GEOFF HALLIDAY – BUSINESS CONSULTANT, WB POWER SERVICES LTD, UK GENERATOR MAINTENANCE
Wet stacking and diesel generator maintenance
Geoff Halliday, business consultant at WB Power Services Ltd, considers the issue of diesel engine ‘slobber’ or ‘wet stacking’ in standby diesel generators.
Many operators and maintainers of standby diesel generators 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 will in a timely fashion – premature failure. This can be caused by a number of reasons, not least of which are: l The generator is oversized for the power required.
l An adequate load is not available during the regular maintenance test running period (healthcare providers frequently have this issue).
l In a hire application where the generating set 24/7 are prime power source and loads are very light for prolonged periods – say over a weekend.
The usual way of overcoming this issue is to connect and external load bank for testing purposes, which of course incurs the additional costs and time to connect with resultant higher emissions and extra fuel costs.
What is wet stacking? 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 either does not achieve or is
unable to sustain its 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 is not high enough to ensure the piston rings expand enough to
Geoff Halliday Geoff Halliday started his career as an apprentice working for
Square D (later part of Schneider) before moving into the critical power sector, where he has now worked for over 40 years, splitting that time equally between both the UPS and standby diesel generation sectors.
During this period, Geoff has held several roles ranging from
customer service engineer, project manager, technical director, sales director through to managing director. Drawing on his management skills, product knowledge and vast application
experience amassed throughout his career, Geoff now enjoys sharing his knowledge with others.
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seal the space between the pistons and cylinder walls. The result of this 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: l Fouls the fuel injectors. l Causes excessive valve guide wear. l Can lead to damaged pistons, piston liners, and rings among other problems (see Fig 1).
Emission considerations In addition to the risks of shortening engine life and running higher maintenance costs, operators could also fall foul of tighter emissions regulations
IFHE DIGEST 2025
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