Hydraulic Ram

1. INTRODUCTION

A hydraulic ram pump is a water pump powered by water with a height difference. In areas where natural flows exist with a height difference of the water over a small distance, hydraulic ram pumps can be used to transport water to higher grounds without using electricity or fuel. The hydraulic ram uses the water hammer effect to develop pressure that allows a portion of the input water that powers the pump to be lifted to a point higher than where the water originally started. Apart from the kinetic energy of the water, no other source of power is needed.

The hydraulic ram pump was invented in 1772 and widely used in the 19th century, but was side-tracked by the advent of the coal-powered steam engine and later by diesel powered pumps. In recent years the hydraulic ram pump has seen a renewed interest, because it is powered by sustainable energy, and can be produced locally.

A hydraulic ram pump is powered by a body of water flowing downhill with a height difference. A general rule of thumb is that the water can be pumped 30 times as high as the available drive head (the height difference of the water driving the pump). So a head of 1 m can be used to pump up water to ~30m, while a 7 m head can pump water up to 210 m.

The capacity of a hydraulic ram depends on the scale of the pump, which is often measured in the diameter of the tube delivering the water to the pump. Pumps exist in the range 1" up to 5".

With height difference, the actual difference in vertical height is meant, not the length measured along the slope.

Advantages	Disadvantages
- Uses renewable energy sources.  - If properly designed, can be produced and maintained locally.  - Very effective in mountainous areas	- Water with height difference is needed

2. PRINCIPLE OF OPERATION

A hydraulic ram has only two moving parts, a spring or weight loaded "waste" valve sometimes known as the "clack" valve and a "delivery" check valve, making it cheap to build, easy to maintain, and very reliable. In addition, there is a drive pipe supplying water from an elevated source, and a delivery pipe, taking a portion of the water that comes through the drive pipe to an elevation higher than the source.

2.1       Sequence of operation

Figure: Basic components of a hydraulic ram:
1. Inlet – drive pipe
2. Free flow at waste valve
3. Outlet – delivery pipe
4. Waste valve
5. Delivery check valve
6. Pressure vessel

A simplified hydraulic ram is shown in Figure. Initially, the waste valve is open, and the delivery valve is closed. The water in the drive pipe starts to flow under the force of gravity and picks up speed and kinetic energy until the increasing drag force closes the waste valve. The momentum of the water flow in the supply pipe against the now closed waste valve causes a water hammer that raises the pressure in the pump, opens the delivery valve, and forces some water to flow into the delivery pipe . Because this water is being forced uphill through the delivery pipe farther than it is falling downhill from the source, the flow slows; when the flow reverses, the delivery check valve closes. Meanwhile, the water hammer from the closing of the waste valve also produces a pressure pulse which propagates back up the supply pipe to the source where it converts to a suction pulse that propagates back down the pipe.

This suction pulse, with the weight or spring on the valve, pulls the waste valve back open and allows the process to begin again. A pressure vessel containing air cushions the hydraulic pressure shock when the waste valve closes and it also improves the pumping efficiency by allowing a more constant flow through the delivery pipe. Although, in theory, the pump could work without it, the efficiency would drop drastically and the pump would be subject to extraordinary stresses that could shorten its life considerably. One problem is that the pressurized air will gradually dissolve into the water until none remains. One solution to this problem is to have the air separated from the water by an elastic diaphragm (similar to an expansion tank); however, this solution can be problematic in developing countries where replacements are difficult to procure. Another solution is to have a mechanism such as a snifting valve that automatically inserts a small bubble of air when the suction pulse mentioned above reaches the pump. Another solution is to insert an inner tube of a car or bicycle tire into the pressure vessel with some air in it and the valve closed. This tube is in effect the same as the diaphragm, but it is implemented with more widely available materials. The air in the tube cushions the shock of the water the same as the air in other configurations does.

2.2 Efficiency

Typical energy efficiency is 60%, but up to 80% is possible. This should not be confused with the volumetric efficiency, which relates the volume of water delivered to total water taken from the source. The portion of water available at the delivery pipe will be reduced by the ratio of the delivery head to the supply head. Thus if the source is 2 meters above the ram and the water is lifted to 10 meters above the ram, only 20% of the supplied water can be available, the other 80% being spilled via the waste valve. These ratios assume 100% energy efficiency. Actual water delivered will be further reduced by the energy efficiency factor. In the above example, if the energy efficiency is 70%, the water delivered will be 70% of 20%, i.e. 14%. Assuming a 2 to one supply head to delivery head ratio and 70% efficiency, the delivered water would be 70% of 50%, i.e. 35%. Very high ratios of delivery to supply head usually result in lowered energy efficiency. Suppliers of rams often provide tables giving expected volume ratios based on actual tests.

2.3 Drive and delivery pipe design

Since both efficiency and reliable cycling depend on water hammer effects, the drive pipe design is important. It should be between 3 and 7 times longer than the vertical distance between the source and the ram.

Commercial rams may have an input fitting designed to accommodate this optimum slope. The diameter of the supply pipe would normally match the diameter of the input fitting on the ram, which in turn is based on its pumping capacity. The drive pipe should be of constant diameter and material, and should be as straight as possible. Where bends are necessary, they should be smooth, large diameter curves. Even a large spiral is allowed, but elbows are to be avoided. PVC will work in some installations, but steel pipe is preferred, although much more expensive. If valves are used they should be a free flow type such as a ball valve or gate valve.

The delivery pipe is much less critical since the pressure vessel prevents water hammer effects from traveling up it. Its overall design would be determined by the allowable pressure drop based on the expected flow. Typically the pipe size will be about half that of the supply pipe, but for very long runs a larger size may be indicated. PVC pipe and any necessary valves are not a problem.

2.4  Starting operation

A ram newly placed into operation or which has stopped cycling must be started as follows. If the waste valve is in the raised (closed) position, which is most common, it must be pushed down manually into the open position and released. If the flow is sufficient, it will then cycle at least once. If it does not continue to cycle, it must be pushed down repeatedly until it cycles continuously on its own, usually after three or four manual cycles. If the ram stops with the waste valve in the down position it must be lifted manually and kept up for as long as necessary for the supply pipe to fill with water and for any air bubbles to travel up the pipe to the source. This may take a minute or more. Then it can be started manually by pushing it down a few times as described above. Having a valve on the delivery pipe at the ram makes starting easier. Close the valve until the ram starts cycling, then gradually open it to fill the delivery pipe. If opened too quickly it will stop the cycling. Once the delivery pipe is full the valve can be left open.

2.5  Common operational problems

Failure to deliver sufficient water may be due to improper adjustment of the waste valve, having too little air in the pressure vessel, or simply attempting to raise the water higher than the level of which the ram is capable. The ram may be damaged by freezing in winter, or loss of air in the pressure vessel leading to excess stress on the ram parts. These failures will require welding or other repair methods and perhaps parts replacement.

It is not uncommon for an operating ram to require occasional restarts. The cycling may stop due to poor adjustment of the waste valve, or insufficient water flow at the source. Air can enter if the supply water level is not at least a few inches above the input end of the supply pipe. Other problems are blockage of the valves with debris, or improper installation, such as using a supply pipe of non uniform diameter or material, having sharp bends or a rough interior, or one that is too long or short for the drop, or is made of an insufficiently rigid material. A PVC supply pipe will work in some installations but is not as optimal as steel.

3. HOW A HYDRAULIC RAM PUMP WORKS

Here's a simplified version of how the hydraulic ram pump actually works, step-by-step:

Water (blue arrows) starts flowing through the drive pipe and out of the "waste" valve (4 on the diagram), which is open initially.  Water flows faster and faster through the pipe and out of the valve.

        

At some point, water is moving so quickly through the brass swing check "waste" valve (4 on the diagram) that it grabs the swing check's flapper, pulling it up and slamming it shut.  The water in the pipe is moving quickly and doesn't want to stop.

All that water weight and momentum is stopped, though, by the valve slamming shut.  That makes a high pressure spike (red arrows) at the closed valve.  The high pressure spike forces some water (blue arrows) through the spring check valve (5 on the diagram) and into the pressure chamber.  This increases the pressure in that chamber slightly.  The pressure "spike" the pipe has nowhere else to go, so it begins moving away from the waste valve and back up the pipe (red arrows).  It actually generates a very small velocity backward in the pipe.

As the pressure wave or spike (red arrows) moves back up the pipe, it creates a lower pressure situation (green arrows) at the waste valve.  The spring-loaded check valve (5 on the diagram) closes as the pressure drops, retaining the pressure in the pressure chamber.

At some point this pressure (green arrows) becomes low enough that the flapper in the waste valve falls back down, opening the waste valve again.  

Most of the water hammer high pressure shock wave (red arrows) will release at the drive pipe inlet, which is open to the source water body.  Some small portion may travel back down the drive pipe, but in any case after the shock wave has released, pressure begins to build again at the waste valve simply due to the elevation of the source water above the ram, and water begins to flow toward the hydraulic ram again.

Water begins to flow out of the waste valve and the process starts over once again.

Steps 1 through 6 describe in layman's terms a complete cycle of a hydraulic ram pump.  Pressure wave theory will explain the technical details of why a hydraulic ram pump works, but we only need to know it works. The ram pump will usually go through this cycle about once a second, perhaps somewhat more quickly or more slowly depending on the installation. 

Each "pulse" or cycle pushes a little more pressure into the pressure chamber.  If the outlet valve is left shut, the ram will build up to some maximum pressure (called shutoff head on pumps) and stop working.