Using a magnetic coupling for pumping applications is common practice. The advantages are well known in as much as the magnetic coupling guarantees a completely hermetically sealed pump. This eliminates the need for external sealing systems including gland packings, mechanical seals, and their associated ancillary equipment, such as thermosyphons or flushing systems. For difficult to seal products such as latex based fluids or toxic substances, magnetic couplings are the ideal pump drive system.
Nemo progressing cavity pumps from Netzsch Pumpen & Systeme GmbH are typically used in challenging applications where the fluid to be pumped is abrasive, contains solid particles, is viscous or shear sensitive, or requires accurate dosing or any combination of two or more of these characteristics.
The combination of these typical characteristics for a progressing cavity pump application means that standard magnetic couplings designed for direct coupling to a centrifugal pump, running at two and four pole motor speeds, are not suitable for such duties.
Netzsch experts therefore had to develop a magnetic coupling specifically designed to meet the requirements of typical progressing cavity applications to combine the significant benefits of the magnetic coupling with the capabilities of the company's Nemo progressing cavity pump.
Challenges
The rotational speeds of the progressing cavity pump are lower than usual for a centrifugal pump, so excessive heat generation within the magnetic coupling when used at lower speeds was not to be expected. There were, however, other challenges for which solutions needed to be found. One such challenge was the torque that the coupling would have to transmit. Due to the interference tolerances of a progressing cavity pump rotor and stator system, any design of a magnetic coupling for a progressing cavity pump would have to overcome starting torques significantly higher than those encountered when operating a centrifugal pump.
Additive manufacturing
In a parallel development program, a new stator and flexible coupling rod were designed and tested using an additive manufacturing process.
Subsequently, Netzsch can provide a solution for transferring and dosing chemicals where elastomer components are not compatible with the pumped fluid. Using a flexible coupling rod produced by additive manufacturing made it possible to maintain the identical pump length of a configuration using a standard pin joint coupling rod.
Following the conclusion of the development and testing phases, Netzsch engineers produced a magnetically driven progressing cavity pump that has proved itself in demanding applications.
These include the chemical, paint and lithium battery industries pumping solid laden slurries, products with viscosities of up to 20,000 cps and toxic or otherwise harmful media.
The increased starting torque was not the only issue the Netzsch engineers had to overcome to produce a magnetic coupling suitable for a progressing cavity pump. As the magnetic coupling needed to be able to handle high viscosities to be successful, the influences of such viscosities within the magnetic coupling needed to be investigated, and where necessary, accommodation made to counter any negative influences.
Separating the inner magnet and the outer magnet is a containment shroud, the component that ensures the coupling is hermetically sealed.
The gap between the containment shroud and the inner magnet needs to be kept as small as possible to maximise the performance of the coupling.
As the gap between the inner magnet and the containment shroud contains the pumped product, this was an area of special interest for the Netzsch engineers. Special consideration needed to be given to any potential viscosity losses between the containment shroud and the inner magnet.
Potential applications for a progressing cavity pump combined with a magnetic coupling precluded using elastomer components such as the pump stator and joint sealings. Consequently, innovative solutions were needed to maximise the potential of magnetic coupling development.
About the author
This article was written by Roger Willis, Global Business Field Manager Chemical, Pulp & Paper, Netzsch Pumpen & Systeme GmbH.
