* the suction channel or tank, that is the "environment" from which the pump takes
       the liquid;

  * the suction duct or pipe through which the liquid is led to the pump (missing only in
       systems where the pump body is immersed);

   * the delivery duct or pipe into which the pump sends the liquid;
 
   * the delivery channel or tank, that is the "environment" into which the pump sends
       the liquid that it has lifted.

The energy required to carry 1 kg of liquid from the suction tank to the delivery tank is:

Hi = (Hm - Ha) + Hg + Hp (m)

where:

* Hm e Ha, expressed in C.L., are the pressures on the free surface of the liquid in the delivery tank and in the suction tank respectively. If both tanks are at atmospheric pressure, the term (Hm - Ha) is equal to zero;
* Hg is the geodetic difference in level, in metres, of the system: the examples given in figure 3 show how it should be measured;
* Hp is the total load losses, in metres, of the suction pipe (if fitted) and of the delivery pipe. Load losses are the energy losses that take place during movement of the liquid; they may be "continuous" or "accidental". The former are those due to friction between the fluid and the pipe whereas accidental losses are due to a variation in speed (widening or narrowing of the section, mouths and outlets, valves, etc.) and to changes in direction (curves, elbows, etc.). To calculate load losses, graphs and/or tables are generally used that may be easily found in good books on hydraulics or on pumping systems.
While Hm, Ha, Hg generally remain constant as the flow rate varies, the load losses Hp increase as the flow rate increases following a fairly quadratic law: as a result the value of Hi also increases as the flow rate increases.

If a graph is drawn showing the values of Hi as a function of flow rate, the so-called "line of total resistance or characteristic curve of the system" is obtained (figure 4). Generally the characteristic curve of the system is plotted by joining a certain number of coordinate points (Q, Hi), bearing in mind that, as we have said, only the term Hp varies as Q varies.


 

Figure 3 - A few examples of installation with respective indication of the geodetic difference in level.




Figure 4 - Characteristic curve of the system.

If on the same diagram we show the characteristic curve of a certain system and the characteristic curve of a certain pump, a point of intersection of the two curves is found which defines the working point of that pump in that system (figure 5).

Figure 5 - Working or operating point of a pump.

 

From this brief description it may already be deduced that the working point does not depend only on the pump's characteristics, but is also a function of the type of system in which the pump is fitted: it is sufficient to manoeuvre a gate valve fitted on the pump delivery, thus varying the load losses there, or to vary the level of the liquid in the suction or delivery tank, thus varying the geodetic difference in level Hg, to shift the working point to a different capacity.