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Magnetic Drive Pumps

What is a magnetic drive pump?

A type of pump driven by the force of magnetism rather than being directly coupled to a motor. Magnetic drive pumps eliminate the mechanical shaft seal that conventional pumps require, and they are completely leak-proof. They’re noteworthy for being one of the only types of pumps that use the force of magnetism.

How do they work?

They generally fall into the category of centrifugal pumps, though there are some that are considered rotary positive displacement pumps. The motor drives a cylindrical shell that contains the outer magnets. There is a thin-walled can separating the outer magnets from a set of inner magnets located inside the can. The inner magnets are attached to the impeller of a centrifugal pump, or to the rotor of a positive displacement pump. The magnetic flux thus passes from the outer rotating magnets to the inner magnets, causing the impeller or rotor to spin without a direct connection to the motor. This makes it a sealless pump, eliminating the shaft mechanical seal, which is often the component of a pump that requires a lot of maintenance. This also guarantees the pump to be leak-proof, and eliminates the sometimes costly barrier fluid system required for double mechanical seals.

The normal magnet materials that are used in the pump are samarium cobalt and neodymium iron boride. These rare earth magnets don’t allow any slip between the outer and inner magnets, which allows for high torques to be transmitted to the rotor, permitting pumps as large as 200 horsepower to be designed in this style.

There are several components within a mag drive pump that require careful consideration in their selection and application. The impeller or rotor of the pump still generates radial and thrust loads on the shaft, just as is the case with any pump. These loads cannot be transmitted via the magnets to a set of ball or roller bearings external to the pump. Therefore, radial and thrust loads generated by the pump must be carried by sleeve bushings and rubbing thrust surfaces located inside the mag drive can. These bushings and thrust surfaces are made of materials that are designed to be rubbing against each other in a pump, such as carbon and ceramic, tungsten carbide, or silicon carbide (the same materials that are commonly used for mechanical seal rubbing faces.) These wear surfaces must be lubricated, so a small portion of the pumped liquid is normally diverted into and through the can, returning to the pump inlet. This fluid serves to lubricate the rubbing surfaces of the radial bushings and thrust surfaces, similar to the way that seal faces are lubricated by a seal flush liquid. This is sometimes a problem if the pumped liquid contains abrasives or is too viscous. Such liquids can limit the use of mag drive pumps, or require a separate external lubricating liquid, rather than relying on the pumped liquid.

Another purpose of the circulating liquid that passes into the can, besides lubricating the radial and thrust bearing surfaces, is to carry away heat generated as the magnetic flux passes across the usually metal can. If flow to the pump is inadvertently lost (due to a tank being emptied or a suction valve being inadvertently closed) the cooling medium is interrupted. The mag drive pump can then fail due to high temperature near the can surface. This can be a very quick and catastrophic failure, so mag drive pumps are normally monitored, sometimes redundantly, to detect the loss of cooling flow and to shut down the pump before it overheats.

Where are mag drive pumps used?

Although there are some magnetic drive pumps that are capable of moving heavier or more viscous fluids, most are primarily designed for moving clear liquids that contain no solids content, because of the concerns mentioned above. Solids in the liquid would cause quick failure of the sleeve bearings and thrust surfaces that are inside the pump. For this reason, they are best for industrial, municipal, or agricultural applications that require the pumping of clean, low viscous fluids. They’re generally not used for heavier industrial applications in which sludges, slurries, or solids-laden fluids are processed.

Mag drive pumps are an excellent choice for transmitting acids or other liquids which cannot be allowed to leak to the environment, and where a mechanical seal would be expensive to buy and expensive to maintain. They are commonly found in chemical plants, and in chemical services in other types of plants such as pulp mills, power plants, water and wastewater treatment plants, and other facilities where acids or other problematic liquids must be pumped.

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