3.2.11 Sealless pumps

Sealless pumps

Sealless pumps are characterised by the fact that their design and construction prevents leakage to the surrounding environment. A mechanical seal type of shaft seal cannot in this context in any way be considered as leakage free. The description glandless pumps is there fore associated with the alternative description hermetically sealed pumps or canned pumps.

Sealless pumps must be used for the most dangerous fluids from the point of view of toxicity and radioactivity, for high or very Iow temperatures and for very high intake pressures. This type of pump has many other applications 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 etc. When ordering pumps for these types of fluids the degree of acceptable leakage should be specified.

The procurement costs for larger sealless pumps are approximately 2 to 3 times greater than for conventional pumps, although the complications and costs of conventional seal maintenance is avoided. Small pumps, up to approximately 100 W are often cheaper than conventional pumps. The main reason being that the drive motor need not be dimensioned to cope with shaft seal starting torques and power requirements.

There are two basic types of sealless pumps, Magnet driven pumps and Canned motor pumps. Both types employ fluid lubricated plain bearings of graphite, hard metal or bronze. For completely pure fluids ball-bearings may even be used. Thrust bearings are avoided as much as possible by hydraulically balancing the axial forces.