The fuel supply system is the most important component of a diesel engine. Its working condition directly affects the performance of the diesel engine, such as power size, fuel consumption, smoke emission, and ease of starting. Therefore, it is of great significance to promptly inspect, diagnose and eliminate faults in the fuel system by means of non-disassembly inspection, nip faults in the bud and keep the diesel engine in good technical condition all the time.
The function of the fuel system is to provide the diesel engine with clean fuel of appropriate viscosity. Heavy oil contains a large amount of impurities, so the heavy oil system must be equipped with a fuel cleaning treatment device to remove the impurities in the heavy oil. Heavy oil has a high viscosity. To facilitate the pumping of fuel and make the viscosity of the fuel meet the usage requirements of diesel engines, it is necessary to heat the heavy oil to reduce its viscosity. To prevent the high-temperature heavy oil from vaporizing, it is necessary to pressurize the heavy oil system. Simply put, the function of the heavy oil system is to clean, heat and pressurize the heavy oil to ensure that it meets the usage requirements of the diesel engine.
Heavy oil purification
Heavy oil is always contaminated to varying degrees during the supply process. Before use, it should be purified to remove all kinds of solid and liquid impurities it contains. The solid impurities in heavy oil mainly include rust, sand, dust and catalyst particles added during the refining process. The main liquid impurities are water, including seawater and fresh water. The presence of these impurities will cause damage to the high-pressure oil pump and injector, accelerate the wear of the cylinder liner, the wear of the exhaust valve sealing surface, and the accumulation of dirt on the turbocharger blades and air channels.
For a certain main engine model: MAN B&W 6L70MCE, a major accident occurred due to fuel problems, resulting in a significant loss of human and financial resources.
Details of the accident:
On January 17, of a certain year, the SHH vessel loaded 1,450 tons of IF380cSt fuel into the No. 5 and No. 7 fuel tanks respectively in Singapore. On January 23 of the same year, during the voyage from Hong Kong to the United States, as requested by the chief mate, the engine room simultaneously used fuel from the left and right fuel tanks of No. 5. That afternoon, a large amount of water was found in the heavy oil sedimentation tank (it was confirmed to be seawater on the 25th), but the cause of the seawater entering could not be identified.
When using the No. 5 right fuel tank alone, the seawater content in the fuel keeps increasing, and the fuel level in the No. 5 right fuel tank shows an upward trend. Even after switching to the No. 5 left tank, there is still seawater. Starting from February 7th, when using the No. 7 right tank fuel, there is still a large amount of seawater in the sedimentation tank. The chief engineer suspected that several fuel tanks contained seawater, so he switched back to using the fuel from the right tank of No. 5. After the ballast water tank of No. 6 was emptied, the situation of seawater in the fuel improved, thus confirming that seawater had entered the right fuel tank of No. 5. During this period, the operating conditions of the main unit began to deteriorate from January 29th, with the exhaust temperature rising, reaching as high as 373℃ in some cases. On February 5th, the exhaust temperature of the main unit reached as high as 400℃. Most of the flue pipes of the exhaust gas boiler were clogged with dirt, and the grid of the turbocharger was also clogged with dirt. Moreover, the situation was deteriorating further. According to the company's instructions, the machine was stopped to clear the exhaust gas boiler and the exhaust pipe to the grid of the booster, but the effect was not significant. The main unit can only maintain operation at 72 revolutions per minute.
On the morning of February 23rd, the exhaust temperature of the 4th and 5th cylinders of the main engine rose again, and the cooling temperature of the cylinders and pistons increased. After parking and inspection, it was found that two piston commands of each of the two cylinders were broken. The cylinders were immediately lifted for emergency repair. After replacing the commands, the navigation continued. On February 27th, due to the continuous clogging of the inlet filter of the main engine fuel delivery pump, the chief engineer decided to remove the filter element of the inlet filter of the fuel delivery pump.
On February 29th, the vessel arrived at a port in the United States to load 468 tons of IF380 cSt fuel. The inlet filter of the fuel delivery pump has never been reinstalled since it was dispatched from the United States. Since March 5th, the fuel installed in the United States has been used, but the working condition of the main engine has not improved yet
On March 18th, the exhaust temperature of the first cylinder of the main unit was too high. The order was to stop the machine, lift the cylinder and change it. On March 26th, during the disassembly inspection of the fuel separator, it was found that the cylinder was clogged with dirt and could not be discharged. On March 28th, the cylinder liner of the 4th cylinder of the main engine cracked, and the machine stopped again to replace the cylinder liner. On April 6th, the cylinder liner of the 2nd cylinder of the main engine cracked. On April 11th, the water jacket of the 2nd cylinder of the main engine cracked. On April 12th, the cylinder liner and water jacket of the second cylinder were replaced in Singapore as a temporary measure
Repair: After changing the hoist command for the third cylinder, the main engine gradually resumed normal operation.
Analyze the cause of the accident
1.There is a 40-centimeter-long crack in the partition between the No.5 right oil tank of this vessel and the No.6 ballast tank above it. The seawater in ballast tank No.6 leaked into the right oil tank No.5 from here. When No.5 left fuel tank and No.5 right fuel tank are used simultaneously, due to the mutual collusion between the two compartments, No.5 left is also mixed with seawater.
2. The Marine engineers failed to fully recognize that fuel containing seawater would deteriorate the combustion conditions. They merely settled and drained the residual water, which could not eliminate the hazards of seawater at all.
3. Two fuel separators failed to drain for two months, resulting in severe dirtiness of the fuel and frequent clogging of the filters of the transfer pumps. In response to this, the chief engineer did not address the issue of the oil separator not being able to drain water. Instead, he removed the filter element of the inlet filter of the fuel delivery pump, causing the main engine to run unclean fuel containing seawater for a long time, which led to serious consequences.
Summarize the lessons learned from the accident
During the voyage from January 23rd to April 11th, due to seawater entering the fuel tank and improper handling, the main engine's operating conditions deteriorated (the cylinder was lifted 7 times, damaging 28 piston rings, two cylinder- liners, and one cylinder head water jacket). Huge losses and profound lessons:
1. Enhance safety awareness, strengthen business and technical training for crew members, improve their comprehensive ability to scientifically analyze faults and handle accidents, and intensify theoretical learning for crew members. When it is determined that there is a large amount of seawater in the right fuel tank of No. 5, its use should be immediately stopped and the cause identified. However, the vessel blindly continued to use it, resulting in serious adverse consequences.
2. Enhance a sense of responsibility and eliminate any violation of operating procedures. If the inlet filter of this round of fuel transfer pump is frequently clogged, the fault of abnormal sewage discharge of the oil separator should be found and eliminated. Then, measures such as operating two oil separators in series and increasing the sewage discharge frequency should be adopted to improve the separation effect of the oil separator. The chief engineer's act of removing the filter element from the inlet filter of the fuel delivery pump is extremely despicable!
Avoid overloading the diesel engine as much as possible
Generally speaking, the thermal load of the main heat components of a diesel engine that burns heavy oil is much higher than that of a diesel engine that burns diesel. That is to say, its operating conditions are harsher than those of a diesel engine that burns diesel. Under such circumstances, to ensure the normal service life of components and the intervals between major overhauls, it is necessary to maintain economic load operation as much as possible and avoid overloading the diesel engine.
Partial load operation
Under normal circumstances, when the load exceeds 20% of the rated power, diesel engines can operate without limit. When the load is lower than 20% of the rated power, due to the low temperature in the combustion chamber, incomplete combustion may occur, and the pollution of the intake and exhaust channels, combustion chamber and turbocharger will intensify. The performance of the diesel engine will deteriorate over time. Therefore, when the load is lower than 20%, operation must be restricted in accordance with the requirements of the manual.
Fuel supply system
The public fuel supply system is a pressurized system, consisting of an HFO supply pump, an HFO circulation pump, a heater and viscosity control equipment.
The fuel is introduced from the daily-use fuel tank into an electric supply pump and pressurized to approximately 4bar before entering the low-pressure side of the fuel system. This prevents the fuel in the vent tank from vaporizing within the heated temperature range.
The fuel is introduced into an electric circulation pump from the low-pressure side of the fuel system, which enables the fuel to pass through the preheater pump and enter the diesel engine.
The vent box is connected to the daily-use oil tank through an automatic vent valve, which will release any existing gas.
To ensure that the high-pressure fuel pump is fully filled with fuel, the capacity of the electric fuel circulation pump is more than three times the fuel consumption of the diesel engine at 100% load. The excess fuel flowing out of the diesel engine is recirculated through the vent box.
To ensure that the fuel pressure entering the high-pressure oil pump of the diesel engine remains constant under various loads, a spring load relief valve must be installed in the fuel system.
The pressure of the circulating pump is approximately 10bar, which ensures that the fuel system will not vaporize or cavitation even at 150℃. The circulating pump will always remain in operation, even if the propulsion main unit and one or several generators set all stop running. This is to ensure that the heated heavy oil circulates through the fuel system inside the engine, thereby guaranteeing that the diesel engine has a heated high-pressure oil pump and a deflated fuel injector and can be started at any time.
This article briefly introduces the function, principle and composition of the diesel engine fuel system from both internal and external system aspects. Since the introduction is based on individual models as examples, it is not comprehensive. There are significant differences among various fuel systems due to the different models, the types of fuel used, and the specific circumstances. Therefore, they should be treated differently based on the actual situation.