Most standby generator systems up to five megawatts use reciprocating internal combustion engine as the power source to drive the generator that produces the electrical power. The engines of choice are diesel, natural gas or LPG fueled. A large percentage of standby power systems use diesel engines. Diesel is a convenient, independent fuel source and the compression ignition systems of diesel Tier 4 engines have a much higher thermal efficiency than the spark ignition system used by gas engines. However, one factor to be considered when selecting a diesel power source is the potential for “wet stacking.”
The National Fire Protection Association (NFPA), in the 1996 edition of their NFPA 110 Code for Emergency and Standby Power Systems, refers to wet stacking as a field term indicating the presence of unburned fuel or carbon, or both, in the exhaust system. The later 1999 edition suggests a more quantitative method for determining the presence of wet stacking, by measuring the exhaust gas temperature (which is explained later in this information sheet). This post discusses the causes of wet stacking, its effect on the engine, why it should be avoided and methods for addressing the problem of wet stacking.
The designer of a Tier 4 generator system must take into account the potential for wet stacking when determining equipment for the system, load calculations and maintenance and service programs.
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.”
The effects of wet stacking
When unburned fuel is exhausted from the combustion chamber, it starts to build up in the exhaust side of the engine, resulting in fouled injectors and a buildup of carbon on the exhaust valves, turbo charger and exhaust.
Excessive deposits can result in a loss of engine performance as gasses bypass valve seatings, exhaust buildup produces backpressure, and deposits on the turbo blades reduce turbo efficiency.
Permanent damage will not be incurred over short periods, but over longer periods deposits will scar and erode key engine surfaces.
Also, when engines run below the designed operational temperature, the piston rings do not expand sufficiently to adequately seal the space between the pistons and the cylinder walls. This results in unburned fuel and gasses escaping into the oil pan and diluting the lubricating properties of the oil, leading to premature engine wear.
Why avoiding wet stacking is essential
In addition to the adverse engine effect, the designer and user of a system have to consider:
- Expense- Excessive wet stacking will shorten engine life by many years and before planned replacement.
- Pollution- Many urban areas restrict the level of smoke emissions wet stacking produces.
- Power- Even before an engine is damaged, deposits will reduce maximum power. A prematurely worn engine will have a lower maximum power than it was designed to produce.
- Maintenance- An engine experiencing wet stacking will require considerably more maintenance than an engine that is adequately loaded.
Wet stacking is a recognized condition with organizations that write codes for standby generator set systems, such as the NFPA, which has issued several guidelines for controlling the effects.
The NFPA guidelines in Level 1 and 2 applications require exercising the unit, at least monthly, for 30 minutes under either of two methods: (NFPA 110 8.4.2)
- Loading that maintains the minimum exhaust gas temperatures, as recommended by the manufacturer
- Under operating temperature conditions and at not less than 30 percent of the EPS standby nameplate kW rating
The Joint Commission on Accreditation of Health Care Organizations (JCAHO), the organization that accredits health care institutions, has taken this testing to a level beyond the NFPA. They require testing of 12 times per year with testing intervals between 20-40 days. Testing generators for at least 30 minutes under a dynamic load of 30 percent or greater of the nameplate rating.
Systems that do not meet the 30 percent load capacity have three options:
- Increase the load to meet or exceed 30 percent of the nameplate rating
- Maintain the minimum exhaust temperature as recommended by the engine manufacturer
- Undertake load bank testing for a total of 2 hours continuous loading as follows:
- Load at 25 percent of nameplate for 30 minutes
- 50 percent for 30 minutes
- 75 percent for 60 minutes.
JCAHO also recommends that all automatic transfer switches (ATS) are tested 12 times per year at 20 and 40-day intervals. The provider of the power system through planned maintenance programs can undertake load testing when testing the ATS.
Solutions to the problem of wet stacking
The most straightforward solution is to always run the generator set with an electrical load that reaches the designed operational temperature of the diesel, or approximately 75 percent 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’s 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. During a regularly scheduled planned maintenance call, the distributor will bring in a portable load bank to run the generator at a load that maintains the designed operational temperature. Portable load banks range from a few kWs to 3 MW units mounted on large trailers.
Avoiding wet stacking is only one aspect of diesel generator maintenance. For more generator maintenance tips you can download our Keys to Running Your Generator Efficiently guide below. For immediate generator maintenance advice, reach out to our parts & services department directly or the leave a message in the chat box on the bottom right.