Hydraulic Ram Efficiency Calculator

A hydraulic ram pump seems incredibly simple in design. It requires no motor, fuel or electronics but only two valves and a steady impulse created by the falling water that forces a small amount of water up hill.

It uses the d'Aubuisson formula used by engineers since the 1800s and also shows more accurate rankine values so this hydraulic ram pump can be understood from two different perspectives.

How a hydraulic ram pump works:

Water flows from the water source through a pipe downwards and gradually accelerates. At the bottom end there is a spring-loaded drain valve which is initially open. When the flow speed becomes high enough, the valve closes suddenly due to pressure.

As there is no outlet for the impulse, a pressure surge, also known as water hammer, suddenly occurs. This surge opens the check valve and forces the stream of water into an air chamber. The trapped air then pushes the water further through the pipeline upwards.

After a while the relief valve returns to its open position and water flow begins again. This cycle is repeated at a frequency of 30 to 100 times per minute. With just two valves, gravity, and a small amount of compressed air this system has worked on farms for over 200 years.

How to use this calculator:

You need to obtain four values from the system. First measure the inlet pressure, i.e. the height difference between the water source and the pump, and the delivery pressure, i.e. the height that the water is raised by the pump to the water tank. A spirit level and a tape measure are sufficient for this.

Next, measure the flow at both ends with a bucket. The drain flow is the amount of water that goes out through the bottom valve while the pump-out flow is the amount of water that reaches the top end. Enter these four values and read off the efficiency.

You can also perform calculations in the opposite direction. Leave one of five fields blank and enter the remaining four values. The calculation tool will then determine the missing value. This is useful for design problems, such as when you want to specify a lift height corresponding to a certain efficiency.

D'Aubuisson formula:

This calculation compares the input energy (all the falling water) to the output energy (the amount of water that is moved to a higher position but has less volume).

Here q is the pumping volume, H is the pumping height, Q_w is the wastewater volume and h is the inlet pressure (level). The sum of the wastewater volume and the pumping volume corresponds to the total volume of water that the hydraulic ram pump extracts from the source.

Suppose the water source is 3 meters above the pump, the collection tank is 15 meters above the pump and there's a drain of 40 liters per minute with 5 liters per minute reaching the collection tank.

The numerator of 75 represents the useful work required to lift 5 liters of water by 15 meters. The denominator of 135 is an energy budget corresponding to moving 45 liters of water falling by 3 meters. The flow rates in the numerator and denominator cancel out, as do the units for height, leaving a simple percentage.

The table below shows each symbol and an example of it.

D'Aubuisson and Rankine comparison

There are two classical methods of evaluating hydraulic ram pumps that give intentionally different results. As D'Aubuisson measures the two heights relative to the pump, it attributes all the water moved by the pump as a complete lift from the pump inlet to the header tank as head.

Rankine uses the water source as a datum. As the amount of water lifted up to the level of the water source is not included in the cost, Rankine only calculates the lift height above the water source and runs the hydraulic ram pump exclusively with waste water.

The Rankine value is always lower than the d'Aubuisson because it requires more pumping. Both values are correct. The d'Aubuisson is a field commonly used value while the Rankine provides a value that better represents thermodynamics. If you turn on "Comparison Method" you can see both values as well as a simplified version of d'Aubuisson where the denominator has been reduced to water flow rate.

This is how you read hydraulic performance:

Efficiency is a ratio but the underlying wattage figures will give you an idea of whether or not a hydraulic ram pump is suitable for the task. If you turn on the "power" switch, then you can convert water flow and lift into hydraulic power.

The input power is the power that would be produced by falling all of the water in the head. The output power is the power that is produced by lifting a small amount of water to the full lift height. The difference between these two values corresponds to the loss of power due to friction, valve actuation and turbulence. The ratio of the output power to the input power is still the efficiency.

Suitable areas of use for hydraulic ram pumps:

It is suitable for any place where there are water losses and the water needs to be transported uphill. For example it can be used to bring water from a farm with spring water as source to a dam on top of a mountain, to supply a small farm with water from a pond above a stream in sustainable agricultural installations using a stream for gravity irrigation or in remote villages without an electricity grid.

It is ideal for tasks that require a steady thin stream of water rather than large sudden amounts. It also works well with water sources that provide year-round water and places where you do not want to have to go often to perform maintenance. The system operates in an efficient range when the ratio of lift to drop is approximately between 3:1 and 15:1.

Factors that affect efficiency:

The most important factor is the ratio between lift height and inlet pressure. When water is lifted to a height that is more than 20 times the head, efficiency falls dramatically while it remains relatively high up to five times the head approaching this upper limit.

The length of the drive hose is also important. If it's too short, the water flow can't develop enough speed to create a strong impulse and if it's too long, friction forces will consume energy. The optimal length for the drive hose is usually between five and ten times the water transfer pressure.

The valve setting is also a major factor. The relief valve must be closed at the right time and the weight and lift of the valve can be adjusted. All are limited by the available flow rate. If the flow in the drive hose is below about 15 to 20 liters per minute, then the hydraulic ram cannot circulate properly. A well-tuned hydraulic ram operates in a range of 50 to 65 percent and thus is a very efficient machine considering that it is powered only by gravity.

Tips for maintaining good performance:

Adjust the size of your pressure pipes to achieve a water flow rate of one to two meters per second. Agricultural operations typically use 1 or 2 inch diameter tubing. Refill the air tank with air every few weeks. This is because trapped air in the water will slowly dissolve and lose its damping effect, which reduces shock absorption.

Set the relief valve to make a clear and regular "bang" rather than irregular interruptions. Use a check valve with a pressure rating at least twice that of your pump, and anchor the pump on a concrete base so it won't gradually shift due to vibration.

Calculate the values regularly. If efficiency slowly decreases, this usually indicates a worn valve seat or air tank leak. In cold climates, either drain the water before the first hard frost or keep it warm.