The stern shaft sealing device is one of the important parts of the ship shafting. Its performance directly affects the operation of the ship, and also plays a very important role in preventing the stern shaft lubricating oil from polluting the marine environment. Therefore, it is extremely necessary to study the stern shaft sealing device. On the ship where the author is working, the engine room stern shaft seal oil tank gave a high level alarm when sailing in strong wind and waves. After inspection, it was found that the poor stern shaft seal caused seawater to leak into the seal oil chamber, causing the seal oil tank to be high and emulsified. There are two common cases of leakage of sealing oil of ship's stern shaft: first, in strong wind and wave weather, the draft of ship's stern is small, and severe vibration of the hull will cause poor sealing of the sealing device; Second, the ship is anchored for a long time, and sometimes the stern shaft seal is not good when the main engine is started, leading to leakage. Leakage caused by poor sealing of the stern shaft is generally divided into: the sea water enters the oil chamber and causes the oil in the seal oil tank to be emulsified, which eventually leads to corrosion of the stern shaft, or the oil in the seal oil tank leaks out of the ship's side and causes oil pollution to the water area. In either case, it will lead to adverse consequences. Therefore, attention must be paid to the sealing condition of the stern shaft of the ship. In case of leakage, it is necessary to take measures in time

Take correct measures.

1. Features of modern stern shaft seal

At present, most of the stern bearings of seagoing ships replace traditional water lubricated bearings such as iron plough wood with oil lubricated white alloy bearings. The oil film bearing capacity is large, and the lubrication performance of oil is good. In particular, its sealing device can effectively seal, making it difficult for seawater and sediment to enter the stern tube, so the wear is very small. The work of the main engine and shafting is relatively stable and reliable.

2. Structure of tail shaft seal

Tailshaft sealing device includes front sealing device (SC2Z) and rear sealing device (SC2B). The front sealing device is located at one end of the engine room to prevent the lubricating oil in the tail shaft tube from leaking into the engine room; The rear sealing device is located at one end of the propeller, which is used to prevent the lubricating oil in the stern tube from leaking to the outboard to pollute the sea, and prevent the sea water from entering the emulsified lubricating oil in the stern tube. The rear sealing device generally has three sealing rings (No. 1, No. 2 and No. 3). The latter two sealing rings (No. 1 and No. 2) are used to prevent seawater from entering the tailshaft tube, and the third sealing ring (No. 3) is used to prevent oil leakage in the tailshaft tube (see Figure 1). The structure of the front sealing device is basically the same as that of the rear sealing device, except that the front sealing device has only two sealing rings (V1 and V2) (see Figure 2).

The sealing ring is made of two sealing rings (NBR) or Viton (VITON) of the front sealing device of NBR Fig. 2, which can be selected according to different service conditions. The inner side of the lip of each sealing ring is provided with a fixed spring ring. This spring ring has two functions: first, it acts on the seal ring with a certain pre tightening force to make the seal ring fit on the chrome steel bushing with appropriate tightness, so as to ensure the sealing between the seal ring and the chrome steel bushing; Second, when the seal ring is worn due to long-time operation, it can be compensated to some extent.

3. Operating principle of tailshaft seal

The sealing ring uses the elbow type sealing shape. Under the combined action of seawater pressure, lubricating oil pressure, self elasticity and the preload provided by the spring, the lip of the sealing ring and the chrome steel bushing maintain a certain radial force, thus forming a sealing property. When the pressure P2 acting on the sealing surface of the sealing ring is greater than the back pressure P1, the lip of the sealing ring is pressed against the chrome steel bushing to achieve the purpose of sealing. When the pressure difference between P2 and P1 is very small or almost no, the sealing performance of the sealing system cannot be guaranteed, which may lead to leakage; When the pressure difference between P2 and P1 is 0.3MPa, it can ensure that the seal ring is well lubricated, has sealing performance, and has little temperature rise and wear, thus extending the life of the seal and maintaining the performance of the seal within the working life; When the pressure difference between P2 and P1 is too large, although the sealing ring can provide better sealing, it also increases the wear between the sealing ring and the chrome steel bushing and the temperature rise due to friction, thus accelerating the aging of the sealing ring.

The seawater pressure and the elasticity of the seal ring itself produce P1, and the lubricating oil pressure and the preload provided by the spring produce P2. In general, the lubricating oil pressure is provided by the height of the rear seal oil tank (1.1m from the oil tank of our ship to the centerline of the tail shaft, see Figure 3).

In the rear sealing device, there are two independent sealing chambers, namely the sealing space composed of sealing ring No. 1, sealing ring No. 2 and intermediate ring, and the sealing space composed of sealing ring No. 2, sealing ring No. 3 and intermediate ring. The two seal chambers are filled with lubricating oil during installation, which is used to increase the back pressure of the seal ring, maintain the balance between the lubricating oil and sea water, and ensure the sealing of the seal ring. There is also an independent sealing chamber in the front sealing device, that is, the sealing space composed of sealing ring V1, sealing ring V2 and, which also needs to be filled with lubricating oil to maintain the balance between lubricating oil and external atmospheric pressure and ensure the sealing of the sealing ring.

3、 Treatment of leakage of tail shaft sealing device

The most common fault of the stern shaft sealing device is that the lubricating oil leaks to the sea or the sea water leaks to the sealing oil chamber. The tail shaft sealing device leaks oil, which not only wastes a lot of lubricating oil and causes direct economic losses, but also causes marine environmental pollution. Therefore, once the tail shaft sealing device is found to have oil leakage, great attention must be paid to it. The most thorough and effective way to deal with the oil leakage of the stern shaft sealing device is, of course, to dock and replace the sealing ring as soon as possible. However, in many cases, the leakage of the tail shaft seal is only temporary, and temporary measures need to be taken to prevent the leakage from spreading.

1. Treatment of oil leakage of tail shaft sealing device

If it is found that the tail shaft sealing device leaks oil to the sea surface, some or all of the lubricating oil can be replaced with high viscosity lubricating oil to reduce or improve the oil leakage by taking advantage of the characteristics of high viscosity lubricating oil, such as high viscosity and poor liquidity.

2. Treatment of seawater entering due to oil leakage of sealing device

If seawater is found to enter the stern shaft seal oil tank, the stern shaft seal oil tank will generally give an alarm at a high level. During inspection, it will be found that the lubricating oil in the stern shaft seal oil tank is emulsified. In this case, the leakage is generally caused by the poor contact between the two seal rings (No. 1 and No. 2) behind the stern shaft seal and the stern shaft due to the great vibration of the ship. At this time, it is not necessary to change the emulsified oil in the oil tank in a hurry, but to inform the captain to take appropriate measures to adjust (generally increase) the draft of the ship's stern shaft or change the ship's speed and course to reduce the ship's vibration. As long as the vibration of the ship is reduced, the seal between the seal ring and the stern shaft will become better, and the sea water leakage will naturally improve, and then the emulsified lubricating oil in the seal oil tank will be gradually replaced. The oil change method is generally as follows: drain the emulsified oil from the residual valve 1 while adding new oil from the filler pipe. This will take a while to complete, and you must always observe the lubricating oil in the oil tank. After the lubricating oil is replaced, check the lubricating oil condition from the residual place of the oil tank frequently. It should be noted that when the oil tank high level alarm occurs, do not block the overflow pipe 3, otherwise the pressure in chambers 1, 2 and 3 will increase, thus changing the pressure of P2, causing the emulsified lubricating oil to leak into the lubricating oil in the tail shaft tube.