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Mar 23

What Is Wet Stacking?

By: Patti Dinneen


Most standby generator systems (up to 5 MW) use a reciprocating internal combustion engine as a primary power source to drive the generator that produces the electrical power. Typically, generator engines are either fueled by diesel, natural gas or LPG. In our experiences, a large portion of standby power systems use a diesel engine. The compression ignition systems of diesel engines have a much higher thermal efficiency than the spark ignition system used by gas engines, which is a reason for their popularity.  However, with diesel power sources, you need to be aware of the potential for wet stacking. This blog post discusses the causes of wet stacking, its effect on the engine, why it should be avoided and methods for eliminating the condition.

What Is Wet Stacking?

Wet Stacking occurs when a diesel engine has to operate below the output level. When this happens, the engine starts to over-fuel or “wet stack.” Diesel engines, typically used in generators, are created with the idea that they will be operating with a load in the 70-80% range of rated output. So, when an engine operates for a long period of time below 40% rated output, it will began to over fuel. Specifically, this happens because the injection tips begin to carbonize and disrupt the fuel spray pattern.

What Causes Wet Stacking?

Like all internal combustion engines, to operate at maximum efficiency a diesel engine has to have exactly the right air-to-fuel ratio and be able to sustain its designed operational temperature for a complete burn of fuel. When a diesel engine is operated on light loads, it will not attain its correct operating temperature. When the diesel engine runs below its designed operating temperature for extended periods, unburned fuel is exhausted and noticed as wetness in the exhaust system, hence the phrase wet stacking.

How Are Wet Stacking Issues Addressed?

The obvious solution is to always run the generator set with an electrical load that reaches the designed operational temperature of the diesel, or approximately 75% of full load. Built-up fuel deposits and carbon can be removed by running the diesel engine at the required operational temperature for several hours if wet stacking has not yet reached the level where carbon buildup can only be removed by a major engine overhaul.

The following load bank solutions should prevent a reoccurrence of wet stacking:

Automatic Auxiliary Loading: This solution is usually used only when the diesel generator set is the primary source of power. The “auxiliary load bank” will be switched into the system when only the lighter loads are present and switched out when the larger load is connected.

Facility Manual Load Bank: Operated as described for the automatic load bank, but a manually operated system for use with light loads and when the larger load is also manually initiated. The load bank can also be used for load testing a system primarily used for standby power.

Portable Load Bank:  The distributor for the diesel generator set is often the best qualified to undertake the maintenance of the system. Today it is very common for the owner of a standby generator system to outsource complete maintenance of the system and have a planned maintenance (PM) contract with a full service generator-set supplier.

Kinsley Power Systems

Our goal is to ensure that your system performs when you need it. Kinsley Power offers customized preventive maintenance programs and extended warranties for all customers - hospitals and cable companies and factories and homeowners alike. Kinsley services all major make and model generators, transfer switches, switchgear and PTOs. Kinsley also offers such specialized services as load bank and infrared testing.

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