The economic operation of modern large diesel engines relies on exhaust gas turbochargers, which can increase the power of diesel engines by about 30%. In order to meet the demand for turbochargers in high-power diesel engines, MAN company has utilized the latest development achievements and manufacturing technology, relying on its experience in independently manufacturing diesel engines and turbochargers, to develop the TCA series of turbochargers that are suitable for diesel engines with power ranging from 5400 to 30000 kW.
A rescue ship is equipped with two MAN6L48/60CR electronic fuel injection diesel engines, with a power of 7200kW × 2, and each main engine is equipped with one TCA55-42W turbocharger. The maximum speed of the turbocharger rotor during short-term operation is 19800r/min, and the maximum speed of the rotor during continuous operation is 19400r/min. The maximum boost pressure can reach 4 bar, and the maximum allowable intake temperature of the exhaust turbine is 600 ℃.
One day while the ship was sailing normally, the right main engine monitoring computer on the engine room control panel suddenly displayed an alarm of "turbine disc cooling air pressure low" and triggered a request to reduce the load on the right main engine. The duty engineer immediately notified the bridge of this situation, and the right main engine must be reduced to below 50% load for operation. After receiving the notification, the driver immediately took a load reduction operation on the right main engine. At this moment, the engineer presses the mute alarm reset button on the right main monitoring computer of the control panel, and the low air cooling pressure alarm of the turbocharger turbine disk disappears.
Subsequently, the instruction manual for the main engine turbocharger was consulted. The manual stipulated that the maximum load of the main engine turbocharger turbine disk cannot exceed 70% without any boost air cooling (the manufacturer sets the cooling pressure of the turbocharger turbine disk corresponding to the turbocharger speed in the monitoring computer program. Once the cooling pressure deviation is greater than the set value ± 50mbar, the alarm will be triggered after a delay of 60 seconds, provided that the turbocharger speed is ≥ 11600r/min, and the turbocharger turbine disk cooling pressure monitoring will take effect). However, the manual did not specifically provide the reason for the low cooling air pressure alarm for the turbocharger turbine disk.
The cooling system of the turbocharger turbine disk is shown in the diagram.
From the diagram, it can be seen that the turbocharger turbine disk cooling system mainly consists of the turbine disk cooling air intake pipe 2 (8), the cooling pipe pressure sensor 4, the cooling pressure sensor intake pipe 3, and the host control system 5.
The main reasons for the low air cooling pressure of the turbocharger disk in the main engine are as follows: (1) The pressure sensor of the cooling pipe is damaged, resulting in false alarms; (2) The intake pipe of the cooling pipe pressure sensor leaked air, causing the pressure sensor to feel low pressure; (3) Cooling pipe leakage causes low cooling pressure alarm.
After the ship docked, the engine supervisor first conducted a calibration test on the cooling pipe pressure sensors of the left and right main engine boosters. The test values of the left and right pressure sensors were the same, and the pressure sensor fault was ruled out. Subsequently, an inspection was conducted on the intake pipe of the cooling pipe sensor of the right main engine, and it was found that the pipeline was normal without any air leakage.
The cooling pipe intake pipe is divided into two parts: the inner and outer parts. The cooling air intake pipe for the turbine disk on the outer side is easy to inspect, well connected, and has no possibility of air leakage. Subsequently, the engine supervisor disassembled and inspected the cooling pipe of the turbine disc in the exhaust gas inlet chamber of the right main engine turbocharger. It was found that the threaded sleeve joint at the right angle elbow of the cooling inlet pipe of the turbine disc in the exhaust gas chamber had fallen off and was stuck at the nozzle ring together with the broken cooling pipe joint at the root of the cooling pipe.
For safety reasons, an inspection was also conducted on the air cooling pipe of the left main engine turbocharger turbine disc, and it was found that the bolts of the air cooling sleeve in the intake chamber of the left main engine turbocharger turbine were only loose and not damaged.
When the above-mentioned faults occur, the ship has been out of the factory for less than 1 year, and the left and right main engines have been running for more than 1000 hours and are still under warranty. The ship management personnel promptly reported the above situation to the fleet maintenance supervisor and the main engine service provider. After receiving the feedback, the main engine manufacturer attached great importance and sent relevant service providers to replace the right main engine intake sleeve assembly and re tighten the left main engine turbocharger air cooling sleeve. The detached turbine disk air cooling pipe component was blocked by the nozzle ring and did not enter the turbocharger exhaust turbine rotor end, which did not cause serious mechanical accidents.
Once the diameter of the detached part is smaller than the aperture of the nozzle ring's air outlet channel, the detached part enters the turbine blades, which can cause damage to the blades or even render the entire turbocharger useless, resulting in significant losses.
Threaded connections are prone to failure at high temperatures due to improper material and assembly, as well as stress relaxation. The stress relaxation of bolts at high temperatures is a typical relaxation phenomenon. The middle corner of the air cooling pipe of the turbine disk on this ship is connected by a threaded sleeve. Once the connected thread becomes loose, the vibration of the loose threaded sleeve will intensify under the transmission of main engine vibration and gas impact vibration. The pressurized air circulating inside the trachea is cooled, while the outside of the trachea is subjected to high-temperature exhaust gas flow erosion, causing not only thermal stress but also pulsating stress on the turbine disk cooling tube.
Based on the damage of the air cooling pipe of the turbocharger disk, it can be inferred that the cause of this malfunction is that the air cooling pipe of the turbocharger disk of the right main engine first loosened at the connection of the middle threaded sleeve. The loose cooling pipe suffered fatigue failure under repeated cyclic thermal stress and pulsating stress.
After the above-mentioned malfunction occurs, the equipment manufacturer sends a service provider to replace the air cooling pipe assembly of the turbocharger disk on the right main engine, and the loose threaded sleeve of the air cooling pipe on the left main engine turbocharger is re tightened. The design of the turbocharger air cooling pipe has serious defects and must be improved before it can be used. Before the manufacturer made any improvements to the turbocharger, the only solution was to shorten the inspection interval for the air cooling pipes of the turbine disc, inspect and tighten the connecting threaded sleeves to prevent them from loosening.
There are two improvement plans: (1) Redesign the air cooling pipes of the turbocharger turbine disk, using materials with stronger fatigue resistance and adopting more reliable connection methods; (2) Remove the air cooling pipes of the turbine disc and replace the turbine disc that does not require cooling.