HistoryThe follwing describes a repair to a large twin screw positive displacement pump. The scrolls where synchronised with a pair of timing gears fitted to one end. This is one of four installed all of which had had a history of failure after relatively short run times over a 25 year period (in the region of 6-24 months). Failure normally took the form of bearing failure, in this case the driven shaft non drive end bearing had catastrophically failed. The following briefly describes the repair methods
InvestigationThe pump was disassembled with the following damage noted.
Repairbearing & mechanical seal landings- due to the failure of previous metal spray repair a different approach was taken. The shaft was machined and thin steel half segments were welded on and final machined.It should be noted that the material selected for the shell has to be galvanically and thermally compatible with the parent metal. The possibility of building up the parent metal with weld was discounted due to the poor weldability of the material . Wear in the timing gear, key and shaft meant that specially offset keys had to be fabricated to ensure the scrolls were properly centralised.
The pump was preassembled with steel fars taking the place of the main body. This alowed the scrollsto be accurately centralised using shims behind the bearings.
With regard to the bearing failures the author informed of the existance of ceramic coated bearings and the use of shaft earthing rings