Applications
The magnetic driven NMW/PRMW pumps are designed for hot oil applications, to improve plant and personnel safety in industrial heating plants. Leak of hot oil at high temperature reacts to the atmosphere and turns into coke or tar. If the vapour pressure at operating temperature is above the atmospheric pressure, leakage vaporizes. In these cases, double mechanical seals are required in conventional pumps.
The containment shell replaces any mechanical seal and saves maintenance cost because 90% of pump failures are caused by leaking seals. NMW/PRMW pumps therefore offer exceptional benefits to the user and fulfill all environment protection rules. Maximum operating temperature without water cooling is 400 deg C.
Construction
The NMW/PRMW pumps are single stage, volute casing pumps with closed impellers, back-pull-out design, with end suction and top centreline discharge flange. Foot mounted casings are supplied as standard. Centreline mounted design is available as an option. 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 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 the outer magnets passing motive force through the containment shell to the internal drive magnets. Inner and outer magnet rings are locked together by the magnetic field lines and are working synchronously without any slip. 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. During operation, the rotating field lines cut the metallic containment shell and generate magnetic losses. The magnetic couplings are sized for direct-on-line starting of the electric motors. The transmissible power of a coupling depends on the axial magnet length. The couplings are sized for the maximum power consumption of the rated impeller at the end of performance curve. The rated coupling power can be increased by mounting an additional series of magnets, required eg. when installing a large impeller respect. motor.
The maximum transmissible power is 132 kw at 2900 rpm.
Internal Clearances
The internal clearance between inner magnets and containment shell depends on the wall thickness of the shell. However, a minimum clearance of 1 to 2 mm 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. Ball bearing monitoring devices are available as an option.
Monitoring
Temperature monitoring, power monitoring, dry running protection devices, leakage detection systems are available.
Outer Ball Bearings
The drive shaft of the NMW type is carried in generously dimensioned antifriction bearings, grease filled for life time and protected against 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 antifriction bearings. The bearings are L10 rated for more than 25000 hrs. The oil bath is protection against the atmosphere by labyrinth seal. The oil level is controlled by a constant level oiler and additionally by a bull's eye sight glass. Oil most lubrication is available as an option.
Secondary Containment
A mechanical stand by seal can be supplied in lieu of the inboard labyrinth seal. This mechanical seal separates the magnet area from the oil bath 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. The secondary containments is standard in PRMW types.
NPSH Conditions
To avoid cavitation, the impellers of the NMW/PRMW pumps are designed to achieve low NPSH values. For pumps with discharge, above 50 mm additional inducers are available for further improvement of NPSH required conditions. The inducers are designed such that NPSH improvement is given from minimum flow up to BEP.
Retrofit of inducers on site is possible by re-machining the volute casing, without changing the suction pipe.
The NMW/PRMW pumps are designed for operating temperatures up to 400 deg C. The bearing housing with the casted cooling fins separates the high temperature parts (volute casing, impeller) from the magnet end. This design creates such a temperature difference that 210 deg C in the magnet area will not be exceeded. Cobalt-Samarium magnets can meet 250 C. 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 deg C. 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 can be installed. the back vanes of the rotor create a constant cooling flow through the cooler and dissipate the heat from the magnets.
Drain Connections
Volute casing and magnet end have separate drain connections.
Venting System
During start-up and filling of the system with open suction valve, the pumped liquid enters the magnet end through holes in the sleeve bearings and the gap between pump shaft and bearing housing. Remaining air in the containment shell will be vented by the two valve venting system according to the manual instructions.
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 grants 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 elastically beared SiC parts to avoid any thermal stress at operating temperature, they can meet any temperature swing.
Balanced Thrust Loads
The thrust loads of the closed impellers are balanced by wear rings, balanced holes in the impeller hub and back vanes. 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 at the rear rotor area and the variable pressure at the front side creates a counter force in direction to the containment shell. The value of this reaction force depends on the variable gap. That means, the internal rotor floats until the forces at impeller and rotor are balanced. The thrust bearings work as start-up rings only.
Containment Shell, PRMW Design
Different from the conventional design, the pressured PRM containment shell consists of two shells. The inner shell which accepts the radial loads, is consisting of centred ring. These rings are insulated from each other by confined Gore-Tex-PTFE gaskets and preloaded by a wave spring to avoid leakage. The outer shell consists of a slotted pipe with bottom and adapter flange welded to it. This shell accepts the axial loads.
This design, together with a special magnet assembly, reduces the magnetic losses by approximately 50% compared to Hastelloy C containment shell of 1.0mm thickness. The maximum allowable working pressure is 35 bar at 200 deg C.
With the reduced magnet losses, the PRMW type can meet applications with rated motor power up to 132 kw at 2900 rpm without water cooling.
Air cooled cooling loop and vent line with thread connections are standard, flanged connections are available as an option. |
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