With the advancement of informatization, vessels are transforming from traditional transportation service support to joint mobile operations. This places new demands on the technical performance of crew members and equipment, especially higher requirements on the maintenance and replacement operations of diesel engine parts due to the increasing maneuverability of vessels.
In the principle of emergency repair, it is difficult to combine professionals with crew members in reality. Therefore, it is necessary to cultivate crew members into professionals, and attach importance to the maintenance and repair of diesel engines, so as to improve the efficiency and quality of repair and enhance the working efficiency of the ship's power equipment.
Exhaust gas turbocharging is commonly used in Marine diesel engines. In recent years, the probability of problems and malfunctions of turbochargers in Marine diesel engines has significantly increased.
Therefore, it is necessary to analyze and study the usage of the turbocharger, determine the main faults that may occur in the turbocharger, combine the fault mechanism, explore effective methods to reduce the failure rate, extend its service life, enhance its reliability, and ensure the navigation performance of the vessel.
I. Structure and Working Mechanism of the Turbocharger
The diesel engine exhaust gas turbocharger is mainly composed of a turbine, a compressor and an intermediate bearing.
Firstly, the intake port of the turbine is connected to the exhaust pipe, and the exhaust port is attached to the exhaust pipe.
The intake port of the turbocharger is connected to the air filter pipe, the exhaust port is connected to the intake manifold, and the turbine and impeller are respectively installed in the coaxial turbine chamber and the turbocharger.
A centrifugal compressor is composed of a muffler, an air intake shell, a compressor impeller and a diffuser.
During the operation of the compressor, the exhaust gas enters the exhaust main pipe and the turbocharger turbine, does work and is then discharged from the exhaust port. After that, it is connected to the exhaust muffler through the exhaust elbow, bellows and the exhaust muffler.
Fresh air enters the compressor through the muffler and air filter.
② The bearings adopt two built-in floating bearings, which are installed in the bearing housing. The lubricating oil enters from the main lubrication system and then flows back to the bottom of the bearing housing.
③ Coaxial operation of the compressor and turbine can increase the pressure and density of the incoming gas. When the rotational speed remains constant, it can provide greater power output, achieving the effect of improving fuel economy and reducing emissions
Ⅱ. Common Faults and Causes
In exhaust gas turbochargers, common problem areas include the compressor, wheel axles, bearings and turbines.
Before starting and after stopping the machine, it is necessary to pay close attention to the changes in the speed, temperature and operating sound of the turbocharger.
The turbocharger operates at high speed in a high-temperature environment, and the thermal stress and alternating stress it bears are very complex.
When operating at high speed, the vibration of the turbocharger impeller, axle and bearings will damage the fit clearance and affect the performance of the turbocharger.
Moreover, the exhaust gas contains oxides and vapors of sulfur and nitrogen. The acid formed after the combination will corrode the turbocharger and shorten its service life.
After investigation and analysis, the turbocharger has the following common faults.
1. Surge
When a diesel engine suddenly stops or the load is removed, the rotor of the turbocharger will not stop due to inertia.
At this point, the intake pressure is still very high, but the diesel engine does not require much air after unloading. The excessive back pressure of the diesel engine causes surging.
When a diesel engine is overloaded, the energy of the exhaust gas discharged shows an explosive trend. At this time, the speed of the diesel engine increases very little or not at all, and the physical phenomenon of surging is also prone to occur. This is because the air entering the compressor swings to a large extent, causing the vibration of its impeller, thus generating a heavy wheezing sound.
For centrifugal compressors, this surge phenomenon will occur when the flow rate is low and the exhaust pressure value is greater than 2.5kPa.
There are mainly three causes of shortness of breath.
Under normal operating conditions, the airflow path of the turbocharger:
Air → Air filter at the compressor intake → Muffler → compressor impeller → compressor diffuser → air cooler → scavenging box → air intake → diesel engine cylinder → Exhaust port → exhaust main pipe → nozzle ring at the end of the exhaust gas turbine → exhaust gas turbine impeller → exhaust gas boiler → Discharge.
The flow area of each part is fixed.
If any of the above links malfunctions, it will affect the gas circulation of the diesel engine, causing surge of the diesel engine turbocharger, a decrease in the performance efficiency of the turbocharger and the boost scavenging pressure, as well as a rise in the exhaust temperature of the main engine. If the exhaust temperature of the main engine rises to a certain value, fault alarms, automatic deceleration or shutdown may occur.
Among the components through which the airflow passes, the dirtiness of parts such as filters, cylinders, and nozzles is the main factor causing surge.
2. Mechanical failure of the turbocharger
Mechanical failures include bearing damage and impeller blade damage.
When a malfunction occurs, first check the filter screen. If it is clean, consider that there is damage to the turbocharger components. If a problem is found, stop the machine immediately for repair.
(1) The turbocharger bearing is damaged
The axle of the turbocharger has broken.
One end of the rotating shaft is a turbine, and the other end is connected to the impeller.
If the axle breaks at high temperatures, the diesel engine will shut down.
The main causes of bearing fracture are:
① Under the action of external forces, the surface of the turbine changes, or the compressor is not fixed tightly and cannot work normally and stably. The deformed wheel shaft breaks under the action of stress and torque.
② When the turbocharger is in operation, the axle vibrates as the turbocharger's rotational speed changes, and fatigue fracture occurs under the action of alternating stress.
③ The rotating shaft lacks lubrication and adheres to the floating bearing. High-temperature and high-pressure exhaust gas impacts the fixed wheel shaft, causing the shaft to break.
If foreign objects fall into the pipe inlet and move around in the turbine box housing, rigid collisions between hard objects such as bolts and the wheel axle may cause the axle to break.
Poor lubrication of the turbocharger.
The lubricating oil of the turbocharger bearing comes from the lubrication system and the oil pan. Poor lubrication and other factors can lead to the erosion of the wheel shaft. The main reasons are:
① When the lubricating oil is insufficient or the oil pressure does not meet the requirements, the axle will be burned due to friction and high temperature when the flow rate is insufficient or the oil pressure is lower than the specified value.
② If the oil quality does not meet the requirements, using lubricating oil of non-specified grades or with a large amount of unclean impurities, the temperature of the bearing will increase.
③ When the turbine seal ring wears out excessively, generating a considerable amount of heat and damaging the bearing clearance, the uneven oil film in the gap between the turbine and the rotating shaft is difficult to achieve a good lubrication effect.
When the turbocharger's rotational speed exceeds its maximum speed, a runaway phenomenon will occur.
As the rotational speed increases, the supply of lubricating oil becomes insufficient, the cooling effect deteriorates, and the wheel axle is burned out due to the inability to dissipate heat.
When the inlet and outlet temperatures of the lubricating oil are too high and the cooling is not timely, the wheel axle will also be burned out.
When speeding, the speed should be reduced or the vehicle stopped for inspection. If the axle is damaged, it should be replaced.
Common damages to bearings include excessive wear, scratching, spalling, cracking and burning.
When the oil pressure is insufficient, vibration problems, shell deformation and even severe wear of the bearings will occur.
When a new turbocharger is installed, the lubricating oil and filter are replaced, the machine has been idle for a long time, has not been started correctly, or the quality of the lubricating oil is low, it is easy to cause faults such as increased resistance or rupture of the oil inlet pipeline. When the oil level in the oil pan is low, air enters the lubrication pipeline and causes air resistance, or the lubricating oil supply in the turbocharger is cut off, it will lead to wear and burning of the bearing surface
(2) Blade damage
When the turbine is running at high speed, even if very small debris enters, its air guide Angle will change, which will have a significant impact on the blades or impeller.
Large and hard objects can leave scratches or even cracks on the leaves.
The main cause of blade fracture is fatigue failure caused by repeated bending stress on the blade.
When the turbocharger surges, the blade tips of the turbine impeller collide with the casing, causing blade deformation and having a significant impact on the rotational speed.
Abnormal knocking sounds often occur when the impeller breaks.
When the vibration is abnormal or the noise is large, the machine should be stopped for inspection.
After replacing or repairing the impeller, static balance and dynamic balance tests need to be conducted.
(3) Abnormal boost pressure
When the air intake is insufficient, the compressed air can be sent to the compressor by the air replenishment device to achieve the purpose of pressurization.
The pressure gauge reading is more obvious when the rotational speed approaches or reaches the rated speed (for example, the rated speed of the NTA855-M350 diesel engine is 1800 r/min).
At high rotational speeds, mechanical efficiency is very high, and various reasons can cause significant changes in pressure.
The pressure rises.
When the load remains constant, the overspeed of the turbocharger often leads to an increase in pressure.
The main reasons are:
① A fuel system failure led to poor combustion, severe afterburning and an increase in exhaust temperature.
② The turbocharger itself is dirty with the turbine, reducing the flow area and accelerating the gas ejection speed, which causes the turbocharger's rotational speed to rise.
③ When a diesel engine is overloaded, the load is too large and the rotational speed is relatively high, the volume of exhaust gas increases, the rotational speed of the exhaust gas turbine rises, the rotational speed of the compressor impeller increases, the volume of compressed air increases, and the diesel burns more fully.
Under this working condition, the fuel injection volume increases and the vehicle may stall at any time. It is necessary to immediately slow down and unload to ensure safety.
The pressure has decreased.
Incomplete combustion affects power.
It is manifested in:
① Dirty intake pipe: Reduced intake volume, insufficient boost pressure and accompanied by whistling. Carbon deposits or oil stains adhering to the diffuser, impeller and turbine nozzle increase the rotational resistance of the turbocharger.
② Inaccurate fuel supply timing: The fuel supply time lags behind, causing aftercombustion, incomplete combustion of diesel, weakened exhaust energy, and a decrease in turbine speed.
③ Exhaust pipe leakage: This leads to a decrease in the available energy of the turbine, affecting its rotational speed, and the pressure of the turbocharger also drops accordingly.
④ Late opening and early closing of the exhaust valve: The exhaust volume decreases, the rotational speed of the turbine rotor drops, resulting in a decrease in the boost pressure. The corresponding table of turbocharger faults and possible causes is shown in Figure 1.
Figure 1 Table of Faults and possible Causes of ship turbochargers
Through the analysis in Figure 1, the relevant phenomena and causes of the turbocharger failure can be determined, and targeted solutions can be taken.