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Hermetically Radial Pumps

Hermetically sealed pumps are being used more and more often in today’s world. This construction offers long intervals between maintenance and also protects the environment from emissions. It makes it possible to convey hazardous media and reliably control particularly high or low temperatures. Where high system and vapour pressures are involved, the hermetic construction can be simpler and more cost-effective than conventional shaft seals. The pumps are fitted with plain bearings to control inner, axial and radial forces. Thanks to their design, canned motor pumps do not require any further bearings in addition to the plain bearings, whereas magnetically coupled pumps are also fitted with conventional dynamic bearings such as ball roller bearings or taper roller bearings. In addition to eliminating dynamic seals (and a possible cause of failure as a consequence), particular advantages of the hermetically sealed drive solution include the high level of safety and the fact that they are essentially maintenance-free. The hermetically sealed drives are divided into two subgroups:

Pumps – hermetic construction

The hermetic leak-free sealing of magnetically coupled pumps is guaranteed by a simple and effective containment shell. The so-called spacer can keeps the liquid separate from the outside environment. A standard motor connected to the magnetic drive by means of a coupling is used to drive the pump, as is the case with conventional, commercially available centrifugal pumps equipped with a mechanical seal. Permanent magnets affixed to the outer rotor transfer the torque generated by the motor to the inner rotor via the spacer can.

Schematic representation:

A partial flow is guided through the magnetic coupling to dissipate the generated heat and lubricate the plain bearings. Depending on the type of medium being pumped, various partial flow guidance solutions have become accepted:

Partial flow return to the inlet and discharge sides:
The pumped medium is fed via the inlet chamber into the impeller, which in turn conveys it to the delivery port. The partial flow used to cool the rotor chamber and lubricate the plain bearings is taken from the periphery of the impeller and returned through the hollow shaft after flowing through the spacer can. This sees a portion of the partial flow conveyed to the inlet side of the impeller, while a further portion is conveyed through the hollow shaft to the discharge side. This construction is suitable for conveying non-critical liquids with low vapour pressure.


Partial flow conveyance to the inlet side:
The pumped medium is fed via the inlet chamber into the impeller, which in turn conveys it to the delivery port. The partial flow used to cool the rotor chamber and lubricate the plain bearings is taken from the periphery of the impeller and returned through the hollow shaft again to the inlet side of the impeller after flowing through the spacer can. This construction is suitable for conveying non-critical liquids with low vapour pressure.


Partial flow return to the discharge side:
The pumped medium is fed via the inlet chamber into the impeller, which in turn conveys it to the delivery port. The partial flow used to cool the rotor chamber and lubricate the plain bearings is taken from the periphery of the impeller and returned again to the discharge side via the spacer can after flowing through the hollow shaft. Additional radial bores at the end of the rotor serve to overcome hydraulic pressure losses that occur along this flow path. Due to the return of the partial flow to the discharge side, the heated cooling flow still has sufficient pressure reserves above the boiling point curve of the medium being conveyed as it reenters the pump. Consequently, when the same conditions prevail it is also possible to convey liquid gases with this construction.

The double containment shell is a specific feature of this construction, which is often used for critical applications; however, it is also increasingly being deployed in standard applications due to its operating reliability and MTBF. The canned motor pump is an integral and compact unit without a mechanical shaft seal. The motor and the pump form a unit that sees the rotor and impeller arranged on a common shaft. The rotor is guided by two identical plain bearings that are lubricated by liquid. A thin stator liner separates the stator winding of the motor from the rotor chamber. For its part, the rotor chamber forms a common cavity together with the hydraulic part of the pump; this must be filled with the liquid to be conveyed before the pump is started up. A partial flow between the rotor and the stator dissipates the waste heat from the motor. At the same time, the partial flow lubricates both plain bearings in the rotor chamber. The motor casing represents a second containment shell in addition to the stator liner that functions as a hermetically sealed component. Consequently, canned motor pumps always offer the highest level of safety, especially when conveying hazardous, toxic, explosive and valuable liquids.

Schematic representation:

A partial flow of the medium being pumped is guided through the canned motor to dissipate the generated heat and lubricate the plain bearings. Various partial flow guidance solutions have become accepted depending on the type of medium being pumped. What they all have in common is temperature control, which guarantees the allowable vapour pressure of the partially boiling liquids or liquefied gases is not exceeded anywhere in the pump:

Partial flow guidance to the inlet side:
This is the standard version of the canned motor pump. It is used for media with temperatures that are sufficiently below the vapour pressure curve. The partial flow used to cool the motor and lubricate the plain bearings is taken from the periphery of the impeller and returned through the hollow shaft again to the inlet side of the impeller after flowing through the motor. This construction is suitable for conveying non-critical liquids with low vapour pressure.


Partial flow guidance to the discharge side:
The version for liquid gases or boiling liquids as media being pumped close to the vapour pressure. The partial flow used to cool the motor and lubricate the plain bearings is taken from the periphery of the impeller and returned again to the discharge side after flowing through the motor. An auxiliary impeller serves to overcome hydraulic pressure losses that occur along this flow path. Due to the return of the partial flow to the discharge side, the heated motor cooling flow still has sufficient pressure reserves above the boiling point curve of the liquid being conveyed as it reenters the pump. Consequently, when the same conditions prevail this construction is also suitable to convey liquid gases with a steep vapour pressure curve.


Partial flow guidance via an external heat exchanger:
This partial flow guidance is often used for pumped media exhibiting high temperatures.  The conveyed liquid is fed through the inlet chamber into the impeller, which in turn conveys it to the delivery port. A thermal barrier prevents the direct transfer of heat from the pump part to the motor part of the unit. The secondary cooling/lubricating circuit dissipates the waste heat from the motor to a separately arranged heat exchanger. This cooling/lubricating circuit cools/lubricates the plain bearings at the same time. As a consequence, it is possible to convey liquid with temperatures up to +400 °C on the pump side, while the temperature of the secondary cooling circuit remains at a low level. This type of construction is also suitable for conveying contaminated liquids and liquids containing solids by adding metered quantities of pure process liquids into the motor circuit, where necessary.

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