Ram pumps explained

In a 12/31/2009 post to the energy options forum, MidKid requested a detailed explanation to the workings of a ram pump. I hope this provides that explanation.

Ram pumps are a centuries old “simple” tool used to move water from a lower elevation to a higher elevation. Like the lever, screw, pulley and inclined plane, it makes use of the laws of physics and nature to make tasks easier to perform. Two of the laws that apply to a ram pump are gravity and the law of inertia, which is the resistance of an object to a change in its state of motion.[cite] The ram pump relies on gravity to cause a flow of water and it then harnesses the energy of that flowing water's resistance to being stopped suddenly and redirects that energy into pushing water to higher elevations. At its simplest level, a ram pump consists of an inlet for water and two outlets; a discharge outlet and a delivery outlet.

Sample ram pump use diagram

The need for an inlet pipe is to confine and control the flow of water. It is not enough to simply place a pump into the flow of a stream or river because, even though the water is flowing under the principles of gravity, it is not constrained and therefore would simply flow around the pump.

The delivery outlet is a pipeline that will carry the water to the higher elevation point which can be a tank, pond, irrigation canal or any number of other places to deliver water. It is important that the receiving vessel needs to allow for overflow. This is required so that the ram pump does not get stopped due to pressures building to the point where it can not push the water through. It is for this reason that a ram pump would not work well to directly feed a system of use that would require constant pressure. Such a system would require that a holding tank at a higher elevation be used to establish a gravity feed to provide the pressure it requires.

The discharge outlet is what allows the pump to establish the flow velocity it requires to work and simply discharges the water out of the pump. Usually this is discharged back into the source that provides the water being pumped and lets it simply continue on its original path.

An important aspect of the ram pump is that the flow through these two outlets is controlled by valves. The delivery outlet valve is a back flow valve that keeps water from returning back through the valve into the pump. The discharge outlet valve is a pressure (clap) valve which slams shut when a specific amount of pressure is placed on it, essentially stopping the flow of water through it.

The inlet will need to be a pipe of sufficient size and length to provide the delivery of water from a point that is sufficiently higher than the pump. The height to the source of water has a direct relationship to the elevation the water can be pumped to. The basic rule of thumb is that a ram pump will pump water to a height that is 10 to 15 times the height from the source to the pump when the inlet pipe is at least 1.5 inches in diameter with a flow rate of at least 15 gallons per minute.

Now, lets look at how the mechanics of the pump come into play. Unless the pump is properly set up, the results would be that the flow through the inlet pipe would be stopped and the water would simply overflow the top of the inlet pipe as if it were just a large glass placed under the flow of a faucet. A ram pump is designed so that the flow of water can pass through a discharge outlet in such a way that the pressure on the pressure valve in the discharge outlet increases to the point where the valve slams shut. That is when things get real interesting with regard to the forces that result.

In an attempt to understand what happens, think of a piece of wood, a nail and a hammer. Hold the nail and hammer as you would to drive the nail into the wood. Place the hammer head on the top of the nail and observe what happens to the nail. Not much is happening. The weight of the hammer is not sufficient enough to overcome the resistance of the wood to the nail passing through it. Now simply raise the hammer and let it fall onto the top of the nail. Do not swing the hammer using the force of your muscles, simply let it fall and strike the top of the nail (you could hang the hammer over the nail on a string that would allow you to raise it and let it drop, but I would get someone else to hold the nail :) ). The point is, you should see that the weight of the hammer in a free fall coming to a sudden stop at the top of the nail causes a compression of the weight in motion such that it transfers enough energy to allow the nail to overcome the resistance of the wood. Granted, the nail will not be driven in very much with this one drop of the hammer, but continuous repetition of this process would result in causing the nail to be driven into the wood.

This same principle of inertia comes into play when the pressure valve in the discharge outlet slams shut under the increasing pressure of the water flowing through it. The energy created by the compression of the weight of the flowing water that has suddenly been stopped is known as a water hammer and it tends to apply great forces on the inlet system. It is the energy created by this water hammer that will be directed to the delivery outlet, opening the back flow valve that is there and allowing the water to flow into the delivery pipe carrying the water up to the higher elevation. The water would then continue into the delivery pipe for as long as the pressure allows it to do so. When the pressure is no longer enough to push the water into the delivery pipe, a back flow starts which causes the back flow valve to close. This stops the delivery pipe water from coming back into the pump. It also causes a bit of a vacuum in the pump as the motion of the back flow pushes water back up the inlet pipe under the momentary equal pressure in the system. This momentary vacuum is enough to allow the pressure valve in the discharge outlet to open which then allows the flow from the inlet pipe to pass through the discharge outlet again, starting the whole cycle over again. This is what results in the pumping of the water.

The discharge outlet and pressure valve have to be such that they allow for the valve to be manually opened in order to facilitate priming the pump. Many times when starting a ram pump, there is not enough water in the delivery outlet to result in enough back flow to create the vacuum required to reopen the pressure valve in the discharge outlet. By manually opening the valve, you initiate another cycle and eventually you will have caused enough cycles to get the pump working automatically.

Simple! And yet, not so simple. Ram pumps have been used for centuries, but the advent of electric powered pumps that transfer much larger quantities of water for very little cost has resulted in ram pumps being all but forgotten. Ram pumps are devices that can only transfer about one gallon of water for every eight gallons delivered through the inlet pipe. This results in a lot of wasted effort and energy, although some of that can be captured and used in other ways. It also places some dependancies on the use of them. The pump needs to be where the discharged water can simply flow away. If not, then it would also need to be piped away and this would need to be done in a way that would not cause back pressures on the discharge system that would stop the pump from working.

And then there are things that tend to complicate the process. Things like how much water, how high and how far the water needs to be delivered as well as the impact of air being drawn into the system as a result of the back flow process that makes it work. These are all things that have implications on the efficiency and construction requirements of a ram pump. Going back to the hammer and nail example for a moment, it would be easy to see that if you use a larger hammer or raised it to a higher height, you could drive a larger nail, or drive a similar nail much further. These are the same with regard to the ram pump. If you can increase the size of the inlet pipe (the hammer) or can have a higher point to draw the water from, then you can increase the height and distance you can deliver water to or increase the amount of water you can deliver. These adjustable aspects of the system also have an impact on the forces created within the system such that the materials used would then be a factor. Larger amounts of water and greater length or size of the inlet pipe would increase the forces such that the pump and pipes used would need to be metal rather than PVC. So, these aspects place limits on the height and size of the inlet pipe that can be reasonably obtained. There are concerns to keep in mind with regard to the other end of the system as well. The elevation of the delivery point has to be high enough to create a back flow pressure sufficient to allow the system to work. If there is not enough back flow pressure, then the back flow valve will not close, no vacuum will be created to reopen the pressure valve in the discharge outlet and the result would be that the water would find its own level and act much like that large glass mentioned earlier.

There are circumstances that might call for pressure chambers to be introduced to the pump to allow it to handle greater forces involved when trying to pump larger volumes of water. There are also circumstances that might require a standing pipe system to be used in conjunction with the inlet pipe. This would come into effect when the water source is such that the height where you tap into it is a long distance from the base and the drop in height is gradual. Your pipe delivering the water to the pump might follow the contour of the land and enter into a chamber that bring the water level to the required height for the inlet pipe and deliver the water to a tee in the inlet pipe at that height. In this type of system, a standing pipe should extend the inlet pipe to a height that is at least a foot higher than the actual source and should not emit any water during operation. A standing pipe lets the inlet system adequately handle the hammer forces caused by the pump.

Another aspect of ram pumps is that they draw water from less than ideal sources and therefore tend to contain sediments no matter how much you try to filter and limit the intake of sediment. This means that the pump should be maintainable, which means it has to be stopped and restarted at times. One thing to consider is to have manually operated shutoff valves within the inlet pipe and the delivery outlet pipe to stop the flow at the pump end of both of these pipes. Having a union between those manual valves and the pump would allow the pump to be removed and serviced ( clean out the sediment ). And the air that gets taken into the system as a result of the vacuum opening the pressure valve in the discharge outlet will have to be dealt with. There are a number of ways to deal with this, but the simplest way is to have the discharge outlet be such that there would be enough of a reservoir of water to allow the vacuum to draw water back into the pump from that side.

With regard to the actual construction of a ram pump, there are a number of web sites that outline the construction process as well as a number of sites that sell commercially produced ram pumps that are meant to handle the needs of many different situations. The price is usually dependent on the situation the pump is designed to handle. A simple search for “ram pumps” will yield a number of such sites.

I hope this has helped to explain the ram pump in a way that you can understand its principles and determine if it is something that you would like to pursue.