‘Wet stacking’ and diesel generator maintenance

Many operators and maintainers of standby diesel generator are probably all too familiar with the problem of engine 'slobber', or 'wet stacking'. This problem is typically the result of the generator engine running for long periods at low levels of load — the outcome of which is a build-up of unburned fuel in the exhaust system that can lead to reduction in engine performance, and, if not dealt with in a timely fashion, premature failure. This can be caused by a number of reasons, including:

The usual way of overcoming this issue is to connect an external load bank for testing, which of course incurs additional costs and time, with the resulting higher emissions and extra fuel costs.

Wet stacking typically occurs when a generator is run at less than 30 per cent of its nominal capacity for extended periods of time. With this level of load the engine doesn't achieve, or is unable to sustain, the optimal operating temperatures needed to fully burn the fuel injected into the engine. The lack of engine temperature means that the pressure inside the combustion chamber falls below the crankcase pressure, and the engine temperature isn't sufficiently high to ensure that the piston rings expand enough to seal the space between the pistons and cylinder walls. The result is incomplete combustion of the fuel, and a propensity for the engine to draw small amounts of lube oil up from the crankcase, which becomes visible as white smoke in the exhaust. A build-up of lube oil in the combustion area can cause glazing on the cylinder wall. Additionally, there can be a build-up of unburnt fuel or soot in the exhaust line, which of itself can also be dangerous. Wet stacking: