25 Feb 2015
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 evaluating the pump curve a point will be identified as the “Best Efficiency Point” (BEP) and it is at this point that will give the optimum delivery using the least power and it is at this point that the pump selected must be operated at.
To confirm that the Submersible Pump is operating at the BEP a pressure gauge can be installed as close as possible to the Submersible Pump discharge and using the pump curve with the discharge pressure known it can be confirmed at what point on the curve the Submersible Pump is operating at. This is also applicable to all centrifugal pumps.
The product being pumped can also influence the total cost. In most cases, submersible pumps are used to pump water and all pump curves are generated for this condition and should the product contains solids or a chemical substance, as previously mentioned, a detailed analysis of the product should be done. With solids present in the liquid the Specific Gravity (SG) and particle size is important as this will have to be taken into account when determining the installed power of the submerged pump, while with chemical substances the Ph and a chemical composition will be important, and must be established to ensure that the submersible pump is constructed from
materials that will be capable of pumping the specific liquid with the minimum wear. When liquid that has both solids and is acidic needs to be pumped, certain Duplex Stainless Steels such as Cd4CMu can be used but is advisable to consult an expert with regards to the selection of the materials of construction. It is of utmost importance that when pumping liquids containing solids that the pump operates at BEP. The wear rate at BEP will be minimal and will contribute to both a cost saving on power consumption and an extended operating life of the pump.
The major cause of failures of submersible pumps is the overheating of the electrical windings and it essential that the heat is dissipated away from the electrical windings. Most submersible pumps rely upon the product being pumped to dissipate the heat from the windings and mechanical seals and when the pumps run dry in most cases the pump will build up the heat and the electrical winding as well as the seals would fail. To improve the heat dissipation the motor housing is filled with oil and the oil will effectively dissipate the heat away from the electrical windings and seals even if the pump runs dry which will ensure that the electrical winding does not overheat and burn out. Damage to the electrical cables also frequently occurs and it is essential that the pump is fitted with a good quality trailing cable with screened conductive conductors.
All electrical submersible pumps will require an electrical control panel that has been specifically designed to protect the pump should the pump be overloaded. With the pump operating in liquid the panel must be fitted with an Earth Leakage. The installation must be done by a qualified electrician with a wireman’s license and a certificate of compliance must be issued.
If the above facts are taken into consideration the use of Electrical Submersible Pumps can be economically viable and can be used in any application where the Submersible Pump will fit into, being either a tank or a sump. The Electrical Submersible Pump is a portable pump and can in most cases be used in place of Horizontal or Vertical Spindle pumps.