Gene Vogel 
EASA Pump and Vibration Specialist 

For most centrifugal pumps, the shaft has packing or a mechanical seal where the shaft enters the wet end. Fundamentally, a sealless pump sub­stitutes a magnetic drive for the shaft seal. The impeller shaft is fitted with a magnetic rotor which is contained in a thin metal cover. The shaft, impel­ler, the rotor and the bearings are all “wet” components; that is, they are completely submersed in whatever liquid is being pumped. Over the out­side of the magnetic rotor and cover is a magnetic drum which is driven by the power source, usually an elec­tric motor. The magnetic coupling between the rotor and drum delivers torque to the shaft and impeller. 

There are a number of variations in design, but all use sleeve type bear­ings lubricated by the pumpage. Some have bearings mounted in the rotor which runs on a fixed shaft instead 
of the more common rotating shaft supported on stationary bearings. In either case, the only lubrication the bearings receive is from the pump-age – a serious weak point that limits sealless pumps to applications where the pumpage is free of any contami­nation, abrasives or debris (Figure 1). Also, bronze or babbitt which are commonly used for sleeve bearings are not suitable for these applications where pumpage, rather than oil, pro­vides the lubrication. Consequently, bearings are constructed from more exotic materials such as carbon or ceramic. 

Axial Thrust 
All centrifugal pumps develop axial thrust that varies with the pump­ing conditions (flow and pressure). Various hydraulic mechanisms are employed to balance or limit the axial thrust, but ultimately the bearings must contain axial movement to pre-vent the impeller from rubbing. Seal-less pumps employ a thrust face on the bearings, or a separate thrust face bearing, which is also lubricated by pumpage. So the dependability of the pump relies heavily on stable pump­ing conditions to supply the necessary axial thrust compensation and bearing lubrication. 

Obviously an upset in the pumping condition can very quickly cause a pump failure. Even though some seal-less pumps are advertised as tolerant of “run dry” conditions, just imagine what happens when a pump starts up dry, then gets a slug of pumpage, then more air and another slug of pump-age. Sealless pumps tend to be much more sensitive than conventional pumps to operation at too high or low flow rates, and to run dry operation. 

The pump may be flange mounted on a C-face motor with the magnetic drum directly on the motor shaft (Figure 2), or the pump may be an independent unit coupled to the driver on a com­mon base plate. While this article focuses on cen­trifugal pumps, the sealless magnetic drive concept is also applied to a variety of PD (pos­itive displacement) pumps, especially gear and piston type pumps. 

For centrifugals, another varia­tion is to use the magnetic coupling between the motor stator and rotor as the sealless magnetic drive. The mo­tor rotor and bearings are enclosed in 
a thin “can” which fits through the air gap of the motor. The motor rotor and bearings are therefore “wet” and ex­posed to the pumpage. This arrange­ment is commonly called a “canned motor pump” (not to be confused with a “vertical can pump,” which is a completely different type pump). 

Cooling The Rotor 
Since the motor rotor generates heat from the electrical current, cool­ing is an important consideration. Normally the pump pressure is used to circulate pumpage around the ro­tor and through the bearings. Some designs use a secondary circulating impeller to provide positive cooling to the rotor (Figure 3), and some also circulate pumpage through a cool­ing jacket to cool the stator. Often the shaft will be hollow to provide a return duct for the pumpage. 

The elimination of the seal avoids the most common source of pump failure, but it comes at a price. Journal type bearings, lubricated only by the pumpage, have a wear factor that increases dramatically when pumpage is not pure and clean. Special materi­als and more complex assembly result in a much more expensive pump. Sealless pumps have their place in pumping very hazardous and sensitive liquids, especially in the chemical and pharmaceutical industries. However, their limitations make them impracti­cal for some of the most common pumping applications in municipal water, waste water and industrial ap­plications. 

Categories: Pumps, Pump seals

Tags: Centrifugal pumps Pump seals

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