Centrifugal compressor
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I’ve read some articles recently about balance discs and thrust discs. I’m a bit dizzy; I’d like to ask you all a few questions 1. Where are the balance disk and thrust disk installed? Many sources state that the balance disk is installed between the compressor stages. But we install it on the high-pressure side of the compressor, not between the stages ； We install the thrust disc on the low-pressure side of the compressor, not on the high-pressure side. So where exactly should it be installed, and are there any regulations regarding this? 2. I have always believed that the axial force of the compressor acts from the high-pressure side to the low-pressure side; therefore, the balance disk and the thrust disk should be installed between the stages or on the low-pressure side. Why are they installed on the high-pressure side? 3. Our centrifugal compressors are driven by turbines. During this maintenance work, when we adjusted the compressor’s position, we pushed it firmly in the direction of the turbine; the axial displacement of the compressor shaft was found to be only 32 microns. In other words, even if the compressor does shift, the displacement is at most 0.32 mm. However, in actual operation, this axial displacement is gradually increasing – it has now reached 0.59 mm, and there is a tendency for it to increase further. I can’t figure out where this additional displacement is coming from Could I ask you all for some advice? I earnestly hope for your guidance
The balance disk relies on a medium to create a pressure difference on both sides of the disk, thereby balancing the axial force of the rotor. The thrust disc relies on thrust bearings to directly bear all the axial forces that remain unbalanced by the rotor’s balancing devices. The balance disk is installed inside the compressor cylinder. The thrust disc is installed in the bearing housing.
This post was last edited by wuchangjian on 2015-8-19 at 12:34. 1. The position of the balance disk and thrust disk is determined by the designers. Based on the power models and such, there are no specific regulations regarding this; I think the users don’t need to worry about these things. Once it’s designed, it’s not possible to change the positions of these components later on ； 2. The serial quantity you measure is based on static testing; the result will be different under dynamic conditions. It’s definitely more accurate in a dynamic setting.
The balance disc is located on the high-pressure side; on one side there is the pressure of the high-pressure gas, while on the other side there is the inlet pressure coming from the balance tube. A pressure difference is created between these two sides, which acts in the direction of the low-pressure side and thus balances the axial thrust. The thrust disc is located on the low-pressure side, with thrust bearings installed on both sides to balance a portion of the axial force.
The changes in values are related to zero-point calibration; you can discuss this with the instrument manufacturer
There are two aspects: one is an instrument issue and the other is a mechanical issue. Since mechanical issues are more common, proper installation is key.
The balance disk must be on the high-pressure side; otherwise, where would the thrust in the opposite direction come from? It comes from the gas pressure behind the impeller on the high-pressure side. The sudden increase in 58 silk levels is probably due to an issue with the instrument
A gradual increase in shaft displacement first rules out any issues with the installation of the instruments. Excessive axial force can cause wear on the shaft bearings, which in turn leads to an increase in their temperature; in such cases, it is necessary to stop the machine and check the bearings
The machine should be shut down to check whether there are any issues with the gap voltage of the instruments; additionally, it is necessary to monitor whether there are any changes in the temperature of the thrust bearings.
The balance disc and the thrust disc are two different concepts. The balance disk primarily serves to balance the axial force of the compressor at the structural level, playing a key role in this balancing function. The thrust disk acts as a supplement to the balance disk in terms of balancing the axial force; generally, any remaining axial force is balanced by the thrust disk. Therefore, the main thrust bearings and the auxiliary thrust bearings are installed on either side of the thrust disk, thereby helping to balance the axial force from left to right. The position of the balance disc depends on the design of the compressor’s body, while the thrust disc is generally installed at the non-driving end. In my opinion, the shaft displacement of the compressor has nothing to do with the turbine; it can be measured using the standard methods for measuring shaft alignment, and then the zero point can be set accordingly.
There is such a situation: for example, the clearance of the thrust bearing in a static state is 0.30 mm, but the alarm threshold set by the control system might be around 0.56 mm. So why is it designed this way? There are mainly two issues. The first is related to the entire unit; at least in terms of the bearing areas, it is necessary to take into account the changes in expansion that occur under hot conditions. The second issue is that during high-speed rotation of the rotor, vortices can form in the lubrication film at the frictional areas, a phenomenon known as suspension; it is also important to consider the behavior of babbitt metal under conditions of overheating.