Magnetic Drive Pumps
Magnetic Drive Pumps:
Magnetic drive pumps are driven by the force of magnets instead of being directly coupled to a motor. Such pumps do not require a mechanical shaft seal, and as an added benefit, they are also leak-proof. They’re typically centrifugal pumps, though there are some that are considered rotary positive displacement pumps.
Mag drive pumps work when the motor drives a cylindrical shell that contains the outer magnets. Inside the cylindrical shell, there is a thin-walled can that separates a set of inner magnet from those outer magnets. The inner magnets are either 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. Without the shaft mechanical seal component, it is a seal-less pump, which offers several advantages; These seal-less pumps require less maintenance, are leak-proof, and eliminate the sometimes costly barrier fluid system required for double mechanical seals.
Magnetic drive pumps use samarium cobalt and neodymium iron boride. That’s because these materials don’t allow any slip between the outer and inner magnets. That allows for high torques to be transmitted to the rotor, and permits pumps as large as 200 horsepower to be designed in this style.
Mag drive pumps include several components that require careful consideration. As with any pump, the impeller or rotor of the pump still generates radial and thrust loads on the shaft. But these loads cannot be transmitted via the magnets to a set of ball or roller bearings external to the pump. Instead the pump relies on sleeve brushings and rubbing thrust surfaces located inside the mag drive can to carry these loads. The bushings and thrust surfaces are made of materials designed to rub 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.) The wear surfaces must be lubricated, so a small portion of the pumped liquid is normally diverted through the can. The fluid lubricates the rubbing surfaces of the radial bushings and thrust surfaces, similar to the way that seal faces are lubricated by a seal flush liquid. If the pumped liquid contains abrasives or is too vicious, a separate external lubricating liquid may become necessary.
The circulating liquid that passes into the can also carries away heat created as the magnetic flux passes across the (usually metal) can. If flow to the pump is lost (due to a tank being emptied or a suction valve being closed) the cooling medium is interrupted. In these instances, the high temperatures near the surface of the can cause the mag drive pump to fail. Because this failure can occur without warning and had catastrophic implications, mag drive pumps are frequently monitored to ensure quick detection of a cooling flow failure.
Although there some magnetic drive pumps are capable of moving heavier or more viscous fluids, they are primarily designed for transporting clear liquids that contain no solids content.. Solids in the liquid would cause quick failure of the sleeve bearings and thrust surfaces inside the pump. These pumps are best for industrial, municipal, or agricultural applications that require the pumping of clean, low viscous fluids, and are generally not used for heavier industrial applications that process sludges, slurries, or solids-laden fluids.
Mag drive pumps are an excellent choice for transmitting acids or other dangerous liquids where a mechanical seal would be expensive to buy and maintain because of its leak-proof design. They are commonly found in facilities such as chemical plants, and chemical services in other types of plants such as pulp mills, power plants, and water & wastewater treatment plants.
Authored by PumpScout staff.