With the increasingly strict requirements of port countries on pollution prevention, major equipment manufacturers have invested a lot of manpower and material resources in the research and development of new equipment. The air type stern shaft seal can better solve the pollution of the marine environment caused by oil leakage, and solve the wear of the seal ring and bushing to the maximum extent. In addition, the device has high reliability and can be easily converted to traditional oil seal in emergency, so air seal has great development space. Taking the air type stern seal of 10000TEU series ships of a company as an example, this paper discusses the working principle and technical key points of the device.

1、 Advantages of air seals

The head seal structure of the device is basically the same as that of the previous CX and DX seals. The tail air seal device is shown in the following figure:

The tail seal has four sealing rings. The air source in the ship is connected to the chamber between # 2/3 rings through the pipeline, and then released from the tail to the sea water in the form of bubbles. According to the classical principles of physics, the pressure at any point below the liquid surface is proportional to the height from that point to the liquid surface. Therefore, the device uses the air pressure released from the tail in the form of bubbles to detect the change of the ship's draft. Any change of ship draft can be automatically tracked by the air control unit and the output pressure and oil pressure in the stern tube can be automatically adjusted, so as to prevent seawater from entering the ship and lubricating oil from flowing out of the ship. The advantages of this device are:

A. High reliability

1) The load borne by each seal ring is obviously reduced, and the service life of the seal ring and the chrome plated bushing is extended;

2) There are two water seal rings and oil seal rings respectively, which improves the reliability of the device. Once the sealing ring is damaged, the lubricating oil or sea water in the stern tube will be recycled to the discharge collection tank inside the ship through the # 2/3 ring chamber to prevent the oil from flowing out of the ship or the sea water from entering the stern tube.

3) A 30mm thick adjusting ring is added between the tail shaft tube end plane and the flange ring of the sealing device. When the contact between the sealing ring and the chrome plated bushing is worn, remove the adjusting ring to stagger the relative position between the sealing ring and the bushing, so as to reduce the number of light turns of the chrome plated bushing and extend its service life.

B. Simple maintenance

After the air flow is set, the system automatically adjusts the pressure according to the change of draft to avoid manual conversion of light and heavy load gravity oil tanks according to the change of draft. C. Low air consumption

The air flow to # 2/3 chamber is set at 45-55N х/min, and the compressed air consumption is low.

D. Various emergency measures can be provided

1) When ring # 3 fails, ring # 3S can be put into operation as long as the inlet and outlet valves to chamber # 3/3S are closed.

2) In case of failure of the air pressure control unit, as long as a temporary gravity oil tank is provided, the air seal (AX type) device can be simply transformed into a double protected oil seal (DX type) device.

2、 Principle of air seal:

The system is mainly composed of rear seal, front seal and the following five parts:

A. Air Control Unit

B. Drain Collection Unit

C. Tailshaft tube oil tank unit (S/T L.O. Tank Unit)

D. S/T L.O. circulation pump

E. Picture of Fwd Seal L.O. Pump Unit

Sealing device system pipeline diagram

According to the piping diagram of the tailshaft air seal device system, the explanation is as follows:

1) The compressed air from the air control unit in the ship enters the # 2/3 ring chamber through the pipeline installed in the stern tube. One part overflows from the sea water through the # 2 ring and # 1 ring, and the other part enters the discharge collection unit from the leakage recovery pipeline.

2) The leaked lubricating oil or seawater can be discharged from chamber # 2/3 to the discharge collection unit installed under the axis of the intermediate shaft.

3) The normal oil level of the stern tube lubricating oil tank is 2-2.5m higher than the shafting center line. The oil tank is a closed type, on which a way of air with the same pressure as the # 2/3 ring chamber is connected.

4) # 4/5 chamber oil is the circulating oil provided by the front seal oil pump unit# The pressure difference between the air pressure of 2/3 chamber and the lubricating oil pressure of # 3/3S chamber is always kept within 0.05MPA, ensuring the service life of the seal ring and bushing; Even if ring # 3 is damaged and leaked, the leaked lubricating oil will enter the discharge collection unit from the lower discharge pipeline and will not flow overboard# The air in the 2/3 chamber rises out of the sea water by slightly lifting ring # 2 and ring # 1, and the air that is always higher than the sea water pressure ensures that the sea water will not enter the # 2/3 chamber; Even if seawater leaks into chamber # 2/3, it will also be discharged to the leakage collection unit.

Discharge and collection unit

1) The discharge and collection unit is mainly used to monitor the working condition of the rear seal of the tail shaft. The flow control valve installed on the unit is used to control the flow of air in the discharge pipeline and ensure that the discharge pipeline is unblocked. In case of seawater ingress or lubricating oil leakage, it will be collected into the unit through the discharge pipeline and a high level alarm will be sent to remind the engine management personnel that the sealing device behind the tail shaft is abnormal. The engine management personnel shall discharge the oil and water in the discharge collection tank according to the following steps when the main engine is stopped.

A. Open the drain valve.

B. Ensure that the collecting cabinet is completely emptied.

C. Confirm that the drain valve is closed.

Remember: Do not open the drain valve when the engine is running.

picture

2) When the pressure difference between the output pressure P4 of the air control unit and the pressure of the leakage collection unit is greater than or equal to 0.03MPA, clean the air pipeline with fresh water. Flushing can also be carried out when the main engine is stopped. The specific methods are as follows:

A. Open the drain valve V56 on the drain collection cabinet.

B. Switch the three-way valve 3MU on the air control unit to the fresh water side, open the fresh water valve V9, and flush the pipeline with fresh water until the water flows out of the drain valve V56 on the drain collection tank.

C. After cleaning, switch three-way valve 3MU to air side and close drain valve V56.