The design of a Centrifugal pump

The construction of a centrifugal pump is illustrated using the example of a monoblock pump with air-cooled motor and with the suction and discharge pipes located on the same axis.

The spiral-shaped housing with a diffuser-shaped flow channel contains a closed impeller with radially curved blades. The impeller is mounted on the motor shaft and fixed with a clamping screw.

The pump housing is rigidly connected to the motor housing with tightening bolts. The sealing between the motor shaft and the pump housing is provided by a mechanical seal.

Housing

The basic construction of centrifugal pumps includes a cast iron housing, but most models can be equipped with housings made of bronze or stainless steel upon special request.

Pumps with stainless steel housings are used in food and chemical industries, as well as for pumping aggressive media. Pumps with bronze housing are used in drinking water supply systems.

Impeller

Depending on the pump power and the properties of the pumped liquid, the impeller can be made of cast iron, bronze, or polymer materials.

In centrifugal pumps with monoblock construction, the impeller is attached to the motor shaft, while in cantilever pumps it is attached to a separate shaft connected to the motor with a coupling.

Closed impellers are used for clean liquids, where radial blades are fixed between two parallel discs, one of which has an opening for the inlet of the working medium, and the other has an opening for fixation on the shaft. Pumps with a closed impeller have higher efficiency, as they have almost no leakage through gaps.

Motor

Centrifugal pumps are equipped with an asynchronous motor with a squirrel-cage rotor for connection to single-phase and three-phase electrical networks. Depending on the number of poles when connected to a three-phase network, the motor can have the following rotational frequencies:

3 phases, Hz	2 poles	4 poles	6 poles	8 poles	10 poles	12 poles	14 poles
~ 50	2900	1450	960	725	580	480	415
~ 60	3500	1750	1160	875	700	580	500

Two-pole electric motors have the simplest design, which is why they are cheaper than others, but the high rotation frequency may not always be acceptable due to the high noise level during operation. Motors with 4 or more poles have a more complex design, but the noise emitted by such a pump is lower, and the efficiency is higher than that of a two-pole analog.

Shaft sealing

As a rule, modern pumps are equipped with sliding mechanical seals, but in some designs of centrifugal pumps, shaft gland seals are also used.

The sliding mechanical seal consists of two graphite rings. One ring is tightly fitted onto the shaft and rotates with it, while the other is fixed in the pump casing. The rings are pressed against each other by a spring. When rotating, a thin layer of water is formed between the polished surfaces of the sealing rings, simultaneously performing a lubricating and cooling function. With this type of sealing, there is no leakage along the shaft.

The shaft gland seal is a stuffing box made of Kevlar, Twaron, PTFE, or synthetic yarn wrapped around the shaft and fixed with a gland nut.