Magnetic Drive & Canned Pump Advantages

Both pump types have many clear advantages so why arent they used more?

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The benefits

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 '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.

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If you don’t know very much about sealless pumps, allow me to shed a little light on why and how these zero emission pumping technologies exist.

1. Sealless pumps have been around for a very long time.While it is hard to pinpoint exactly when magnetic drive pumps were invented, the technology has existed since the early ’s; though the type of magnets originally used were of poor quality, meaning they had to be very large in order to transmit the required torque. Sometimes, instead of two magnets, one was used against a magnetic mass (a piece of iron, for example). This original arrangement required very large motors and was not really economically feasible. Later in the ’s, with the use of rare earth magnets such as samarium cobalt, the magnets were much smaller and more efficient, leading to the units being more economically feasible.

2. Mechanical seals always leak. To explain it simply, mechanical seals have 2 parts. One part is bolted to the housing (stationery), and the other part is attached to the shaft (rotates with the shaft). The two parts of the mechanical seal are what keeps the fluid from leaking out; but because one part is moving, and one part is not, the mechanical seal eventually wears out and leaks. Wear and the resulting leaks are inevitable – making sealless pumps critical in many hazardous applications.

3. Mag drive pumps are ‘zero emissions’ pumps. Mag drive pumps (short for magnetic drive) do not require mechanicals seals because they rely on a containment shell to form a liquid pressure boundary. In a mag drive pump, the shaft from the motor is connected to an outer magnet carrier. Attached to the other part of the pump is a containment shroud, and inside of that containment vessel is another shaft – which is not same as the shaft connected to the motor. This shaft inside the containment shell is connected to the impeller, and inside the shroud is another magnet. So the outer magnet in the carrier is connected to the motor, and when it spins, the magnetic forces go through the containment vessel to the inner magnet (which is attached to shaft which is attached to impeller) to rotate the shaft.

4. Sealless Pumps keep people and the environment safe. Sealless pumps typically transport hazardous materials that may sometimes be considered ‘environmental contaminants’, making zero-leakage sealless pumps critical for employee and environmental safety.  While petrochemical plants and refineries in the U.S. have been slow to adopt canned motor pumps and mag drive pumps, the establishment of API standard 685 has provided the assurances needed to motivate many to install these types of pumps into their zero emissions applications.

5. Sealless Pumps often surprise owners by showing extended MTBR (mean-time-between-repair). People typically choose sealless pumps for their applications, and eventually see that their MTBR frequency and maintenance costs improve (see figure 1). Typically, the mechanical seals in a centrifugal pump only last about a year and a half. Sealless pumps, on the other hand, can operate 2 to 3 years without maintenance (unless bearings need to be replaced). If MTBR is increased, the total life cycle cost of the pump ends up being much more cost effective.

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