FIGURE 1 Typical canned motor pump

FIGURE 1 Typical canned motor pump assembly. The stator liner (F) is supported on the outside diameter by the steel lamination of the motor. Back-up sleeves (G) are provided to strengthen those areas of the stator liner not supported by the stator laminations. The stator liner is, in effect, a cylindrical "can" placed in the stator bore and welded to the rear end bell and front end bell shroud to hermetically seal off the windings from contact with the liquid being pumped. Terminal leads (H) from the windings are brought out through a pressure tight lead connector (I) mounted on the stator band and terminated in a standard connection box.

Motors are either designed and manufactured specifically for use in canned motor pumps or components of conventional motor are modified. A variety of motor insulation types is available ranging from temperature limits of 266°F (130°C) to above 482°F (250°C).

Because the pump and motor are one unit, the complete assembly must be tested and approved for explosion-proof applications. Explosion-proof pumps are rated as either Class 1, Group D, Division 1 or Class 1, Group C & D, Division 1 locations. It is not uncommon to operate canned motor pumps with variable speed drive controllers.

Rotor Assembly The rotor assembly is a squirrel cage induction rotor constructed and machined for use in canned motor pumps (see Figure 3). It consists of a machined corrosion resistant shaft (A), laminated core (B) with copper or aluminum bars and end rings, corrosion resistant end covers (C), and a corrosion resistant can (D). Various methods are used to attach the impeller to the motor shaft (E).

The rotor end covers are welded to the shaft and to the rotor can that surrounds the outside of the rotor, thus hermetically sealing off the rotor core from contact with the liquid being pumped.

Some manufactures offer replaceable shaft sleeves (F) and axial thrust surfaces (G) for longer service life and ease of maintenance.

Bearings Only two bearings are required for canned motor pumps. These bearings are normally cooled and lubricated by the pumped fluid; therefore, they must be compatible

FIGURE 2 Stator assembly
(C) End Covers FIGURE 3 Rotor assembly

with the process fluid. A multitude of materials is available such as various grades of carbon graphite, silicon carbide, aluminum oxide, and many polymers. Bearing selection is dependent on the compatibility with the process fluid, amount of solids present, and pumping temperature.

A hydrodynamic bearing is the most common type of bearing used. The bearings can be either stationary or rotating with the rotor assembly. In either case, the fluid passes between the bearing and shaft journal, resulting on the rotating assembly running on a thin film of liquid, not the journal and bearing. Most bearings have helical grooves in the inside diameter to increase the flow of process fluid through the journal area, thereby decreasing the temperature of the bearings.

Internal Clearances The determination of the overall gap between the stator windings and the rotor armature is paramount in the design and operation of the pump. The wider the distance between the iron of the motor winding and the rotor armature, the less efficient the motor becomes. The material of construction of the stator liner and rotor sleeve also effects motor efficiency. Stainless steels and Hastelloy are the most common materials used for stator liners and rotor sleeves. Although stainless steel is less expensive, Hastelloy C has higher corrosion resistance, is a stronger material, and offers lower electrical losses. Motor efficiency in canned motor pumps is not only important for energy cost considerations, but also for the amount of heat input to the recirculation fluid.

The stator liner (a wetted, pressure boundary component) ranges in thickness between 0.010 to 0.040 in (0.254 to 1.016 mm). For high-pressure applications, the liner remains at the same thickness, but the outside diameter is supported by the motor laminations and by back up sleeves located on both sides of the motor. Canned motor pumps have been designed to withstand working pressures up to 5,000 lb/in2 (345 bar) with 0.015 in (0.381 mm) stator liners and heavy walled back-up sleeves.

The rotor armature (a wetted component) is also protected from the process fluid by a sleeve and two end covers. The thickness of the rotor sleeve ranges from 0.010 to 0.25 in (0.254 to 6.35 mm). The radial running clearance between the rotating motor armature and the stationary stator liner is usually about 0.020 in (0.508 mm). The total diametral clearance can range from 0.040 to 0.075 in (1.016 to 1.905 mm), or higher, depending on the manufacturer's design.

Secondary Containment Canned motor pumps offer a level of safety and process fluid containment unavailable with any other type of pump. Positive, secondary containment of the process fluid is a built-in feature with canned motor pumps when the motor lead wires are housed in a pressure retaining lead seal. In case of a failure of the primary containment shell (stator liner), the outer stator band becomes a secondary containment vessel, preventing the process fluid from entering the environment.

The outer stator band is far removed from the rotation element, making it impossible for the rotating element to make contact. When secondary containment is required, the stator band assembly should be designed and tested to the same pressure and temperature rating as the pump.

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