The corroded gas casing on the turbocharger caused a cooling water loss. Due to a lack of cooling water treatment, the fault was common on all the turbochargers in the multi engine installation.
The Main Engine header tank low level alarm sounded on a multi Sulzer ZA40 installation fitted with ABB VTR turbochargers. Refilling of the tank and observation showed a constant loss. A check of the cooling system pipework, pumps etc. showed no leaks. LO tests showed no appreciable water in the main engine sumps and no water from the air cooler drains.. The main engines were stopped in turn and checked over.
On closing the CW inlet and outlet on one engine, the header tank stopped dropping. When the the turbocharger gas casing drain was opened, a large quantity of water was found to run out. Reopening the CW valves caused water to continuously run. The leak had been found!
The gas casing was changed as soon as a new one was received. The cause was due to corrosion because of inadequate nitrite levels in the cooling water. This was traced to bacterial contamination, the nitrite being the food source for the bacteria.