The Hazards of Prolonged Low-Load Operation in Diesel Generator Sets

The Hazards of Prolonged Low-Load Operation in Diesel Generator Sets

There is a common misconception among diesel generator users that the smaller the load, the better it is for the generator. However, this is a significant misunderstanding, as long-term operation at low loads can be detrimental to the generator set. The following are some of the hazards associated with low-load operation:

1. Poor Sealing of Pistons and Cylinder Liners: When the load is too low, the pistons and cylinder liners may not seal properly, causing engine oil to leak into the combustion chamber, where it burns and produces blue smoke, polluting the air.

2. Reduced Turbocharger Pressure: In turbocharged diesel engines, low or no load reduces the turbocharger pressure, which can lead to a decline in the sealing efficiency of the non-contact oil seals in the turbocharger. This may cause engine oil to leak into the turbocharger chamber and subsequently enter the cylinders with the intake air, shortening the lifespan of the generator.

3. Increased Carbon Deposits: At low loads, some of the oil that leaks into the cylinders may burn, while some may not burn completely, leading to carbon deposits on the valves, intake ports, piston tops, and piston rings. Some oil may also be expelled with the exhaust, causing oil accumulation and carbon deposits in the cylinder liners and exhaust passages, thereby reducing the generator's power output.

4. Oil Leakage from Turbocharger: If the generator operates at a low load for an extended period, the oil may accumulate in the turbocharger chamber, eventually leaking from the turbocharger's joints.

5. Accelerated Wear of Moving Parts: Prolonged low-load operation can exacerbate the wear and tear on moving parts, deteriorate the engine's combustion environment, and lead to an earlier need for an overhaul. Without the ability to adjust the fuel system, insufficient load leads to insufficient power demand. However, the combustion system continues to supply fuel normally, resulting in incomplete combustion due to the excess fuel. This increases the carbon content in the system, which can affect the system's efficiency and functionality, potentially leading to equipment and valve failures. Many customers report oil leaks in their generator sets, which are often caused by long-term low-load operation.

Why Should Diesel Generators Not Run Unloaded for Long Periods?

Operating a diesel generator below 50% of its rated power increases oil consumption, causes carbon buildup in the engine, raises the fault rate, and shortens the overhaul cycle. Generally, the no-load operation of a diesel generator should not exceed 15 minutes. Typically, a 3-minute warm-up, followed by increasing the speed to the rated RPM, and then 5 minutes of stabilized voltage before load application is recommended. The generator set should operate at a load of at least 30% to ensure the engine reaches the necessary operating temperature, allowing optimal clearance between components, preventing oil burning, reducing carbon deposits, avoiding early cylinder liner wear, and extending the engine's lifespan.

Upon successful start-up, the diesel generator should have a no-load voltage of 400V, a frequency of 50Hz, and balanced three-phase voltage without significant deviation. If the voltage deviates too much from 400V, or the frequency falls below 47Hz or rises above 52Hz, the diesel generator should be inspected and repaired before load operation. The coolant in the radiator should be fully saturated, and the temperature of the coolant should be above 60°C before engaging the load. The load should be gradually increased from a small load in a controlled manner.

The health and reliability of each piece of equipment are crucial to your business operations. This is especially true for your diesel generator—it is essential to keep it in optimal condition. The generator is a critical component of your power system, ensuring that your business continues to operate smoothly. However, a malfunctioning diesel generator can have a significant negative impact on your business. You may need to consider restarting your diesel generator to assess its condition. If necessary, repairs or replacements should be made.

Causes of Diesel Generator Failure

One of the main causes of diesel generator failure is operating in a low-load mode. What is low load? Manufacturers typically specify the minimum allowable load for a diesel generator (as a percentage of its rated capacity) to guide operators on the recommended load. This does not prevent the generator from being shut down; it only prevents it from operating at too low a load. For example, if the minimum load is set at 40%, and the required power for the diesel generator is 50% of its capacity, it will operate at 50% efficiency. Conversely, if the required power is 30%, it will still operate at 40% efficiency. The excess power may be used to charge the battery, serve as a delayed load, or simply be discarded. In such cases, any value below 40% is considered a low load. Operating at a low load not only reduces the generator's performance but, in the worst case, can cause it to fail completely.

Impact of Low Load

There are several impacts of operating a diesel generator at a low load, including:

1. Excessive Wear of Moving Parts

2. Low Fuel Temperature and Increased Percentage of Unburned Fuel in Oil: This unburned fuel passes through the crankcase, reducing the performance and quality of the lubricating oil.

3. Insufficient Cylinder Pressure: This can cause engine oil to enter the combustion chamber and form ash deposits. These deposits can affect the compression ratio and reduce detonation resistance, leading to component damage, failure, or unexpected shutdown.

4. Loss of Power and Poor Equipment Performance: This leads to unplanned downtime, increased failure rates, and higher maintenance costs.

Preventive Measures

To avoid uninterrupted low-load operation of your diesel generator, you should either minimize usage during low-load periods or limit it to the shortest possible time. It is recommended that the duration of low-load operation for a diesel generator set does not exceed 15 minutes. After that, you should quickly increase the load to raise pressure and temperature, helping to avoid some of the aforementioned issues. You can always contact your manufacturer for advice on safe low-load operation durations and values.

On the other hand, it is important to note that an annual "load" test should be performed. The diesel generator should run at full load for several hours to thoroughly clean the engine and remove carbonized oil deposits from the exhaust system and engine.