Magnetic pumps for solid and gas containing liquids

Paul Boughton

The selection of suitable sealing systems for pump shafts between the atmospheric environment and the working liquids is made depending on the potential hazard which this medium entails for the environment.
Statutory regulations control the permissible concentrations of gaseous components of the escaping medium in the respiratory air and thus define the required performance of a suitable shaft seal.
Only the sealing systems double mechanical seal, magnetic drive and canned motor have been approved for media which entail a medium to high potential hazard for the environment.
Magnetic drive pumps and canned motor pumps fulfil the highest safety standard, because the liquid compartment is hermetically sealed off from the environment with an encapsulating thin-walled, usually metallic, partitioning element. The torque transmission between the pump shaft and the motor is effected by magnetic force through this closed mantle.
The shaft bearings of these pumps are slide bearings which are lubricated by the pumped product. However, these bearings are sensitive to contact with coarser solids because these impair the lubrication of the bearings and produce bearing wear which can relatively soon lead to destruction of the bearings.
If the pump runs dry, eg if the feed stream is interrupted, the bearing temperature increases very strongly in a short time, and when the liquid flows again this commonly produces a temperature shock leading to rupture of the bearing. A further problem is the heat generated in the can by the rotating magnetic field. Without continuous cooling here by the flowing product the can could be destroyed. The consequence is escape of product into the environment.
The horizontal magnetic pumps MPCH presented here have a dry running bearing system and are suitable also for pumping media which contain solids and gases.
The first development of the dry running magnetic drive started in 1997 with the immersion pumps MPAT. The task was to develop a safe to run dry pump for feeding products containing large amounts of solids and for complete container discharge. This resulted in a pump whose grease-lubricated anti-friction bearings and magnetic drive run in a nitrogen atmosphere, whereby the magnetic drive is designed free from eddy currents with a ceramic can. Contact between the solid material and the bearings is avoided by feeding-in a small stream of nitrogen to produce a gas bell under the bearings. In this version it can be used irrespective of the nature of the pumped medium.
A further development was the dry erected vertical pump MPCV which is also suitable for high temperatures up to 350°C. Furthermore, through combination with the pump design V-AN, a self-regulating pump with a very small NPSH value, a pump MPCVAN was devised which is also suitable for pumping liquids with very high gas or vapour fractions, as well as for plant conditions in which NPSHa is close to zero. Through the development of a labyrinth seal, which is interposed before the bearing unit and through which sealing gas flows, the concept of the vertical pump described above was transferred to the horizontal pump MPCH.
The bearings and the magnetic drive run in a nitrogen atmosphere irrespective of the pumped medium.
The pump with this concept can run dry without any restriction and, in contrast to the standard magnetic drive pumps, is suitable for all liquids.
Application fields of the pumps MPCH/MPCV(AN):
v Complete discharging of vessels.
v Toxic fluids, liquids containing solids and hot liquids.
v Pumping media containing gas or vapour fractions.
v Ensuring high operational safety and dependability.
The horizontal pump is available up to a shaft power rating of 75kW at 1450rpm (or a nominal torque of 500Nm) with standardised dimensions.

Hans-W Moellmann is with Paul Bungartz GmbH & Co KG, Düsseldorf, Germany. www.bungartz.de

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