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By: B. Thrasher
There are a few basic system characteristics that must go into the design of every hydraulic system.
The energy loss of a hydraulic system based on the Bernoulli equation is of the form,
where K is a resistance coefficient found in hydraulics books, V is flow velocity, and g is gravity. This equation accounts for valves, orifices, turns, and pipe friction. Pipe Flow can be one of the major sources of loss, and losses from flow are primarily governed by flow rate. In fact, pressure loss can be approximated by the square of flow, Q.
System operation point
Similar to an electrical system, the pressure drop across the pump must be equal to the system resistance. Due to this fact the systems operation point will be the intersection between the system curve and the pump curve. The system curve is that of hydraulic losses as a function of flow, and the pump curve is the relationship between flow and pressure (see Figure 1). As the pump curve is specific to the rpm, the system operation point shifts as the rpm is varied. One key relationship to note is that of pressure and head,
Viscosity is one of the main parameters that determine losses, and the more viscosity in a system the more losses but less component wear. See Figures 2-6 for a few flow optimization and fluid relationship charts.
There are many different configurations of pumps, but all pumps fall into three main categories, centrifugal, rotary, and reciprocating. Each of these pump type will be discussed with details on some of the sub-types basic performance curves.
Usually used in applications demanding large quantities of fluid for rapid machine movements, centrifugal pumps depend on inertia to produce pressure by accelerating the fluid about a circle. This fluid entered the eye, or center of the impeller, and is rotated around the pump until it reaches the outlet. At this time the centripetal forces is removed and the fluid flows off at a tangent to the circle of rotation. General centrifugal pump curves can be seen in Figure 7-9.
The five main types of Rotary pumps are spur-gear, internal-gear, generated-rotor, sliding-vane, and screw the pump curves and diagrams can be found in Figures 10-20.
Spur-gear, Internal-gear, and generated-rotor pumps are constant displacement with discharge depending mainly on shaft speed. Sliding-vane pumps can be variable displacement and operate mainly at high rpm. On the other hand, screw pumps operate at lower rpm and are fixed displacement.
There are four main type of reciprocating pumps: radial-piston, axial-piston, duplex, and triplex. For specific characteristic pump curves and diagrams see Figures 10 and 20.
EEUA Guide to the Selection of Rotodynamic Pumps. London: Constable & Company Ltd. 1972.
Garay, Paul. Pump Application Desk Book. Linburn, Ga: The Fairmont Press, 1993
Harkay. G and others. Fundamentals of Hydraulic Power Transmission. New York: Elsevier, 1988.
Khaimovich, E.M. Hydraulic Control of Machine Tools. New York: The MacMillan Company, 1965.
Nelik, L. Centrifugal and Rotary Pumps Fundamentals With Applications. New York: CRC Press, 1999.
Pippenger, J. , Hicks, T., Industrial Hydraulics. New York: McGraw-Hill, 1979.
Figure 10: General Pump Performance
Figure 13: Generated-rotor Pump
Figure14: Sliding-vain Pump
Figure 15: Screw Pump
Figure 16: Characteristic Curves for a Spur-gear Pump
Figure 17: Effect of Fluid Viscosity on Rotary Pump Performance
Figure 18: Characteristic Curves for a Sliding-vain Pump
Figure 19: Characteristic Curves for a Screw Pump
Figure 20: General Rotary and Reciprocating Characteristic Pump Curves
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