Guo Kaifang
(Xiamen Shipbuilding Industry Co., Ltd.)
Abstract: As one of the main equipment of the ship, the main generator has a highly automated management system. However, due to the failure of the equipment itself or misoperation, the failure rate is relatively high. How to troubleshoot promptly and accurately to ensure the safe operation of the ship has become one of the focuses of widespread concern. The article analyzes the troubleshooting of typical faults of the L23/30H type main generator.
The main generator of a ship is the heart of the ship's power system. The normal operation of the main generator is crucial to the safety of the ship when the ship is sailing and docking. Therefore, timely and accurate detection and diagnosis of various faults of the main generator is the key to safe ship operation. The article analyzes and introduces the diagnosis and treatment solutions for several important faults that occur most frequently in the L23/30H type main generator.
1 Main generator cannot start
On-site inspection results: During the vessel's high-power load startup test, main generator No. 2, the first standby unit, failed to start (the alarm monitoring system in the central control room indicated a startup failure for main generator No. 2). Main generator No. 1, the second standby unit, started and ran. At that time, the vessel's power management system in the central control room was in automatic mode. Main generator No. 3 was running with a load of 645 kW. The standby indicator lights for main generators No. 1 and No. 2 were on. Main generator No. 2 was first standby, and main generator No. 1 was second standby.
The main generator's local control panel showed that No. 2 main generator was in remote-control mode. Fuel pressure was 0.8MPa and fuel temperature at the engine inlet was 105°C. Low-temperature water-system pressure was 0.12MPa and high-temperature water-system pressure was 0.12MPa. The lubricating-oil priming pump was running, with a priming pressure of 0.06MPa and a lubricating-oil pressure of 0.025MPa at the main-bearing inlet. Auxiliary-engine air-receiver pressure was 2.8MPa, and pressure downstream of the main generator's starting-air pressure-reducing valve was 0.8MPa.
Open all the power indicator valves and use the cranking lever to check the main generator for cranking. After confirming that there are no other problems with the main generator, start it next to the machine. It is found that the starting resistance of the main generator is very large and the starter motor cannot move. Then the emergency start method is used to start the main generator 2. The main generator 2 can be started. At the same time, everything works normally after the main generator is running.
Fault analysis: field conditions indicated that all conditions required to start the main generator were present. Local starting behavior initially suggested worn or broken air-starter blades. Starting air enters the starter and drives its impeller, causing the starter gear to engage the generator flywheel and crank the generator. If the impeller blades are worn or broken, the impeller cannot produce enough force to start the generator.
Therefore, further analysis confirmed that the blades of the starter motor impeller were worn or broken.
Troubleshooting plan: Remove and disassemble the starting motor for inspection. Troubleshooting result: After removal and disassembly, all impeller blades of the starting motor were found broken at their roots, with a considerable quantity of powdery particles present. Analysis indicated that a foreign object in the piping had struck the starting motor's impeller while it was rotating at high speed, breaking one blade. The broken blade then caused further damage to the other blades. After a new starting motor was installed, main generator No. 2 started normally.
The operation is working normally and the fault is eliminated.
Preventive measures: Before starting the ship's main generator, the control of pipeline cleanliness should be strengthened. Before the main generator is started for the first time, the starting air pipeline should be blown out. At the same time, a rubber hammer should be used to tap the pipeline during the pipeline blowing work to ensure the removal of impurities attached to the pipeline. At the same time, pay attention to the ventilation of the engine room to reduce dust in the air and reduce the possibility of dust entering the starter motor through the air compressor and causing starter motor failure.
2 Black smoke comes out of the chimney exhaust pipe when the main generator is running
On-site inspection results: The main generator was operating at a load of 545kW, with the governor fuel-quantity switch set to maximum fuel delivery. Exhaust temperatures for all cylinders were around 300–310°C, the exhaust manifold temperature was 300°C, and the air temperature after the air cooler was 45°C. Cooling-water outlet temperatures for all cylinders were around 73–76°C. Low-temperature water-system pressure was 0.12MPa, high-temperature water-system pressure was 0.3MPa, lubricating-oil pressure before the filter was 0.4MPa and pressure after filtration was 0.34MPa. Black smoke was coming from the chimney exhaust pipe.
Failure analysis: There are many factors that cause black smoke to emit from the exhaust pipe. For the main generator with normal parameters to emit black smoke from the exhaust pipe, it can be initially judged as follows:
① The filter of the supercharger is too dirty, which increases the flow resistance and reduces the supercharging capacity, causing insufficient air in the combustion chamber and resulting in poor afterburning.
(2) Incorrect valve clearance. When a diesel engine is cold, a specified clearance must remain between the rocker-arm end and valve stem in the mechanical valve gear to allow for thermal expansion during operation. If the clearance is too small, the heated valve may not close fully. Leakage then reduces the air available in the combustion chamber, causing afterburning and black smoke. If the clearance is too large, the valve opens late and closes early, leaving exhaust gas in the combustion chamber and reducing air quality. This leads to poor combustion and afterburning with black smoke.
③ The fuel injection pressure of the fuel injector is too low or the fuel injection hole is blocked and dripping oil, which causes the average diameter of the fuel atomization droplets to be too large and the atomization uniformity is not good, and cannot be completely mixed with the air in the combustion chamber, resulting in a rough combustion process, black smoke, and carbon deposition.
④ When operating under rated load, if the power distribution of each cylinder is uneven, the fuel injected into some cylinders will not be burned and will not perform work, causing the remaining cylinders to be overloaded and black smoke will be emitted.
⑤ If the piston ring assembly gap is too large or the elasticity is not enough, a large amount of lubricating oil will enter the cylinder combustion chamber, and similar situations will occur if the piston ring sticks or breaks.
⑥ The fuel injection start time is too late (that is, the injection advance angle is too small), so that part of the fuel injected into the cylinder is discharged into the exhaust pipe together with the exhaust gas before being burned, and is burned only after being affected by high temperature (afterburning phenomenon). Therefore, not only the exhaust gas appears dark black, but the exhaust temperature at this time will also exceed the rated temperature.
Troubleshooting options:
①Clean or replace the filter of the supercharger.
② Use a feeler gauge to check the size of the air valve clearance and adjust it to the specified requirements.
③ Remove the fuel injection nozzle from the cylinder head, remove all the fuel injection nozzles, and verify the injection pressure on the fuel injection nozzle calibration bench. If the fuel injection nozzle is found to be dripping or the hole is blocked, disassemble the fuel injection nozzle and check whether the needle valve and the needle valve body cone seat are deformed or damaged. They can be reground or replaced with new ones. For cracks, magnetic particle inspection can be used to check. If cracks are found, they must be replaced. For clogged fuel injectors, a special needle can be used to clean the blocked fuel injection hole of the fuel injector.
④ Measure the load of each cylinder. If the load of each cylinder exceeds the allowable deviation, make corresponding adjustments.
⑤ Lift out the piston and check the piston rings. If any do not meet the requirements, replace them with new ones.
⑥ Check the fuel injection timing and adjust it.
Troubleshooting results:
After troubleshooting measures ① and ② were implemented, the main generator was run, but its exhaust pipe still emitted black smoke. Troubleshooting measure ③ was then implemented and the main generator restarted, after which the exhaust color returned to normal. While the main generator was operating, the load was increased from 255 kW (approximately 25% load) to 485 kW (approximately 50%), then to 750 kW (approximately 75%), and finally to 950 kW (approximately 100%). It ran for 30 min at each load level. Measurements showed that peak combustion pressures in all cylinders were essentially consistent at each load level, and the exhaust manifold temperature fell to 280°C. The exhaust color remained normal throughout, confirming that the fault had been eliminated.
Precautions:
The daily piping system of the main generator must pass a qualified inspection. The oil used by the main generator must be separated by an oil separator. The temperature of the fuel used must meet the corresponding requirements. Ensure that the cleanliness, viscosity, and temperature of the oil used by the main generator meet the corresponding requirements. When conditions permit, try to use high-quality fuel or diesel. At the same time, the filter of the supercharger should also be cleaned frequently to keep the air intake smooth.
3 The main generator speed is unstable after starting.
On-site inspection findings: Before the main generator was started, the indicator light on the local start button of the local control box was on, and the lubricating-oil prelubrication pump was running. The prelubrication oil pressure was 0.07 MPa, the prelubrication pressure at the main bearing inlet was 0.025 MPa, and diesel fuel pressure was 0.4 MPa without fluctuations. Low-temperature water system pressure was 0.12 MPa and high-temperature water system pressure was 0.3 MPa. The governor's fuel quantity knob was set to the maximum position of 10. After the main generator started, its speed fluctuated unstably between high and low values.
Failure analysis:
Judging from the parameter indicators, all parameters are normal. Preliminary judgment is that the speed of the main generator fluctuates high and low after starting. There are two possibilities. One is that the fuel pipeline is leaking, causing the fuel pressure to be unstable; the other is that the governor is malfunctioning. However, the on-site diesel pressure is 0.4MPa and there is no fluctuation, which can be basically ruled out. Therefore, it can be confirmed that the governor is faulty.
Troubleshooting options:
Check the oil level of the governor, cleanness of the throttle strip of each cylinder, jamming of the valve stem, etc., check the operation of the transmission and servo motor inside the governor, and check whether the feedback system of the governor has malfunctioned.
Troubleshooting results:
Troubleshooting revealed that the governor oil level was too high, water droplets covered the entire governor top cover, and the shutdown solenoid valve stem was sticking. The corrective procedure was as follows. ① Repair the shutdown solenoid valve stem, drain the old oil from the governor through the drain cock, and refill with governor oil. ② Bring the governor oil level to its normal position. Start the diesel engine at low speed, then unscrew the compensating needle valve several turns to induce severe speed fluctuations and force air out of the oil passages through the oil outlet. Maintain this pronounced hunting for at least 2 min, then slowly close the compensating needle valve until the hunting disappears completely.
Restart the main generator. After the main generator starts, the speed is 720r/min and the speed is stable. There is no more high and low speed phenomenon. The load is increased from 255kW (about 25% load) to 485kW (about 50% load) and then to 750kW (about 75% load) and then to 950kW (about 100% load). Each load section runs for 30 minutes. There is no more high and low speed phenomenon. The fault is eliminated.
Preventive measures: During the monitoring and inspection process, it should be noted that the oil level of the speed regulator must be maintained between the scale lines of the oil level glass meter, and cannot be too high or too low. If the oil level drops too fast, it means there is oil leakage or oil seepage in the speed regulator, which should be found and dealt with immediately. If it is too high, attention should be paid to purging the internal oil passage of the speed regulator. At the same time, pay attention to the sealing of the oil filling port of the speed regulator to prevent the lubricating oil from being contaminated, ensure the cleanliness of the lubricating oil, and pay attention to the oil temperature of the speed regulator not being too high.
Regularly check whether the fuel gauge of each cylinder is clean and add lubricating oil in time.
4 Conclusion
Although the operation and management of modern ship main generators are highly automated, for the safe operation of the equipment, monitoring and inspections are intensified, abnormalities are discovered in a timely manner, and effective measures are taken to eliminate the faults in the initial stage at the first time, providing a strong guarantee for the safe, economical and reliable operation of the ship. The main methods and measures in the process of handling main generator failure can be used for similar troubleshooting.
Source: "China Ship Repair" 2007 Issue 5
(Copyright belongs to the original author or institution)