Accident analysis of hydraulic motor shell rupture of marine winch (anchor)
Accident analysis of hydraulic motor shell rupture of marine winch (anchor)
Because the hydraulic motor system has the characteristics of simple structure, good low-speed performance, impact resistance and reliable operation, it is widely used in ship winch (anchor). In recent years, there have been several consecutive hydraulic motor shell rupture accidents, and similar accidents have also occurred in other ports.
2. Basic information of the accident
The motor model of the hydraulic winch system used by a company on the 3088 kW tugboat is mrh-750. During the tugboat berthing assistance operation, the shell of the hydraulic motor of the winch is broken, mainly in the following three cases.
(1) The brake slips and the shell is broken. When the tug is pulling away from the operation, the towline is released, and the winch is in the braking state. When the wind and waves are large, the main cable is impacted by the shaking of the hull due to the influence of the wind and waves, or when it is used as the jacking, the buffer length released by the towline tied to the big wheel is less than the wave height. When the wave trough, the towline is affected by the weight of the hull, so that the force on the cable is greater than the braking force, and the winch brakes slip, The hydraulic motor housing is broken.
(2) The brake fails and the shell breaks. For the winch with hydraulic brake, when the winch is towed away, the brake system fails or the hydraulic pump stops suddenly, and the brake cannot be self-locking and fails, so that the force of the main cable acts directly on the hydraulic motor, resulting in the rupture of the hydraulic motor shell.
(3) Shell rupture occurred during cable laying. In the process of berthing assistance, the high-power tugboat adopts the combined operation of jacking and pushing. In the process of berthing assistance, the main cable is always tied to the large wheel assisted by berthing. During jacking, the main cable shall be recovered; When towing away, the tugboat shall reverse, the winch shall release the cable, and the towing operation shall be started when it is put to a certain length. In this way, during each berthing aid operation, the winch needs an average of about 10 times to collect and release cables. If it is necessary to change from pushing to towing, the tugboat will reverse quickly and the winch will release the cable quickly. When the reverse speed of the tugboat is higher than the cable laying speed of the winch, the shell of the hydraulic motor of the winch will be broken.
3. Shell rupture
Judging from the damage of the five times hydraulic motor shell, the broken part and shape have certain rules. According to the cylinder numbers distributed by the five shell fractures of the hydraulic motor, three times appear in the fourth cylinder (see Figure z) and two times appear in the first cylinder. For example, based on the laying out of the winch, when the hydraulic motor enters the oil for pipe B and discharges the oil for pipe a, whether the shell rupture occurs during the laying out of the cable or after the brake slips and brake fails, it is in the fourth cylinder. If pipes a and B are opposite, the shell is broken in the first cylinder.
From the shape of the shell fracture, the shape of the five fractures is the same. The cracks appear at the inlet of the oil passage of the cylinder block and are distributed outward with the inlet of the oil passage as the center, but the length and number of cracks are different.
After the hydraulic motor was broken, the cylinder head, piston parts and transmission parts were normal except the shell crack. The piping, safety valve and control valve in the system are slightly abnormal, and others are normal. As long as the shell of the hydraulic motor is replaced, the system will return to normal.
4. Cause analysis
From the analysis of the rupture of the hydraulic motor shell, the external force is greater than the output force, resulting in the pump working condition of the hydraulic motor. Based on the analysis of the phenomenon of shell fracture, the main reasons are as follows.
(1) When the brake slips and fails, the hydraulic motor shell breaks, mainly in the hydraulic system without fluid replenishment. When the winch brakes, the control valve is interlocked and closed, and the system does not supply oil to the hydraulic motor. When the brake slips, the main cable slides out and drives the hydraulic motor to rotate in the direction of cable laying into a pump state. The oil discharged from pipe a enters the oil through pipe B through the safety valve. Generally, the volumetric efficiency of the hydraulic motor is 90% ~ 95%. During the circulation process, nearly 10% of the hydraulic oil enters the low-pressure oil chamber of the hydraulic motor and returns to the oil tank through the low-pressure pipe system, so as to continuously reduce the oil volume of the system and generate vacuum. Similarly, the gas-liquid two-phase flow first appeared in the fourth cylinder with the highest relative position, resulting in the rupture of the hydraulic motor shell. If the connection of a and B pipes is opposite, the fracture occurs in the first cylinder.
(2) In the process of cable laying, the reverse speed of the tugboat is higher than the cable laying speed of the winch, and the external force of the winch is greater than the output force of the hydraulic motor, so that the hydraulic motor is in the working state of the pump. According to the five times of shell rupture, the cable laying speed of the winch is relatively low. In actual use, the high-speed gear is 20 ~ 50M / min. during normal operation of the tugboat, the main engine reverses with the main cable at the lowest stable speed (400R / min). At 0 ~ 50m, the speed can reach 3 ~ 4 knots (93 ~ 120m / min). During normal operation, the laying length is 50 ~ 100m. Therefore, during the berthing assistance operation, when the tug is transferred from pushing to towing, the reverse speed of the tug is not well controlled. Finally, the tug will drag the winch to release the cable at the speed of 90 ~ 120m / min, so that the hydraulic motor of the winch is in the pump state, and the corresponding required absorption flow is 0 ~ 140L / min. The output flow of the hydraulic pump in the hydraulic system is not enough, resulting in the vacuum state of the system. According to the theory of gas-liquid two-phase flow, when the vacuum state reaches a certain value, the hydraulic oil of the system will release air. During the cable laying, the oil is usually fed from pipe B and discharged from pipe a. the working sequence of each cylinder of the hydraulic motor is 1-2-3-4-5 circulation, which will make the gas-liquid two-phase flow first appear in the fourth cylinder at the highest position in the system. When the piston of the fourth cylinder goes down, there is an atmospheric mass at the inlet of the oil passage in the cylinder. When the piston goes up, the air mass will break and explode rapidly, resulting in high pressure, This caused a crack in the cylinder block of the hydraulic motor centered on the inlet of the oil passage. If the connection of a and B pipes is opposite, the fracture occurs on the first cylinder of the hydraulic motor, and the causes are the same. However, the possibility of overspeed of the hydraulic motor can be ruled out, because the speed range of the hydraulic motor is 1 ~ 400R / min. when the corresponding hydraulic motor is 400R / min, the cable laying speed is l50m / min, and the speed should be higher than 6 knots. When the tug is reversing with cable (the speed of the main engine is 400 ~ 450R / min), it is difficult to reach this speed, so the possibility of overspeed damage can be completely ruled out.
5. Preventive measures
From the cause of the hydraulic motor shell rupture accident, it is caused by the system vacuum when the hydraulic motor is in the working state of the pump. In order to prevent the hydraulic motor shell from cracking, the vacuum of the system must be prevented. The main measures are as follows.
(1) Measures in designing the hydraulic system of winch.
1) Full consideration shall be given to the need for rapid cable laying in actual operation. From the performance of the hydraulic motor and the shipping size of the winch, the cable laying speed can reach 150m / min, which can meet the requirements of emergency cable laying. The key is the output of the hydraulic pump. During the design, the oil supply of the hydraulic motor at full speed shall be ensured. The variable displacement pump shall be selected, and the maximum displacement shall be about 150 l / min;
2) An oil replenishment system capable of rapid one-way oil replenishment shall be added. The oil replenishment pipe shall be connected to the oil inlet pipe when the hydraulic motor is laying off the cable, and the oil replenishment volume shall be greater than the maximum possible pump oil volume of the hydraulic motor, so as to prevent the pump state of the hydraulic motor and vacuum the system;
3) The safety valve shall be installed as close as possible to the hydraulic motor;
4) The clutch of the winch can be remotely and conveniently disengaged, so that when releasing the cable, the clutch can be disengaged at the console, and the cable can be released freely without affecting the hydraulic motor; When braking, disconnect the clutch. Even if the brake slips, the hydraulic motor will not be affected by external force, or adopt one-way output clutch to prevent external force from being transmitted back to the hydraulic motor;
5) The braking system adopts spring hydraulic braking. Even if the braking system fails or the hydraulic pump suddenly stops, the braking can be self-locking.
(2) Measures during normal operation. During normal operation, the tug shall be prevented from reversing at high speed, so that the reverse speed of the tug is the same as that of the winch.
(3) Measures for jacking operation in strong wind and waves. During the pushing operation in strong wind and waves, the buffer cable with corresponding length shall be set out on the deck according to the wave height to prevent the tugboat's hull gravity from acting on the main cable and causing brake slip in the trough.
(4) Measures during towing. When towing away, lengthen the streamer appropriately, and the length should be more than 70m to prevent being affected by wind and waves. The impact force of the main cable is greater than the braking force, resulting in braking slip. If necessary, pull the main cable on the cable pile or manually disengage the clutch.
The above improvement and popularization and application have achieved good results and effectively prevented the occurrence of such accidents.