Two Wärtsilä 8L46C main engine exhaust gas booster system on a ship use booster air bypass valves and wastegate valves to improve the efficiency of the supercharger and improve the working condition of the main engine.
Abnormal opening fault of wastegas bypass valve
1. Fault phenomenon
One day, the booster pressure of the left main engine of a ship decreased under low load, and the exhaust temperature of each cylinder increased, of which the exhaust temperature of 1#, 4#, 7# and 8# cylinders exceeded 520 °C, the average temperature exceeded 450 °C, and the exhaust temperature of the supercharger exceeded 430 °C; The speed of the supercharger decreases, the temperature difference between the front and rear exhaust of the supercharger is too small, and the exhaust smoke of the main engine turns black.
The inspection found that the wastegate was open.
2. Failure hazards
The main engine works under low load, the wastegas bypass valve is abnormally opened, and part of the exhaust gas is bypassed; The kinetic energy of the turbocharger exhaust gas turbine rotor is insufficient, and the speed of the compressor decreases, resulting in a decrease in the booster pressure and insufficient air supply.
At the same time, the exhaust temperature of the multi-cylinder increases, resulting in the automatic load reduction of the main engine.
At this time, when the speed of the main engine is less than 415r/min and the scavenging pressure is greater than 0.35bar, the booster air bypass valve is opened, which is equivalent to the pressurized air bypass valve and the wastegas bypass valve are opened at the same time.
In this case, the exhaust gas enters the scavenger box in large quantities through the booster air bypass valve, and the scavenging temperature rises rapidly, and the oxygen content of the inlet air decreases rapidly, resulting in the rapid aging and deformation of the inlet and exhaust valves of the main engine.
3. Fault solving
The wastegate was dismantled and inspected, and it was found that the booster air pressure sensor of the main engine was damaged, resulting in an abnormal control signal (sending an error signal to the system when the booster air pressure was less than 3bar), and the wastegate valve was opened abnormally.
Troubleshoot after replacing the charge air pressure sensor.
4. Countermeasures
Perform functional tests on the wastegate to ensure that the equipment is in good condition.
The boost pressure is simulated by the test valve 6 to the pressure transmitter 5 on the main engine, and the action of the wastegate valve is observed.
When the simulated boost pressure is less than 3 bar, the wastegate starts to act, indicating that the charge air pressure sensor is damaged or malfunctioning and should be replaced or calibrated.
When the simulated boost pressure is greater than 3 bar, the wastegate should start acting, at which point 2 anomalies occur:
(1) The electronic control system is able to give the correct signal (24 V DC on the terminals of the wastegate), but the wastegate does not operate.
Check whether the control air pressure is too low to drive the cylinder action;
If the control air pressure is normal, but the wastegate valve, positioning device and pneumatic power cylinder are stuck, the wastegate valve should be disassembled, and the damaged seal should be cleaned and replaced.
(2) If the electronic control system does not give the correct signal, check whether the wiring of the electronic control system or the pressure transmitter is normal.
Once the simulation test is complete, a running test is required.
Slowly raise the load of the main engine above the opening set point of the wastegate valve, and observe whether the function of the electronic control system and the position of the wastegate valve are normal.
When the pressure of the booster air is less than 3 bar, the wastegate valve acts, or when it is greater than 3 bar, it is necessary to stop the machine to find out the cause and eliminate the fault in time.
Concluding remarks
With the use of charge air bypass valves and wastegate valves, the Wärtsilä 8L46C main engine exhaust gas booster system can effectively improve the efficiency of the supercharger.
