Pumps are mechanical devices used to convey liquids from point A to point B. In particular, they can be used to take a liquid at pressure P1 and raise it to pressure P2
(where P2 > P1).
To convey a liquid from one place to another, the pump must supply a certain pressure called the total manometric height, which depends on the suction and discharge conditions.
Schematic diagram of transfer of liquid from one vessel to another
Pumps are equipment used to transfer energy from the fluid to the appropriate mechanical device. According to the conditions of use, pumps transfer to the liquid energy that is either essentially potential, due to the increase of the downstream pressure, or kinetic, by the movement conveyed to the liquid.
The energy required to operate these machines therefore depends on the many factors encountered in the study on flows:
The fluid properties: density, viscosity, compressibility,
The characteristics of the installation: length, diameter, roughness, peculiarities…
The flow characteristics: velocity, flowrate, elevation height, pressure…
Confronted with the wide variety of solutions, we can see that many of these machines can be classified into two main groups:
Volumetric (or positive displacement) pumps
The liquid moves as a result of
an energy increase derived
from the centrifugal force.
Example of an Axial
Suction type centrifugal
Positive displacement pumps
The flow develops from the variation in the
volume occupied by the liquid.
Figure 3: Example of a displacement pump with
Figure 4: Cross-section of a displacement pump with diaphragm
THE VARIOUS APPLICATIONS
The simplest and most common means of conveying liquids (Crude, Oil, Water) is the centrifugal pump, and it is also the most economical means.
However, there are at least three types of applications for which centrifugal pumps lose their effectiveness:
Pumping viscous products; in fact, as from a certain degree of viscosity, using a centrifugal pump would require using an oversized pump which would deliver a flowrate outside of its optimum specifications, and so make for very poor efficiency, and consequently a very high power consumption.
The problems of accurate instantaneous dosing, for which using a centrifugal pump would require the facility designer to use a flowmeter to control the speed of the centrifugal pump, with the risk of the pump operating outside of its optimum specifications.
Pumping liquids deemed “sensitive”, i.e. fragile, which react poorly to the internal eddies present in a centrifugal pump (milk, wine, beer, volatile liquids, etc.).
These three types of application require the use of a positive displacement pump.
The choice of design technology will primarily depend on the properties of the product to be conveyed:
and on the type of application:
Pumps, whether they are centrifugal or positive displacement, are used for conveying liquids of all types: water, hydrocarbons, more or less viscous liquids, chemicals or toxic substances such as benzene. However, vacuum pumps such as liquid-ring rotary pumps can convey gas.
As we have already seen, it is often the composition of the effluent that will determine the type of pump to use.