Exposing the common mag drive misconceptions

Over the past 20 years, magnetically driven centrifugal pumps (mag drives) have become more common in the chemical processing industries.

Unlike traditional centrifugal pumps with mechanical seals, mag drives come equipped with a full hermetic seal, making them ideal for processing corrosive or hazardous fluids.

The absence of mechanical seals prevents leakage and emissions in hazardous services— increasing mean time between failure and decreasing overall life-cycle costs.

One misconception related to mag drive pumps is that the technology is complex in design or more expensive than traditional models.

However, this perception is often based on customer experience with older mag drives, which had more than a dozen components and required additional maintenance time.

The next generation of sealless technologies offers a simpler design and requires less time spent on installation, maintenance and inventory.

Some operators, engineers and other plant personnel believe that magnetically driven pumps are inherently different than the sealed centrifugal pumps they are often more familiar with.

However, mag drives operate much the same as other centrifugal pumps, albeit with a few fundamental differences due to the mag drive’s sealless design.

The motor transfers power through the drive magnet assembly to the driven magnet assembly, which is connected to the impeller and ultimately moves the fluid through the pump system.

Between the two assemblies is a containment shell that keeps all of the fluid within the pump and serves to maintain pressure, acting as a hermetic seal that prevents fluids and vapors from escaping to the atmosphere.

The magnets interact through magnetic flux lines that are translated across the containment shell.

Within these two assemblies are alternating rings of north and south magnets, which both attract and oppose each other based on positioning, preventing slippage from occurring.

This type of design is known as a coaxial synchronous magnetic drive, and ensures that both the pump and motor will spin at the same rate.

Metal Mag drives are also offered in an alternate high-temperature (above 260 degrees Celsius) model for petrochemical applications, which employs an eddy current torque ring design.

Although similar to a traditional design, this type of mag drive does not have magnets on the inside, but rather magnetic metaldriven components that couple to the drive magnets through the containment shell.

Applications

Some plant operators are often unaware of the many applications that are well-suited for mag drive pumps.

When choosing a pump for services that process hazardous or valuable fluids, it is important to understand some of the key benefits of using mag drive technology.

Hazardous or regulated services: Since the rotating leaking point in a mag drive is eliminated, the pump is better equipped to process harmful substances like strong acids, bases and solvents without the risk of leaking toxic emissions.

Replacing a traditional sealed pump with a mag drive helps to protect employees and the environment by preventing leakage.

Valuable pumpage: When pumping expensive substances such as Mercury or printer ink, any fluid leakage could cost thousands of dollars in lost production.

A mag drive pump prevents fluid leakage and seals without any additional maintenance, making it a more attractive option over single- or double-sealed centrifugal models.

In addition to the design and application differences from sealed pumps described above, there are also operational factors, such as temperature limitations and torque limitations, that are often missed when operating a mag drive pump.

Temperature Limitations: When exposed to temperatures above their threshold, the magnets in a mag drive begin to lose their magnetism.

Over time, this loss of magnetism results in pump failure due to degradation of the magnetic coupling between drive and driven component.

Torque Limitations: When specifying a mag drive pump it is important to make sure that the magnets are sized correctly so the magnetic coupling breakaway torque is not exceeded during unit start up or steady-state operation as this can cause the magnets to decouple.

An appreciation of the current state of mag drive technology can provide specifiers and operators an array of new options for various pumping applications, particularly for hazardous or regulated services and high value pumpage.

Rick Tym is Global Product Manager for ITT Goulds Pumps’ non-metallic and mag-drive product lines, based in the US state of New York.