The inspection and maintenance of centrifugal compressor bearings (i.e. sliding bearings) is the core link to ensure the safe, stable and long-term operation of the unit. The inspection content is systematic and precise, mainly covering appearance inspection, gap measurement, contact status assessment and defect detection.
1. Core contents and methods of bearing inspection
1. Macroscopic and appearance inspection
This is the most basic inspection and is designed to detect obvious defects on the bearing surface.
(1) Inspection focus: Carefully observe whether there are traces of pull marks, burns, alloy peeling, cracks, corrosion or wear** on the surface of the bearing bush (especially the Babbitt alloy layer).
(2) Bonding fastness detection: Check whether the alloy layer and the steel tile backing are peeled off (poor bonding) by tapping the Babbitt alloy layer and listening to the sound, or using the kerosene penetration method.
2. Accurate measurement of critical dimensions
The clearance and tightness of the bearing bush are the most critical parameters affecting the formation of oil film and the stability of the rotor.
(1) Radial clearance measurement: Radial clearance refers to the gap between the journal and the bearing bush. If the gap is too small, the oil film will be thin and the bearing will easily overheat; if the gap is too large, the oil film will have poor rigidity and the rotor vibration will increase. Commonly used measurement methods are:
(2) Axis lifting method: The most common method. Use a tool to slightly lift the spindle and read the two dial indicators installed on the shaft and bearing seat to calculate the clearance value. For tilting pad bearings, different tile numbers have corresponding correction coefficients (such as 5 pad clearance = 0.894 × shaft lifting amount).
(3) Lead pressing method: Place a soft lead wire with a diameter 30%~50% larger than the measured gap on the journal, reinstall and tighten the upper tile, then disassemble it, and measure the thickness of the flattened lead wire. This method is simple, but the error is large when used for tilting tiles.
(4) Dummy shaft (sample shaft) method: Make a dummy shaft that is very close to the real journal size (with a tolerance within 0.05mm), install the bearing on the dummy shaft to simulate real working conditions for measurement, the results are accurate, and it can avoid frequent lifting damage to the main shaft.
(5) Thrust bearing (thrust bearing) clearance measurement: This refers to the axial movement of the rotor between the working surface and the non-working surface. Usually, the dial indicator is fixed on the stationary part, the meter head is against the smooth end surface of the rotor, and then the rotor is pushed and pulled back and forth along the axis. The difference between the dial indicator readings is the axial clearance.
(6) Inspection of bush back-tightening force (remaining surface height): refers to the amount of interference between the outer diameter of the bearing bush and the inner hole of the bearing seat to ensure that the bearing bush will not rotate during operation. It can be calculated by placing lead wire in the back of the tile and the bearing seat, and measuring the thickness difference after tightening.
3. Contact status check
(1) Pad back contact: The bearing pad back and the bearing seat hole should be in even contact. It is usually required that the contact area is greater than 85% and the contact points are evenly distributed. When inspecting, the coloring method is commonly used. Apply red red powder on the back of the tile, then grind it and observe the distribution of contact spots.
(2) Pad contact: For bearings with a pad structure, when not bearing the weight of the rotor, the pads on both sides should not be inserted with 0.03mm feeler gauge**, and the lower pad should have a gap of 0.05~0.07mm.
(3) Fulcrum contact: For tilting pad bearings, it is also necessary to check the contact spots of the fulcrum (ball head) on the back of the pad to determine whether the load distribution is uniform.
4. Non-destructive testing
For critical or suspicious parts, more precise flaw detection is required.
(1) Coloration (Penetration) Detection (PT): Specially used to detect defects such as fine cracks, pores and peeling on the surface of Babbitt alloy and the joint surface of the alloy and steel back. The principle is to use penetrating liquid to penetrate into surface opening defects and then display them through the developer. During operation, steps such as pre-cleaning, penetration, cleaning, and imaging must be strictly followed.
2. Special inspection for tilt pad bearings
Modern large centrifugal compressors widely use tilting pad bearings. In addition to the above routine inspections, you also need to pay attention to:
1. Tile swing flexibility: Check whether each tile can swing flexibly on its fulcrum.
2. Thickness consistency of tiles in the same group: The thickness difference of tiles in the same group should be controlled within ≤0.01mm.
3. Inspection of the inner and outer arcs of the thrust tiles: Check whether there are any scratches on the inner and outer arcs of the thrust tiles.
3. Common faults and solutions
If the following problems are found during maintenance, they need to be dealt with accordingly:
1. The bearing temperature rises abnormally: Possible reasons include too small clearance, deterioration of lubricating oil, damaged bearing, etc. It is necessary to adjust the gap, clean the oil circuit or replace the bearing bush.
2. Bearing wear or burning: often caused by excessive vibration of the rotor, impurities entering the bearing, and poor lubrication. It is necessary to dynamically balance the rotor, clean the system thoroughly, and check the oil circuit.
3. Bearing carbon deposit: formed by the oxidation of lubricating oil at high temperatures. Carbon deposits need to be removed, and the lubrication and cooling systems need to be checked.
4. Maintenance cycle and precautions
1. Maintenance cycle: usually during overhaul, it is generally recommended to check once every 8000~12000 hours. If there are abnormalities such as bearing temperature exceeding 95°C, increased vibration, or oil pressure drop, inspection should be arranged immediately.
2. Key notes:
(1) Accurate measurement: All clearance and tightening force measurements must be accurate and must comply with the equipment technical documentation after adjustment.
(2) Standard operation: It is strictly forbidden to use sandpaper to polish the bearing alloy to avoid abrasive embedding. When reassembly, it must be installed in place according to the markings.
(3) Cleaning management: During the maintenance process, strict precautions must be taken to prevent foreign objects from falling into the bearings. The oil circuit must be thoroughly cleaned before reassembly.
(4) Detailed records: Record all maintenance data promptly and clearly.
Centrifugal compressor bearing inspection is a task that requires extremely high accuracy and standardization. The core is to discover surface problems through macro inspection, ensure dimensional matching through precision measurement, and detect hidden defects through special inspection to ensure the safe operation of the unit.
Source: Basic knowledge of benchwork
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