I have had the occasional cooling water
this year so decided to investigate further. The engine
is a 20hp Bukh
DV20Me with direct sea water cooling
via a Johnson pump (rubber impeller). The engine
spec states the cooling requirement is 11 litres/min at 2000rpm. A new impeller was fitted 3 months earlier.
I measured the cooling water flow at 5.8 l/m at 2000rpm using a bucket and stop watch.
I next cleaned the strainer but this is a coarse strainer anyway and checked the supply hoses for restrictions (found none).
Next checked the the flow with the fridge/freezer heat exchanger
bypassed and this did make a 0.5 l/m change but no more than I would expect. Conclusion that the Fridge /freezer heat exchanger
was not the cause.
Next checked the cooling water into the engine block, this was slightly contaminated with a calciferous deposit. This was cleaned out and the pipework cleaned.
Next came the exhaust
manifold cooling water outlets, the thermostat housing and the thermostat bypass loop. These all showed some deposit build up and were all cleared out.
Finally the pump casing was removed for overhaul
. The back face wear seems excessive but the cover is good(recently replaced) as there is a marked step between where the impeller runs and the area under the vac cam which is untouched. The unworn section measures 0.748 ins so have assumed that the casing needs to be machined to recover this dimension. The new impeller width is 0.751 ins and with a new gasket
on the cover of 0.010 ins would mean a final axial clearance for the pump of 0.007 ins. My thinking is that as these pumps function by creating a small depression then any axial clearance is undesirable, therefore I favour reducing this clearance to zero for the rebuild
Do any of you guys have experience on these type pump builds and the axial clearance requirements?