3.2.13 Canned pumps
Canned Pumps
Canned pumps or canned motor pumps are centrifugal pumps, where pump and motor shaft is one unit single shaft design, assembled in a hermetically sealed cylinder or annular tube. The shaft is holding both the motor rotor and impeller, the motor stator is separated by a sealed, non-magnetic sleeve. The rotor package including bearings are immersed in the pumped fluid and part of the fluid is pumped through the rotor package in the hermetically sealed annular tube to lubricate and cool the rotor package before pumped out. Due to it’s design, there are no rotating parts to seal off from atmosphere and in case the connecting flanges are welded to suction and pressure pipes, canned pumps can’t leak.
Canned pumps are used for the most dangerous fluids from the point of view of toxicity and radioactivity, for high or very low temperatures and for very high intake pressures. This type of pump was historically developed for nuclear power plant applications but chemical industries were fast to recognize the advantages as well. At that time mechanical seals were not as well developed as today and to seal off a tough chemical fluid efficiently was difficult. During the nineteen sixties the pump type was evolving and since the many other industries and applications has been motivated by its zero leakage characteristics. Leakage may occur however when handling certain types of highly penetrative fluids due to penetration through minute faults in the actual cast material, via static seals (packing’s, o-rings) etc. When ordering pumps for these types of fluids the degree of acceptable leakage should be specified.
Canned motor pumps, also called wet motor pumps, generally have the stator sealed from the pump fluid by means of an annular tube, hence the name wet-rotor pump see figure 3.2.13. Since the design is hermetically sealed it is important to fit sensors on the pump to monitor rotational speed, bearings, temperature etc. Canned motor pumps require strict control of bearing wear. If the bearings wear, uncontrolled failure can be catastrophic because it includes all the mechanical parts of the pump and the electric rotor winding.
However, among the advantages for canned motor pumps compared with other pumps in similar applications, that is API 610, ISO 13709 equivalent pumps we count:
- No leakage, no mechanical seal or seal plan (incl. lubrication and cooling)
- No external lubrication system, nor is an internal lubrication program needed
- No foundation or grout
- No alignment
- High efficiency
These benefits can all translate to safe, reliable, low maintenance, low installed cost which in turn are related to canned pumps high MTBF and low LCC.
There are many types available with large power outputs, 500-600kW for high heads of over 900 m and flows of 2500 m³/h. Smaller sizes, up to about a few hundred Watts are generally used as domestic hot water pumps, see also group Domestic heating and sanitation pumps. Canned motor pumps are produced as both single and multi stage types with various types of drive motor. Fluids with temperatures ranging from minus 200°C to plus 500°C are available and fluid pressures (pressure ratings) up to PN1000 can be dealt with.
Material are normally stainless steel 304 and 316 but most manufacturers can offer tougher materials like Hastelloy, Titanium and other materials up on request.
Heating jackets are available for handling fluids with high melting points, whilst built in filters are primarily used to cope with contaminated fluids although trickle feeding to the rotor chamber may also be necessary for fluids containing high concentrations of solid particles.