Gas compression is essential in petrochemical and fertilizer industries, and as a reciprocating compressor, it is more widely used in petrochemical and small and medium-sized nitrogen fertilizer industries because of its wide pressure range, high efficiency and strong adaptability. Disadvantages are large appearance and weight, need a large foundation, air pulsation, and many wearing parts, which increases maintenance work. Based on the above characteristics, the correct maintenance of reciprocating piston compressor in use is particularly important. Correct maintenance is very important for the operating rate, output rate and cost reduction of tons of finished products. Four new large reciprocating compressors in our company, with a single wind capacity of 1.90 m3/min, have had a series of accidents due to various reasons during use. Here, we will introduce them for your discussion.
First, eccentric wear of crank pin bearing:
The connecting rod transfers the thrust acting on the piston to the crankshaft, and then converts the rotary motion of the crankshaft into the reciprocating motion of the piston. A compressor in our company has frequently suffered eccentric wear accidents of the first-stage crank pin bush for a period of time, and the eccentric wear direction remains unchanged. Mainly from the following aspects of analysis and processing.
1. Carefully check the bearing clearance of the crank pin, the clearance between the crosshead pin and crosshead, the clearance between the connecting rod and the head shoe, the six-point clearance between the crosshead and the slideway, and the ellipticity of the crank pin shaft. Replace the crosshead pin with a new one to ensure the clearance of each part.
2. The parallelism of the large and small head holes of the connecting rod is detected by special tools, and the parallelism of the crosshead pin hole and the primary crank pin shaft is also detected by special tools. Parallelism is not more than 0.02—0.03mm, which is within the allowable range.
3. The design value of piston rod runout shall not exceed 0.07mm/ full stroke, and the safety engineer shall add favorites within the design scope on site.
After the above three points are confirmed, the crank pin bushing is still eccentric wear only three days after the overhaul unit is started. Finally, it is found that there is something wrong with the alignment of the crankshaft coupling, which leads to the deflection of the farthest end of the crankshaft, which is the most obvious, thus causing the eccentric wear of the crank pin bushing. By readjusting the coaxiality of crankshaft and motor and adjusting the clearance of main bearing bushing, the problem of eccentric wear of crank pin bushing is completely solved.
Second, the damage of the crank pin journal
Because of the original design error of the compressor in our company, the first-stage piston force and the first-stage crank pin shoe specific pressure are too large after transformation. The main bearing, main piston and connecting rod are often damaged.
The specific pressure bear by that bearing shoe is:
Pmax=Rmax/db'N/mm2
Rmax-bearing support pressure n;
D—— bearing inner diameter mm;
B'—— the calculated width of bearing is mm, for thin-walled tile b'= b (bearing width), for thick-walled tile b'= b-2r, and r is the radius of fillet on both sides of bearing;
For large and medium-sized compressors, the allowable specific pressure of main journal and crank pin journal bearing is 4-5 MPa respectively; 9.0 MPa. Under the condition of ensuring the maintenance quality, by adjusting the reasonable clearance of crank pin bushing, the oil supply situation and oil pressure adjustment are improved, and an effective oil film is formed. By adjusting the inlet pressure and selecting the bearing bush with appropriate specific pressure, the damage of bearing bush caused by imbalance is solved, and then the damage of crank pin journal is solved.
Third, the handling of crosshead pin
When the compressor is running, the compression screw on the end face of the crosshead pin breaks and the crosshead pin falls off, which will cause very serious accidents. Therefore, special attention should be paid to the clearance between the crosshead pin and the connecting rod small head tile. More importantly, there can be no gap between the crosshead pin cone and the crosshead body, because in theory, once there is a gap, contact is line contact, which has a great influence on force transmission and the stability of the unit. Therefore, the contact area required for research and development should be above 80%. If there is a gap between the new spare pin and the crosshead pin hole, it is not allowed to repair the crosshead taper hole, because once the crosshead taper hole is repaired, it is likely to change the axial position of the future pin, thus affecting the positioning and the supply of lubricating oil. If the deviation of the pin is too large, you can measure the radial dimension, file and locate it on the lathe, and then grind it, but still use the appropriate spare parts taper pin.
Four, the connection of the piston rod tail:
Five, the piston and piston rod damage:
In use, the first-stage pistons of four compressors are all broken, and the piston rods are also broken. The connection between piston rod and piston must be firm and accurate. The perpendicularity between the positioning shoulder of the piston rod and the center line of the piston meets the requirements. The shoulders at both ends of the piston should match the supporting surface of the piston rod and be fastened according to the specified fastening torque. The joint surfaces of the piston halves (cast aluminum) should be closely attached, and there should be no gap between the joint surfaces of the inner and outer rings. This point should be highly valued. Because the clearance of the inner ring joint surface will produce alternating stress, thus shortening the service life of the piston, and the clearance of the outer ring will make compressed gas enter the piston cavity, making the internal volume become the clearance volume of the cylinder to a certain extent, which will have adverse effects on the efficiency of the compressor and the service life of the piston. Therefore, all parts of the assembled piston should be carefully inspected and studied. In addition, the mechanical properties and smoothness of the bearing surface of the piston tail rod also have great influence on the life of the piston rod. Due to the specific pressure on the bearing surface:
q=4p/π(D2-d2)
Where p is the piston force, and d and d are the diameters of the outer circle and the inner circle of the end contact surface of the tail rod under the action of pressure volume respectively.
The smoothness and hardness of this surface are strictly required, so it should be carefully checked before use. The smoothness is generally required to be above Ra0.8, and the carburized layer should be fine martensite structure, and needle-like or net-like free cementite is not allowed. Otherwise, it is easy to cause the stress surface to be hairy, which seriously affects the use of the piston rod.
Six, piston rod jump abnormal processing:
Generally speaking, the piston rod jump is the final verification result of compressor alignment, which should be within the allowable range. When the cylinder and crosshead slideway are correctly aligned, the allowable horizontal and radial runout of the piston rod should be a tolerance zone, that is, the stroke of 0.00015 mm/mm, not exceeding 0.064mm, and the slight variation of the piston rod deflection should also be considered for vertical and radial runout. Even if the out-of-tolerance is from one side to the other, the value will continue to increase or decrease, which can be eliminated by readjusting the horizontal and all-directional positioning. However, the following anomalies were found during the detection. During the movement of the piston, the vertical jump of the piston rod is always good, but a wide range of jump changes suddenly appear at the dead points on both sides, and the horizontal jump is good. After eliminating the influence of the clearance between the connecting rod and the cylinder head gasket, the clearance between the crosshead and the dead center of the cylinder, it is found that the piston rod jumps suddenly. The treatment method is to remove the wear shoulder on the inner wall of the cylinder, adjust the piston ring runout value, and then eliminate the piston rod runout.
Seven. Concluding remarks
In addition to the above summary, in order to make the compressor run normally and prolong its service life, a regular maintenance system should be established in strict accordance with the random instructions. Compressor maintenance should be carried out in a planned and step-by-step manner, and methods suitable for this enterprise unit should be constantly summarized, which is believed to achieve satisfactory results.