31-03-2014, 02:36 PM
Electrical Energy Equipment: Pumps and Pumping Systems
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What are pumps and pumping systems?
Pumping systems account for nearly 20% of the world’s electrical energy demand and range
from 25-50% of the energy usage in certain industrial plant operations (US DOE, 2004).
Pumps have two main purposes:
Transfer of liquid from one place to another place (e.g. water from an underground
aquifer into a water storage tank)
Circulate liquid around a system (e.g. cooling water or lubricants through machines and
equipment)
Pump performance curve
The head and flow rate determine the performance of a pump, which is graphically shown in
Figure 5 as the performance curve or pump characteristic curve. The figure shows a typical
curve of a centrifugal pump where the head gradually decreases with increasing flow.
As the resistance of a system increases, the head will also increase. This in turn causes the
flow rate to decrease and will eventually reach zero. A zero flow rate is only acceptable for a
short period without causing to the pump to burn out.
Pump suction performance (NPSH)
Cavitation or vaporization is the formation of bubbles inside the pump. This may occur when
at the fluid’s local static pressure becomes lower than the liquid’s vapor pressure (at the
actual temperature). A possible cause is when the fluid accelerates in a control valve or
around a pump impeller.
Vaporization itself does not cause any damage. However, when the velocity is decreased and
pressure increased, the vapor will evaporate and collapse. This has three undesirable effects:
Erosion of vane surfaces, especially when pumping water-based liquids
Increase of noise and vibration, resulting in shorter seal and bearing life
Pumps in parallel to meet varying demand
Operating two pumps in parallel and turning one of when the demand is lower, can result in
significant energy savings. Pumps providing different flow rates can be used. Parallel pumps
are an option when the static head is more than fifty percent of the total head. Figure 15
shows the pump curve for a single pump, two pumps operating in parallel and three pumps
operating in parallel. It also shows that the system curve normally does not change by
running pumps in parallel. The flow rate is lower than the sum of the flow rates of the
different pumps.