Considering canned and mag drive pumps

Mag drive pumps (Fig.1) transfer power using a drive magnet attached to the motor shaft which rotates an internal magnet attached to the impeller via the magnetic field between the two magnets.  This means that the motor shaft does not pass through the rear pump housing, eliminating the need for any seals. As a result they offer advantages over traditionally sealed pumps. 

Canned pumps (Fig.2) integrate with the motor itself thereby using the electromagnetic field between the motor stator and the rotor (which is positioned within the rear pump chamber) to turn the impeller. Canned pumps are therefore also 'seal-less' and can offer additional benefits over magnetically driven pumps.

As there are no seals, mag drive and canned pumps require little maintenance, offer complete containment of the liquid, can handle temperatures from 350 to -150°C and contain very high system pressures. For many API applications they are cheaper than mechanical sealed pumps containing expensive seal support systems. If a mag drive pump requires maintenance, the pump chamber assembly can be easily disengaged from the motor bracket. Should the pump become jammed, the magnets de-couple thereby protecting the motor. The pumps can be manufactured from wide range of chemically inert plastics preventing metal from coming into contact with the liquid.

Canned pumps offer double liquid containment because the liquid is contained within the pump chamber which is surrounded by the motor casing. With the rear pump chamber being contained within the motor casing noise is reduced, plus the pump’s footprint is much reduced because the pump is incorporated within the motor itself. Although usually more expensive than other pumps,  mag drive and canned pumps  offer lower total life time costs, reduced maintenance and provide a safer option than mechanically sealed pumps.

So why aren’t they used more?

There are several reasons why these pumps are not used more, with ignorance being the main one. Many people believe that mag drive and canned pumps are by definition centrifugal pumps. This is not the case as the term only applies to the coupling and the nature of the construction. At AxFlow our pump portfolio incorporates rotary vane, gear and regenerative turbine pumps. In fact any pump that uses a rotating shaft (non-elliptical) could be mag drive or canned.

The second reason is a belief that both pump types are new and relatively untested technologies, where in fact they have been around since the mid 1950s. Virtually every home has a canned pump in the guise of the central heating circulator pump, whilst mag drive pumps have been used by NASA on all space missions. Most nuclear plants use canned pumps as the main pump type, and they are generally used for the most toxic of applications. 

An inability to handle solids of any size is often cited as a generic disadvantage of mag drive and canned pumps. True, there is a need to have close tolerances at the rear of most pump types, so they can’t be considered as 'solids' handling pumps. However, they can cope with a small degree of solids and are no less susceptible than mechanically sealed pumps – that is unless the solids include ferrous particles. Slightly higher energy costs are a valid down side due to the losses across the magnetic and electromagnetic couples, but for most applications these are trivial and far outweighed by the maintenance and reliability savings.

That leaves us with the age-old obsessions of finding the lowest initial cost and an inability to consider the long term direct and indirect costs of a product. If reduced breakdowns and their associated costs, plus negligible maintenance are considered, then I believe mag drive and canned pumps win hands down.

The truth is that in most chemical process situations mag drive or canned pumps offer easily the best solution in terms of cost and safety, but yet many people try everything to avoid their use. Perhaps we should move to an environment wherein people have to justify why they haven’t used a mag drive or canned pump.