Mag Drive & Canned Pump Advantages
Both pump types have many clear advantages so why arent they used more?
Magnetically driven pumps transfer power using a drive magnet which is fixed to the motor shaft, which rotates an internal magnet attached to the impeller via the magnetic field which exists between the two magnets. This means that the motor shaft does not pass through the rear pump housing eliminating the need for any seals and because of this they offer many advantages over traditionally sealed pumps. Canned pumps integrate with the motor itself thereby utilising 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 to magnetically driven pumps.
As there are no seals, mag drive and canned pumps require virtually no maintenance, offer complete containment of the liquid, can handle very high temperatures (up to 350 oC in our case), low temperatures down to – 150 oC and contain very high system pressures. While for many API applications they are cheaper than mechanical sealed pumps with expensive seal support systems.
In the case of mag drive pumps; if for some reason maintenance is required the pump chamber assembly can be easily disengaged from the motor bracket and if the pump becomes jammed the magnets de-couple thereby protecting the motor and they can be manufactured from wide range of chemically inert plastics making it possible to eliminate any metal from coming into contact with the liquid. Canned pumps offer double liquid containment as the liquid is contained within the pump chamber around which the motor casing then provides an additional containment vessel, reduced noise as the rear pump chamber is contained within the motor casing and for the same reason boasts an extremely small foot print as the pump is incorporated within the motor itself. Although usually more expensive mag drive and canned pumps clearly offer a much lower total life time cost, reduced maintenance and a safer option than mechanically sealed pumps.
So why aren't they used more?
There are several reasons all of which seem to be caused through old fashioned human nature in the form of ignorance. 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 in addition to centrifugal pumps, we offer rotary vane, gear and regenerative turbine pumps. In fact any pump time 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. Mag drive and canned pumps have been around since the mid 1950's, just about every home has a canned pump in the guise of the central heating circulator pump, mag drive pumps have been used by NASA on all space missions, most nuclear plants use canned pumps as they 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. It is true to say that there is a need to have close tolerances at the rear of most pump types and that they can't be considered as "solids" handling pumps however, they can cope with a small degree of solids and are no less susceptible to problems that a mechanical sealed pump – that's unless the solids include ferrous particles which are for obvious reasons a complete "no no". Slightly higher power consumption costs are a valid down side to both technologies due to the losses across the magnetic and electromagnetic couples however, for most applications these are trivial and far outweighed by the maintenance and reliability savings which are all too often ignored. That leaves us with the age old problem of an obsession with finding the lowest initial cost and an inability to look at the long term direct and indirect costs of a product as if reduced breakdowns and their knock on costs along with negligible maintenance were also considered then there really is no contest 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 in their powers 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.