One of the most important components in magnetic coupled units is the containment shell. It provides a safe barrier between liquid and atmosphere. However compared to units with mechanical seals, a significant disadvantage arises in the high performance range regarding energy efficiency. The induction of an electric voltage through the rotating magnetic field generates eddy current losses in the containment shell which is normally made of metallic materials. The total efficiency of magnetic coupled pumps decreases, especially for high transmission powers. Losses above 20 kW are not uncommon, but avoidable as this article shows.
The use of non-metallic materials for these applications has failed so far not only because of the universal applicability and interchangeability, but unfortunately also due to higher costs. Not only higher material costs must be considered, also the required special tooling before manufacturing is quite expensive.
An energy-efficient option for high transmission capacities was so far the so-called sandwich containment shell according to. With this design a reduction of more than 50% compared to normal metallic containment shells could be obtained. However, problems appeared occasionally in the past with pump units in operated with variable frequency drive. It is possible to be operated in the natural frequency range of this containment shell because of the external straps load, the inner shell´s initial load and the rotating magnetic fields and their excitation frequency. These have been solved meanwhile through design changes.
The aim was now to combine proven technology with the current possibilities of modern materials and more than 25 years of experience in magnet coupling design. In order to maintain safety and operating limits the advantages of different containment shell designs had to be combined and weak points needed to be eliminated.
The result is a new segment containment shell of different designs ‘SCS’ and ‘SZCS’. The single segment rings assembled together with a retainer and a cover form the shell. Electric isolation of the single segment rings is done by gaskets. Centering of retainer, cover and segment rings is guaranteed. An internal pressure-resistant and liquid-tight connection is obtained with tension rings and their frictional connection through high-strength studs and nuts. The internal components are additionally held by a threaded ring and sealed by a grooved ring.
Advantages of this design:
·Reduced manufacturing and assembly costs
·High-strength, removable and non-wetted stud bolts
·Internal pressure-compensated connection
·Non-wetted spring loaded bottom cover
·High-strength stud bolt material with high electrical resistance
·Standard material Duplex. Other high-strength materials are available on request
With this design ‘SCS’ the magnetic losses could be reduced by 20% compared with the sandwich containment shell.
An exterior rubber tape prevents that the stud bolts are vibrating. With this measure unlimited application of frequency converter is possible.
The design was also checked with a numeric stress-analysis - e.g. for the critical area of the tension ring – it shows a 60% utilization of the yield strength of the selected material, at a test pressure of 60 bar. The FEM-analysis proofs further that the stress on the sealing elements which is important for a liquid-tight execution, is within an uncritical range.
The highlight of this segment-containment shell is the possibility to replace the single segments by a compact intermediate pipe of ceramic, e.g. zirconium oxide, or fiber-reinforced composites, the so called ‘SZCS’-design. Sections to bottom- and flange area, which are critical for these materials, do not apply. The simple form of a pipe can be manufactured easily and is optimal concerning stability and compressive strength.
Coupling efficiency of different containment shell design options depend on the circumferential speed of the driven rotor. The design options are Hastelloy C4 with 1 mm wall thickness, sandwich-design and the new designed segment containment shells ‘SCS’ and ‘SZCS’. This figure demonstrate the advantage of the segment containment shell for applications with rated motor output above 90 kW / 120 HP at motor speed 2900/3500 rpm.
Yet, the biggest benefit is the repeated reduction of the power loss which reduces 40% compared to the sandwich-containment shell. 90% of the power losses come from friction losses through the rotating internal magnet rotor and the liquid around it. The eddy current losses contribute only about 10% to the power loss. The result of this is, apart from the reduced production- and assembly costs, further energy savings through higher efficiency as shown in the following example:
The sandwich-containment shell, installed in a pump unit operating at a speed of 2900 rpm, is replaced by a segment containment shell with zirconium oxide tube. Due to the reduced power loss, the power consumption after one year of continuous operation is about 28000 kWh less. Based on an industrial electricity rate of 10 cents/kWh, cost savings are € 2800,per year.
This new design offers furthermore following benefits:
Reduced containment shell temperature when handling volatile liquids
Allows design of hot oil circulating pumps with higher motor rating without water cooling system.
A patent application has been filed for this energy-efficient segment containment shell which can not only be applied in magnetic coupled pumps, but also in agitators or dry-running compressors.