The main shaft seal of the turbine is an important protection for the turbine.
It is installed under the water guide bearing to prevent the pressure water from leaking into the pit between the main shaft and the top cover (or support cover), flooding the water guide bearing, destroying the work of the water guide bearing, and affecting the operation of the unit.
According to the working nature of the main shaft seal of the turbine, it can be divided into maintenance seal and working seal.
1. Maintenance seal
The maintenance seal is a seal that works during the maintenance of the turbine.
It is a bearing seal required when shutting down or overhauling the working seal and water guide bearing of the turbine.
It prevents backflow into the pit when the tail water level is high.
The maintenance seal can be divided into air belt type, mechanical type and lifting type maintenance seal according to its structural type.
But in practice, most of them use air belt maintenance seal.
It is mainly composed of a fixed ring and a rubber air belt.
During operation, fill the air belt with 0.4-0.7Mpa air pressure to expand the rubber of the air belt, tighten the main shaft and prevent water from entering.
When the machine is turned on, the compressed gas in the air belt is discharged to shrink it, so that a gap of 1.5-2mm is maintained between the air belt and the main shaft.
The maintenance seal has a simple structure, easy operation and good sealing effect. Most unit maintenance seals use this structure.
2. Working seal
According to the sealing structure, the working seal can be divided into:
● Flat plate seal
Flat plate seals include single-layer flat plate seals and double-layer flat plate seals.
Single-layer flat plate seals mainly use a single-layer rubber plate to form a seal with the end face of the stainless steel rotating ring fixed on the main shaft, and seal by water pressure.
Its structure is simple, but the sealing effect is not as good as that of the double flat plate seal, and its service life is not as long as that of the double flat plate seal.
The double-layer flat plate seal has a good effect, but its structure is complex and leaks when the machine is lifted. It is currently still used in small and medium-sized axial flow units.
●Radial seal
Radial seal is composed of several fan-shaped carbon blocks pressed tightly against the main shaft by springs in steel fan-shaped blocks, forming layers of seals.
There are small drainage holes in the sealing ring to drain the leaked water.
It is mainly sealed in clean water, and its wear resistance is poor in water containing mud and sand.
This seal has a complex structure, difficult installation and maintenance, difficult to guarantee the performance of the spring, and small radial self-adjustment after friction.
So it has been basically eliminated and replaced by end face seals.
●Packing seal
The packing seal is composed of bottom seal ring, packing, water seal ring, water seal pipe and gland.
It mainly relies on the bottom seal ring and gland to press the packing in the middle to play a sealing role.
This seal is commonly used in small horizontal units.
●End face seal
There are two types of end face seals: mechanical and hydraulic.
Mechanical end face seal It relies on the spring to pull up the disc with the round rubber block, so that the round rubber block and the stainless steel ring fixed on the main shaft are in close contact to play a sealing role.
The rubber sealing ring is fixed on the top cover (or support cover) of the turbine. This kind of sealing structure is simple and easy to adjust.
However, the spring is unevenly stressed, prone to bias, wear, and unstable sealing performance.
● Labyrinth ring seal
The labyrinth ring seal is a new type of seal that has appeared in recent years.
Its working principle is to provide a pump plate device on the top of the turbine runner.
Due to the suction effect of the pump plate, the main shaft flange is always in the atmosphere, the shaft and the shaft seal are not in contact and there is only a layer of air, and the seal has an extremely long life.
The main shaft seal is a non-contact labyrinth type, consisting of a rotating sleeve close to the shaft, a sealing box and a main shaft seal drain pipe.
Under normal operation of the turbine, there is no water pressure on the sealing box within the entire load range.
The pump plate on the runner rotates with the runner to prevent water and solids from entering the main shaft seal.
At the same time, the pump plate drain pipe prevents sand or solid matter from accumulating under the water machine top cover, and discharges a small amount of water leaking through the upper leak-proof ring to the tail water through the pump plate drain pipe.
When the start-up and shutdown speeds are reduced, the water pressure may reach the shaft seal.
At this time, the leakage pressure is greatly reduced due to the diffusion effect of the labyrinth ring seal box.
At the same time, the main shaft seal drain pipe located between each labyrinth ring can effectively prevent water from leaking on the seal box and discharge the leaking water to the power station water collection well.
This new type of seal has been gradually promoted and applied in many power stations, and the operation effect is good.
