Friday, 22 June 2012

Cargo Pump

Cargo Pump
Cargo Pump

The repairs described below are to aid maintenance planning and not a forum for denigrating machinery, companies or personnel


The 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


The pump was disassembled with the following damage noted.
The scrolls were in generally good condition with the exception of the bearing and mechanical seal landing surfaces. These had been repaired previously using metal spray
Their was general erosion to the casing to about 0.5%of the diameter. More significant damage was noted to the cutwater beween the two concentric bores. Due to the advance age of the vessel only the damage to the cutwater was addressed
Typically the metal spray coaring had detached from the parent metal under the bearing landings. Thus demonstrating the unsuitablility of this repair under high impact lading conditions, although it is quite satisfactory under normal conditions for bearing landings
Normally metal spray repair is satisfactory for mechanical seal landing. Here is shown a typical poor application of the process. A steel landing surface should have been available for the seal locating grub screws otherwise mechanical damage occurs which then allows further destruction of the coating
It is difficult to show clearly the damage on the drive end bearings but it was not of the form of pitting and did not appear to be due to the permenant set damage known as brinelling. A repeat to a lesser extent was seen on the rolling elements. This was unusual in form and did not fit the authors understanding of rolling element bearing failure and caused some confusion. The author offered that very similar damage is seen in large electrical motors and is caused by electrical discharge through the bearings. It is noted that this pump is driven by a diesel engine which also drives an electrical generator via a clutch arrangement. Similar poor levels of reliability had been seen in the clutch.
The above and below show a before and after picture of the mechanical seal. after crack detection two of the four carbons had to be replaced as well as all rubber seals


bearing & 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

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