Machinery's Handbook, 31st Edition
2802 PUMPS systems. These pumps are comprised of a centrifugal pump located above, with a jet assembly down in the well. The centrifugal pump pressurizes the water, which is then passed through a nozzle. The jet created by the nozzle entrains additional water and lifts it up toward the pump inlet. In the suction line there is also a diffuser, which slows the water and converts velocity to pressure energy. The Effect of Viscosity on Pumps: Viscosity affects positive displacement pumps, result ing in higher friction head values for a given flow rate. When centrifugal pumps handle fluids more viscous than water, the pumping efficiency decreases and the flow for a given head also decreases. Centrifugal pump performance is typically rated at values achieved when pumping cool water. For computing centrifugal pump performance, some correc- tion factors for viscous fluids can be found in Fig. 8. To use the chart, enter at capacity, go straight up to the head curve, over horizontally to the viscosity curve, and then straight up to the correction factor curve.The correction factors are applied according to the fol- lowing equations: Q v = C Q Q water where Q v is the corrected viscous capacity, C Q is the capacity correc tion factor, and Q water is the flow rate achieved while pumping cool water. h v = C h h water where h v is the corrected viscous head, C h is the head correction factor, and h water is the head achieved while pumping cool water. h v = C h h water where h v is the corrected viscous efficiency, C h is the efficiency cor rection factor, and h water is the efficiency achieved while pumping cool water. P v = P water C η -------- where P v is the corrected viscous pumping power, and P water is the power achieved while pumping cool water. Pump Capacity: Flow rate through a pump is known as capacity. For positive displace ment pumps, flow rate is simply related to cavity volume and cycle rate. For centrifugal pumps, a change in impeller diameter or speed results in a capacity change according to the affinity laws discussed later in Affinity and Similarity on page 2808. Centrifugal pumps are intended to be used at or near rated capacity. When operated below this capacity, the pump experiences higher axial thrust loads, elevated tempera- tures, and there is a potential for gases to become trapped in the pump case. Another effect, called recirculation , can also occur in which flow reversals occur at the inlet or discharge tips of the impeller. This can result in pressure pulsations which can damage the pump. In cases where demand is likely to fall below pump capacity, a parallel pump arrangement can be considered, or a bypass installed in the discharge line of the pump to allow the pump to operate at higher flow than demanded. Total Suction Head: This is the sum of all the heads imparted on the fluid in the suction line supplying a pump. It includes static and dynamic heads, unless otherwise specified. The general equation for total suction head is: h t s ( ) h z h p h v h f – = + + where h z is static head, h p is pressure head, h v is velocity head, and h f is friction head. All are taken with respect to the suction line. Acceleration head can also be subtracted when dealing with reciprocating pumps. Total Discharge Head: This is the sum of all the heads imparted on the fluid in the discharge line exiting a pump. The general equation for total discharge head is: h t d ( ) h z h p h v h f = + + + . Here the terms are taken with respect to the discharge line. Acceleration head can also be added for reciprocating pumps.
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