Temperature rise in sealless magnetic driven pumps
In sealless pumps with magnetic couplings and metallic containment shells, eddy currents are generated which lead to heat and cause temperature rise of the pumped liquid in the containment shell. In order to prevent inadmissible temperature rise, this heat must be dissipated through an internal cooling flow.
The internal cooling flow causes a nearly constant temperature rise when operating the pump right of the thermal stable minimum flow. However, below minimum flow, the temperature will rise remarkably. Hence, these pumps cannot operate against closed discharge valve without an additional bypass, because heat cannot be removed properly.
Dry running is the worst of these flow related conditions. The heat built up in the containment shell can cause temperature rise of more than 600°C (1110°F) which can also cause the magnets to demagnetize if the temperature is not reliably monitored.
Sleeve bearing, allowable temperature
Sealless pumps require sleeve bearings which work in the pumped liquid. The sleeve bearing material in DICKOW-pumps is Silicon Carbide with diamond layer, providing dry running capability. The widely used term "process lubricated bearing" is not quite correct, since there are no lubrication grooves applied. There is no defined flow through these bearings and pumped liquids as LPG or Methylene Chloride provide no lubrication capability. Similar to the situation between the faces of mechanical seals, a stable fluid film is required between the slide faces. If temperature rise in the magnet end causes vaporization of the pumped liquid, this fluid film breaks down and the sleeve bearing runs dry and fails sooner or later. Although the diamond coating can accept dry running in an empty pump because no hydraulic loads are acting, it cannot save the bearings if dry running occurs under normal operating conditions. Only a reliable temperature monitor can avoid such upset conditions. When handling volatile liquids, the relation between temperature and pressure in the magnet end and the boiling point of the liquid should be considered in any case.
During operation, the containment shell is pressurized with pressure Pgp. The pressure rise A Pgp depends on the pump's differential pressure and on the pump design. The intersection point between containment shell pressure at rated conditions and the vapour pressure curve determines the boiling temperature TD of the liquid. With a certain safety margin ATg, we find the setting temperature of the temperature monitoring device to avoid vaporization in the magnet end.
Sealless magnetic coupled pumps are not considered as electrical equipment. Level detectors or temperature monitoring devices are not required by any safety regulation or by the authorities, even if the pumps are installed with explosion proof motors in hazardous areas. Usually, if seal- less pumps are selected for a particular service, it's because the process fluid is toxic, flammable or both. Since the goal is to keep the liquid within the system, paying careful attention to the prevention and detection of failures is vital for a sound operation. Experiences in applying such pumps have proved that the main reason for pump failures - besides worn out ball bearings - was the flow-related increase in temperature. This has led several manufacturers and end users to incorporate temperature measurement into their designs.
PT100 - Temperature probes
The most common temperature monitoring systems are the PT100-elements. The disadvantage of these elements is the location outside of the magnets.
This is proved by the test results shown in the graph below. The graph shows the temperature rise (T2) at the PT100 and the temperature rise (T1) in the center of the magnets during dry running of a pump over a period of 4 minutes.
In the center of the magnets the temperature
rises very fast and can reach, depending on the
magnetic losses, 450 - 500°C (840 - 930°F)
already after 30 seconds.
The temperature reading at the PT100 after
4 minutes is 40°C (105°F) only.
These results prove that the PT100-probe cannot
act as a dry running protection.
To obtain reliable readings from the PT100- probe, the pump must be vented respectively properly filled with pumped liquid and the internal circulation flow must transport the heat from the magnet center to the measuring spot of the PT100. This is provided in our NM-pumps with circulation from discharge to discharge by rotor back vanes and the PT100 located at the return of the internal cooling flow (after passing the magnet area).
Problems with temperature rise can also occur through unreliable temperature reading of the PT100 when handling volatile liquids in pumps with cooling flow circulation from discharge to suction, or in case of decoupled magnets and starved cooling flow.