| HOT OIL PUMPS TYPE NMW / PRMW
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Containment shell
Magnetic coupling
Internal clearances
Containment shell protection
Monitoring
Outer ball bearings
NPSH-Conditions, Inducer
Double sleeve bearings
Balanced thrust loads
General
Magnetic coupled DICKOW-pumps of the series NMW/PRMW are of sealless design. The containment shell forms a closed system with a hermetically sealed liquid end.
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Applications
The magnetic driven NMW/PRMW-pumps are designed for hot oil applications, to improve plant reliability and personnel safety in the Oil & Gas, Petro-chemical, Chemical and Waste Heat Recovery industries.
The containment shell replaces any mechanical seal and saves maintenance costs as leaking seals cause 90% of pump failures. NMW/PRMW-pumps offer exceptional benefits to the user and exceed all environmental protection requirements. Maximum operating temperature without water-cooling is 400 degrees C (750 degrees F).
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Construction
NMW/PRMW-pumps are single stage volute casing pumps with closed impellers, back-pull-out design, with end suction and top discharge DIN or ANSI flanges. Foot mounted or centreline mounted API 610 / (API 685) design is available.
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Design Features
The NMW/PRMW-pumps are designed for operating temperatures up to 400 degrees C. The bearing housing with the cast cooling fins separates the high temperature parts (volute casing, impeller) from the magnet end. This design creates such a temperature difference that 210 degrees C (410 degrees F) in the magnet area will not be exceeded. Cobalt-Samarium magnets can meet 250 degrees C (480 degrees F). The temperature in the magnet end depends also from the magnetic losses in the containment shell. Increase of losses from 2 to 2,9 kW at same pumping temperature creates a temperature rise in the shell from 160 to 210 degrees C (410 degrees F). The pumps can operate with dead ended magnets at losses up to 3 kW. For higher losses (depending on motor rating), an air-cooled heat exchanger is installed. The back vanes of the rotor create a constant cooling flow through the cooler and dissipate the heat from the magnets.
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Containment shell
The containment shell is designed as a pressure vessel to separate the pumpage from the atmosphere. The containment shell is bolted to the bearing housing in a manner that allows in-situ removal of the anti-friction bearing bracket without removing the rotating element from the pipework or exposing the pumped liquid to the atmosphere.
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Magnetic coupling
The single elements of the multipolar magnetic coupling are manufactured of Cobalt-Samarium (Rare Earth), a permanent magnet material with unlimited life. The internal magnets are completely encapsulated and no contact with the liquid occurs.
Energy is transmitted to the hermetically sealed liquid end by a bank of external magnets passing motive force through the sealing shroud to a bank of internal magnets. Inner and outer magnet rings are locked together by magnetic forces and work as a synchronous coupling. The inner magnet ring transmits the required torque direct to the impeller. No slippage occurs between inner and outer magnets. The magnetic drives are designed for electric motors with direct on line starting. Should a subsequent increase of motor power be required, the nominal power of the coupling can be increased by adding further rows of magnets.
The maximum transmissible power is 132 kW at 2900 rpm (225 HP at 3500 rpm).
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Internal clearances
The internal clearance between inner magnets and containment shell is a minimum of 1 to 2 mm (0.039” - 0.078”). This, together with wear resistant sleeve bearings, allows handling of fluid with solids particles.
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Containment shell protection
The different clearances between the rotating outer magnets and stationary containment shell, and the rotating magnet holder and bearing bracket adapter, prevent rubbing of the magnets on the containment shell in the case of ball bearing failure. Ball bearing monitoring devices are available as an option.
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Monitoring
Temperature monitoring, power monitoring, dry running protection devices and leakage detection systems are available. Ask DICKOW engineers for further details.
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Outer ball bearings
The drive shaft of the NMW-type is carried in generously dimensioned anti-friction bearings, grease filled for life and protected against the environment by radial seal rings. The bearings are located behind the containment shell with C3-clearance and filled with high-temperature grease, type L12.
The NMWR/PRMW pumps have the drive shaft carried in oil bath lubricated anti-friction bearings. The bearings are L10 rated for more than 25000 hrs. The oil bath is protected against the atmosphere by a labyrinth seal. The oil level is controlled by a constant level oiler and additionally by a bull's sightglass. Oil mist lubrication is available as an option.
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NPSH-Conditions, Inducer
To avoid cavitation, the impellers of the NMW/PRMW pumps are designed to achieve low NPSH-values. For pumps with discharge above 50 mm (2"), an inducer is available for further improvement of NPSH-required conditions.
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Double sleeve bearings
The double internal bearings are of the sleeve type, positioned in the pumpage. The stationary bearings are located centrally in the common bearing housing, which ensures proper alignment for true running. Standard material is Silicon Carbide, highly resistant against corrosion and wear. The stationary sleeve bearings, the shaft sleeves and also the start-up rings have SC-parts mounted resiliently to avoid any thermal stress at operating temperature.
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Balanced thrust loads
Wear rings, balance holes in the impeller hub and back vanes balance the thrust loads of the closed impellers. Residual forces on the impeller act towards the suction flange. These forces are balanced by the rotor design. The difference between the constant pressure at the rear rotor area and the variable pressure at the front side creates a counter force towards the containment shell. The value of this reaction force depends on the variable gap 'S'. That means that the internal rotor floats until the forces on the impeller and rotor are balanced. The thrust bearings work as start-up rings only.
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