By Sal Pitti
Although magnetic drive pumps (mag-drive pumps) have been around for almost 70 years, it wasn’t until recently that the general industry lowered their resistance and started embracing the use of mag-drive pumps. Plant managers, engineers, and maintenance supervisors all had the same arguments for why they would not use mag-drive pumps in their plants:
- Mag-drive pumps are too expensive
- Catastrophic failure upon “dry-run”
- Pump and part deliveries are too long
There was no arguing of these points a decade or two ago, but today things have changed. Corporations are looking at additional key driving points than just the initial cost of the pumps. Safety, environment, skill set and reliability are now part of the decision making process.Today the reverse arguments can be made to the above:
Mag-drive pricing has been reduced to a point that the smaller-sized pumps are less expensive than equivalent sized mechanically sealed pumps. Most of the Teflon lined mag-drive pumps sold today range from $4,500 to $6,500. Most 316 SS ANSI process pumps fitted with a single cartridge mechanical seal will exceed that price.
In addition to comparison pump pricing, end users are taking a look at the overall cost and value of a project using mag-drives. Mag-drives do not require mechanical seal support which can require additional utilities, seal pots, heat exchangers and more. By adding all these required life support systems, mechanically sealed pumps often exceed the price of even the larger sized mag-drive pumps. The value of magnetic drive pumps comes in the form of plant safety, house keeping and elimination of monitoring.
Yes, catastrophic failures can occur upon “dry-run” from system upsets or operator errors. Instead of replacing a $1,000 to $2,500 mechanical seal, you could have complete pump replacement cost. To eliminate this problem, a properly programmed power monitor will automatically shut down the pump upon measuring low motor load when in a “dry-run” situation. Power monitors range from $400 to $1,500 and should not only be limited to protecting mag-drive pumps.They can also be used to protect damage to mechanically sealed pumps.
In addition to power monitors, pump manufacturers are working on vapor detectors that sense when internal bushings are no longer lubricated by the process fluid and are running on vapor. When vapor is sensed, the pump is essentially in a “dry-run” situation and can be shut down.
Mag-drive pumps are extremely popular in difficult-to-seal applications (i.e. caustic, sulfuric acid, HCL acid, solvents, heat transfer fluids, etc.) but are gaining popularity in general services today, even on water applications. The increased number of installed pumps has forced manufacturers’ distribution channels to increase inventory levels of mag-drive pumps and parts, allowing for quick shipments for end users.
Sealless magnetic drive centrifugal pumps are categorized into two general classifications: synchronous and asynchronous mag-drive technologies.
The synchronous mag-drive pump utilizes permanent magnets for both the outer and inner magnetic rings. The outer magnet ring is attached to a motor, directly or by a coupling, and rotates at the motor speed. Magnetic attraction occurs through a containment shell causing the inner magnetic ring to rotate in sync with the outer magnetic ring. Magnets are made of either neodymium iron boron or samarium cobalt for up to a maximum temperature rating of 500 F with no external cooling required.
The asynchronous mag-drive pump, also referred to as an Eddy current design, utilizes permanent magnets for the outer magnetic ring only. A torque ring is used as the inner magnetic ring and is constructed of copper bars encased in a sheath instead of the permanent magnets. The outer magnetic ring is attached to the motor, directly or by a coupling, and rotates at the motor speed. As the outer magnetic ring rotates around the torque ring, its magnetic field induces a current to flow within the torque ring copper bars; hence creating its own magnetic field. This induced magnetic field causes the torque ring to rotate with the outer magnetic ring, but at a slightly slower speed. The speed difference is called “slip.” The outer magnets are normally made of AlNiCo for up to a maximum temperature rating of 750 F with no external cooling required.
Contact IPEG, your industrial processing equipment expert, to learn more about the benefits of sealless magnetic drive pump technology.