Are there any special operational considerations for magnetically driven pumps and what are their implications?
In terms of the pumping action, there is no difference between normally sealed pumps and magnetically driven pumps. There are, however, a number of operational situations that could cause significant damage to magnetically driven pumps.
- Dry running - Magnetically driven pumps must not be run dry. There are two types of magnetic couplings on the market; synchronous couplings and torque ring couplings. Both couplings generate heat with the torque ring coupling by far the more significant. Magnetically driven pumps have internal bearings that are lubricated by the product pumped. Running the pumps dry will cause serious damage to internal bearings with subsequent possible contact between the inner magnet ring and the containment chamber It may also cause contact between the impeller and the casing if the rotating impeller assembly moves forward toward suction due to axial bearing failure. Some manufacturers may make pumps that have dry run capability but this is only for short run periods.
- Operation with a closed discharge valve - The heat generated in the magnetic coupling will increase the temperature of the liquid with subsequent vaporisation. This will cause bearings to run in vapour with subsequent damage similar to dry running. Additional damage may be caused due to increased internal pressures from the vaporisation with subsequent ballooning of the containment chamber causing contact with the outer magnet ring.
- Entrained gases - The problem caused by entrained gases is similar to that of dry running. The gases interfere with lubrication of internal bearings causing bearing failure with damage is detailed earlier.
- Cavitation - Apart from the normal mechanical damage to the impeller and casings, additional problems will be encountered with the internal bearings. The mechanical shocks from the cavitation may cause the shattering of internal bearings particularly if bearings are made of ceramics such as alumina, silicon carbide and tungsten carbide. Contact between internal components as previously detailed may occur. The shattered ceramic bearings may also cause severe erosion of internal components.
- Temperature -Temperatures above the recommended limits from manufacturers can cause damage to the magnetic coupling. The magnet strength and permancy is affected by increased temperature. Magnets retain most of their strength up to the manufacturer’s maximum temperature. As the temperature increases the magnet will lose some force, possibly resulting in the decoupling of the pump. Once the Curie temperature is reached, all magnetic strength is lost.