1, Ship condition and accident process
The "Qin Gang 12" is a coastal tugboat built by Ishikawa Island Shipyard in Japan in 1983. The main engine is two large diesel engines 8DSM-26s with a power of 1176 × 2KW, and the rotary engine is DP40E.
Over the course of 20 years, the ship's main engine has accumulated more than 28000 hours of operation, and the ring groove on the piston head is severely worn, exceeding the upper limit value (0.30mm) stated in the manual.
Therefore, on October 19, 2003, five piston heads in the left main engine were replaced, and on February 8, 2004, during the ship repair at Huangpu Shipyard, five piston heads in the right main engine were replaced.
On February 19th, the ship was repaired and ready to return to Shenzhen.
At 1420 in the afternoon, prepare the car, and at 1430, the slow car leaves the dock and gradually accelerates to around 500rpm.
When the ship left the dock, abnormal operating parameters were found in the main and auxiliary engines as well as other auxiliary equipment in the engine room.
After departure, there were blockages in the subsea valves of the auxiliary engine and DP cooling seawater pump, as well as leaks in the cooling fresh water pipe of the exhaust valve of the NO.4 cylinder of the right main engine.
The chief engineer organized 4 personnel from the engine department to handle the abnormal phenomenon on duty.
After sailing for 30 minutes, the left engine accelerated to 600rpm, while the right engine did not accelerate due to the need for break in as it had just replaced 5 piston heads and rings, and kept sailing at 500rpm.
About 10 minutes later, the chief engineer heard two "explosions" from the turbocharger of the left engine. The chief engineer immediately stopped the left engine but did not notify the driver's console due to the emergency situation.
Afterwards, the chief engineer arranged personnel to turn the engine and found that the left main engine could not rotate. The lubricating oil pump was turned on for cylinder lubrication and cooling.
Drive again, and the left main engine can now rotate.
Afterwards, a collision was carried out and a temporary inspection was initiated.
Open the rocker arm box cover of the left host and find oil mist.
Open the guide door of the left engine and find aluminum shavings on the lower part of cylinder NO.7, and traces of cylinder pulling on the cylinder liner wall, indicating that cylinder NO.7 is pulling.
2, Fault analysis and argumentation
In theory, there are several reasons that can cause cylinder pulling:
(1) Poor lubrication of the cylinder;
(2) Insufficient running in;
(3) Poor cooling;
(4) Piston ring fracture;
(5) Using inferior fuel;
(6) Long term overload operation of diesel engines leads to increased thermal load and overheating expansion.
Port 12 uses No. 0 light diesel, and the ship has just left port. The two main engines have been replaced with piston heads, and the engines have just started accelerating without overloading.
Therefore, the 5th and 6th reasons were excluded;
The left engine that experienced cylinder pulling was the piston head that was replaced in October last year, and the running in has already been completed. The second reason can also be ruled out;
Lifting the NO.7 cylinder out for inspection, no piston ring fracture was found, and the fourth cause has been ruled out;
Before lifting the cylinder, the lubricating oil pump was used to circulate the oil, and the injection amount of each cylinder was basically the same. No leakage was found. The large end of the connecting rod was checked for displacement, and the piston pin and liner were both normal.
In addition, during the inspection of the NO.7 cylinder piston extraction, it was found that two oil rings were stuck, and three air rings were normal with oil film. The above situation indicates that the lubrication system is normal.
Therefore, we focused on the reason of poor cooling and conducted a detailed analysis and inspection based on the characteristics of the ship's main engine system.
The seawater cooling system of the two main engines of the "Qin Gang 12" ship is a parallel system, which means that the seawater of the two main engines cools their respective lubricating oil coolers, and then converges with the air neon cooler to enter the shared fresh water cooler of the two engines, and is discharged outside the ship.
In this system, when the discharge pressure of two seawater pumps (centrifugal pumps) is unbalanced, or after system repair, the system is not separately ventilated, or when there is a change in the flow field during operation, it is easy for the high-pressure end to flow backwards to the low-pressure end, causing interruption of seawater on the low-pressure side, resulting in an increase in seawater temperature, gasification, and corresponding heating of pipelines and seawater pumps.
But observe that the pressure value on the seawater pressure gauge is almost equal to the pressure value on the other side, and there is seawater discharged from the outside of the ship.
The two main engine oil coolers and air coolers of the "Port 12" vessel during this factory repair have been cleaned and pressurized.
In addition, the air cooler of the 8DSM-26S diesel engine of Daihatsu is located above the water surface outside the ship, and the air in the air cooler cannot be discharged by the water outside the ship on its own.
After the ship was docked, the newly replaced piston head of the right main engine needed to be run in. The right main engine was continuously run in after docking, and the left main engine hardly started.
Due to the lack of inspection by the engineer on the water and exhaust of the left engine and the discharge of seawater outside the ship, the air in the air cooler of the right engine was never discharged, resulting in an air blockage.
And usually, the seawater pressure on the right host is slightly higher than that on the left host.
Start the right engine first when preparing the car, and then start the left engine.
Taking into account the above reasons, the backflow of seawater from the right unit to the left unit resulted in the interruption of seawater from the left main unit.
After sailing, it has been running at 500rpm with low load. In addition, it has been suspended for a period of time, and the temperature of the lubricating oil and the body is low. After sailing for about 40 minutes, the intake temperature and fresh water temperature of the left main engine have actually increased, causing the working cycle temperature of the left main engine to rise, resulting in an increase in fresh water temperature.
But the personnel in the cabin were busy dealing with the abnormal phenomenon and neglected the inspection of the left main engine.
Because there is seawater at the outboard seawater outlet, and the seawater pressure is also normal.
It was mistakenly inferred that the cooling systems of the two main engines were normal, but in reality, the left main engine never had seawater. When the left main engine was driven at 600rpm, due to the increase in speed, the boost increased, and the air did not have seawater cooling. As a result, the air intake temperature sharply increased, causing the working cycle of the left main engine and the temperature to rise, resulting in an increase in freshwater temperature.
The fresh water temperature field of this model is relatively small, generally at 65 ℃ under rated load speed, not exceeding 75 ℃, and an alarm is triggered at 80 ℃.
After sailing in this way for more than ten minutes, the actual fresh water temperature has already exceeded the limit of fresh water temperature, causing an increase in the thermal load on the main engine.
The piston of "Port 12" adopts a combination piston with steel top, aluminum skirt, thin top, strong back and internal support. When the thermal load is high, the aluminum piston skirt rapidly expands due to heat, causing the cylinder with the smallest clearance between the piston skirt and the cylinder liner to lose its motion clearance and experience cylinder pulling phenomenon.
3, Experience and lessons learned:
(1) Request the marine engineering personnel to strengthen the patrol inspection of duty, especially the inspection of equipment and various system functions after repairing the ship.
(2) Engineers of the same type of vessel are required to pay close attention to the issue of unilateral suction of seawater in the main engine's seawater system while on duty, strengthen inspections, and promptly address the problem of unilateral suction of seawater to prevent accidents from occurring.
(3) Regularly inspect and calibrate the alarm values of the protective devices for the main unit, auxiliary unit, and rotary unit.
