Main content: This article mainly introduces the process of finding and troubleshooting a sticking exhaust valve fault on a WARTSILA RT-flex50-B main engine, so as to draw inferences from one example for other similar faults.
1. Introduction
In February 2017, I was assigned to work on our company's vessel TX. During the handover with the previous chief engineer, it was mentioned that during the return voyage, the second cylinder exhaust valve of the main engine had repeatedly triggered alarms for closing too fast and too slow. Usually, after stopping and resetting, restarting would make it work normally. The exhaust valve position sensor had been replaced, ruling out a sensor fault. It was suspected that the FC-20 control board for that cylinder was faulty. A control board had just been replaced, and it had been working normally for a period of time. If the fault did not occur again, it would be confirmed as a control board problem.
2. First Encounter with the Fault
In April 2017, while returning to China from the Mediterranean Sea, I received a report from the duty engineer that a second cylinder exhaust valve fault had occurred on the main engine and the main engine had automatically reduced speed. I quickly went down to the engine control room and called up the alarm list. As shown in the two figures below: there were a series of alarms including main engine second cylinder position sensor fault; main engine second cylinder exhaust valve fault; main engine automatic speed reduction warning; main engine remote control fault speed reduction; and main engine second cylinder exhaust gas temperature difference alarm. The main engine control and monitoring screen also displayed a main engine second cylinder exhaust valve fault alarm.
After checking the main engine control system parameters, it was found that the VCU and ICU of cylinder 2 were in automatic lockout state. That cylinder was not injecting fuel, and the exhaust valve was not working either. I immediately requested the bridge to stop the engine for troubleshooting.
3. Troubleshooting
3.1 Initial troubleshooting and handling: Checked the wiring terminals and fixing screws related to the exhaust valve position sensor; no abnormalities were found. Considering that the control board had just been replaced on that cylinder recently, I wondered whether a sudden fault was caused by loose internal terminal posts. I opened the box cover and checked and tightened all terminal posts. Considering the high temperature in the sea area during the return voyage, I wondered whether poor heat dissipation of the control box had caused the control board to freeze. I then opened the control box cover and used a portable fan for forced cooling. After taking the above measures, I powered off and restarted the control board, releasing the lockout. The main engine was restarted for a trial run, but the same fault still occurred during acceleration, so the engine was stopped again for troubleshooting.
3.2 Second round of troubleshooting: The filters related to the VCU were dismantled and inspected, with no abnormalities found. The check valve at the air spring inlet of the exhaust valve was also dismantled and inspected, with no abnormalities found. Since the control board was a spare part that had just been replaced not long ago, the VCU was listed as the primary suspect. To confirm the fault point, the main engine was started. When the fault occurred, the electrician was immediately asked to measure the pulse voltage of the control solenoid valve on the No. 2 cylinder VCU and compare it with other cylinders. It was found that the voltage of the VCU control solenoid valve was normal, and it was immediately confirmed that it was not a control board fault. The fault point should be in the VCU itself or the exhaust valve itself. At that time it was breakfast time, so it was planned to first replace the complete VCU spare part after breakfast. During breakfast, I kept thinking about this problem. Suddenly it flashed through my mind that among the VCU-related parameters on the main engine monitoring screen, the current fed back by the exhaust valve position sensor after shutdown was different from that of other cylinders. Theoretically, after shutdown, all cylinder exhaust valves should be in the closed position under the action of the air spring, and the current fed back by their position sensors should be roughly the same. Could the No. 2 cylinder exhaust valve stem be stuck? To confirm this idea, I immediately went down to the engine room after finishing the meal. The First Engineer was asked to stop the main engine lube oil pump and close the control air to all cylinder air springs. After waiting for a period of time, it was found that the valve heads of the exhaust valves in the other cylinders could fall freely, and the current fed back by the position sensors all changed and were basically consistent. Only the No. 2 cylinder still showed no change in the fed-back current. At this point it was confirmed without doubt that the No. 2 cylinder exhaust valve was stuck.
3.3 Dismantling the exhaust valve to confirm the sticking position: The exhaust valve was dismantled and inspected in place. After lifting off the upper actuator head, it was found that the actuator piston and the air spring piston had no signs of sticking, and it was confirmed that the exhaust valve stem and the guide bushing were stuck. As the schedule was tight at that time, it was decided to free up the valve in place and then reassemble it. A small amount of lubricating oil was injected into the air spring cylinder. By hammering and repeatedly opening and closing the air spring air, after about half an hour of repeated knocking and freeing, the valve stem was basically free with no sticking. After reassembly, the engine was started and the voyage resumed, returning to normal condition.
3.4 Recurrence of the fault: After the fault was handled in April, for a long period of time the cylinder did not have an exhaust valve sticking fault. On the morning of June 8, 2017, while sailing from China to South America, the duty engineer called to report that the main engine had automatically reduced speed and an alarm for the No. 2 cylinder exhaust gas temperature difference appeared. I immediately went down to the engine room to check the relevant parameters and alarms to confirm that the No. 2 cylinder exhaust valve stem was stuck, causing the automatic speed reduction. This time it was decided to replace the exhaust valve of that cylinder and thoroughly investigate the cause of the sticking.
3.5 Dismantling the faulty exhaust valve to find the cause of sticking
From the two photos taken after dismantling, it can be seen that:The root cause of the valve stem sticking is excessive carbon deposits on the matching surface between the lower valve stem and the guide bushing.
3.6 Analysis of the cause of carbon deposits
3.6.1 Lubrication mechanism of the exhaust valve stem: Generally, from a design perspective, the valve stem and guide bush should be designed with appropriate lubrication. The purposes are, first, to reduce wear between the valve stem and guide bush, improve sealing, and extend the service life of the valve stem seal; second, appropriate lubrication helps remove carbon deposits and impurities in the valve stem and guide bush. For the MAN B&W ME-C engine, a dedicated sealing oil pipe is designed to lead to the valve stem and guide bush. The WARTSILA RT-flex50-B engine has a similar measure (not explicitly stated in the manual).
3.6.2 Lubrication mechanism of the WARTSILA RT-flex50-B engine:As shown in Figure 1 above:In the lower space of the air spring cylinder, a small amount of lubricating oil from the air system (from the compressor crankcase) usually accumulates. The oil passes through the gap between the guide bush and the exhaust valve housing (indicated by the red arrow in Figure 1), seeps along the threads of the hexagon socket screw in Figure 2 (installed on the side of the guide bush in Figure 3, with a drilled hole in the middle of the screw), then enters the guide bush through the drilled hole on the guide bush, lubricating the guide bush and the valve stem.
3.6.3 Cause analysis of carbon deposition and seizure:Figure 4 above is the original design lubrication mechanism diagram, as indicated by the red arrows:The lubricating oil seeps along the first thread of the hexagon socket throttle screw into the middle of the screw, and then enters the valve stem surface through the middle drilled hole.The problem is that the lubricant only seeps through the thread contact surface between the screw and the guide bush. How good is its permeability?Is the amount sufficient?I think even the engine manufacturer has no idea either.There may be differences between cylinders; cylinders with poor permeability may cause seizure.I personally have discussed this with the engine manufacturer's after-sales service engineer, who said their design department also realized there was a problem.
4. Preventive and corrective measures
4.1 Method 1:As shown by the screw in Figure 2, file a groove about 1-1.5 mm deep at the first thread to ensure an appropriate amount of air or lubricating oil enters. The appropriate amount of air helps clean dirt on the valve stem and distribute oil. This method is relatively simple. But be careful not to make the groove too large, otherwise it will cause excessive leakage of spring air.
4.2 Method 2:As shown in Figure 5, drill an orifice of about 1-1.5 mm in the middle of the screw. If conditions permit, finally machine an oil distribution groove at the corresponding position on the guide bush; this is the best way. However, it is difficult to achieve with the lathe on board at present (the MAN B&W MEC engine has designed an oil distribution groove at this position).
4.3 Usually pay more attention to checking the real-time indicator diagram and strengthen monitoring of combustion conditions in the cylinder. Regularly replace the fuel injector to maintain good combustion. This helps prevent carbon deposition at the valve stem.
Conclusion: Troubleshooting an electronically controlled injection engine is relatively troublesome. When judging faults, one should have a relatively clear thought process and make full use of the elimination method. At the same time, using existing fault information and monitoring parameters, from simple to complex with flexible approaches, helps to quickly solve the problem.
