaning that you can’t fit more water into a container by cramming it in under pressure than you can just by pouring it in. So let’s imagine 200 L of water in an oil drum free falling under the influence of gravity. Next let’s imagine it hitting a concrete floor. For one thing the stop would be sudden and the force and the noise would be great. This is exactly what a ram pump does — the water falls through the pump gaining velocity until that velocity is sufficient to flip the waste valve shut. Suddenly you have a lot of water — which an instant earlier was traveling at quite a speed — stopping instantly. As you can imagine, with nowhere to go (like the concrete floor in our example), we have a lot of energy to dissipate.

By putting a one-way valve in a much smaller line coming off the main pipe we can use the energy in the main flow of water like a battering ram to punch the water past the one way valve and up the smaller pipe.

The energy of the sudden stop starts to dissipate as more water is pushed up the small pipe. Eventually the energy is exhausted and the water flow stops, and at this point the one-way valve shuts, trapping water in the smaller supply pipe, waiting for the next hammer blow of our watery battering ram. At this point the water in the main (drive pipe) is totally stopped and even starts to ‘bounce’  back just a little from the shock wave of the initial ‘hit’.

It is this reversal of flow which creates a small negative pressure in the pump which allows the waste or ‘clack’ valve to drop open via gravity and start the process all over again.

Now I want to go back to the point in the cycle where the water is pushing past the one-way valve and go into a little more detail, as I have, for the sake of clarity, left out a vital component of the system.

Attached to the outlet side of the one-way valve and branching of the delivery line we have a tank with air trapped in it. This tank’s job is to act as a shock absorber and smooth out the pressure spike of the ram. Without this tank the pressure spike can be so great it will impact on the service life of the pump. After the one-way valve is shut the pressurized air in the tank re-expands and continues to push water up the delivery pipe — greatly increasing the efficiency of the pump. Those tempted to do without the air tank be warned. Just to see what would happen I ran a 100mm diameter ram with only 1 meter fall and a lift of three meters and managed to tear a 50mm check valve to bits. Now the valve was a good quality, glass-reinforced plastic one rated to over 100psi. While I should have been mortified at the destruction of the valve I couldn’t help but feel an evil glee at the clear demonstration of the forces involved.

If ram pumps have a downside, the one that most often comes to mind is the need for a certain amount of fall, below which they won’t operate. Depending on size and other factors, minimum fall is between 500mm and 1000mm. We also need to realize that when it comes to energy there is no such thing as a free lunch, so how much fall we have directly effects how high we can pump. Typically most commercial rams are quoted as being capable of lifting water ten times the fall, so 1 meter of fall will give you 10 meters of lift. It also stands to reason that the higher we pump the less we will pump as we are having to use more energy simply overcoming the greater weight of water in the delivery pipe.

Recently I was presented with a challenging site, where the creek the client wanted to pump from had a maximum fall of 1 meter and the height we needed to lift the water simply to get out of the creek gully was 20 meters. From the numbers given earlier the upper range we could hope to pump to with a 1 meter fall is 10 meters, so it usually wouldn’t have been deemed possible. Then add in the factor that I was hoping to build the ram pump myself and that performance figures for other homemade rams was generally given to be in the region 5 meters lift per meter of fall — so the task looked all but impossible. The icing on the cake was that the creek was prone to flash flooding and to get any fall at all the pump would have to sit directly in the creek bed, right in the way of logs and such when it flooded.

It didn’t look good till I hit on the idea of staging two pumps to get us up and out of the gully.

The idea goes something like this: first we use our low fall (1m) to run a large ram pump and pump to a low delivery head (5m) and then let it fall through a smaller ram pump from the height of 5 meters back to the creek. This effectively gives us a small pump running with a fall of 5 meters which even on the lower end of the drive to head ratio (5:1) gives us enough to get up and out of the gully.

Luckily I even had a small pump that I had previously built and tested that I knew would do the job with the given fall. Perfect.

All that had to be done then was work out the volume of water required to run the small pump and then build a bigger pump to supply that volume with a little spare capacity. After a lot of research and some mid-range number crunching (not one of my strengths) I came up with a pumped sized to use a 100mm drive pipe. My target was 40 liters a minute delivered to 5 metres above the creek.

In the photo above you can see the pump design i came up with. Ram pump size designation normally comes from the diameter of the drive pipe — in this case 100mm. Going from left to right we can see where the 100mm drive pipe attaches then the main body of the pump (the big steel box) and then the waste valve (square opening) on the right.

An important design consideration is that we want the water to gain the greatest velocity possible, as the faster the water the more energy. This is especially important when we have very low falls as we have less energy to start with, so an important consideration is a straight flow path from the drive pipe inlet to the waste valve outlet. 90 degree turns and such will rob us of potential energy. Also we want the waste valve outlet to be at least as big as the pump inlet (remember we want maximum flow for maximum energy). To do this I worked out the cross sectional surface area of a 100mm pipe (78.5 cm2) and then cut the waste valve hole to be slightly bigger — 100mm x 100mm (100 cm2). I did this before actually sourcing the pipe, and, because we were using scrap, when it turned up it was actually 112 mm internal diameter, which is 113 cm2 cross sectional surface area. Unable to resist getting the absolute best out of the pump I remade the waste valve and opening larger so that once again it was larger than the inlet — so no flow restriction.

Again under the theme of getting the most energy, a waste valve that shuts early won’t allow the water to gain the greatest velocity and as we already know we have limited energy to start with so anything that interferes with that is a big no no. To that end we need a waste valve we can tune so that it closes just as the water is gaining its maximum velocity. Anything less is wasted energy.

It’s a bit hard to see in the photo but our waste valve is a heavy steel flap hinged at the bottom so it lays back into the oncoming flow of water. You can see the row of four small shiny bolts in the square waste valve opening — these attach the valve plate to a stainless steel door hinge. The valve drops open under gravity because the plate it is bolted onto is angled back. The adjusting bolt simply passes through the center of the valve plate (single big bolt in the square opening) and can be adjusted so the angle the water passes over the valve plate can be adjusted. We’ve all stuck our hands out a car window and made those wavelike motions up and down and you will have noticed it’s easy to keep you hand horizontal to the air flow but the instant you angle up or down a larger force flips your hand up or down rapidly. This is exactly what happens with our valve.

Of equal importance to all I’ve written about so far is the need to use rigid materials for the drive pipe and the pump body. The reason these pumps work at all is to a large degree due to the incompressibility of water. So imagine our 200L liter barrel falling towards the floor except this time it’s a large tractor tube filled with water. For a start we will get more of a splat than a crash, and there won’t be any chipped concrete, that’s for sure, and that’s because the energy of the falling water is to a great degree dissipated by the elasticity of the rubber tube. So it is with our drive pipe and pump body. If we make them out of materials that have some ‘give’  in them, then we are wasting energy by flexing those materials. You will see lots of ram pumps on the internet using plastic parts for the drive pipe and the pump bodies. These certainly work well if we are in a situation where we have enough energy, so we can afford to waste some of it. Ultimately, however, if we are talking maximum efficiency, then rigid materials like steel are best. In the end it comes down to what you need and what it costs. For us we needed steel because of our low head and also for durability as this thing will, over the course of its life, have whole trees coming downstream and smashing into it — so plastic pipe just doesn’t cut it.

Most of the designs you will see on the internet tend not to be designed to sit right in the stream bed, and certainly most aren’t designed to have logs and boulders crashing into them, so they tend to have air tanks that stick straight up from the pump body so that they can trap air in the top of the chamber to act as a shock absorber — generally with a device called a snifter valve that lets a tiny gulp of air into the chamber each time the pump cycles. This is so that air under pressure isn’t gradually dissolved into the water in the pump (just like CO2 into a bottle of soft drink). This would then make the pump act like it had no air chamber, with the result of lower performance and far greater stress on the pump parts.

Anyway, as you can imagine in a flood, air chambers that stick up will tend to get damaged far easier than ones that lie flat, so we designed ours to lie flat. This required that instead of a snifter valve we trap air in the chamber using a different method, because, for one, a snifter valve doesn’t work well with a low air chamber. Secondly our pump is sitting right at water level so the minute the water level rises and the snifter valve goes under, the air chamber will fill with water and the pump stops. We got around this simply by filling a car tube with air and stuffing it in our air chamber so the air was trapped in the car tube and couldn’t dissolve away, with the result that the pump will continue to run under water as long as we still have sufficient fall.

The air tank in the photo is salvaged from an old LPG vehicle tank, and the fitting on top of the tank is just the original from when it was in a car — it is retained purely to seal the tank. You can also see water starting to build up velocity as it exits the waste valve.

Another important factor in getting the most out of a ram pump is the length of the drive pipe — too short and you don’t get enough mass going . Think of it like this; you have two battering rams, one is a telephone pole and the other is a pool cue. Both are made out of wood, but we know which one will bash down a door. So you need the biggest reasonable mass of water you can. I say ‘reasonable’ because over a certain length and you will actually start to lose performance again. It’s a Goldilocks thing — not too big, not too small, but just right. This has to do with the interactions of the various shock waves traveling up and down the pipe and certainly is not something I’m going to go into great detail on. Luckily we have a rule of thumb calculation that we can use to come to the correct lengths. Take the internal diameter of the drive pipe and multiply it by 150 and that will give you your minimum length. Next take the internal diameter of the pipe and this time multiply it by 1000, which gives you your maximum length. Now if you have a longer drive pipe run, because of site conditions, don’t fret. You can put in a stand pipe, which is simply a pipe coming vertically off the drive pipe. If we do this towards the effective maximum length of the drive pipe as you’ve calculated it, then this will allow the pressure waves to dissipate and the pump retain its maximum efficiency.

Let’s recap regarding factors effecting efficiency.

Fall. Get the maximum fall of water into the pump. At 1m we’re right at the bottom of what is considered viable. In fact, for 100mm ram pumps generally, 1.5m is considered the minimum viable fall.

Rigidity. The pump body is vastly overbuilt because the scrap steel we had was very thick (12mm) so is very rigid, however other factors can effect rigidity — like any leaks in the drive pipe and the material the drive pipe is made of. Here we had some issues. Firstly the gasket material for fitting between our lengths of drive pipe was salvaged conveyer belt and was too hard and didn’t seal properly. Also after our initial run the pressure spike in the drive pipe found some internal weak points where the pipe must have been nearly rusted through. Every time the waste vale shut we had water from various leaks shooting skyward. We now have better gasket material ready for instillation and our local scrap guy has his eye out for some replacement pipe.

Tuning. We have adjusted the long bolt so that the water builds to nearly maximum velocity before shutting. A simple test is to observe how far the water is gushing out the waste valve with the valve held fully open with a stick — this is our maximum velocity. Next, observe how far the water gushes out when the pump is running. If it’s substantially less than when the waste valve is held open, then adjust till it’s nearly as much. Too much though and it won’t shut reliably. A word of caution — don’t hold the waste valve open with you finger…. Here we can go back to my original analogy and imagine what would happen if your finger was caught between a falling 200kg drum of water and a concrete floor. Ouch!

Drive pipe length. Here we’re actually under our minimum length, and, as already mentioned, the pipe is leaking, so it’s definitely sub-optimal. So, we’re waiting on our scrap guy!

Pressure/Air tank. Here we’re good. Best practice is to size the tank to 50 times the pumped volume of one cycle of the pump, which for this size pump is around one liter. The tank is about 60 liters, so that’s good to go.

I suppose the next question is how did it all go, considering our target of 40LPM and a few suboptimal factors?

As I stated at the start, due to boyish enthusiasm we fired it up prior to finishing the air tank and destroyed the check valve, so we knew we had harnessed some substantial energy. Or should I say we hadn’t harnessed it, hence the rupture. Once the air tank was finished and installed we primed the pump and started it up. A flow test at 5 meters gave us 36LPM. Considering the performance tweaks we know we have to address we will easily hit our target of 40LPM. Happy is not the word — we were stoked!

Next we installed a pressure gauge in the delivery line to see what our maximum delivery pressure was. We topped out at over 70 PSI (sorry for the imperial measurement but I’m a PSI kind of guy).

This gives us a maximum theoretical pumping height of 49 meters. This means we could actually get up and out of the gully in one lift. Before you get too excited though it is still to be assessed wether we could match the flow rate of the second stage pump to get the ultimate flow rate we required without the staging. But if its over 4LPM we’re in business!

It was about this time that a loud whooshing sound was heard and the air tank took off down the creek spraying a jet of water out the back. We had salvaged the old damaged valve and put it back together, and under the strain of the pressure test it finally gave up its life for the greater good. A quick trip to town and a nice new brass valve was soon installed.

This shot shows removing the gauge after the pressure test.
The water was shooting at least 15m vertically.

Considering all the adverse factors, some of which we will shortly correct, I think it’s fair to say that the pump construction and installation is and will be a resounding success and will become a vital part of the property’s infrastructure.

On my next post i want to explore some intriguing possibilities for ram pumps. Hint. What set of circumstances would allow you to take water out of a creek or stream and yet increase the volume of water flowing in the creek?