Wet Stacking: The Effects & Prevention

Wet Stacking: The Effects & Prevention

Load bank testing your generator not only verifies system capability and performance, it also removes the excess, unburned fuel created by wet stacking.  Wet stacking is a condition that affects diesel engines and is caused by unburned fuel passing into the exhaust system.

When diesel engines or generators are run under lightly loaded conditions, it can result in low combustion temperatures. This leads to inefficient combustion and an incomplete burn of the available fuel. The unburned fuel can be identified by the presence of a black, oily substance, which comes from the exhaust manifold(s), exhaust piping, and turbo charger(s).

Effects of Wet Stacking

  • Loss of engine performance
  • Fouled injectors
  • Carbon buildup
  • Premature engine wear or failure
man working on open cat panel

How to Prevent Wet Stacking

The effects of wet stacking can be rectified by running the generator set at or above 75% of the generator’s nameplate rating. This allows for increased exhaust temperatures which vaporize the unburned fuel in the exhaust system. Annual load bank testing can alleviate heavy carbon buildup that can occur is wet stacking goes unaddressed.  Unaddressed wet stacking can result in component damage that can ultimately lead to component repairs, replacements or in severe cases, a major engine overhaul.

NFPA Guidelines

The NFPA also has guidelines to reduce the effects of wet stacking on critical power systems. In Level 1 and 2 applications, it is required to exercise the unit at least monthly, for 30 minutes under either of two methods:

  • Loading that maintains the minimum exhaust gas temperatures, as recommended by the OEM
  • Under operating temperature conditions and at not less than 30% of the EPS standby nameplate kW rating

For further information on NFPA guidelines, click here.

The Joint Commission on Accreditation of Health Care Organizations (JCAHO) accredits health care institutions and provides feedback on their requirements for testing generators. You can learn more here.

Contact us at info@LHCPS.com or 844.LHPOWER to learn more about our preventative maintenance program. For further information about LionHeart and the additional services we provide, click here.


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