Centrifugal pumps are used in industries to move liquids. The motor applies kinetic energy to cause liquid flow. Centrifugal pumps are applied in –
- Heating and ventilation
- Water processing plant
- Oil & energy companies
- Chemicals & process industries
- Refrigerants & cryogenics
- Agriculture sector and more
Centrifugal pumps mechanism
Centrifugal pumps include a rotating shaft-driven impeller within its casing. When liquid flows inside the casing’s suction port, it gets thrown outside the casing and finally exits from the discharge port. The impeller imparts velocity to the liquid and gets converted into pressure energy [head].
Flow rates of centrifugal pumps vary with fluctuation in the TDH [Total Dynamic Head] of the specific piping system. A simple valve can be placed within the discharge piping to throttle the varying flow rate without triggering excessive pressure buildup within the piping. There is no need for a pressure relief valve. Centrifugal pumps are popular as they offer high or very high flow rates and cover a wide range of pumping applications.
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Throttling flow rates
Centrifugal pumps as mentioned above are throttled with the discharge valve to slow motor speed. The discharge valve is not energy efficient like Variable Frequency Drive but is cheaper to install. However, throttling centrifugal flow rates has its limits. Throttling must not be below the minimum safe flow rate. If ignored, then this can cause heat buildup in the liquid causing shaft deflection and affecting the seals and bearings.
Best Efficiency Point is the ideal flow rate, which includes the impeller diameter and speed [RPM]. Operating a pump at a speed of 1750 RPM rather than the 3500 RPM helps to reduce the deterioration of the bearings and seals. The pump will even cavitate less when suction conditions are less favorable. If the centrifugal pump has to run at 1750 RPM speed than very large casing as well as impellers are needed, which can cost more than the 3500 RPM.
Viscosity & horsepower
Viscosity has to be low like light oil or water. If the viscosity exceeds 30 wt. oils than extra horsepower is needed or else the centrifugal pumps efficiency will decrease. In such cases, positive displacement pumps are recommended to keep energy usage and horsepower requirements lower.
Water’s density is 8.34 lbs. /gallon. When a centrifugal pump is used for moving liquid denser than water it will need extra horsepower.
For example, a fertilizer has gravity 1.40, which is -1.4 times denser than water. Therefore, the increased horsepower needed to operate the pump efficiently will be 1.4 times the horsepower needed to pump water using the same pump. In this example, for pumping water 20HP motor was used, then to pump fertilizer there will be a need for 30HP motor [1.4 x 20HP = 28HP]. The nearest motor available is 30HP and if you choose 25HP, then the power will not be sufficient.