What is Wet Stacking and How do Load Banks Prevent it?

What is wet stacking?

The term ‘Wet Stacking’ originates from the fact that fuel is still ‘wet’ in the exhaust system when in operation. Wet Stacking occurs in diesel gen-set engines operating under no or lightly loaded conditions (less than 30% of rated performance) for extended periods. The low running load means the engine does not reach optimal operating temperature for peak performance leaving unburnt fuel and carbon deposits within the engine.

Consequences of wet stacking

Wet stacking conditions cause unburnt fuel deposits to collect on the combustion chamber, injector nozzles, piston rings, turbocharger, and exhaust system. Some issues associated with wet stacking include:

Increased emissions: In areas of stringent emissions standards, the effects of wet stacking can cause reduced efficiency of the engine and excessive pollution.

Increased cost: Without maintenance, wet stacking can reduce the working life of a diesel generator by years increasing potential capital expenditure for replacement units.

Power: Wet stacking reduces efficiency and power of the engine and may not be able to cope with power demands required causing a stall or failure.

Maintenance costs: A correctly loaded gen-set will need much less maintenance than a wet stacked unit.

National fire protection agency (NFPA) standards

The NFPA has guidelines to reduce the effects of wet stacking on backup power systems:

The NFPA guidelines in Level 1 and 2 applications require exercising the unit, at least monthly, for 30 minutes under either of two methods: (NFPA 110 8.4.2)

• Loading that maintains the minimum exhaust gas temperatures, as recommended by the manufacturer
• Under operating temperature conditions and at not less than 30 percent of the EPS standby nameplate kW rating

How to prevent wet stacking

The best way to alleviate the effects of wet stacking is to apply additional load to the gen-set, increasing operating temperatures to burn off accumulated unburnt fuel and carbon. The amount of minimum load varies per engine manufacturer, but the typical range is 30% to 50% of the kilowatt rating. It is proven diesel engines operate more efficiently in the 70% to 80% range of rated kW output.

Load banks are the best practical means of applying load to gen-sets and preventing wet stacking. There are a variety of load banks that can be used to prevent wet stacking:

Radiator mounted load banks (Avtron 1000 Series) are fitted directly to the diesel generator radiator to provide supplemental load from 50%-70% kW rating size.

Portable load banks (Avtron 2000 Series) can be moved into position when required; often used by power rental and maintenance companies.

Permanently Installed Load Banks (Avtron 3000 or 4000 Series) are integrated into the facility as part of a general maintenance schedule.

Avtron offers a wide range of load bank types to ensure gen-set performance and prevent wet stacking.