by John Hall of Viking Pump
December 31, 2007

 Sealless magnetically-coupled (mag-drive) pumps are found throughout the process industries for a variety of hazardous liquids. Like pumps with conventional shaft seals, there are more mag drive centrifugals than PDs, but PD pumps offer a broad range of capabilities . . . and you might be surprised at some of the applications where mag drive PD pumps are used. 

Magnetic Drives

Magnetic drives solve the problem of shaft leakage by eliminating the dynamic seal where the rotating shaft passes through the pump casing to connect with the drive. Liquids are hermetically sealed inside a containment canister with a static seal.

Typically an inner magnet assembly is connected to the pump shaft within the canister, and an outer magnet assembly is mounted outside of the canister. The outer magnet assembly is connected to the drive so that as it rotates, magnetic force passes through the canister and, in turn, causes the inner magnet assembly and the pumping elements to rotate.

Applications 

Innovative pump users everywhere have found many reasons to adopt mag drive technology, including:

The Obvious

  • Prevent leakage of liquids that can be hazardous to humans, like strong acids, bases, oxidizers, very hot or very cold liquids, and carcinogens.
  • Prevent leakage of flammable liquids that could be a fire hazard.
  • Eliminate routine housekeeping from cleanup of seal leakage, especially where leakage might require reporting to regulatory agencies.
  • In the U.S., eliminate regular monitoring and reporting under the Clean Air Act. Some regulations under the National Environmental Standards for Hazardous Air Pollutants (NESHAP) require weekly monitoring and reporting of pumps handling certain chemicals with packing or single mechanical seals, but they exempt mag drive and double-sealed pumps. The fewer points that require monitoring, the lower your ongoing compliance costs.

The Not So Obvious

  • Ensure uptime on critical applications. Pareto analysis of reasons for pump failure has shown that the single most frequent cause of unplanned pump downtime is seal leakage or failure. Unplanned downtime is lost production, with the complexities of shutdown and start-up, as well as wasted product. 
  • Handle liquids that react on contact with air or moisture, like cyanoacrylate adhesives, by preventing air and humidity from entering through the seal gland. Similarly, handle liquids that crystallize on seal faces and cause excessive wear, like isocyanates for polyurethane manufacturing.
  • Handle hydrocarbons and other liquids that "coke" on hot seal faces, then cause the seal faces to stick together when the pump is started up, destroying the seal.
  •  Handle very costly liquids to prevent loss of revenue due to leakage

The Downright Oblivious

  • Handle high viscosity liquids that are difficult to seal with conventional sealing technologies.
  • Accept high inlet pressures, like pipeline sampling.
  • Deal with low inlet pressures, like suction lift or vacuum vessel applications, to prevent air infiltration through the seal that can cause air binding or loss of prime.

Types of Mag Drive PD pumps

Selecting a mag drive pump starts with the technologies used with traditional sealed pumps. If it works with packing or mechanical seal, it will generally work with a mag drive. Some, but not all, rotary PD technologies are available with sealless magnetic drives.

Internal gear pumps are commonly provided with magnetic drives, and offer adjustable clearances for use across a broad range of viscosities, from very thin and non-lubricating (like ammonia) to very thick (like caulking compounds). Pressure capabilities are usually limited to about 200-psi (14-Bar).

 

1 cast iron mag drive.jpgCast iron mag drive internal gear process pump, 130-gpm at 200-psi.

 

External gear pumps with magnetic drives can allow higher pressures (to 2500-psi (170-Bar) or more) because the gears are supported at both ends of the shafts. Applications range from high pressure isocyanate and polyol blending to injection of fuel additives. Completely non-metallic mag drives made of engineered plastics and ceramics can safely meter or transfer corrosive liquids that would otherwise require very costly alloys.

2-non-metallic mag drive.jpgNon-metallic mag drive external gear pump for corrosive liquids, 20-gpm at 150-psi.

Three-screw (three spindle) pumps, with a center drive screw, are often available as mag drives for lubricating liquids, and some are capable of generating pressures to 1500-psi (100-Bar). Rotary vane pumps are also available with mag drive models, offering high efficiency on very thin liquids.

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