Main content: This article mainly introduces the troubleshooting and handling process of a seized exhaust valve fault on a WARTSILA RT-flex50-B main engine, so as to draw inferences for other similar faults.
1. Preface
In February 2017, I was assigned to work on our company's TX vessel. During the handover with the previous chief engineer, it was mentioned that on the return voyage, the main engine had repeatedly experienced alarms of the No. 2 cylinder exhaust valve closing too fast and too slow. Usually, after stopping and resetting, it would be fine upon restarting. The exhaust valve position sensor had been replaced, eliminating position sensor failure. The cylinder's FC-20 control board was suspected, and a control board had just been replaced; it had been normal for some time. If the fault does not recur, the control board is confirmed as the problem.
2. First encounter with the fault
In April 2017, on the return voyage from the Mediterranean to China, the duty engineer reported that the main engine had a No. 2 cylinder exhaust valve fault and had automatically slowed down. I quickly went down to the engine control room and called up the alarm list. As shown in the two figures below: a series of alarms appeared, including main engine No. 2 cylinder position sensor fault; main engine No. 2 cylinder exhaust valve fault; main engine automatic slow-down pre-warning; main engine remote control fault slow-down; main engine No. 2 cylinder exhaust temperature deviation alarm. The main engine control monitoring screen also showed a main engine No. 2 cylinder exhaust valve fault alarm.
Checking the main engine control system parameters, it was found that the No. 2 cylinder VCU and ICU were in automatic lockout state; the cylinder was no longer injecting fuel, and the exhaust valve was not working. I then requested the bridge to stop the engine for troubleshooting.
3. Troubleshooting
3.1 Initial troubleshooting: Checked the exhaust valve position sensor related terminals and fixing screws, no abnormalities found; recalling that the cylinder's control board had been replaced not long before, wondering whether a sudden fault was caused by loose internal terminals, opened the box cover and checked and tightened all terminals; considering the high temperature in the navigation area during the return voyage, whether poor heat dissipation of the control box caused the control board to freeze. Immediately opened the control box cover and used a portable fan for forced cooling. After taking the above measures, powered off and restarted the control board, and released the lockout. Restarted the main engine for trial operation, but the same fault occurred again during acceleration, so the engine was stopped again for troubleshooting.
3.2 Second troubleshooting: Dismantled and inspected the VCU related filters, no abnormalities found; dismantled and inspected the exhaust valve air spring inlet check valve, no abnormalities found; considering that the control board was a recently replaced spare part, the VCU was listed as a key suspect. To confirm the fault point, started the main engine, and as soon as the fault occurred, immediately had the electrician 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 VCU control solenoid valve voltage was normal, immediately confirming that it was not a control board fault. The fault point should be in the VCU itself or the exhaust valve itself. It was then breakfast time; after breakfast, the plan was to first try replacing the entire VCU with a spare. 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 feedback from the exhaust valve position sensor after stopping was different from other cylinders. Theoretically, after stopping, all cylinder exhaust valves should be in the closed position under the action of the air spring, and the current feedback from their position sensors should be roughly the same. Could the No. 2 cylinder exhaust valve stem be seized? To confirm this idea, after the meal, I immediately went down to the engine room. Had the second engineer stop the main engine lubricating oil pump and shut off all cylinder air spring control air. After waiting for a while, it was found that the exhaust valve heads of other cylinders could fall freely, and the current feedback from the position sensors all changed and were basically consistent. Only the No. 2 cylinder feedback current remained unchanged, confirming that the No. 2 cylinder exhaust valve was indeed seized.
3.3 Dismantling the exhaust valve to confirm the seizure position: Dismantled and inspected the exhaust valve on site, lifted off the upper driving head and found that the driving head piston and air spring piston had no seizure; confirmed that the exhaust valve stem and guide sleeve were seized. Due to the tight schedule at that time, it was decided to take measures to free it on site and reassemble. Injected a small amount of lubricating oil into the air spring cylinder, used hammering and repeated opening and closing of the air spring air measures; after about half an hour of repeated hammering and freeing, the valve stem was basically free without seizure. After reassembly, started the engine and set sail, and it returned to normal.
3.4 Recurrence of the fault: For a long time after the fault was handled in April, the cylinder did not experience exhaust valve seizure. On the morning of June 8, 2017, on the voyage from China to South America, the duty engineer reported by phone that the main engine had automatically slowed down, and at the same time a main engine No. 2 cylinder exhaust temperature deviation alarm appeared. I immediately went down to the engine room to check relevant parameters and confirm the alarm: the No. 2 cylinder exhaust valve stem was seized, causing the main engine to automatically slow down. This time it was decided to replace the cylinder's exhaust valve and thoroughly investigate the cause of seizure.
3.5 Dismantling the faulty exhaust valve to find the cause of seizure
From the above two pictures after dismantling, it can be seen that: The root cause of the valve stem seizure is excessive carbon deposits on the mating surface of the lower valve stem and guide sleeve.
3.6 Analysis of carbon deposit causes
3.6.1 Exhaust valve stem lubrication mechanism: Generally, from a design perspective, the valve stem and guide sleeve should be designed with appropriate lubrication. Its purpose is firstly to reduce wear between the valve stem and guide sleeve, promote sealing, and extend the service life of the valve stem sealing ring. Secondly, appropriate lubrication helps remove carbon deposits and impurities in the valve stem and guide sleeve. MAN B&W ME-C main engines specially design a sealing oil pipe leading to the valve stem and guide sleeve. WARTSILA RT-flex50-B main engines have similar measures (not explicitly stated in the manual).
3.6.2 Lubrication mechanism of WARTSILA RT-flex50-B main 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 air compressor crankcase) usually accumulates. The lubricating oil passes through the gap between the guide sleeve and the exhaust valve housing (indicated by the red arrow in Figure 1), penetrates along the threads of the hex socket screw in Figure 2 (which is installed on the side of the guide sleeve in Figure 3, with a drilled hole in the middle of the screw), and then enters the guide sleeve along the drilled hole in the guide sleeve, lubricating the guide sleeve and valve stem.
3.6.3 Analysis of carbon deposit seizure causes: Figure 4 above shows the original design lubrication mechanism, as indicated by the red arrow in the figure: Lubricating oil penetrates into the middle of the screw along the first thread of the hex socket throttle screw, and then enters the valve stem surface along the middle drilled hole. The problem is that the lubricating fluid only penetrates through the thread contact surface of the screw and guide sleeve. How is the penetration capability? Is the amount sufficient? I think the engine manufacturer also has no idea. There may be differences among cylinders; cylinders with poor penetration capability may cause seizure. I once communicated this matter with the main engine manufacturer's after-sales service engineer, who said their design department was also aware of the problem.
4. Preventive and corrective measures
4.1 Method one: 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. An appropriate amount of air entering helps clean debris on the valve stem and distribute oil. This method is relatively simple. But note that the groove must not be too large, otherwise it will cause excessive leakage of spring air.
4.2 Method two: As shown in Figure 5, drill a throttle hole of about 1-1.5 mm in the middle of the screw. If conditions permit, finally machining an oil distribution groove at the corresponding position on the guide sleeve is the best way. However, it is difficult to achieve with the lathe on board at present (MAN B&W MEC main engines are designed with an oil distribution groove here).
4.3 Pay more attention to checking the real-time indicator diagram and strengthen monitoring of combustion conditions in the cylinder. Replace fuel injectors regularly to maintain good combustion. This helps prevent carbon deposits at the valve stem.
Concluding remarks: Troubleshooting electronic fuel injection main engines is relatively troublesome. When judging faults, one should have a relatively clear train of thought and make full use of the elimination method. At the same time, using existing fault information and monitoring parameters, from simple to complex, and being flexible and adaptable, helps to quickly solve problems.
