Underloading Diesel and Gas Generator Sets

System health and reliability are critical to backup and prime power solutions for any installation—from mission critical data centers to neighborhood grocery stores. While power systems vary in operation, application and load profile, they’re all designed to provide reliable power and maximized efficiency. The generator set is a key piece of the power system that requires special attention for long-term system reliability, availability and uptime. To achieve these, it’s important to understand system operation, load profile and required maintenance. This blog will focus on generator set operation in low-load scenarios, and what can result if they are used outside of specific parameters.

First, it is important to understand that generator sets are designed to run with load. This may seem trivial, but loading a generator set properly is essential to availability and a long engine life. Manufacturer service intervals and projected component life are based on operation in load ranges to deliver an ideal mix of product performance, power density and long-term operational life.

Incorrect generator set operation can result in reduced output, component damage, reduced lifecycles, and unscheduled downtime. The ideal operation targets of each generator set will depend on the application and rating.

Underloading Diesel Generator Sets

Generally speaking, standby- and prime-rated diesel generator sets are designed to operate between 50 and 85 percent load, while continuous-rated diesel generator sets optimize between 70 and 100 percent load. Operating diesel generator sets at loads less than 30 percent for extended periods can impact uptime and engine life.

The most prevalent consequence of underloading is exhaust manifold slobber, or wet stacking, which is the black oily liquid that can leak from the exhaust joints when the engine does not reach minimum temperatures and pressures. Visible engine slobber doesn’t necessarily indicate a problem, but it signals possible underloading concerns, low ambient temperatures or jacket water temperatures that are too low. Additionally, long periods of light loading can lead to deposit build-up behind the piston rings or inside the cylinders, which can cause power loss, poor performance, accelerated wear and in extreme cases, cylinder liner polishing.

Underloading Gas Generator Sets

Natural gas and biogas generator sets, independent of application and rating, are typically designed for operation between 60 and 100 percent load. Without enough cylinder pressure to maintain oil control at low loads, gas engines can develop ash deposits, a reduced detonation margin and damaged engine components. Similar to diesel generator sets, deposit build-up on valves, spark plugs and behind piston rings can occur—which may cause cylinder liner polishing, power loss, poor performance and accelerated component wear—ultimately increasing the likelihood of higher maintenance costs and downtime.

Managing Low Load

If maintained properly, diesel and gas generator sets can operate at light loads for long periods without harmful effects. After underloading, the generator sets should run at increased load to raise the cylinder pressure and temperature, which will clean the deposits from the combustion chamber. Regular low load operation requires a more aggressive maintenance plan to reduce excessive component wear and may require engine modification.

Diesel engines should run at a minimum of 30 percent load for about 30 minutes for every four hours of light-load operation, and operators should measure exhaust temperature prior to the turbo for analysis. Natural gas engines are more sensitive to low loads, so there are specific underloading time limits required to maintain optimal performance, see the chart above. After the time limit has expired, the engine should run at a minimum of 70 percent load for at least two hours.

Power system underloading impacts individual components as well as overall performance, so it should not be taken lightly. While the simple solution is to operate generator sets at a load that meets design requirements, the reality is that system needs sometimes change. This makes underloading common in the realm of power generation—especially in standby applications. However, the effects of underloading can be minimized with a thorough operation and maintenance plan to preserve system health and avoid extra costs down the road.