Centrifugal Pumps ( Part 2)

We talk in the part one about Centrifugal Pumps . Now we complete in the second part the other Centrifugal Pumps parts.



 Coupling And Drive

The impeller is usually driven by an electric motor. The coupling between motor and hydraulics is a weak point because it is difficult to seal a rotating shaft. In connection with the coupling , distinction is made between two types of pumps: 

Dry-runner pumps and canned rotor type pump. The advantage of the dry-runner pump compared to the canned rotor type pump is the use of standardized motors. The disadvantage is the sealing between the motor and impeller.

 In the dry runner pump the motor and the fluid are separated either by a shaft seal, a separation with long shaft or a magnetic coupling. 

In a pump with a shaft seal, the fluid and the motor are separated by seal rings. Mechanical shaft seals are maintenance-free and have a smaller leakage than stuffing boxes with compressed packing material. The lifetime of mechanical shaft seals depends on liquid, pressure and temperature.

 If motor and fluid are separated by a long shaft, then the two parts will not get in contact then the shaft seal can be left out. This solution has limited mounting options because the motor must be placed higher than the hydraulic parts and the fluid surface in the system. Furthermore the solution results in a lower efficiency because of the leak flow through the clearance between the shaft and the pump housing and because of the friction between the fluid and the shaft.

In pumps with a magnetic drive, the motor and the fluid are separated by a non-magnetizable rotor can which eliminates the problem of sealing a rotating shaft. On this type of pump, the impeller shaft has a line of fixed magnets called the inner magnets. The motor shaft ends in a cup where the outer magnets are mounted on the inside of the cup. The rotor can is fixed in the pump housing between the impeller shaft and the cup. The impeller shaft and the motor shaft rotate, and the two parts are connected through the magnets. The main advantage of this design is that the pump is hermetically sealed but the coupling is expensive to produce. This type of sealing is therefore only used when it is required that the pump is hermetically sealed. 

In pumps with a rotor can, the rotor and impeller are separated from the motor stator.  the rotor is surrounded by the fluid which lubricates the bearings and cools the motor. The fluid around the rotor results in friction between rotor and rotor can which reduces the pump efficiency

 Impeller seal

A leak flow will occur in the gap between the rotating impeller and stationary pump housing when the pump is operating. The rate of leak flow depends mainly on the design of the gap and the impeller pressure rise. The leak flow returns to the impeller eye through the gap,  the impeller has to pump both the leak flow and the fluid through the pump from the inlet flange to the outlet flange. To minimise leak flow, an impeller seal is mounted. 

The impeller seal comes in various designs and material combinations. The seal is typically turned directly in the pump housing or made as retrofitted rings. Impeller seals can also be made with floating seal rings. Furthermore, there are a range of sealing with rubber rings in particular well-suited for handling fluids with abrasive particles such as sand.

Achieving an optimal balance between leakage and friction is an essential goal when designing an impeller seal. A small gap limits the leak flow but increases the friction and risk of drag and noise. A small gap also increases requirements to machining precision and assembling resulting in higher production costs. To achieve optimal balance between leakage and friction, the pump type and size must be taken into consideration. 

For More information you can review Grundfos Pump