Reducing the hidden cost of electrical submersible pumps
PUMPS
Electrical Submersible pumps, after being installed and in operation, are in most cases completely submerged and out of sight with little attention being paid to the actual operating costs involved. When purchasing a submersible pump or any Capital Equipment the main consideration should be the total cost of ownership and not only the purchase price of the pump, the total cost of power usage, maintenance, as well as the cost involved in the loss of usage must all be taken into account. There are many factors that will have a drastic influence on the life expectancy of the submersible pump and if not properly addressed will result in unnecessary costs which could quite easily have been avoided. The submersible pump is part of a pumping system and it is essential that the system is evaluated to ensure that that the system has the ability to achieve the desired result at the minimum total costs. When designing the pumping system certain information is required to ensure the optimal performance, starting with evaluating the liquid that needs to be pumped and the
following information would be required before selecting the submersible pump. Evaluation of Pump System: 1) The composition of the liquid needs to be known and it is important to establish if there are any solids present in the liquid, and should there be what are the size of these solids as well as the specific gravity of the liquid. If the pH of the liquid is not neutral the chemical composition will also be required. The above information is required to determine the material of construction of the pump. 2) What volume of liquid that needs to be pumped and this must be determined accurately in order to select a pump with the correct capacity? The pumping capacity will be directly proportionate to the cost of the pump. The volume required to be pumped is calculated to be slightly greater than the maximum inflow of the liquid into the sump or tank. 3) The Vertical height which the liquid must be pumped to relative to the pump. 4) The pipe length as well as all the pipe fittings to be used also needs to be known in order to determine the friction loss which is determined by the flow rate and the diameter of the discharge pipe. For borehole submersible pumps the maximum volume that can be pumped cannot be more than the inflow of water entering the borehole from the aquifer. If the borehole pump is also to be used to supply water to an irrigation sprayer then the pressure that the sprayer would require also needs to be included when determining the total discharge pressure of the borehole pump. The evaluation of the pipe system will also be a factor to be reckoned with when the total discharge pressure is calculated. Incorrect pipe diameter will contribute to both unnecessary wear and high energy costs when pumping a specific volume and it is essential that the pipes are sized to achieve the minimum energy usage while achieving the optimum flow. Once the pipe diameter has been determined other fittings such as valves, bends, and etcetera must also be taken into account to determine what is commonly known as friction losses. The total discharge pressure is made up of three components being the sum of the vertical pressure plus the pressure to overcome the friction loss and the required discharge pressure. Once the required flow rate and total pump discharge pressure of the pump have been determined the pump can be selected. All pumps are tested and a pump curve is generated with the pressure – referred to as the HEAD (H) measured in “kPa” or “Meter” on the Y axis and the flow rate in “m3/h” or “l/s” on the X axis. The Power Absorbed, and the efficiency is indicated on the right-hand side of the curve’s Y axis. When
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Pipes, Pumps and Valves Africa - Jan/Feb 2022
