The most commonly used pumps are centrifugal, both in an industrial and a civil environment. This type of pump can provide flow rates that range from a few l/m to a lot of m3/h, and a head ranging from a few metres up to more than a thousand.
The pump converts the energy of a motor first into speed (kinetic energy) and then into pressure energy. It is based on the principle according to which a liquid closed in a container, if exposed to a high angular speed, tends to become more dense towards the extremities, with a resulting increase in energy.
More in detail, the fluid contained inside the pump body is made to rotate by an impeller with blades that is fastened to a shaft. The shaft, which rotates, pushes the liquid towards the edges as a result of centrifugal force (for example a spoon in a coffee cup).
The movement of the fluid in a centrifugal pump is axial-radial. The curved blades of the impeller rotates around the axis at a peripheral speed of Vp, and the fluid exits in a radial direction from the end of the propeller, at a speed of Vt. The composition of the vector speeds defines the flow rate and head that represent the hydraulic characteristics of the centrifugal pump. These are typically expressed by the characteristic pump curve.
• A pump from which a strong flow rate and small head are requested will have an impeller with a small diameter and wide passages, to permit the elaboration of large volumes of liquid; a pump suitable for supplying modest flow rates and high heads will have a very narrow impeller with a wide diameter.
• Several centrifugal pumps with an equal flow rate can be positioned in series in order to attain a head that is a multiple of the value obtained with a single impeller. In this manner, heads up to various hundreds of metres can be attained, against the 80-100 that are typically reached by a single-impeller pump.
• The pump body is designed accordingly with conservation of constant total energy principle (Bernoulli’s equation) in order to convert kinetic energy component into static energy, increasing the head.
SELECTING A PUMP
When selecting a centrifugal pump, it is always a good idea to choose a model whose working point falls in the descending part of the curve. In this manner, in addition to obtaining higher efficiency, the flow rate is easy to control through the interposition, at outflow, of a loss of variable load (usually a valve).
The pumps can be installed:
- With pump suction below the liquid level (positive displacement);
- With pump suction above the liquid level or inflow (negative displacement).
Installations are defined as having pump suction below the liquid level (positive displacement) when the pump axis is below the minimum level of the water to be sucked in; all the other installations are defined as having suction above the liquid level or suction pipe.
In these installations, the suction pipe must be horizontal or have an upward slope towards the pump, and must be realized in a way that prevents the formation of air pockets.
With negative displacement installations, to prevent the phenomenon of cavitation, the difference in height between the minimum level of the liquid to be sucked and the pump axis must not be higher than the NPSH (Net Positive Suction Head) value of the pump.
The suction pipe of an installation with pump suction above the liquid level must be equipped with a check valve because otherwise whenever the pump stops the pipe empties and the pump would not prime when started again (unless the pump is self-priming). To prime, both the pump body and the suction pipe must be kept constantly full of water.
“Dry run” starting as it is called causes the pump to overheat, with subsequent failure.
Almost all centrifugal pumps have axial inflow and radial or tangential delivery, with tangential heading upwards. Generally, centrifugal pumps have a back pull-out construction, therefore the union complete with mechanical seal or the stuffing box and shaft can be extracted from the rear without having to dismantle the pump casing. This gives the advantage of not having to disconnect the pump casing from the piping while carrying out maintenance.