The necessary flow and pressure of any system can be defined with the help of a graph called System Curve.
Pump manufacturers try to fit the system curve, provided by the user, to the performance of a pump that satisfies these needs as closely as possible. A pumping system operates as the point of intersection of the pump curve with the system's resistance curve.
The intersection of the two curves defines the operating point of both the pump and process (see figure below). It is, however, impossible for an operating point to meet all the desired operating conditions. For example, when the discharge valve is throttled, the system's resistance curve shifts to the left, and is accompanied by the shifting of the operating point.
SYSTEM CURVE
The system's resistance curve or system load curve is the variation of the flow related to the system load.
It should be developed by the user based on the service conditions. These conditions include the physical layout, the process conditions, and the characteristics of the fluid.
It represents the ratio between the flow and the hydraulic losses in a system, in the form of a graph, and, as the friction losses vary with the square of the rate of flow, the system curve has the form of a parabola. The hydraulic losses in piping systems are composed of friction losses in the pipe, valves, elbows and other fittings, inlet and outlet losses, and losses from changes in the size of the pipe, as a result of increase or reduction of the diameter.
PUMP PERFORMANCE
The performance of a pump is shown by its characteristic performance curve, where its volumetric flow is plotted against the load developed. The pump's performance curve also shows its efficiency, the input power required (in HP), NPSHreq, the revolutions (in rpm), and other information such as the size and type of the pump, size of the rotor, etc. This curve is drawn for a constant speed (rpm) and a certain rotor diameter (or series of diameters).
It is generated by tests performed by the pump manufacturer. The curve of a specific pump is drawn based on a fluid with a specific mass equal to 1.0 (water at 0ºC and 1 atm). Other densities should be considered by the user.
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