API 685 Seal-less magnetic coupled centrifugal pumps

General

Magnetic coupled pumps are of sealles design. The containment shell forms a closed system with hermetically sealed liquid end.

Applications

Magnetic driven pumps are designed to improve plant and personnel safety, especially when handling toxic, explosive or other dangerous liquids which react on contact with the atmosphere. For all these services the containment shell replaces double acting mechanical seals with external fluid reservoirs and the necessary control equipment. These pumps therefore offer exceptional benefits to the chemical, petrochemical and allied industries and fulfill all environment protection rules.

Construction

These pumps are single stage, volute casing pumps with closed impellers, back-pull-out design, with end suction and top centerline discharge flange. Rigid support foot mounting is supplied as standard. Centerline mounted design is available on request. Capacity and flange to flange dimensions comply with EN 22858 (ISO 2858).

Design Features

Containment shell, Disassembly

The containment shell is designed as a pressure vessel to separate the pumpage from the atmosphere only. The containment shell is not used as an additional bearing holder. No dynamic stress occurs. The containment shell is bolted to the bearing housing in a manner that allows removal of the bearing bracket including outer magnets and ball bearings without exposing the pumped liquid to the atmosphere.

Magnetic coupling

The single elements of the multipolar magnetic coupling are manufactured of a permanent magnet material "Cobalt Samarium-Rare Earth" with unlimited lifetime. The magnets in the internal rotor are completely encapsulated, no contact with liquid occurs.

Energy is transmitted to the hermetically sealed liquid end by a bank of external magnets passing motive force through the containment shell to a bank of internal magnets. Inner and outer magnet rings are locked together by magnetic power and are working as a synchronous coupling. The inner magnet ring transmits the required torque direct to the impeller. Overload of the magnetic coupling and slipping will not effect demagnetization if a reliable monitoring device prevents overheating of the magnets.

The magnetic drivers are designed for electric motors, direct-on-line starting. Should a subsequent increase of motor power be required, i.e. when installing a larger impeller, the nominal power of coupling can be increased accordingly by an additional series of magnets. The maximum drive power is 165 kW at 2900 min-1 (265 HP at 3500 min-1).

Internal clearances

The internal clearance between inner rotating magnets and containment shell depends on the wall thickness of the shell. However, a minimum clearance of 1 to 2 mm (0.039-0.078") will be provided in any case. This, together with wear resistant sleeve bearings, allows handling of fluid with solids.

Containment shell protection

The different clearances between the rotating outer magnets and stationary containment shell, and the rotating magnet holder and bearing bracket respectively adapter, prevent rubbing of magnets on the containment shell in case of ball bearing failure.

Internal circulation, pressurized magnet end

When pump is in operation it generates eddy currents which heat up the containment shell and the pumpage in the magnet area. This heat is dissipated by internal circulation. The internal circulation flows from discharge, direct behind the impeller, through the pump shaft to the rear impeller. Pressure increase by rear impeller leads circulation back to discharge. The capacity of the circulation flow, together with pressurization of the magnet end with the sleeve bearings, prevents vapourizing or flashing of pumpage in this area when handling boiling liquids.

Casing drain

Complete drainage of casing and magnet end is possible through casing drain. No additional drain required.

Double sleeve bearings

The double internal bearings are of the sleeve type, positioned in the pumpage. The stationary bearings are located central in the common bearing housing that grants proper alignment for true running. Standard material is diamond coated pure Silicon Carbide, highly resistant against corrosion and wear and providing also dry running capability.

NPSH-Conditions

Due to the internal circulation from discharge to discharge, no elevated temperature will re-enter the impeller eye. Handling of boiling liquids is possible without an increase of NPSH-required.

Balanced thrust loads

The thrust loads of the closed impellers are partially balanced by wear rings and balance holes in the impeller hub. Residual forces on the impeller are acting in suction flange direction. These forces will be balanced by the rotor design. The difference between the constant pressure P4 at the rear rotor area and the variable pressur P5 at the front side creates a counter force in direction to the containment shell. The value of this reaction force depends on the pressure P5, resp. the variable gap S. That means, the internal rotor floats until the forces at impeller and rotor are balanced. The thrust bearings work as start-up rings only. Measurement of thrust loads as part of the performance test is possible.

Outer ball bearings

The drive shaft of the NMR-type is carried in generously dimensioned antifriction bearings, grease filled for life and protected by radial seal ring against environment. Secondary containment If desired, a mechanical stand-by seal can be supplied in lieu of the inboard labyrinth seal. This mechanical seal separates the magnet area from the oilbath respectively the atmosphere and forms, together with the closed bearing bracket, a secondary containment behind the containment shell.

The secondary seal complies to the rated working pressure of the pump. The seal is lubricated and cooled by the oilbath. Containment shell leakage monitoring is recommended in this case.

Inducer

To improve NPSH-required conditions, additional inducers are available. The inducers are designed such that NPSH-improvement is given from minimum flow up to BEP. Retrofit of inducers on site is possible by remachining the volute casing and without change of suction pipe.

Hot Oil Design

These pumps are designed for handling heat transfer fluids with temperatures up to 400°C. The bearing housing with the casted cooling fins separates the wetted high temperature pump parts (volute casing, impeller) from the magnet end. This design, together with an external cooling loop for higher power consumption, creates such a temperature difference that 210°C in the magnet area will not be exceeded. Cobalt-Samarium magnets can meet 250°C.

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