The design of a Centrifugal pump

Centrifugal Pump Construction is considered using the example of a monoblock pump with air-cooled motor and with connections located on the same axis.

In the spiral-shaped body with a flow channel in the form of a confuser, a closed impeller with radially curved blades is located. The impeller is mounted on the motor shaft and fixed with a pressure screw.

The pump body is rigidly connected to the motor body with clamping bolts. The seal between the motor shaft and the pump body is ensured by a mechanical sliding seal.

Body

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

Pumps with a stainless steel body are used in the food and chemical industries, as well as for pumping aggressive liquids. Pumps with a bronze body are used in cold and hot drinking water supply systems.

Impeller

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

In monoblock centrifugal pumps, the impeller is mounted on the motor shaft, while in cantilever pumps, it is mounted on a separate shaft connected to the motor by a coupling.

For clean liquids, closed impellers are used, where radial blades are fixed between two parallel disks, one of which has an opening for the entry of the working medium, and the other for fixing on the shaft. Pumps with a closed impeller have a higher efficiency as there are practically no clearance flows.

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, three-phase motors can have the following rotation speeds.

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 motors have the simplest design, so they are cheaper than others, but their high rotation speed may be unacceptable due to the high noise level generated during operation. The design of motors with 4 or more poles is more complex, but the noise they generate is lower, and their efficiency is higher than that of two-pole counterparts.

Shaft Seal

Modern pumps are usually equipped with a mechanical sliding seal, but in some centrifugal pump designs, a shaft seal based on a sealing ring is also used.

A mechanical sliding seal consists of two graphite rings. One ring is tightly seated on the shaft and rotates with it, while the other is fixed in the pump body. The rings are compressed against each other by a spring. When rotating, a thin layer of water forms between the polished surfaces of the sealing rings, performing lubrication and cooling functions. With this type of seal, there is no leakage along the shaft.

The shaft seal based on a sealing ring consists of a filler made of Kevlar, Twaron, PTFE, or synthetic cord, wound around the shaft and secured with a compression nut.