Last time we had started with definition of pump and discussed two of the fundamental factors that describe its operation. Here we know the remaining other factors. Those are as below:
a. Power:
There is the power supplied by the pump to the liquid, expressed as:
Pu[W] = g[m/s2] * ?[kg/m3] * Q[m3/s] * H[m C.L.], where g[m/s2] is the acceleration of gravity, generally equal to 9,81 m/s2. Then there is the power Pnom absorbed by the pump, that is, in the case of electropumps, the power transferred by the electric motor to the pump axle. Then there is the electric power Pabs absorbed by the electric drive motor from the power mains.
b. Efficiency:
There is the efficiency ?p of the pump, defined as the ratio between the power Pu supplied to the fluid and the power Pnom aabsorbed by the pump (that is the mechanical power transferred by the electric motor): ?p = Pu / Pnom. Then there is the efficiency ?mot of the electric motor, defined as the ratio between the power absorbed by the pump and that absorbed by the motor: ?mot = Pnom / Pass. In the case of electropumps we frequently speak of the efficiency of the unit, defined as the ratio between the power supplied to the fluid and the power absorbed by the motor:
?gr = Pu / Pass = ?p* ?mot. It must be stressed that the efficiency ?gr of the unit is a very important parameter for an electropumps: the higher its value the less the cost, in terms of electric energy and in money in the long run, that must be borne to have the electropump perform a certain job
c. Speed:
The rotation speed is the number of revolutions performed by the pump in the time unit; this is generally indicated with the letter n and measured in rpm. All KIWI PUMPS electropumps are fitted with a 2-pole induction motor; considering the average running of the motors and the fact that the electric energy distributed in the mains generally has a frequency of 50 or 60 Hz, this gives roughly
n(50 Hz) = 2750 - 2950 rpm and n(60 Hz) = 3300 - 3550 rpm.
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d. NPSH:
This parameter indicates the pump's inability to create an absolute vacuum, that is the inability of all centrifugal pumps to suck at a height equal to or higher than 10.33 m (which generally corresponds to the value of atmospheric pressure at sea level). From the physical point of view, the NPSH indicates the absolute pressure that must exist at the pump intake to prevent the occurrence of cavitation phenomena. When a pump tries to suck up a certain amount of liquid from a depth greater than that allowed by its characteristics, cavitation occurs: the impeller interrupts the flow of liquid and, as a result, small vapour bubbles are formed; these bubbles implode shortly after being formed, making a loud noise similar to metallic hammer and causing severe damage to the hydraulic parts of the pump. That is why it is important for every pump manufacturer to indicate clearly, among the characteristics of his machines, the maximum suction depth, or to supply the curve of the NPSH as a function of flow rate. The maximum suction depth Hmax and NPSH are linked by the relationship:
Hmax = A - NPSH - Hasp - Hr (m)
dwhere: A = absolute pressure in m on the free surface of the fluid in the suction tank; if fluid is being sucked from an "open" tank, that is in contact with the atmosphere, A is equal to the atmospheric pressure;
Hasp = load loss in the suction pipe in m;
Hr = vapour tension of the liquid transported in m.
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