In 1999, the technical committee for NFPA 110 adopted a substantial change in the procedure to check for wet stacking conditions present in diesel engines not being loaded sufficiently. The modification involved comparing exhaust gas temperatures with manufacturers’ recommended minimum temperatures. In contrast, the wording in previous editions stated that evidence of wet stacking was indicated “by the presence of continual black smoke during engine-run operation.” Clearly, using exhaust temperature data will yield more objective information with which to determine if wet stacking exists.

NFPA 110 (2019 edition), 8.4.2* Generator sets in service shall be exercised at least once monthly, for a minimum of 30 minutes, using one of the following methods:

  1. Loading that maintains the minimum exhaust gas temperatures as recommended by the manufacturer
  2. Under operating temperature conditions and at not less than 30 percent of the EPS standby nameplate kW rating

The 2019 edition removed the word “Diesel” from the paragraph text. The 2010 edition states “8.4.2* Diesel generator sets in service…”

The alternative method of testing in #1 is of interest to those facilities whose generators are unable to be tested consistently at the minimum 30% of nameplate rating. To obtain the data necessary for this method, however, facilities will need to develop a system to accurately track exhaust temperatures. In some cases, facilities will have to convince manufacturers to cooperate by providing data regarding recommended temperatures in several locations on the piping downstream from the exhaust port. To take accurate exhaust temperature readings there are a few options.

Option A: Thermocouple

On the high-end side of the expense scale is a temperature probe unit, or thermocouple, which is embedded in some portion of the exhaust system or cylinder head and sends data automatically. Once installed, tracking exhaust temperatures is automatic. The drawback to this system is the price: they’re rather expensive to retrofit – upwards of several thousand dollars per engine.

Option B: Infrared Equipment

A less costly, and more practical option involves the use of infrared testing equipment. With this approach, however, the issue is where to obtain the readings. The best place to get readings is at the exhaust manifold-to-cylinder head connection, which is closest to the combustion chamber where the manufacturer takes their readings.

Unfortunately, these connections are not easily accessible on some engines, and attempting to obtain readings here may present a real threat to personal safety and should be avoided. We recommend the safer option of specifying a location on the exhaust manifold where all exhaust ports empty their gases and obtaining temperatures in the same spot. Since your specified location may be some distance from the combustion chambers, the temperatures you obtain will be cooler, and the data must be adjusted for this difference.

Manifold temperature data can provide a very accurate picture of the condition under which an engine is running. Readings that are too low – after adjusting for the temperature differential between the combustion chamber location the manufacturer used for their recommendations and the location the facility chooses to obtain their data – can be a clear indication that wet stacking conditions are present, and appropriate action can be taken to alleviate the problem.

Supplemental Loads

If a facility is unable to make the 30% of nameplate rating and does not have the exhaust temperatures that demonstrate the generator is sufficiently loaded, an annual supplemental “load run” will need to be performed. As stated in NFPA 110,,  installations that do not meet the requirements of 8.4.2 shall be exercised monthly with the available EPSS load and shall be exercised annually with supplemental loads at not less than 50 percent of the EPS nameplate kW rating for 30 continuous minutes and at not less than 75 percent of the EPS nameplate kW rating for 1 continuous hour for a total test duration of not less than 1.5 continuous hours.”

While the monthly test loading of a generator above the 30% level is not required by NFPA 110 beyond the above circumstances, we recommend loading the generator to 75-80% of the nameplate “on occasion” to benchmark exhaust temperature readings starting at a no-load level. The graph below is a record of such Exhaust temperatures.

ATTENTION MEMBERS: Please click here to see maintenance requirements and procedures for the EPS exhaust system.