PLANT MAINTENANCE
‘Wet stacking’ and diesel generator maintenance
In this whitepaper, WB Power Services Business consultant, Geoff Halliday, considers the priorities for regular maintenance testing of generating sets and their associated exhaust aftertreatment equipment.
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: n The generator is oversized for the power required. n An adequate load is not available during the regular maintenance test running period that data centre providers frequently have, or in a hire application where the generating set is the ‘24/7’ prime power source, and loads are very light for prolonged periods, such as over a weekend.
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: n fouls the fuel injectors. n causes excessive valve guide wear. n can result in damaged pistons, piston liners, and rings, among other problems, as shown in Figure 1.
Emission considerations In addition to the risks of shortening engine life and running higher maintenance costs, operators of such generators could also fall foul of tighter emissions regulations due to the discharge, via the exhaust system, of partially burnt lube oil and additional particulates from
the unburnt fuel. The introduction of
ever more onerous emissions targets, has, in recent years, seen the introduction of diesel oxidation catalysts (DOC), diesel particulate filters (DPF), and – more recently via the Medium Combustion
Fuel drip
Carbon deposits
Plant Directive (MCPC), selective catalytic converters, or ‘SCRs’ (see Fig 2). The introduction of these additional elements into the exhaust gas stream further adds to the problems of wet stacking, elevating its importance as an issue that needs addressing. All of these emission treatment devices only function correctly when they reach their optimal operating temperature/operational temperature range. It is heat generated by full and effective combustion in the engine which generates the necessary heat in the exhaust system. The emissions aftertreatment devices are also susceptible to becoming clogged by unburnt fuel and soot resulting from wet stacking, reducing performance
Figure 1: Wet stacking can result in damaged pistons, piston liners, and rings, ‘among other problems’.
Figure 2: A selective catalytic converter.
oxidation catalyst (DOC)
Diesel
Selective catalytic reduction (SCR)
Diesel particulate filter (DPF) October 2025 Health Estate Journal 91
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