What is a centrifugal pump?
A centrifugal pump operates by moving fluid using rotational energy from rotors known as impellers. The fluid being pumped enters the high-speed impeller along its axis. Centrifugal force on the impeller casts the fluid out along its circumference. The rotation of the impeller improves the liquid’s flow velocity and pressure. The pump has a casing that helps constrict fluid from the suction side, directs it to the impeller, and reduces flow velocity while increasing pressure.
Components of a centrifugal pump
A centrifugal pump has several components to ensure fluid flow and maximum efficiency. The proper working condition of these components ensures that the pump’s efficiency remains high.
Figure: Components of a centrifugal pump.
What is pump efficiency?
A pump’s efficiency measures how well it can convert useful energy from its source and use it to move fluid. For example, if a pump motor produces X amount of power, and the output power is half the motor power, then the pump’s efficiency is 50%. The 50% shows that half of the power has been wasted, and only the remaining 50% has been used to pump the fluid.
There are various ways a pump can have “wasted energy.” Some of the ways in which a pump energy is wasted are:
- Noise levels.
- Overheating.
Importance of maintaining pump efficiency
Enhancing pump efficiency is important to save money used in running the pump. This includes power bills, that is, electricity or diesel bills. It also saves the user since a highly efficient pump does not break down frequently. Where the pump has low efficiency, chances are that some components will fail quite often, such as couplings, motors, shafts, and impellers. Replacement and or repair of such components is quite expensive, and thus, there is a need to ensure the pumps are always functioning correctly.
Factors that affect the efficiency of a centrifugal pump
- Fluid viscosity. Viscosity refers to the shear resistance when the pump applies energy. Centrifugal pump manufacturers design these pumps for use in low-viscosity fluids. Using the pump for highly viscous liquid will lower efficiency.
- Specific density and gravity of the fluid. The specific density of the liquid affects the input power that the pump needs to transfer the fluid. As such, centrifugal pump manufacturers design a variety of pumps, each meant for transferring specific fluid. Using the pump for the undesignated fluid will impair efficiency.
- Operating pressure and temperature. The pumping pressure and temperature always affect the pump’s efficiency. Where pumping involves high temperatures, it may need special seals, mounting, and gaskets to enhance efficiency. Also, where high pressure is required, the pump casing must be designed appropriately to retain such high pressure to ensure high efficiency.
- Vapor pressure of the fluid. Vapor pressure refers to the pressure at a certain temperature where a fluid would change to a vapor state. The centrifugal pump manufacturers design pumps for such applications by determining the vapor pressure to prevent cavitation and bearing damage due to dry running when the liquid evaporates.
- Shaft alignment. The shaft connecting the pump and the motor/diesel engine must be properly aligned. If the shaft is misaligned, it will waste input power, appearing as heat and damaging the coupling and bearings.
- Condition of the pump bearing. The pump’s bearings must be adequately lubricated to prevent overheating and noise as they contribute to the waste of the input power, thus low efficiency. Also, old bearings need to be replaced as they contribute in wasting energy.
- Condition of the wear rings. If the rings are worn out, there is leakage and, thus, a loss of volumetric efficiency.
- Blockage. A blocked pump will have low efficiency since the pump would struggle to achieve its flow rate.
- Motor power. The motor used should be according to the specifications given by the centrifugal pump manufacturer. If a low-power motor is used, it will not achieve the intended pump output. Also, the motor is likely to fail, causing unnecessary expenses for replacement.
Types of pump efficiency
Mechanical efficiency
Mechanical efficiency is defined as the ratio of power output to the power input. The power output is obtained by subtracting the mechanical power loss from the input power. Power loss mainly occurs due to friction from bearings, stuffing box, coupling, shaft and seals. Centrifugal pump manufacturers recommend frequent inspections to ensure that the bearings and seals are in proper condition to reduce mechanical power losses.
Volumetric efficiency
Volumetric efficiency refers to the power lost due to fluid leakage via the wear rings, balancing holes on the impeller shroud, pump casing, and front clearances on the vanes of semi-open impellers.
Hydraulic efficiency
This is the efficiency associated with losses on the impeller, volute and liquid friction.