Comparison of Performance between Centrifugal and Positive Displacement Pump at Low Flow Rate

In general, centrifugal pump is more preferable to industrial applications. When a
centrifugal pump operates at its off-design condition; operating at low capacity and
require high pressure conditions, pump will experience some unfavorable condition such
as temperature rise effect. Positive displacement pump is more suitable to be used when
low capacity and high pressure are necessary. Thus, this project is carried out to make a
comparison between centrifugal and positive displacement pumps at low flow rate. The
objectives of the project are to determine the best efficiency points for both centrifugal
and positive displacement pump by using data obtained from the experiments, to suggest
the suitable pump used at low flow rates and to monitor the temperature rise effects of
both pumps when they are running at low flow rates for several hours.
Experiments on single stage centrifugal pump and a gear pump are conducted. The
objective of the experiments is to determine the efficiency of centrifugal and gear pump
at low flow rate with four different types of liquids; water, heavy, medium and light
crude and two different conditions; constant and variable speeds. From the experiments
for both centrifugal and gear pump, the efficiency of centrifugal pump is nearly 0% as
pump operates at low flow rate. As the viscosity of liquid increases, the efficiency
decreases and temperature increases at constant speed. When the speed increases, the
efficiency also increases whereas the viscosity of liquid is decreased. When gear pump
operates at low flow rate, its efficiency value is higher than the centrifugal pump. As the
viscosity ofliquld increase, the efficiency increases and temperature increases at constant
speed. If the speed of the pumps increases, the efficiency increases.
Positive displacement creates flow and centrifugal pump creates pressure. Thus, positive
displacement pump such as gear pump is more preferable when the system runs at low
capacity.