The slow turning system includes a pipe connected to an automatically controlled valve, which bypasses the main air start automatic valve. The slow turning automatic valve provides just sufficient air for the engine's crankshaft to turn slowly on air to prevent any damage in case water or fuel accumulated on the piston due to a possible leak. Liquid accumulated in the combustion chamber would cause the hydraulic hammer effect, damaging head and running gear had full air throughput been used.
The slow turning may be used:
On a slow speed engine during maneuvering. If the engine has been stopped awaiting a movement for more than a set period (usually 20-30 minutes) then when an order is given and the engine control moved to start, instead of the main air start opening, the slow turn valve will open. If the engine satisfactorily completes a revolution, then the main air start opens allowing the engine to start.
On a medium speed diesel electric installation where engines are on standby for immediate start, a slow turning valve may be timed to turn the engines every hour, an alarm sounding if they fail to complete 2 revolutions. The air used for this slow turning must be dry to prevent corrosion of liners over a period of time when the engines are not needed.
