A mechanical seal pump’s shaft passes out through the casing to the motor and is sealed there by a mechanical seal, while a magnetic drive pump has no shaft through the casing at all: magnets turn the impeller through a closed containment shell. With no seal, the mag drive has nothing at the shaft to leak or replace, but its bearings run in the pumped liquid, so it needs that liquid clean, steadily supplied and moderate in temperature.
A sealed pump takes dirtier, hotter and less predictable duties, and a dual pressurized seal brings its emissions down to zero as well.
What Is a Mechanical Seal Pump
A mechanical seal pump is a pump whose shaft leaves the casing through a seal chamber, where a mechanical seal stops the pumped liquid from following the shaft out. A single seal runs on a film of the pumped liquid between its faces; a dual seal adds a second seal behind the first, with a buffer or barrier fluid between the two.
What Is a Magnetic Drive Pump
A magnetic drive pump is a sealless pump: the motor turns an outer ring of magnets, which turns an inner ring of magnets on the impeller shaft through a closed containment shell.
Inside the shell, the impeller shaft runs in sleeve bearings and thrust collars, usually of silicon carbide, tungsten carbide or carbon. Pumped liquid circulating through the rotor chamber lubricates and cools them.
The Difference Between a Mechanical Seal Pump and a Magnetic Drive Pump
A mechanical seal pump’s weak point is its seal; a magnetic drive pump’s is the bearings that run in the pumped liquid. Most of the differences depend on two things: whether the sealed pump carries a single or a dual seal, and whether the mag drive’s bearings get clean, flowing liquid.
Leakage and Emissions
A magnetic drive pump does not leak to atmosphere in normal operation, while a single mechanical seal always lets a little through. A low-emission single seal holds its emissions to a few hundred ppm. A dual pressurized seal reaches zero as well.
Solids and Abrasives
A mechanical seal pump with a dual pressurized seal handles suspended solids, and a standard mag drive does not.
A single seal handles solids poorly on its own; a clean external flush helps.
Some mag drives are designed for limited solids. Before accepting one, give the pump maker the particle size, percent solids and size distribution.
Dry Running
Neither pump survives dry running in its standard form. A mag drive’s silicon carbide bearings can be damaged within seconds of losing suction.
A dual pressurized seal lasts a limited time dry. If the pump can lose suction, a dual-sealed pump is the more forgiving choice.
Temperature
Magnets limit a mag drive and the seal limits a sealed pump, and the seal runs far hotter. Neodymium magnets are recommended for less than about 300 °F, while metal bellows seals with flexible graphite gaskets are rated up to 800 °F.
Samarium cobalt magnets are recommended for somewhat more, and above 500 °F some builders replace the inner magnet ring with a copper torque ring.
Viscosity, Flow and Pressure
Viscosity limits a mag drive at both ends, while the two pumps share the same minimum flow and neither has a general pressure ceiling.
A mag drive needs a viscosity of at least 0.2 cP, unless carbon-graphite bearings are approved for thinner liquid. On thick liquids its magnet coupling must also carry the extra torque, most of all at a cold start; past the coupling’s capacity, the magnets slip out of step and the impeller stops.
Neither pump should run below about 25 to 35 percent of best-efficiency-point flow. Mag drives are not built in the largest sizes and power ratings.
Maintenance and Repair
On a sealed pump, the seal wears and is replaced or repaired; a mag drive has no seal, and its wear parts are the bearings and thrust collars that run in the pumped liquid.
A mag drive can be stripped and rebuilt in the field, usually without alignment. Maintenance staff used to sealed pumps may need retraining.
Support and Protection Systems
A sealed pump needs a seal support system, while a mag drive needs none but must be protected against running dry and monitored for shell failure.
The seal’s environment is controlled by an API piping plan, from a simple recirculation flush to, for a dual pressurized seal, a barrier fluid system with a reservoir, pressurization and monitoring.
A mag drive needs a power monitor that trips it when it loses suction. On hazardous duty it should also have leak monitoring to show that the containment shell has failed.
How to Choose the Right One
A magnetic drive pump is the better choice for a liquid that must not escape and is clean, steadily fed and moderate in temperature; for other duties, a mechanical seal pump is.
- Toxic, flammable or expensive liquid, clean and steadily fed: a magnetic drive pump, as long as the liquid will not flash when drive heat warms it in the rotor chamber.
- Caustic and other liquids that crystallize where they leak across seal faces: a magnetic drive pump.
- A liquid that must not escape, in a duty with solids, high temperature or a risk of losing suction: a mechanical seal pump with a dual pressurized seal.
Neither design is cheaper in every case. A mag drive saves seal replacements, but a metallic containment shell turns part of the drive power into heat through eddy currents. A dual pressurized seal adds a barrier system to buy and maintain.
If a seal that keeps failing is the reason to look at a mag drive, find out why it fails first: dry running, low flow and solids destroy a mag drive’s bearings too.
