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Engine over speed failure
Engine over speed failure is a phenomenon that can happen at any time. Sometimes it’s easy to miss the warning signs. Other times, it can be hard to catch the issue before it causes a severe problem. In both cases, it’s essential to take action.
Due to engine overspeeding, I have received major breakdown reports on Diesel generators and Main Diesel Engines. In most cases, I have found the breakdowns due to the malfunctioning speed device. Due to the over speed of engines, severe damage to the Engine and associated alternator have been observed. In most cases, the Over-speeding of the Engine was caused due to the malfunctioning of the engine governor and fuel injection pumps. In a few instances, Governor’s drive has been the cause of overspeeding.
An over the speed of the Engine will significantly damage the main components like the Crankshaft, Engine bed plate, cylinder liners, and complete power pack of the Engine, including the valve drive mechanism, Turbochargers, and Generator.
In this post, we will cover Engine over-speed failure and try to help the readers to understand Engine over-speed failure.
Operating Principle of mechanical over speed device
Over speed, trip device stops the engine completely when the safe speed of the Engine exceeds.
Mechanical over speed device works on the Principle of centrifugal forces. Mechanical over speed trip has only one flyweight, also working on the Principle of centrifugal force. Flyweight remains in close position during the safe speed of the engine with the force of spring. During the over speeding of the engine, fly weight opens out against the spring tension and releases the tripping latch. Fuel supply is cut off by the release of tripping latch, and the engine comes to stand still position.
The flyweight is held to the drive shaft by a spring when the engine over speeds, the centrifugal force overcomes the resistance of the spring, and the flyweight moves away from the shaft.
During this position, the latch is tripped by the rotating flyweight, which releases the spring-loaded plunger. The plunger pushes against the fuel cutout lever, which moves the fuel pump control to the no injection position, and the engine stops.
Always put the latch in re-set position before re-starting the engine. The action is completed by the arm in re-set position with the help of re-set lever on the shaft. Make use of fuel cut out cam installed on the control shaft of the engine. During manually stopping operation of the engine, starting lever of the engine moves toward the stop position. Cam turns off the fuel cut off lever and shuts the fuel supply off.
Electronic over speed trip
The Magnetic Speed Pickup (MPU)
MPU is an instrument for detecting the speed of an Engine.
Magnetic speed pickup works on the principle of Faraday’s law.
The Engines flywheel is of magnetic material. Installing magnetic pickup is effortless without any additional attachment to a shaft. The following three factors affect the output voltage of magnetic pickup.
- Voltage increases when the speed of the controlled magnet increased.
- The increase in the air gap between the magnetic pickup and the gear tooth will decrease the voltage of the magnetic pickup.
- Voltage waveform will be determined by the size of the gear teeth relative to the size of the pole piece.
The gap between the end of the Magnetic Pickup and the gear teeth should be at-
0.25mm to 1.02 mm at the nearest point
Zero gaps between the MPU and flywheel teeth will damage the MPU. RMS value of an adequately installed MPU will be 50 Vac.
What causes Engine over speed
In most cases, negligence in the maintenance of the over speed was the cause of failure.
The cause of Engine over speed failure is due to the malfunctioning of the Engine Governor and Fuel Injection pumps. Governor is installed on an engine to control the engine’s speed irrespective of load on the Engine. The governor controls Engine speed, with the control of fuel supply to the engine.
If the Governor cannot perform its function for some reason, then the Engine will over speed. Malfunctioning of the Governor is due to the failure of the Governor’s internal parts. Failure of the governor drive is the common reason for the over speeding of engine.
If the Governor loses control of the Engine and the Engine gets over speed, then, in that case, the over speed device must operate and stop the engine by cutting off the fuel supply to the Engine.
The Main causes of Engine over speed
- Faulty magnetic Pick up unit
- Sudden removal of load
- Increased throttle due to mechanical or electrical device
- Disturbed setting adjustments in governor
- Increased starting fuel limit
- Speed gain setting out of limit
- Faulty Injectors
- Disturbed setting in control panel
Over speed trip in Generator (Engine over speed failure)
Over speeding of a Generator is the most undesired situation in any Diesel Generator. If a Diesel generator continues to over speed, It will cause the most disastrous results. This situation will result from improper installation, commissioning of over-speed devices by unauthorized and unqualified technicians/Engineers.
Damages due to over speed of Engine
Over speed of an engine cause the major damage to the crankshaft, valve actuating mechanisms, valve, and valve seats, Camshaft, Turbocharger, Piston, Cylinder Liners and the Generator.
Consequent due to the over speed of engine could be very costly affair.
As per the recommendations of international standards, over speed device of any Diesel generator engine is supposed to operate at 115% of the Engine operating system. In the old design of engines, mechanical over speed device was in use. But in the latest design of the Engine, mechanical over speed device is obsolete. . Engine over speed control is through the electronic device,
If an over-speed device fails to control the speed and the Diesel generator continues to run above the permissible limit of speed, the over speeding of the diesel engine driving an electric generator could cause most Diesel generators to fly apart.
Testing procedure of over speed tripping device
There are different procedures for the other kind of over-speed devices on Diesel Generators. These devices are Electronic, mechanical, and a combination of both methods. Always conduct the tests on the offload condition of the diesel generator.
To test the electronic over speed device is to re-set the tripping set point to the lower limit for safety reasons. Start the engine and let the Engine operating conditions normalize. After the normalization of engine parameters, slowly increase the speed of the Engine. Observe the alarm and tripping RPM of the Engine. On completion of the test, re-set the tripping set back to the specified limit of over-speed.
For the Diesel generating sets with mechanical over speed, increase the engine’s speed and observe the speed at which the engine trips.
Carry out the re-setting operation on the device.
Engine over-speed warning (Engine over speed failure)
The Engine’s electronic device will give the engine’s warning signal over speed signal at 110% above the nominal speed. The warning signal is always set at 5 % less than the tripping speed of the Engine. Attend to the Engine over-speed warning signal without waiting for the Engine to trip.
Real-life incident
This incident occurred in a factory with SKL make Diesel generator. The engine was operating at full load. The engine started over speeding. Warning signals for the over speed was mal-functioning. Operate didn’t hear the abnormal sound from the engine soundproof control room. Engine speed increased slowly at 100 % load. The operator only came to know after the Engine was in a standing still position. The operator rushed to the engine room after the blackout. Operator observed the following significant damages to the engine and the alternator
- The crankcase door of the no8 unit was flown entirely apart.
- The connecting rod was protruding out of the crankcase.
- The piston got separated after getting damaged from the piston pin area.
- Alternator-damaged parts were flown out in various directions.
Engine repairs were done. All the damaged parts were replaced. The engine was made operational and re-commissioned.
I have attended the case of failure to carry out the Failure analysis. The customer had tried the Engine and loaded the Engine before I reached the site.
During the Engine at load, I observed the Engine and monitored all the parameters. After about 1 hour of engine operation, I heard the obnoxious sound from the Engine due to the engine over-speeding. I rushed for the emergency stop button and immediately stopped the Engine.
After cooling down the Engine, I started investigating the cause of the Engine over speeding. During the investigation, I got the Governor removed to send the Governor for the functional test to the workshop. Governor Sent the necessary tests to the workshop. No abnormality was detected in Governor.
During my further investigation, I have observed the splines of the governor drive shaft were worn entirely out except three splines slightly in good condition. The reason was the poor transmission of the drive to the Governor. The Governor’s drive shaft was not transmitting the proper drive, and the Governor was slowing down. The slowing down of Governor Speed, Governor, was increasing the fuel supply to maintain its speed.
Over speed, the device was not in contact with the crankshaft gear due to the missing device drive gear. This was the main cause of Engine over speed failure for this case.
On further investigation revealed that the operator removed the gear as the device was tripping the Engine frequently. The operator didn’t realize the maladjustment of the over-speed device.
Ignorance and poor operator knowledge have caused significant damage to the Engine and massive loss to the factory production due to Engine over speed failure.
I have observed that Engine over speed failure has taken place in many instances due to insufficient knowledge of the equipment. Factory operators don’t get proper attention for the engine operations. Factory owners are sometimes Pound Foolish pennywise. They purchase the costly Diesel generator but employ operators with poor operation and maintenance knowledge for the operation and upkeep and face Engine over speed failure.
Blog Conclusion
We hope that you enjoyed our article about engine over speed failure. In this article, we provided information about engine over speed failure and how it affects the engine. We also shared some tips to help you with your engine over speed failure. If you have any comments or questions, please feel free to contact us athttp://sh057.global.temp.domains/~arinedie/contact-us/. Thank you for reading, we hope that you found the article helpful.
Good luck!!!
