Definition of a Pump

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Definition of a Pump

When a fluid, be it hot or cold, has to be "moved" in a system, pumps are used. In other words, in a more technically appropriate manner, the pump is a machine which has the function of increasing the total (mechanical) energy of a liquid; this means that the pump transfers energy to the fluid that it receives from the driving motor.

At this point we can already make an important distinction based on the driving motor:
- When we speak of an electric pump then the mechanical energy necessary for the pump to turn is provided by an electric motor;
- When we speak of a motor pump then this mechanical energy is provided by a heat engine (combustion engine, diesel engine, etc.).

Here we shall deal exclusively with electric pumps and further on we shall pause to describe the most important characteristics of the electric motors that drive them as electric motors are the most widely used with manufacturers.

Considering the definition, we may proceed with our description of the pump, starting with the fundamental factors that describe its operation:

Flow Rate
Head
Power
Efficiency
Speed
NPSH (Net Positive Suction Head)

Flow Rate

The flow rate of the pump is defined as the useful volume of liquid distributed by the pump in the time unit. It is generally indicated with the letter Q and is measured in m3/s, or in m3/h, or in l/min.

Head

The (total) head of the pump represents the increase in energy acquired by 1 kg of liquid between the input and the output section of the pump itself; this is generally indicated with the letter H and is measured in J/kg or in metres of carried liquid (m. C.L.). It is much more convenient to speak not of the head but of the manometric head, indicated as Hman and measured in m C.W. (metres of column of water): saying that a certain pump gives a flow rate of 3 m3/h with a manometric head of 12 m C.W. means that pump can lift a quantity of water amounting to 3 m3/h up to a maximum height of 12 m. The applicable equation is: Hman [m C.A.] = H[m C.L.] * ?[kg/dm3], where ? = volume of the liquid transported.

All pumps are provided with a data plate which clearly indicates, among the other data, the flow rate, manometric head and their interconnection. However these two parameters are not fixed, but vary inversely to one another: when one increases, the other decreases and vice versa. If the various points of operation of a pump are plotted on a graph, on which the X-axis represents the flow rate and the Y-axis the manometric head, the so-called characteristic curve Q-Hman of the pump is obtained. (fig. 1)

Figure

1 - Characteristic curve of a centrifugal pump.

The characteristic curve may be "flat" or "steep", depending on how the pump has been designed and on the system in which the pump is to be fitted. As may be seen in figure 2, the pumps that have a flat characteristic curve give rise to slight variations in head for strong variations of flow rate, while pumps with a steep characteristic curve give rise to slight variations in flow rate for high variations in head. So pumps of the first type will be preferable when a more or less constant head is desired with a flow rate varying within ample margins (this is the case, for example, of pumps for fire-fighting installations); vice versa, pumps of the second type will be preferable when a more or less constant flow rate is desired with a head varying within a relatively wide field (for example in the case of pumping from wells, where constant flow rates are generally desired even in the presence of high variations in the geodetic difference in level).

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 electric pumps, 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.

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.

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 PENTAX 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.

NPSH (Net Positive Suction Head)

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 a 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)

where:
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.
The NPSH is influenced by the flow rate value: it grows as the latter increases; as a result, in order to return the pump to regular operation it is often sufficient to choke the delivery gate valve suitably, thus reducing the flow rate of the pump.
As may be seen from the equation above, to increase the maximum suction depth of a certain pump the load losses Hsuc of the suction pipe may be decreased: that is why it is always convenient to fit a pipe with the largest possible internal diameter at suction.

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