Wet stacking

Wet stacking is a condition in diesel engines in which all the fuel is not burned and passes on into the exhaust side of the turbocharger and on into the exhaust system.

In generator sets, it is usually because the diesel is running at only a small percentage of its capacity. The accreditation body for hospitals JCAHO has been very concerned about this and has over the past few years dinged numerous hospitals for not running generator sets under at least 30% load as specified by the nameplate, or 50% of the normally connected emergency load, whichever is greater.

Some hospitals purchased large generators when they had the chance, anticipating expansion of the facility. As a result, some facilities fail to meet the percentage limits. Some of those that don't meet the load requirements have connected load banks to load up the generator to 80% of nameplate for a four hour annual run.

Wet stacking is detectable when there is a black ooze around exhaust pipe connections and around the turbocharger. Continuous black exhaust from the stack when under a constant load is also an indication that all the fuel is not being burned - good preventive maintenance is critical for this type of generator application.

NFPA 110 [ [http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=110 NFPA 110: Standard for Emergency and Standby Power Systems ] ] speaks to the generator and wet stacking issues, and the 1997 JCAHO environment of care standards [ [http://www.jointcommission.org/Standards/Manuals/ Joint Commission Accreditation and Certification Manuals | Joint Commission ] ] also address some change in the thinking on load bank testing.

Load bank use can have unexpected consequences; care must be taken if it is desired to connect an external load bank to a required emergency power system, one must make sure that there is some automatic disconnect included that will take the load bank off-line if the generator is needed by the facility during the load bank run. If not, it is possible to seriously overload the system if the emergency load from the hospital is added to the 80% load of the bank, causing a failure of the system when it is needed most. [Raymond Forsell, Clinical Engineer, University of Vermont Technical Services Program]

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