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Energy-efficient containment shell materials

Easy installation in existing magnetic coupling pumps
Energy-efficient containment shell materials

The energy efficiency of pumps and pump systems is set to be one of the central topics of the next few years. According to an EU study, pumps account for approximately 20 % of the total energy consumption of motor operated systems. This corresponds to an annual consumption of about 160 TWh. The German Energy Agency estimates that more than 10 billion kWh per year could be saved by optimising pump systems. Against this background, magnetic coupling pumps with a metal containment shell are predicted to move increasingly into the limelight.

Jürgen Konrad

The basic design of a magnetic coupling is very straightforward: the pumped liquid is hermetically sealed to the atmosphere by means of the containment shell. The input power of the motor is transmitted through the outer magnets to the inner magnets. The magnet losses are eddy current losses caused by the magnetic field flux rotating with the magnets and flowing through the containment shell. They are influenced by the containment shell thickness, the electrical conductivity of the material and the rotational speed. Depending on the magnitude of the coupling’s eddy current losses, the efficiency of the pump is reduced to a greater or lesser degree. To avoid eddy currents, the containment shell material or design should have very low – or preferably zero – electrical conductivity. In addition, the general conditions and constraints of the European Atex Directive 94/9/EC regarding electrostatic charges have to be considered.
The transmissible torque of the magnetic coupling is determined by the gap between the inner and outer magnets as well as by the gap ratio s/D. The willingness to accept an energy-efficient containment shell material is therefore linked to the need to modify the standard design of the surrounding parts. Only if the additional costs and effort can be kept within reasonable limits will the replacement of an existing metal containment shell be recommended by the operator. Ideally, the old containment shell should be abandoned and replaced by a new one.
Various energy-efficient solutions for containment shells in magnetic couplings are described in the following.
Ceramic containment shells
Ceramic containment shells have been around for a long time. Zirconium dioxide (ZrO2, also known as zirconia) – which belongs to the group of oxide ceramics – is the established material here, partially stabilised with MgO. High wear and corrosion resistance, low thermal conductivity and good temperature shock resistance are among the material’s numerous advantages. Due to its very high specific resistance of 1010 Ωmm2/m, zirconium dioxide has no eddy current losses. With a wall thickness of 1.9 mm and a suitable flange geometry, it can be retrofitted in an existing pump unit without difficulty. The permissible operating pressure is 16 bar in the temperature range from -40 to +250 °C.
Peek composite containment shells
A Peek composite containment shell was presented by Dickow at the Achema 2009 exhibition. This GreeneTweed material, in combination with a special production process, allows thicknesses that are amenable to retrofitting in existing designs. The permissible operating pressure is 24 bar for a wall thickness of 2 mm and a maximum temperature of +150 °C. In spite of the relatively low specific resistance of 157 Ωmm2/m, no eddy current losses occur owing to the alignment of the inserted carbon fibres. The application was confirmed by comprehensive internal and external tests prior to the release for series production. The first field tests under real service conditions have been under way for several months. As far as the Atex directive is concerned, there are no restrictions on the installation of the Peek composite containment shell in a group II, category 2 device for applications in zone 1. The surface resistance Ro is significantly less than 1011 Ω. According to BGR 132, there is consequently no risk of ignition.
Cast titanium containment shells
Another new development likewise unveiled at Achema 2009 was a cast titanium containment shell. Titanium was first used as a containment shell material many years ago, though only in deep drawn designs. This production process is very cost-intensive. Despite its manifold advantages, the material has so far failed to become established for this reason. Thanks to the use of castable grade 5 titanium, however, its excellent mechanical properties and good corrosion resistance can now be achieved at a competitive price. Operating pressures up to 60 bar are possible with a wall thickness of 1.0 mm. Due to the specific electrical resistance of 1.62 Ωmm2/m, eddy current losses can be reduced by between 30 and 50 % compared to a Hastelloy containment shell.
The molecular structure is modified again in the semi-viscous state in order to eliminate the micro-shrinkage that occurs as a result of the casting process. This is done using a hot isostatic pressing technique, in which high pressure is applied to the containment shell for up to several hours under inert gas atmosphere at temperatures close to the transit point. Selective cooling for a further period of several hours then follows.
To limit the amount of machining to a minimum, a cast wall thickness of 1.5 mm is used; the containment shell flange is integrated and the bores for the containment shell screws are cast at the same time.
Sandwich containment shells
The so-called Nova Magnetics or sandwich containment shell is another well known type. In contrast to the standard single-shell design, it consists of two shells. The inner shell that absorbs the radial loads is comprised of several centred rings insulated from one another by non-conductive sealing elements. The outer shell takes the form of a slotted pipe that absorbs the axial loads. Together with the special arrangement of the magnet rows, this design cuts eddy current losses by more than 50 %. The permissible operating pressure is 35 bar at +200 °C. Due to the containment shell thickness of 7.5 mm, retrofitting into existing standard pumps is extremely complicated.
Caution should be exercised if a pump with a Nova Magnetics containment shell is used for variable-speed operation with a frequency converter. The shell can be operated in the natural frequency range, owing to the load on the external straps that results from the initial load on the inner shell as well as the rotating magnetic field and its excitation frequency. A natural frequency analysis of the containment shell is essential to tell the operator which speed range of the pump has to be avoided. Dickow therefore also offers a containment shell with a modified design. Measurements have shown that the resonant frequency of the shell can be substantially reduced in this way. Breakage of the external straps is hence ruled out.
The energy-efficient containment shell materials currently available and their technical data are compared in the table. Problem-free retrofitting into an existing system is possible with these variants. They provide a safe and modern alternative to traditional metal materials. The additional cost of the new materials is recovered within a relatively short time. It is up to the operator to leverage the potential energy saving.
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