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Why diesel generator manufacturers do not recommend you to run the unit at low load for a long time?

July 12, 2022
Diesel generator sets are a critical part of the electric power system, and proper operation and maintenance are essential to ensure long-term system reliability in terms of availability and uptime. While the operation, application and load distribution of power systems vary by use and complexity, all power systems are designed with a common goal in mind: to provide reliable power and maximize system efficiency.
 
Diesel generating sets are a critical part of the electric power system, and proper operation and maintenance are essential to ensure long-term system reliability in terms of availability and uptime. While the operation, application, and load distribution of power systems vary by use and complexity, all power systems are designed with a common goal in mind: to provide reliable power and maximize system efficiency. To achieve these design goals, it is important to understand system operation, load profiles and scenarios, and required maintenance. Generally speaking, diesel generator manufacturers do not recommend that you run diesel generator sets at low loads for extended periods of time, why is this?
 
First, it is important to understand that diesel generator sets are designed to operate, specifically, they are designed to operate with a load. This may seem trivial, but properly loading a genset is critical to availability, healthy engine operation and long engine life.
 
The ideal operating target for each diesel genset will depend on the application and rating. Generally, standby and primary rated diesel gensets are designed to operate between 50% and 85% of full nameplate, while continuous rated diesel gensets are optimized between 70% and 100% load.
 
The manufacturer's service intervals and expected component life are based on operation within these ranges to provide the ideal combination of product performance, power density and long-term operating life. This makes the design phase critical to ensure that the generation system is sized to operate within the manufacturer's recommended load levels while meeting facility requirements. Misusing a generator set by underloading it for extended periods of operation will compromise product health, operation and uptime while increasing the chance of unplanned events and downtime.
 
Running diesel gensets at load levels below 30% of rated output for extended periods of time can negatively impact the unit. The most common consequence is engine exhaust port fluid, also known as exhaust manifold port fluid or wet buildup. Engine exhaust port fluid is a black, oily liquid that may leak from the exhaust manifold joints due to prolonged low or no-load conditions. Running at high idle speed under low load or no load conditions reduces the heat in the cylinders, which allows unburned fuel and oil deposits to leak through the exhaust sliding joints.
 
Visible fluid flow does not necessarily indicate an engine problem, but it does indicate a possible underload, low ambient temperature or low water jacket water temperature problem. In most cases, running engine fluid alone, while unsightly, will not immediately damage the engine. However, running water is a sign of insufficient load and may be a sign of other underload effects. Prolonged light loads can cause deposits to build up behind the piston rings, deposits to form inside the cylinder, and in extreme cases, cylinder liner polishing may occur. These conditions can lead to power loss, poor performance and accelerated component wear, resulting in increased maintenance costs and unplanned downtime or failure.
 
Low Load Management
 
If properly maintained, diesel generator sets can be operated at light loads for extended periods of time without harmful effects. After operating at low load levels, each affected genset should be operated at increased loads to raise cylinder temperatures and pressures to remove deposits from the combustion chamber. In addition, if low-load operation is expected to occur periodically, a more aggressive maintenance program will help ensure that excessive component wear does not occur and the chance of unplanned downtime is minimized.
 
The first major consideration in managing low loads is how to add load to the system if there is not enough load on the building, or if the customer does not want to use critical loads for genset maintenance. This problem can be solved by accessing an installed system load bank or a quick connect system that allows load banks to be easily connected to the electrical system for testing or maintenance. Taking these requirements into account during the design phase can be seamlessly integrated into the system, which is more cost effective than having to modify the site after construction and installation is complete.
 
For diesel gensets, Topper Power recommends loading the genset to at least 30% load every four hours for approximately 30 minutes with light load operation. Exhaust temperature measurements should be taken at the exhaust manifold prior to turbocharging or in the exhaust pipe after turbocharging to confirm compliance with recommended exhaust temperatures during operation.