Among the operating machines designed to lift a liquid, centrifugal pumps are the most commonly used in practice. One of the most important advantages of these machines is the possibility of direct coupling, that is, without complicated transmission devices, with fast motors of current use, both thermal and electric.
The motor-pump unit thus constituted has a fairly limited size. Due to its design, a centrifugal pump does not have delicate organs, and this gives greater guarantees for operation, and it is also light. Newly built centrifugal pumps do not prime themselves but need to be filled with water at the start of service. Pump manufacturers have solved the problem in different ways, with automatic priming devices which act in the sense of retaining in the pump body, during the shutdown periods, the volume of water necessary for priming.
Centrifugal pumps in current use are built according to different configurations which can be classified in relation to the place reserved for the machine for its installation. The great variety of machines does not allow a complete description of all types, for which one must necessarily refer to the manufacturers’ catalogs. Generally speaking, the most common centrifugal pumps belong to one of the following three categories:
- surface pumps
- submerged pumps
- submersible pumps.
Surface pumps
Surface pumps are obviously designed to be installed out of the water to be pumped.
Installation of a surface pump
A surface pump can be installed both under load and in suction. Installation under load has the very important advantage of avoiding any risk of disengagement between two consecutive services. It is enough to open the valve completely at the suction for the pump to be ready for operation.
Suction installation implies that the pump is at a height above the liquid level practically no higher than 7 m (maximum suction capacity of the machine). Remember that when negative relative pressures close to 10 m of water column are reached in the section immediately upstream of the pump in theory, at about 7 – 8 m in practice, the gases dissolved in the water begin to be released, therefore I start the cavitation and the pump is de-primed.
On aspiration
- The check valve C, generally comprising a strainer to prevent the entry of foreign elements in the case of non-clear waters. In some cases anti-sand strainers are installed, which can possibly be replaced by sand filters mounted on the suction duct.
- The suction line approx.
- One or more elbows as appropriate.
- The last section of the suction pipe must have a constant slope of at least 2% in the direction of motion and must not be very long (no more than 10 m).
- A DC convergent whose upper generatrix must be horizontal, in order to avoid salient points that would occur with a normal convergent, in which there can be an accumulation of air and therefore a danger of disengaging the pump. It is also necessary to avoid provisions such as the one indicated in the margin of fig. 6, in which due to the centrifugal force there is an uneven distribution of the liquid threads at the inlet of the pump which cause it to operate irregularly.
- The pump is generally equipped in important installations with a vacuum gauge inlet V and a M pressure gauge on delivery.In installations with automatic operation the vacuum gauge, if a check valve is not inserted in the outlet, risks being subject to the shutdown of the pump, at a pressure that can damage it. Therefore, as a general rule, it is good to isolate the vacuum gauge and also the pressure gauge by means of three-way taps that allow the insertion of these devices only when you want to carry out the measurements.
At the delivery
- A divergent CD.
- A one-way check valve CR. This element is not necessary if the delivery pipe is short or if the difference in height between the two tanks is of limited entity. Otherwise the valve is essential, and then there is a need for a small by-pass duct for priming the pump.
- An RV control valve. This element, always appropriate, becomes mandatory when the pump flow rate exceeds 10 m 3 / h. The function of this valve is to be able to adjust the flow rate by creating an additional pressure drop which allows in certain cases to bring the machine operating point back to its normal limits of use. In fact, we will see that if the head decreases for any reason (for example due to a rise in the upstream level), the lifted capacity increases and the motor is forced to develop a power for which it has not been sized. The valve allows to reduce the flow to the normal value, thus avoiding the inconvenience.
- The delivery line CR.
Submerged pumps
The submersible pumps are designed to be installed with a short suction section in the immediate vicinity of the free surface of the liquid to be pumped. Vertical axis type, for installation the pump, equipped with a very short suction section, is fixed to the lower end of the delivery pipe and is lowered to the liquid level of the well. The motor is placed on the ground surface and transmits motion to the pump impeller by means of a shaft placed inside the delivery pipe. The suction tract may also be missing and in this case the pump works immersed in the liquid.
Submersible pumps
Submersible pumps are generally used for lifting from more or less deep wells. A submersible unit is essentially constituted by a centrifugal pump with vertical axis, single-impeller or even multiple, whose impellers are keyed on the same shaft as the electric motor that drives them. The latter is placed in the lower part of the group and is of special construction. The power supply of the motor is ensured by a special cable with waterproof coating. Above the motor, between it and the pump, is the perforated intake section. After this stretch is the actual pump. At the end of it, immediately before the delivery duct, a check valve is generally mounted. The submersible pump is simply suspended from the delivery column and there is no danger of the pump being disengaged.
