Canned Motor Pumps, Magnetic Couplings & Sealless Rotodynamic Pumps

Q. What primary and secondary containment considerations are recommended for canned motor pumps? A. A canned motor pump (CMP) is a type of sealless pump that has a common shaft to link the pump and motor in a single sealed unit (see Figure 1). The pumped liquid is circulated through the motor, but it is isolated from the stator by a corrosion-resistant stator liner. The motor rotor is protected by a corrosion-resistant rotor liner.

The principal design consideration for sealless pumps is to prevent leakage to the atmosphere. Because the primary containment may develop leakage as a result of wear by adjacent rotating parts, corrosion or abrasion, secondary leakage containment or control may be required if specified by the purchaser. In these cases, there is design emphasis on prevention of leakage to the atmosphere in the event of a breach in primary containment. The secondary pressure boundary’s material is evaluated to ensure compatibility with the pumped liquid. The requirements and solutions for secondary containment and control will vary based on the application and pump configuration, so the purchaser and pump supplier must discuss these points. Primary and secondary containment or control must have design pressure ratings equal to, or greater than, the maximum working pressure of the pump at rated conditions. When specified by the purchaser, the CMP’s secondary containment barrier is designed to contain allowable working pressure for a minimum of 48 hours in the event of stator liner leakage and have a minimum standby life of three years. If an oil-filled stator with a pressure relief valve is used, then provision must be made for safe containment of any leakage past the stator liner. When specified by the purchaser, the manufacturer will provide a means for periodically checking secondary containment. In the event of leakage through the stator liner, immediately discontinue the pump’s operation. The purchaser is responsible for providing shutdown devices and procedures required for safety. For more information about sealless rotodynamic pumps, refer to the newly revised standard ANSI/HI 5.1-5.6 Sealless Rotodynamic Pumps for Nomenclature, Definitions, Design, Application, Operation, and Test. Q. What are the design recommendations for magnetic couplings in a magnetic drive pump? A. A magnetic drive pump (MDP) is a type of sealless pump that uses an outer ring of permanent magnets to drive an internal rotating assembly through a corrosion-resistant containment shell (see Figure 2). Permanent magnets have high attraction forces to ferrous metals and high attraction or repulsion forces to companion magnets, depending on polar position. Users must exercise extreme care when disassembling or assembling the drive unit to prevent damage to components and injury to personnel. Methods of piloting and separating flanges are provided in the design to allow for safe disassembly and reassembly of the magnet drive section. The manufacturer makes allowance for torque required to accelerate the inner rotor assembly, considering the acceleration rate of the drive motor. The manufacturer also makes allowance for maximum specified specific gravity, viscosity, temperature and flow. Deterioration of magnetic strength also must be considered. The manufacturer will establish application guidelines for specific designs considering these factors. Heat generated by eddy current and windage losses is removed by the pumped liquid or by the supply of external cooling liquid. Torque capacity is dependent on the coupling mechanical design, temperature and magnet material used. The pump user must maintain the magnetic material temperature at or below rated values for the material used for safe operation. Loss of cooling flow or decoupling will result in rapid heating of an electrically conductive containment shell. If the pump rotor becomes decoupled, continued turning of the outer magnet assembly will cause rapid heating of the inner-magnet assembly. Even though explosion-proof rating of magnetic couplings is not normally required, users must take precautions if potentially flammable vapors are present that might contact the containment shell. Handling liquids that contain magnetically attracted particles must be avoided unless particles can be removed effectively by a magnetic filter, because such particles are collected by the permanent magnetic fields and can cause erosion and blockage of liquid flow passages. The manufacturer must identify materials used for parts that come in contact with the pumped liquid in the event of failure of the primary containment shell or inner magnet liner. The outer magnet carrier (the outer steel portion of the outer magnet assembly) must be coated with a corrosion/heat-resistant paint or coating, with the exception of any close-clearance register fits. If the outer magnets are neodymium, they are coated or painted to prevent corrosion, and the outer magnet assembly is designed to be easily cleaned of stray ferrous particles during assembly. For more information about sealless rotodynamic pumps, refer to the newly revised standard ANSI/HI 5.1-5.6 Sealless Rotodynamic Pumps for Nomenclature, Definitions, Design, Application, Operation, and Test. Q. What material considerations are recommended for sealless rotodynamic pumps? A. When users are selecting a manufacturing material for sealless rotodynamic pumps, careful consideration of corrosion is important. The manufacturer should offer the purchaser guidance regarding selection of materials particular to the design involved (such as primary containment device and rotor encapsulation members). The purchaser must specify any corrosive chemicals used in the particular design or application. Compounds that may cause stress corrosion cracking or hydrogen embrittlement should be included. If austenitic stainless steel parts exposed to conditions that promote intergranular corrosion are to be fabricated by welding, then they must be made of low-carbon or stabilized grades. Minor wetted parts that are not usually identified—such as nuts, bolts, springs, washers and keys—should have corrosion resistance at least equal to that of major parts. If requested, the manufacturer must supply chemical and physical certifications for impeller, shaft and pressure-containing parts as mutually agreed on by the purchaser and manufacturer. Plugging, peening or impregnation should not be used to complete any repair on pressure-containing or wetted metal parts. Operators must perform welding of piping and primary pressure boundary welds, and the purchaser and manufacturer must agree on procedures qualified in accordance with an industry-recognized welding standard. Weld repairs must be made according to the applicable material specification, standard, code or as agreed upon by the purchaser and manufacturer. For more information about sealless rotodynamic pumps, refer to the newly revised standard ANSI/HI 5.1-5.6 Sealless Rotodynamic Pumps for Nomenclature, Definitions, Design, Application, Operation, and Test.