for the pump efficiency, , may be stated for the pump itself or as a combined efficiency of the pump and motor system. The energy usage is determined by multiplying the power requirement by the length of time the pump is operating. Problems of centrifugal pumps These are some difficulties faced in centrifugal pumps: Cavitation—the net positive suction head (NPSH) of the system is too low for the selected pump Wear of the impeller—can be worsened by suspended solids or cavitation Corrosion inside the pump caused by the fluid properties Overheating due to low flow Leakage along rotating shaft. Lack of prime—centrifugal pumps must be filled (with the fluid to be pumped) in order to operate Surge Viscous liquids may reduce efficiency Other pump types may be more suitable for high pressure applications Large solids or debris may clog the pump Centrifugal pumps for solids control An oilfield solids control system needs many centrifugal pumps to sit on or in mud tanks. The types of centrifugal pumps used are sand pumps, submersible slurry pumps, shear pumps, and charging pumps. They are defined for their different functions, but their working principle is the same. Magnetically coupled pumps Magnetically coupled pumps, or magnetic drive pumps, vary from the traditional pumping style, as the motor is coupled to the pump by magnetic means rather than by a direct mechanical shaft. The pump works via a drive magnet, 'driving' the pump rotor, which is magnetically coupled to the primary shaft driven by the motor. They are often used where leakage of the fluid pumped poses a great risk (e.g., aggressive fluid in the chemical or nuclear industry, or electric shock - garden fountains). They have no direct connection between the motor shaft and the impeller, so no stuffing box or gland is needed. There is no risk of leakage, unless the casing is broken. Since the pump shaft is not supported by bearings outside the pump's housing, support inside the pump is provided by bushings. The pump size of a magnetic drive pumps can go from few watts of power to a giant 1 MW. Priming The process of filling the pump with liquid is called priming. All centrifugal pumps require liquid in the liquid casing to prime. If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To ensure that a centrifugal pump remains