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Things to consider before purchasing a pump

1. What type of pump do you require, a bore pump, centrifugal/jet pump, petrol pump, sump pump or a transfer pump?

There are many different types of pumps to suite different applications.  Domestic water systems draw their water from various sources at different levels, depending on the water table and terrain conditions. Bore pumps are ideal for drawing water up from a deep well of between 50m and 100m.  The multistage submersible pumps are often also called turbine pumps.

A jet pump or centrifugal pump like the Onga JMM100 is better suited to drawing water up from shallow wells/bores or from water tanks.

A petrol pump is normally used for off-site applications where a power source is unavailable.  Petrol pumps can be high pressure pumps like the Onga fire fighter pumps or transfer pumps with lower pressure and higher flow rates for transferring water over long distances. A good example of this is the Onga Miniblaze pump.

Sump pumps or grey water pumps are ideal for emptying grey water / effluent tanks. Good examples of these pumps include the:

2. How much pressure does your pump system require?

Pressure is the driving force responsible for the movement of a fluid. It is measured in PSI and is an important consideration to take into account when designing a pump system. The term pressure loss or pressure drop is often used and refers to a decrease in pressure usually caused by friction or a leaking pipe.

The average pressure available in most domestic houses is between 30 and 70 psi.  Make sure your pump is suitable for your pressure requirements.

3. What is the elevation height your pump needs to pump the water / fluid to?

Energy is required to lift water from a lower level to a higher level.  This is called elevation energy.  Your pump needs to supply energy to pump the water from its source to its destination.  The greater the difference in height from the water source to its final destination, the greater the amount of energy required and the larger the pump required. The elevation energy required is also referred to as static head and can be found by measuring the height difference between the water source and its destination.

4. What is the friction force of your pump system?

Friction is a force that is always present, even in liquids.  It is the force that resists the movement of objects. In fluids, friction occurs between fluid layers, traveling at different velocities in the pipe, and between the fluid and the pipe wall. In household water systems, friction can account for as much as 50% of the pump’s energy output.  This is because small household pipes produce much greater friction than the larger diameter pipes used in industrial applications.  The smaller the diameter of the pipe, the larger the friction force and the more energy is required to maintain a higher flow rate of water.

Another cause of friction are the fittings used in the pump’s system, for example, elbows, tees etc.  The more fittings used in the system, the greater the friction force. As a general rule of thumb, fittings rarely represent more than 30% of the total friction force calculated from the overall pipe length.

Friction tables are available on the internet which provide friction calculations for different diameter pipes and different fittings.

5. What flow rate do you require?

Flow rate is a measure of the speed at which the fluid moves through the system, and is often measured in litres per minute. For identical pump systems, the flow rate will vary with the static head, the higher the static head the lower the flow rate.

It is important to note that at a pump’s maximum rated head, the flow rate is zero. In this situation the pump will deliver its maximum pressure but no water flow.  As the pump’s discharge pipe is lowered, therefore lowering the pump’s static head height, flow will increase and total head will decrease. This is because when there is a flow of water there is also friction.  Maximum total head is therefore reduced by the amount of energy the pump needs to produce to overcome friction.

Different plumbing fittings and appliances require different flow rates to operate properly. Before purchasing a pump you need to find out the flow rate you require for each item which  will be running simultaneously.  You can then add up the flow rates to give you a total flow rate requirement for your pump.

6. How do you select the correct pump based on head and flow requirements?

When you buy a pump you don’t specify the maximum head required as this occurs at zero flow. You instead specify the head you require at the flow rate you need to run your appliances, shower, toilet etc. simultaneously. Below is an easy step by step process to follow before choosing a pump.

  1. Determine the flow rate for your application. Remember to add the flow rate requirements of all the appliances / applications which will be running simultaneously on your system.
  2. Determine the static head required for your pump by taking the measurement of the height between the suction tank fluid surface and the highest discharge point in the system.
  3. Determine the friction head.  This will depend on the length of your pipe, diameter of your pipe and number of fittings in your system.  As discussed above you can determine the friction head from tables on the internet. Multiply the friction value for the required pipe diameter to the total length of pipe in the system.  Add a further 30% to this value for your fittings.
  4. Calculate the total head by the adding the figures for static and friction head, calculated in steps 2 & 3 above.
  5. You now have the required head and flow measurements for your system which you can use to purchase your pump.  Make sure you purchase a pump with a head and flow rating 15% greater than that which you calculated.

7. What is the viscosity of the fluid you will be pump?

Different types of fluid have different types of viscosity levels.  The higher the viscosity of the fluid, the greater will be the friction force and the more pump head will be required.  The pumping of oil or salt water will require a much stronger pump to produce the same head and flow rate required for a less viscous fluid like water.

8. Are replacement parts easily available for your pump?

Most pumps perform the vital function of pumping water for household supply, firefighting or livestock. When they break down you need them repaired in a timely manner. Make sure you buy a pump with a brand name like Onga to ensure spare parts availability and timely repairs.

9. What type of environmental conditions will your pump be operating in?

If you pump will be standing outside in the weather, we strongly recommend you consider purchasing a pump with a stainless steel pump housing. This will ensure the pump does not rust and will increase the pump’s longevity.  The following Onga pumps all have steel outer pump casings:

10. Is the pump energy efficient?

This is a very important consideration to take into account with the rising costs of electricity. Pool pumps are one category of pump which can use allot of power. Accordingly, if energy consumption is one of your concerns, you should look for a programmable variable speed pump which will reduce energy consumption and your electricity bills.  A good example of a pump like this is the Onga ECO800 pool pump.

Note that  Energex offers a rebate for some energy efficient pumps, check their website before buying your pump to see if you are eligible for a rebate.


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