Most rainwater systems will require a pump of some sort to get the water from the water tank or water tanks to where it is needed. An important consideration when choosing between a conventional external pump and a submersible pump for your water tank is that submersible pumps are less prone to theft (especially important in South Africa).
In this world of options, one of the choices you will need to make is the type of pump you use in your rainwater harvesting system or water tanks. There are several different types to choose but a primary differentiator is how the pump draws water.
Most people are familiar with the types of pumps used in swimming pools; centrifugal pumps which are situated above water level. These particular pumps are freely available, inexpensive and reliable, but they are certainly not the only option available. External pumps work best when they are situated below water level, adjacent to the water tank. In this way there is positive pressure supplying water to the pump and the efficiency of the pump is greatest.
Following on this line of thought the logical next step was to manufacture pumps that could be submerged in the water they are pumping, thereby ensuring the benefits of positive pressure. In order to operate underwater the motor of the pump is hermetically sealed to exclude water and avoid an electrical short circuit. This significantly increases the cost of a pump relative to the volume of water it is capable of pumping, and seems to shorten their life span at the same time.
The Achilles heel of a centrifugal pump is that it requires priming. In other words, there must be water around the impeller when the pump is activated in order for it to pump water. Submersible pumps neatly avoid this problem by being submerged in the water, whereas pumps situated outside the water tank and above water level must be primed. Failure to do so can result in the pump overheating and burning out.
Another risk associated with external pumps is that they suck water into the pump housing, and if there is any amount of leakage on the suction side, they can also suck in air. Submersible pumps do not experience this problem as they only suck in water.
On the balance of considerations, many applications are based on using external pumps, primarily due to their lower cost and free availability. However, given the dual advantages of being self priming and avoiding the risk of sucking air into the system, perhaps one should be giving submersible pumps a closer look.
Source: Water Rhapsody (via Africa Green Media)
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