National Fire Protection Association (NFPA) code 110 defines wet stacking as the accumulation of unburned fuel or carbon on a diesel generator's exhaust side. To measure wet stacking, emergency power technicians measure the temperature of a generator's exhaust gas. Once the extent of wet stacking is confirmed, technicians suggest ways to eliminate it. If left unaddressed, wet stacking could have the following effects on generator systems:
• Fouled injectors due to carbon accumulation
• Decrease in engine performance due to gases that escape from valves
• Parts corrosion due to carbon accumulation
• Premature engine breakdown due to gasoline diluted with oil
• Significantly shortened engine life
• Significantly increased maintenance expense due to engine damage
These conditions typically occur when emergency generator systems operate below their intended operating temperature for an extended period, although a decrease in engine performance is often seen immediately. To correct the problem and avoid premature engine breakdown, building owners should follow NFPA 110 testing guidelines for diesel generators. Prime power specializes in performing these tests.
Monthly and Annual Testing
During a monthly test, a generator should be operating under available load at (a) a minimum of thirty percent of its nameplate kilowatt rating, or (b) no less than the minimum exhaust gas temperature recommended by its manufacturer. If a generator does not pass its monthly test, it should be operated for two continuous hours under a supplied load (i.e. load bank testing) at the following levels at least once a year, beginning after a failed monthly test:
• At 25% of nameplate rating for 30 minutes
• At 50% of the nameplate rating for 30 minutes
• At 75% of the nameplate rating for 60 minutes
Auxiliary load bank testing and facility manual load bank testing are also used for annual tests, but portable load bank is the preferred method for two reasons: it eliminates the expense of buying and maintaining a load bank, and it places testing in the hands of emergency generator systems experts, who often perform this type of testing as part of a larger preventative maintenance regimen.
Conclusion
Wet stacking can happen to any diesel generator, and its effects can extend beyond generator damage. In extreme cases, it can also lead to environmental concerns, especially for facilities located in low emissions zones (LEZ). The biggest concerns are two problems it creates for building owners: the high cost and the potential disaster associated with generator failure, especially in buildings that have generator powered backup lights or require constant electricity.
At Prime Power, we understand the seriousness of wet stacking. That is why we deploy hundreds of mobile load banks each year to exercise emergency generator systems that experience wet stacking. If your generator fails its monthly test, load bank testing is the best way to prevent wet stacking or eliminate it before it leads to expensive repairs. Wet stacking could occur subtly, and exist for months or years before it is detected. To learn more about wet stacking and what you can do to prevent it, call Prime Power today.
