3.2.12 Magnetic driven pumps

Magnetic driven pumps

Magnetic driven pumps are widely applied for the pumping of hazardous or sensitive fluids in different industries but mainly in the chemical and food processing industries. Magnet driven pumps transfer torque via magnetic field provided by two permanently magnetized rings on each side of a sealed non rotating sleeve, also called can, see figure 3.212. The magnetic ring on the atmospheric side (open to air) is connected to the motor shaft, the other ring/permanent magnet is connected to the pump shaft which is the impeller (in case of centrifugal pump) rotor inside the sleeve. The sleeve is a non magnetic can made from stainless steel or composite material.

Maximum torque capacity between two magnetic rings depend on type of material, gap between inner and outer rings, axial length of the rings and the sleeve material. Stronger magnets has the benefit of slimmer design but the pump rotor must run in bearings lubricated by the fluid being pumped, therefor pumps must be designed with a gap allowing circulation of fluid to keep the fluid around the magnet at an acceptable temperature and maintain it’s lubricating properties. There are designs suitable for slurries but special requirements applies since the gap is a limiting factor for particles which can get stuck between surfaces and damage the pump.

If maximum torque is exceeded the two magnets will slip. In case of slip, the pump will not perform according to it’s pump curve, slip will also lead to other problems, there’s likely a temperature increase, harmful imbalance and forces not accounted for during motor and pump design. In short, slip between outer and inner magnets can not be accepted.

Apart from the normal hydraulic and mechanic considerations that must be taken into account selecting any pump in particular, there are special consideration to be accounted for when selecting a magnetic driven pump:

• What is the torque needed to operate without slip?
• What will be the nominal and maximum temperature at the magnets?
• Is it a clean fluid without particles, and if particles what is the size of particles?

Maximum torque capacity between two magnetic rings depend on type of material, gap between inner and outer rings, axial length of the rings and the sleeve material. Due to advantages in torque transfer capacity, traditional iron ore magnets has been replaced with magnets made from rare-earth material elements such as Neodymium magnets and Samarium Cobalt. Neodymium and samarium cobalt magnets are much more expensive but rare earth magnets are not only much stronger, increased performance due to higher efficiency and less risk for slip, but can also handle higher ambient temperatures and longer life time in higher temperatures.

Another disadvantage with magnets is their poor resistance to corrosion, especially if we consider that one main benefit with magnetic driven pumps is that they are often used in applications for corrosive fluids. To prevent magnets used in a pumps from corrosion, magnets must be plated or coated with some form of protective resin or enclosed in a corrosion resistant casing.

Stronger magnets has the benefit of slimmer design but the pump rotor must run in bearings lubricated by the fluid being pumped and magnetic driven pumps must be designed with a gap allowing circulation of fluid around the pump shaft and magnets to keep the fluid around the magnet and bearing at an acceptable temperature and to maintain it’s lubricating properties. There are designs suitable for slurries but special requirements applies since the gap is a limiting factor for particles which can get stuck between surfaces and damage the pump.

Magnetic driven pumps are available as centrifugal and mixed flow end suction pumps, multi stage pumps, propeller or turbine pumps but also as positive displacement pump types e.g. vane pumps and gear pumps.