I. Fault Overview
The diesel engine of a certain vessel is a MAN B&W 6L 60MCE direct current scavenged engine, featuring hydraulically actuated exhaust valves and air springs for closure. Manufactured in Japan, this engine has a rated power output of 15,000 horsepower*** its normal operating temperature of 330°C. This anomaly triggered an alarm for significant deviation in exhaust temperature and resulted in automatic speed reduction. The knocking sound from this cylinder was notably different from the other cylinders. Given that the vessel was sailing through a busy channel, it was not feasible to stop for immediate inspection; thus, the vessel continued at a reduced speed until it could anchor safely outside the strait for repairs.
Upon disassembly, it was discovered that the valve disc and seat exhibited severe wear and had two areas of significant thermal damage. Throughout its operational history, the engine typically operated at loads below 75%, with cylinder liner cooling water outlet temperatures around 85°C and lubricating oil temperatures within 45°C. These operating conditions ruled out the possibility of valve damage due to overload or excessive thermal stress. However, the valve clearly produced an abnormal knocking sound, and there was evidence of oil leakage at the high-pressure oil pipe joint responsible for actuating the valve. By replacing the valve assembly and reconditioning the mating surface of the oil pipe joint to ensure proper sealing, the knocking issue was temporarily resolved. Nevertheless, during the subsequent voyage, similar incidents occurred twice, posing safety concerns and significantly increasing spare parts costs. Therefore, a thorough analysis of the root cause is essential.
II. Analysis of Fault Causes
1. Working Principle of the Exhaust Valve and Its Opening and Closing Mechanism
Figure 1 illustrates a simplified diagram of the exhaust valve structure for the L60MC diesel engine.
Figure 1 Simplified Diagram of the Exhaust Valve Structure of L60MC Diesel Engine
This valve adopts a hydraulic lift and pneumatic close device. It replaces the original relatively complex spring force closing mechanism with a simple compressed air closing mechanism, which not only greatly reduces the maintenance work of the exhaust valve device, but also reduces the knocking sound during its opening and closing. This exhaust valve device has two features: First, a turbine 9 is installed at the lower part of the exhaust valve stem, which rotates the valve stem under the action of exhaust kinetic energy, thereby improving the contact of the valve surface, making the wear of the valve surface uniform and the temperature distribution around the valve surface uniform, and preventing local overheating of the valve head. Second, a valve opening hydraulic cylinder 1 and a hydraulic cylinder piston 2 are installed in the top of the valve stem valve housing, and a valve closing air cylinder 5 and an air cylinder piston 4 are installed in the upper part of the valve stem valve housing. The air cylinder piston is locked on the valve stem through two half conical clamping blocks 3, a pressure plate and a ring groove on the valve stem. Working air from the control air source is reduced to 7 kg/cm and enters the valve closing air cylinder through a check valve and maintains this pressure in the closed state to ensure a tight seal between the valve and the valve seat. When the exhaust valve is opened according to the timing requirement, high-pressure working oil enters the valve opening hydraulic cylinder, acts on the hydraulic cylinder piston, overcomes the force of the working air in the valve closing air cylinder and the exhaust gas in the diesel engine cylinder, and lifts the exhaust valve open. At this time, the working air pressure in the valve closing air cylinder rises to 17 kg/cm (exceeding 20 kg/cm, the safety valve 6 opens). When the exhaust valve is closed according to the timing requirement, the working oil in the valve opening hydraulic cylinder returns, and the air cylinder piston rises under the action of the working air pressure and gradually closes the exhaust valve.
The working oil for opening the valve comes from the exhaust valve drive mechanism, as shown in Figure 2.
Figure 2 Exhaust Valve Drive Mechanism
The lubricating oil from the crosshead bearing lubrication system, after being depressurized, enters the drive mechanism cylinder 4 through the suction check valve 2. The plunger 5 in the cylinder is controlled by the exhaust cam 9, roller 8, push rod 7 and spring 6. When the exhaust valve is required to open, the rising edge of the exhaust cam valve pushes the roller, and through the push rod overcomes the spring force to push the plunger upward. After the plunger moves up to the point where it closes the throttle hole of the adjusting screw 1, it starts to compress the working oil in the cylinder, causing its pressure to rise rapidly (the safety valve 3 opens when the oil pressure exceeds 300 kg/cm). Then, it enters the top of the exhaust valve through the high-pressure oil pipe and pushes the piston of the hydraulic cylinder to open the exhaust valve [2].
2. Factors Affecting the Knocking of the Exhaust Valve
The knocking sound of the exhaust valve usually occurs when the valve is closing. There are generally two situations. One is that the valve disc or valve seat of the exhaust valve is excessively worn, causing knocking due to the small gap at the top of the valve stem when closing. This can be solved by replacing the exhaust valve or valve seat.
The more common situation is caused by severe leakage in the hydraulic system.
Under normal maintenance conditions, this type of exhaust valve device does not produce obvious knocking sounds when closing. There are two reasons for this. First, when closing, the working air pressure in the closing air cylinder gradually decreases, and the closing speed gradually slows down. Second, when closing, the plunger in the driving mechanism cylinder descends along the descending edge of the exhaust valve cam with the roller, and the working oil in the opening hydraulic cylinder is gradually returned due to the suction effect of the plunger, which acts as a buffer similar to an oil cushion.
When the plunger descends to the opening of the adjusting screw hole, it is close to the end of the closing process. Some of the working oil is released through the throttle hole to ensure that the valve and valve seat are tightly closed.
Therefore, severe oil leakage in the hydraulic system, causing a rapid drop in oil pressure during the closing process, or opening the adjusting screw hole too much, will destroy the buffer effect similar to an oil cushion, resulting in a knocking sound when the valve head impacts the valve seat under the action of the closing air pressure.
From this, it can be known that the knocking sound when the exhaust valve closes can be eliminated by adjusting the total oil discharge volume in the hydraulic system. The total oil discharge volume of the hydraulic system includes the release volume of the adjusting screw hole and the leakage volume of the hydraulic system. It is necessary to avoid the two extreme phenomena of the total oil discharge volume being too small and too large.
III. Troubleshooting Methods
After the abnormal knocking sound occurred in the exhaust valve, by touching the high-pressure oil pipe from the exhaust valve drive mechanism to the exhaust valve opening hydraulic cylinder, a strong impact force could be felt, which was much stronger than that of other cylinders, and there was oil leakage at the oil pipe joint. Thus, it could be concluded that the oil pressure was too high when opening the valve. At the same time, based on the sound, it was judged that the knocking sound mainly came from the closing of the valve, and it was initially concluded that it was caused by the valve disc colliding with the valve seat due to too fast seating. The chief engineer first adjusted the adjusting screw in the exhaust valve drive mechanism, gradually closing the valve to reduce the working oil release through the adjusting screw hole until it was completely closed, but the abnormal knocking sound still did not disappear. This indicated that the knocking was not caused by the rapid return of the hydraulic system. Finally, the exhaust valve assembly had to be replaced after the engine was stopped, and the oil leakage was eliminated by grinding the oil pipe joint. After reinstallation, it finally returned to normal. Later, when the removed valve assembly was disassembled, apart from the severe wear and two severe burn marks on the valve disc and valve seat due to the abnormal knocking, no other abnormalities were found. The sealing conditions of the exhaust valve opening hydraulic cylinder and the closing air cylinder were both good. Although the fault was eliminated, the real cause was not found at that time. It was not until the next voyage when the phenomenon occurred again that everyone paid close attention and analyzed it intensively, and finally found the crux of the problem. It turned out that the fault was in the safety valve of the closing air cylinder. When opening the valve, the valve failed to open normally, resulting in excessive back pressure and a sharp increase in the driving oil pressure, causing oil leakage at the joint. When closing the valve, due to the excessive air pressure in the closing air cylinder, the valve closed too quickly, causing a strong impact and abnormal knocking sound, and the valve disc and valve seat were severely worn, which further led to poor sealing, affecting combustion and increasing the exhaust temperature of the cylinder. The previous time, the safety valve was opened under very high pressure due to the elimination of the oil pipe leakage, thus restoring normal operation. After disassembling and inspecting the safety valve, it was found to be very dirty and had some rust. Finally, the valve was replaced, and the phenomenon has not occurred again in subsequent operations, and the fault was completely eliminated.