About MAG Drive Pumps

Magnetically coupled pumps

Magnetically coupled pumps, or Magnetic drive pumps, vary from the traditional pumping style, as the motor is coupled to the pump by magnetic means rather than by a direct mechanical shaft. The pump works via a drive magnet, 'driving' the pump rotor, which is magnetically coupled to the primary shaft driven by the motor. They are often used where leakage of the fluid pumped poses a great risk (e.g., aggressive fluid in the chemical or nuclear industry, or electric shock - garden fountains). They have no direct connection between the motor shaft and the impeller, so no gland is needed. There is no risk of leakage, unless the casing is broken. Since the pump shaft is not supported by bearings outside of the pump's housing, support inside the pump is provided by bushings. The materials of construction of these bushings and the required clearances of the parts may restrict the kinds of fluids for which this kind of pump may be used.

Advantages

• There are no drive seals, therefore the risk of leaks is completely eradicated. This means that hazardous liquids can be pumped without spillages.
• No heat transfer from the motor—the pump chamber is separated from the motor by an air gap; this provides a thermal barrier.
• Complete separation of the liquid means that liquid cannot seep into the motor from the pump.
• Reduced friction.
• Magnetic coupling can be broken—if the load of the pump is too great. By the magnetic coupling 'breaking', it means the pump does not overload and get damaged.

Eliminating the drive seals gets rid of leaks, friction loss, wear and noise. It provides complete separation of fluid from the pump drive, and nearly 100% transfer of motor power into pumping power.

Disadvantages

• Liquids containing ferrous particles are problematic when a magnetic drive pump is used. This is due to the particles collecting on the impeller magnet, and over time causing the pump to stop working.

Principle of operation

The impeller of such a pump is magnetically coupled with the motor, across a separation wall which is resistant to the fluid pumped. The motor drives a rotor carrying one or several pairs of permanent magnets, and these drag around a second pair(s) of permanent magnets attached to the pump impeller.

Priming

Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function. If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To ensure that a centrifugal pump remains primed and does not become gas-bound, most centrifugal pumps are located below the level of the source from which the pump is to take its suction. The same effect can be gained by supplying liquid to the pump suction under pressure supplied by another pump placed in the suction line.