Pump types--part 2 (Rotodynamic, part 5)

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__3.37 Magnetic drive rotodynamic pumps

Magnetic drive pumps are a type of glandless pump. Glandless pumps are characterized by the fact that their design and construction physically prevents leakage to the surrounding environment. Packing or a mechanical seal type of shaft seal cannot in this context in any way be considered as leakage free, even if it’s barrier liquid or very dilute process liquid/ vapor in minute quantities which escapes. The description glandless pumps is therefore associated with the alternative description hermetically sealed pumps or canned pumps.

Glandless pumps must be used for the most dangerous liquids from the point of view of toxicity, environmental hazards and radioactivity. In some cases for high or very low temperatures.

Leakage may occur however when handling certain types of highly penetrative liquids due to penetration through minute faults in static seals. Magnetically driven pumps can avoid this problem by having the containment can welded to the casing.

When ordering pumps for these types of liquids the level of acceptable leakage should be specified.

Two types of magnetic drive are available:

--Synchronous magnetic couplings

--Magnetic induction couplings

In both cases the pump is driven by a standard motor. However the motor does not drive the pump shaft carrying the impeller(s) directly. The motor drives a rotating magnet assembly. In synchronous magnetic couplings, Fig. 53, the rotating magnetic field drives another set of magnets coupled to the impeller(s). In magnetic induction couplings the rotating magnets create a magnetic field in a rotor of copper bars, very similar to a standard squirrel cage motor rotor. The magnetic properties of materials decay as temperature increases so that synchronous magnetic couplings suffer from a loss in torque. Magnetic induction couplings are better at elevated temperatures because the magnets are not in contact with the liquid.

The pump shaft proper is totally immersed in the process liquid and runs in bearings which are process lubricated. Silicon carbide ceramics and some carbon are popular as bearing materials. Solids in the liquid can cause high bearing wear and in some designs can accumulate in the can and cause obstructions. This problem can be solved by using an external feed to the bearings. Process liquid can be diverted from the pump discharge, filtered and cooled if necessary, and re-injected into a lubrication circuit. This extra complexity will be worthwhile when considering the enhanced reliability and extended bearing life.

Bearing wear monitoring is a useful facility on any magnetically driven pump.

A variety of magnetic materials are used of which aluminum, nickel and cobalt are probably the most widely and successfully employed. Some designs use rare earth elements, such as samarium cobalt and neodymium iron boron for the upper end of the torque range, the higher costs involved being more than offset by improved performance.

Magnetic drive pumps cannot run dry. The pump and can must be thoroughly vented and primed before start up. The process liquid provides lubrication and cooling for the pump internal bearings. The process liquid also removes heat generated by the magnetic coupling. Failure to provide the appropriate liquid filling will cause significant wear of bearings and may cause the pump to seize before venting and priming is accomplished. Dry running monitoring could save major maintenance expenses.

Close coupled magnetically driven pumps, Fig. 54, have become increasingly popular for critical applications where unscheduled process stoppages create additional plant problems as well as loss of revenue. Mechanical seals are seen as a maintenance intensive item, difficult to install and set-up, with costly spares and attendant inventory costs. Removing the mechanical seal and operating pumps of a modified design can save overall costs. Cases have been cited where a magnetic drive pump costs 30% of a conventional pump over a ten year operating life.

Magnetically driven pumps are available from several manufacturers, including non-metallic versions, pumps up to 350 kW are currently built. Hydraulic capacities of 550 m3/h at 500m are possible, temperatures of-80 to 450 ~ are promoted. API 610 and ISO 2858/ANSI B73.1 conditions can be met.

NOTE: Rotary positive displacement pumps can be driven by sealed magnetic couplings. Gear and triple screw pumps are typical examples. If a centrifugal pump is not the best selection for a particular liquid there may be a suitable positive displacement pump.

__3.38 Canned motor rotodynamic pumps

Canned motor, "wet motor", driven pumps generally have the stator windings sealed from the pumped liquid by means of an annular tube hence the name "canned", Fig. 55. There are many types available with power outputs of up to several thousand kilowatts. Smaller sizes, up to about 500 W are generally used as domestic hot water pumps and central heating circulating pumps, see Part. 3.1.

Fig. 54 Close coupled magnetically driven pump

Fig. 55 Canned motor pump

Fig. 56 A large vertical canned motor pump KSB A G

Canned motor pumps are produced as both single and multi-stage types with various types of drive motor. Liquids with temperatures ranging from -200 ~ to +500 ~ and pressures up to 1000 bar can also be handled. Heating jackets are available for handling liquids with high melting points, whilst built-in filters are primarily used to cope with contaminated liquids; although trickle feeding to the rotor chamber may also be necessary for liquids containing high concentrations of solid particles.

Canned motor pumps are selected when the liquid handled is hazardous and/or the user requires the highest possible availability. The canned motor pump is a sealed unit with static pressure seals; no mechanical seal. There is no coupling between motor and pump so there are no alignment problems. When selected properly the canned motor pump can provide an availability of 0.9997 which evaluates to a loss of running time of 2.6 hours per year. Very high availabilities are achieved in systems which operate continuously for months or years. Starting and stopping, which can create hydraulic, mechanical and thermal shock, tends to reduce reliability if not considered thoroughly.

Larger canned motor pumps were primarily developed for boiler circulation pumps and liquid metal circulating pumps for nuclear reactors. Circulating pumps are required on modern large multi-tube boilers to ensure a reasonable water velocity inside the tubes to improve heat transfer and eliminate hot spots and problems with localized vaporization. The suction pressure of the pumps is boiler pressure, 150 barg or more, and the differential only 20 or 30 meters. The high suction pressure produces a large axial thrust requiring enormous, (by comparison to the standard), thrust bearings and also causes mechanical seal problems. Liquid metal pumps for nuclear reactors must be positively leak-free because of the radioactive nature of the liquid. The canned motor pump solves these problems.

The canned motor pump is also available as a chemical and process pump, very similar to magnetic drive pumps and in special variants for sewage, effluent and paper stock. Flows of 300 m3/h at 110m with pressure ratings of 25 bar are fairly common. Some canned motor pumps are available to DIN 24256 standard pump performance and dimensions. See Fig. 56.

Current designs include those which have no sliding bearings.

The motor/pump shaft is raised magnetically and held centrally within the stator/casing. Axial location is also controlled magnetically. The mechanical efficiency is improved because mechanical bearing losses are eliminated. This style of construction allows very high pump speeds to be used without worrying about bearing performance or life.

NOTE: Magnetic drive and canned motor pumps. The American Standards body, the Hydraulic Institute, has issued a Standard covering sealless pumps, ANSI HI 5.1-5.6.

This Standard is designed to eliminate misunderstandings between purchasers and manufacturers and to help the purchaser specify a pump correctly. The Standard covers; pump types, definitions, applications, installation, commissioning, operation, maintenance and testing. Details of monitoring devices unique to sealless pumps are given; testing specific to sealless pumps; can integrity and winding Integrity for canned motor pumps. The application section deals with gaskets and joints, venting and draining and internal process liquid lubricated bearings.

The Standard includes descriptions of piping arrangements for process liquid circulation and recommendations on how to prevent problems caused by solids, magnetic material and gas/vapor evolution. B73.3 covers sealless versions of the ASME/ANSi B73.1 and B73.2 style pumps.

__3.39 Rotodynamic pumps without drivers (for specific applications including mobile agricultural applications and mobile fire pumps)

Pumps specially adapted to be driven from the power take-off of a tractor are used for temporary installations, during the preparation period for a permanent installation and for mobile applications. The speed of the tractor power take-off is too slow, approximately 500 rpm, for a small centrifugal pump. The specially adapted pumps have built-in gears to increase the speed to 3000 to 4000 rpm, see Fig. 57.

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