an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

SEALLESS MADE SIMPLE

There certainly hasn't been any lack of print lately about centrifugal sealless pumps, but I've yet to see a dead simple explanation of the three drive systems reduced to the level of those of us who don't have doctorates in physics. Since the writer's degree in Mechanical Engineering was received before we had computers, I'm forced to communicate at a more basic level.

There are three drive systems used on centrifugal sealless pumps:

(1) Magnet to Magnet
(2) Eddy Current
(3) Canned Motor

The magnetic drive has one magnet inside the pump, fastened to the impeller, and another outside rotated by motor. They chase each other and turn the impeller — simple enough?

The eddy current is a little more complicated. The magnet attached to the impeller is replaced by a ring of mild steel and copper. When the outer magnets attached to the motor shaft rotate around the ring, they induce an eddy current in the ring which makes it follow the outer magnets. That's not so bad is it?

The canned motor is really simple. It's just an electric motor with a pump mounted right on its shaft. Since they're fastened together the leakage out of what would be the stuffing box runs right into the motor air gap and through the motor bearings. To keep the pumpage from ruining the stator windings, they are protected by a "can" (thus the name) that completely encloses the inside diameter of the stator windings.

Now that we have a general idea of how they work, let's look at the pros and cons of each.

 

TYPE	PROS	CONS
All Types	Absolutely can't leak (barring massive failure) A lot cheaper than repairing mechanical seals	Some efficiency loss VERY intolerant running dry VERY intolerant of solids in pumpage
Magnet To Magnet	Can use standard motor Less expensive Simple and rugged Horsepower limited	Can decouple magnetically and must be stopped to recouple Temperature limited (approx. 350° F)
Eddy Current	Can handle high temperatures (+800° F) No horsepower limit Rugged, but less simple Can use standard motors Can't decouple	Expensive Less efficient
Canned Motor	Can't decouple No motor size limit Relatively cheap	VERY fragile Expensive to repair Takes special motor Less efficient Temperature limited (without external cooling)

A full discussion of the details of the above pros and cons would fill a book, but suffice it to say that every major pump manufacturer now rushing to build sealless pumps has weighed them carefully and come down on the side of magnet to magnet or eddy current drive. Even the major producer of canned motor pumps is now building a mag drive!

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