Centrifugal compressor produces axial displacement, firstly because of the existence of axial force. The generation process of axial force is as follows:
After the gas passes through the working wheel, the pressure increases, making the working wheel bear different gas pressures. Because the axial force on both sides of the wheel from the external diameter D2 to the diameter Df of the wheel cover sealing ring counteracts with each other, its axial force consists of the following three parts:
1) F1-the gas force on the area from the diameter Df to the diameter df of the shaft seal ring on the back of the wheel plate;
2) F2-the gas pressure on the area from diameter Df to d at the inlet part of the working wheel;
3) F3 -- the impact force of the inlet airflow on the wheel plate at a certain speed.
In general, F1> (F2 + F3), so the oil thrust direction of each impeller points from the wheel plate side of the impeller to the inlet side (the wheel cap side ). If all impellers are installed in the same direction, the total axial thrust is considerable.
In order to reduce the axial thrust, the balance plate is usually used, and the pressure difference on both sides of the balance plate is used to generate the opposite force of the above-mentioned axial force to balance 70% of the axial force ~~ 90%. In addition, it can also make the air intake directions of working wheels of different levels opposite during design, or adopt double-sided air intake impellers to reduce axial force. The rest is borne by thrust bearing.
During the operation of the compressor, when the sealing of the balance plate is destroyed, or the small tube of the low-pressure cavity behind the balance plate is blocked, which loses the ability to offset part of the axial thrust, then the axial force of the rotor will increase sharply, which makes the thrust bearing unbearable, and finally causes larger axial displacement.
In addition, when the thrust bearing alloy is excessively worn or melted due to other sudden accidents (such as sudden oil break in the lubrication system), excessive axial displacement will also occur.
For safety, centrifugal compressor is equipped with axial displacement safety indicator. When the axial displacement exceeds the allowable value, the indicator will give out sound and light warning or stop automatically.
Remember! In any case, if the axial displacement is too large, it should be stopped immediately to avoid the occurrence of major accidents of the collision between the rotor and the fixed parts.
