The magnetic pump (figure “B”)
A magnet is attached to the shaft. An electric motor turns some magnets outside of the containment shell and the magnetic field is transferred to the magnet inside the containment shell causing it to turn. The magnets are covered with a corrosion resistant covering.
Although this version costs more money than the canned pump, it has the advantage of being able to use a containment shell with a larger cross section. Depending upon the design, this can be as much as 0.125″ or 3 mm.
The magnetic drive pump has limitations:
- The pumped fluid must provide lubrication to the To be considered a lubricant the fluid must have a film thickness of at least one-micron at operating temperature and load, or the sleeve bearings will experience severe wear. Many fluids including hot water and most solvents are not considered lubricants.
- The pumped fluid must be clean or the solids will collect in the close passages surrounding the armature or magnet as well as in the close tolerances between the sleeve bearing and the shaft. This will interfere with the pump performance and cause premature bearing failure. Acids and solvents clean the lines they are pumped through and as a result the fluid often ends up full of solids and oxides removed from the passivated piping.
- This pump is less efficient than conventional centrifugal pumps and therefore generates more internal heat.
- It operates in a narrow window.
- It uses sleeve or journal bearings instead of precision bearings with correspondingly more radial movement.
- Because of the close internal clearances, dynamic balance of the rotating components is critical to reliable operation
- If you do not have a positive method of pumping the fluid through the bearings and around the containment shell, overheating of the area will become a problem. The result can be flashing of the product and a potential loss of lubricating ability as the fluid increases in temperature and decreases in viscosity.
- For the pump to operate satisfactorily you must pump a cool, clean, lubricating liquid and that is not where most people want to operate these pumps. Cool, clean, lubricating liquids have never been a problem for mechanical seals.
- Do not run the pump dry; you will trash it
- Because of the close internal clearances, dynamic balance of the rotating pump components is very important.
- Be careful of pumping products that are sensitive to an increase in temperature. The fluid will get warmer in the close clearances you find in magnetic drive pumps.
- If the containment shell ruptures you could have a catastrophic failure. Here are some precautions you can take to avoid a rupture:
- The containment shell material should be selected to resist corrosion and erosion
- Metal containment shells should be meet the specifications laid out in Section VIII of the ASME Boiler and Vessel code.
- There should be a minimum ratio between bursting pressure and design pressure of 2 to 1 for the pressure/ temperature in the pump.
- The gasket between the containment shell and the pump cover casing must be confined on the atmospheric side to prevent blowout.
- The design should consider thermal cycling that could occur in normal operation
- Sensors should be provide that could detect the magnets rubbing on the containment shell.
- A secondary containment shell should be provided.
- A leak detector should be installed to detect any leakage to this secondary shell
CAUTION: Be very careful when you dissemble a magnet pump. The magnets can pull the pump pieces together with enough force to damage your fingers severely.
- On February 16, 2018