The optimum point, is such a point on the flow characteristic H= f(Q), at which the pump obtains its maximum efficiency. The operating point, is the point where the characteristics of the pump and the characteristics of the pipeline being fed intersect. The mutual position of the two points indicates the quality of the selection carried out. In short, the closer the operating point is to the optimum point, the better the selection. It should be remembered that it is at the optimum point that the pump works best in terms of both energy and motion. The efficiency of the pumping process (the transmission of energy from the motor to the pumped liquid) is the highest, while the flow of the liquid is the most homogeneous, stabilized and organized, that is, free of harmful disturbances and turbulence. If, as a result of faulty selection, the operating point is far from the optimal point, the following unfavorable phenomena should be expected: The article is based on the results of the project "Model multistage pumps with enhanced suction capacity", co-financed by the European Union from the European Regional Development Fund.

• operation of the pump with low energy efficiency, that is, a significant part of the energy transferred from the drive motor is converted into useless thermal energy,
• the occurrence of axial and radial forces with significant values, which will have an impact on reducing the life of the bearing system,
• the possibility of cavitation, which will quickly lead to the destruction of the hydraulic components of the pump, as a consequence, may even cause damage to the pump shaft and bearing system,
• there are vibrations of the pump and noisy operation.

The simplest case is when the pump operates at one well-defined operating point. For example, it is feeding a water tower or a storage tank. The capacity Q and head ΔH are practically constant. Then the operating point on the characteristic curve can be determined very precisely, and the pump can be selected in such a way that it always works with the highest efficiency at the optimum point. The condition is that we have reliable information about the installation:

• actual difference in ordinates,
• lengths, diameters of discharge pipelines,
• the course of the pipeline (number of elbows, valves and other fittings),
• the material and condition in which the discharge pipelines are.

All this makes it possible to calculate flow losses and determine the head at the required capacity. The matter becomes more complicated when simultaneously, depending on the time of day, a given pipeline feeds several pumps connected in parallel. In such a situation, it is necessary to recalculate the position of the operating point for all possible configurations of cooperation of individual pumps on the basis of predetermined characteristics of the pipeline. It may then be necessary to take into account additional regulation of pumps by means of speed (frequency converter). When looking for the correct position of the working point (PT) on the pump characteristics, it should be remembered that, as a rule, centrifugal, single-stage pumps have relatively flat characteristics. This means that to the left of the optimum point, the pump's characteristics begin to approach a horizontal line, where minimal changes in head cause significant changes in efficiency. Under these conditions, operation is at low efficiency, and it is very easy to choke the pump (H = max, Q = 0) when conditions change in the discharge pipeline. The selection of the pump in this area should be avoided. This is shown in Figure 2.

The situation is significantly complicated when the pump operates under suction conditions. This is because to all the unfavorable conditions is added the fact that the water table is below the axis of the pump shaft. Then there is a vacuum in front of the pump inlet, and another parameter characterizing the hydraulic properties of the pump begins to play a significant role. This is the anti-cavitation surplus, denoted as NPSHr (abbreviation adopted from English: Net Positive Suction Head), or suction surplus. For pumps operating with suction, the NPSH value should be as low as possible. Each centrifugal pump has its own characteristics NPSHr = f(Q). The course of an example of the characteristic curve is shown in Figure 3. The value of NPSHr increases with the pump capacity, and therefore it should be avoided to look for the operating point at the end of the catalog characteristics. In certain situations, cavitation will almost always occur there, leading to damage to the pump. The greatest danger of cavitation is before the pump inlet. Therefore, this place of installation requires special attention during the design and selection of equipment. All changes in fluid velocity should occur here as smoothly and calmly as possible. In some cases, cavitation can also be induced by placing the operating point near the choke point of the pump (H = max, Q = 0).

What is cavitation?

At any temperature of a liquid, there is a boiling pressure of that liquid, where there is a rapid change in its state of aggregation. If the pressure under which the liquid is above the boiling pressure, it remains in the liquid state. If this pressure falls below the boiling pressure, there is a transition from the liquid phase to the gas phase. This is manifested by the appearance of vapor bubbles. This is the beginning of cavitation. These bubbles are lifted by the pumped liquid to the places where its pressure increases (the inlet edges of the rotor blades). There the bubbles disappear implosively (the inverse of explosion). This process is so rapid that the metal parts of the pump are damaged. Cavitation can damage the impeller even within a few dozen hours of pump operation. As a consequence, the pump loses hydraulic parameters, begins to vibrate damage the pump bearings up to and including fatigue fracture of the shaft. Initially, the pump works uneconomically, not achieving the required parameters, and then it is only suitable for repair. Therefore, operation under cavitation conditions is unacceptable. The following photograph shows a damaged pump impeller due to developed cavitation.