Preventing wet stacking

Prevention of wet stacking drives many of the details of generator testing requirements.


Figure 4: Unburned liquid fuel mixed with carbon particles result in a gooey mess known as wet stacking. The liquid tends to run along the joints in the engine, which looks like engine seals have sprung a leak. Courtesy: Smith Seckman Reid Inc.

Generally, any mention of wet stacking generates a lot of questions. Wet stacking is a phenomenon that affects diesel engineson the steel side of the unit. Articles like this are read by people with electrical expertiseon the copper side. Prevention of wet stacking drives many of the details of generator testing requirements, so a brief description is presented here.

When a diesel engine runs at light load, its fuel rate is low, and it doesn't generate much heat with each power stroke. The cylinder walls stay relatively cool and don't support complete fuel combustion. The unburned fuel exits the cylinder as vapor and carbon particles. Vapor condenses in the exhaust system,and appears as a thick black liquid at the system exit ports-typically, the engine exhaust outlets and the turbocharger. Historically, the exhaust outlets were called "stacks." When liquid appears at the stack outlets, the engine is said to be "wet stacking."

Figure 4 shows a 1,000 kW diesel generator in this condition. Unburned liquid fuel mixed with carbon particles result in a gooey mess, and drip from the stacks and from the turbocharger. The liquid tends to run along the joints in the engine. The appearance is a bit frightening; it looks like every seal in the engine has sprung a leak. In its early stages, though, wet stacking isn't particularly serious. The cure is to run the engine at reasonable loads for an extended period, using a load bank or available building load, to evaporate the liquid and blow the soot out of the exhaust system.

Extended periods of operation at light load can have serious and expensive consequences. Piston rings don't scrape away the oil film on the cylinder walls, and the remaining oil above the rings is exposed to high combustion temperatures. Under those conditions, the residual oil forms a hard, nonuniform film,disrupting the close mating between the rings and the cylinder wall. Fuel can slip past the rings to contaminate the oil, and the gaps in the film can result in a loss of compression. The piston rings can no longer support the high pressures of the power stroke, and the engine can no longer develop its maximum output power. This phenomenon is called "cylinder wall glazing," and when it's severe, it can't be resolved with a high-load run. Severe glazing requires an engine rebuild, with re-honing of the cylinders or liners, to restore engine performance.

Repetitive operation at light loads results in unwarranted confidence in the generators, in that they continually pass tests but often can't deliver adequate power to their connected loads when the utility fails.

Tom Divine is a senior electrical engineer and project manager at Smith Seckman Reid Inc. He is a frequent contributor to Consulting-Specifying Engineer magazine and Pure Power, and is often a speaker at CSE Webcasts.

Anonymous , 04/08/15 08:46 AM:

My Wife is a School Bus Driver, and she has to watch for a warning light, which her engine Computer turns on when the engine needs a highway run for between 20, and 40 minutes, to clean out the soot builds up from low power use, on city streets. What is available in California to eliminate this ??
Drew , MA, United States, 04/22/15 07:53 AM:

Diesels run best when run hard. Unfortunately, many are "oversized" in the design process to minimize voltage dip for motor starting, or in consideration of the deleterious effects of triplen harmonics, or for future spare capacity, or all the above. (Voltage dip is even more of a concern, with a smaller voltage dip allowed, when feeding sensitive electronic loads such as UPS's, to prevent them cycling on and off the generator, and six-pulse VFD's generate more harmonics than 18-pulse VFD's. Someone can write a separate article on generator sizing, explaining that just because you have 600kW MAXIMUM load on a 1000kW generator does not mean that you have 400kW spare capacity. Engineers have to design for worst case load - all pumps running etc; how often is the generator tested under such conditions?). And sometimes when the generators are exercised, the building load is intentionally not transferred to the generator(this is typically a user-selectable option in the ATS). So it is a good practice to transfer the load to the generator when exercising it, and supplement that load with a load bank to bring it up to 80% or more of the generator's nameplate rating. And the designer can make provisions for adding a load bank, temporary or permanent, of fixed or variable kW, during the design process.
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