1, Working principle of bypass valve
1. Boost air bypass valve
The boost air bypass valve is installed on the connecting pipe between the scavenging box and the exhaust gas inlet end of the turbocharger, providing sufficient surge margin for the turbocharger when the engine is under low load.
When the host is running at low load, the air consumption of the host is small and the boost pressure increases, which can easily cause high back pressure in the compressor and surge in the turbocharger.
Therefore, open the boost air bypass valve to reduce compressor back pressure, increase rotor kinetic energy, and increase surge margin.
The bypass system is controlled by the speed and load of the host.
The boost air bypass valve is controlled by one electrically controlled pneumatic valve, and the control signal comes from the speed relay and boost air pressure sensor connected in series.
The boost air pressure represents the load of the engine. When the engine speed is less than 425r/min and the boost air pressure is greater than 0.35bar, the boost air bypass valve opens, and some of the boost air enters the exhaust pipe before the turbocharger from the compressor, increasing the speed of the turbocharger.
2. Exhaust gas bypass valve
The exhaust bypass valve is installed on the connecting pipe at the exhaust inlet of the turbocharger to maintain optimal operating conditions of the main engine under high loads.
When the host is at 85% load, it is in optimal working condition, with high boost pressure, low exhaust temperature, and low fuel consumption rate;
When the load exceeds 85%, the boost pressure and cylinder combustion pressure are too high, and the exhaust bypass valve allows some exhaust gas to bypass the turbocharger, thereby reducing the flow of exhaust gas entering the turbine end.
The exhaust bypass valve is controlled by one electrically controlled pneumatic valve, and its control signal comes from the boost air pressure sensor.
When the system's boost air pressure exceeds 3 bar, the exhaust bypass valve will activate, and a portion of the exhaust gas will be directly discharged from the exhaust manifold without passing through the turbine blades, controlling the boost pressure and cylinder combustion at the optimal level.
2, Abnormal opening fault of boost air bypass valve
1. Fault phenomenon
On September 12th of a certain year, the right main engine of a certain ship was unable to reach its rated power under high load, with an average exhaust temperature exceeding 450 ℃ and an alarm (the normal exhaust temperature of the main engine should be lower than 430 ℃). The boosting pressure of the main engine increased, the exhaust gas turned black, and the scavenging temperature increased. Upon inspection, it was found that the boosting air bypass valve was abnormally open.
2. Fault hazards
The host is working under high load, and the boost air bypass valve is abnormally opened. At this time, the exhaust gas pressure is greater than the scavenging pressure, and the high-temperature exhaust gas enters the scavenging box, mixes with the boosted fresh air and enters the cylinder, and mixes with fuel for combustion.
The entry of exhaust gas leads to a severe shortage of fresh air. Due to the presence of a large amount of nitrogen and sulfur compounds in the exhaust gas, the oxygen content in the intake is reduced, combustion deteriorates, and ignition stagnation occurs, causing an increase in exhaust temperature and directly affecting the service life of the exhaust valve.
At the same time, some of the exhaust gas enters the scavenging box, affecting the amount of exhaust gas entering the exhaust turbine and causing a decrease in the speed of the exhaust turbine;
The coaxial compressor speed decreases, further reducing the intake air volume.
The mixture of high-temperature exhaust gas and fresh air after pressurization causes an increase in the temperature of the pressurized air; The entry of high-temperature gas causes the valve seat and valve line of the intake valve to age at high temperatures, ultimately resulting in the intake valve not closing tightly, and in severe cases, causing the valve spring to break.
3. Troubleshooting
Upon disassembling the boost air bypass valve, it was found that the solenoid valve core diaphragm was damaged, resulting in the inability to close between pipeline A and pipeline B.
After replacing the solenoid valve, the problem was resolved.
4. Countermeasures
Regularly conduct functional tests on the boost air bypass valve to ensure that the equipment is in good condition.
(1) When the host is parked, the boost air bypass valve should be in the closed state, and there should be no voltage on the terminal connected to the boost air bypass solenoid valve in the junction box; Visually inspect the position of the butterfly valve disc through the actuator to see if the valve position indication corresponds to the actual position.
(2) Connect the pressure sensor inspection tool to inspection port 6.
Slowly increase the pressure and check if the pressure switch opens at 0.35 bar.
At this point, if there is 24V DC on the terminal connected to the boost air bypass valve in the measurement junction box, it indicates that the boost air bypass valve has been opened.
Conclusion
By using a boost air bypass valve and an exhaust gas bypass valve, the W ä rtsil ä 8L46C engine exhaust gas boost system can effectively improve the efficiency of the turbocharger.