Since I am no diesel expert but merely a knowledge-hungry acolyte, I will quote. These descriptions are concurrent with my beliefs on wet-stacking, which is simply a conditiion where the engine load is so light that excess fuel is unburnt and can collect in the exhaust system (less common with electronic injection), and also can later condense and dribble into cylinders, washing the oil off the walls, as well as simply washing the oil off the walls due to low temperature operation caused by light loads and failure to allow the engine to come into full heat. Load banking is the process of applying approximately an 80% load (generally resistive is easiest to apply) to a generator set in order to assure that the machine's heat is raised enough to burn out all the old fuel and gunk in the exhaust and evaporate it from the oil as well and then let the engine be shut down clean after a cooldown of perhaps 10 minutes. In my case I have a 40KW genset with a 6-11KW load, a prime candiate for wet-stacking! but it was cheap, what can I say?:
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quotes:
Wet stacking
From Wikipedia, the free encyclopedia
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 4 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. There should be no surprises in the dark of night if the maintenance is being done correctly.
NFPA 110 speaks to the generator and wet stacking issues, and the 1997 JCAHO environment of care standards also address some change in the thinking on load bank testing.
A caution on load banks. If you choose to connect an external load bank to a required emergency power system, 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, you may 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.
also:
There are a lot of "opinions" about what wet stacking is, but basically in a diesel, it's unburnt fuel and oil that collect in the exhaust from light or no load, as in idling too long. I think in rare cases/application, gas motors will wet stack but it's in the form of carbon building up in the cylinders. Back to diesels, not enough heat to completely burn the fuel. In spite of a lot of people "opinions", it's not near the issue with newer electronically controlled diesel motors.
and:
Wet stacking is when unbent fuel in the cylinder washes off the oil coating on the cylinder walls.
and:
Q. Can the engine on a diesel-driven generator set be damaged due to light-loading and if so, what would be the likely result?
A. Yes, if a diesel engine is operated below about 30% of its rated output a condition known as “over-fueling” or more commonly: “wet stacking”, can occur. This happens when some of the diesel fuel passes unburned through the engine and into the exhaust system producing the tell tail sign of a black gooey secretion around the exhaust connections and turbocharger. If black smoke continues to plume from the exhaust, even though the generator set is under a constant load, this can also be an indication of wet stacking. The result of this condition is a decrease in engine efficiency and performance which will ultimately result in failure. A standby generator is designed to operate ideally at about 80% of its rated power output and at this rate the diesel engine prime mover should be operating efficiently at about the same level. This should be taken into account at the design stage when selecting the correct generator for your application is critical. However, wet stacking can occur if the engine is run for a prolonged period of time (in excess of one hour) below about 30% of its rated output. If minimal over fueling has already occurred, it can be corrected by running the engine (in excess of 80% full load) for several hours to remove the unburned fuel from the exhaust system. When Generac generators are not being used for emergency back-up power, they are run (exercised) for ten to fifteen minutes each week to keep all the moving parts lubricated and the engine ready to go. We have never heard of this exercise period having any adverse effects on the engine despite the no load condition. This is because the engine is not running long enough to cause any wet stacking (under one hour). For applications where running the diesel set for prolonged periods of light loading is unavoidable, the use of a load bank or the application of an “artificial load”, should be considered.
