SUBJECT : The non seal pump 3-11

Non-leak pumps have been the dream of pump designers ever since the first pump was invented back in the dark ages. We all want a non leaking pump for the obvious reasons:

Because of this desire there are a number of companies willing to supply just about any kind of "Voodoo" you're looking for. Let's take a look at the sealess pump subject and see if this type make sense in your application.

Most designs fall into three categories:

The first two designs are very similar in operation. Look at the following illustration.:

THE CANNED PUMP ( FIGURE "A")

An electric motor stator is attached to the shaft and the magnetic fields are placed outside of the "can". Current flows from the windings through the product and the "can" to the stator, causing it to rotate. The pumped fluid flows through the pump bearings and around the stator. Since the pump is in a "can" the fluid cannot leak out.

THE MAGNETIC PUMP ( FIGURE "B")

A magnet is attached to the shaft. An electric motor turns some magnets outside of the can and the magnetic field is transferred to the magnet inside the "can" causing it to turn. The magnets are covered with a corrosion resistant covering.

Although this version costs more money, it has the advantage of being able to use a "can" with a larger cross section meal skin. Depending upon the design this can be as much as 0.125" or 3 mm. larger in cross section

Both versions have the same limitations

Where are canned or magnetic drive pumps a sensible choice?

CAUTION when handling mag drive compoinents

Another approach to eliminating seal problems with the standard centrifugal pump is to install some type of a non-contacting seal and "repeller". There are a lot of different designs but they all share the same basic concept:

When asked why the pump engineers felt it was necessary to come up with this complicated concept the answer was always the same, "To prevent premature wear of the seal faces"

One look at the mechanical seals that had been removed from conventional centrifugal pumps would have proved that this concept is flawed. There was plenty of wearable carbon face left on 90% of the mechanical seals that were removed when they began to leak. A simple inspection would show that for a variety of reasons the faces have opened and solids have penetrated between them causing face damage. These solids penetrate into the softer carbon face where they're trapped. The impregnated carbon face will then act like a grinder causing wear and damage to the hard face.

The trouble with all of these repeller designs is that they open the faces on purpose allowing the solids to penetrate between the faces as the pump slows down at shut off.

All is not bad however. To install these seals most manufacturers need an oversized stuffing box. This means that you can throw these "funny seals" away and there's plenty of room to install a real seal with plenty of clearance.

To design a good seal for pumps, or any other piece of rotating equipment you only have to follow a couple of simple rules:

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