Typical Failure Analysis and Research of Oil-free Screw Air Compressor

Oil-free screw air compressor belongs to rotary compressor, which is mainly composed of compressed air system, condensate discharge system, lubrication system, cooling system (air cooling), cooling system (water cooling), electrical system. The working principle of air compression is that air enters into the compressor through the air intake port of compressor shell, and then enters into the air filter through the channel lined with sound-proofing material, and then enters into the first-stage spiral rotor and compresses it along with the round elastic hose through the air intake valve. Then along the round flexible hose, through the inlet valve enters the first stage spiral rotor and is compressed. The first stage compressed air passes through the intermediate cooler to reduce the temperature, and then the second stage compressor further compresses the air to the required pressure.

Oil-free screw air compressor can realize that the air compressed by the unit does not contain oil, and the oil circuit system is eliminated compared with the oiled air compressor. In order to realize that the compressed air discharged from the unit does not contain oil, in the oil-free screw compressor air end, use air seals, oil seals and other ways to ensure that there is no lubricant between the compression chamber or rotor, compressor bearings, gears and other parts are still used in the normal lubrication mode. Normally, the male rotor of the air end is connected to the motor and drives the female rotor to rotate, because the rotors are not in contact with each other, there is a pair of meshing synchronous gears at the end of the rotor, and the air compressor drives the two-stage rotor to rotate at different speeds through the driving gear in the center and outputs compressed air with different pressures at the same time.

Oil-free screw air compressor is easy to operate and maintain, and the feature of no oil circuit system saves a lot of daily maintenance cost, which is widely used in the field of oil and gas long-distance pipeline. In fact, oil-free screw air compressors also have more operational failures during operation. Taking a natural gas long-distance pipeline pressure station field as an example, our company analyzes the reasons for frequent air compressor failures in depth and improves the air compressor operation mode in view of the head rusting, aftercooler bellows breaking and main motor overloading failures that have occurred in two sets of SM110 oil-free screw air compressors in the industrial frequency, in order to provide a new way of thinking about the typical troubleshooting and prevention of oil-free screw air compressors.

1. Typical failure of Oil-free Screw Air compressor

1.1 Head corrosion

The air compressor has been in production operation for a number of times with high first stage exhaust temperature and high second stage inlet pressure alarms, as well as low rattling noises inside the air compressor during loading operation. During routine testing, a fault trip occurred after starting the unit, with an alarm message indicating insufficient motor speed. Open the rear cover of the air compressor and manually coiled the unit by driving the motor fan, which is confirmed to be a head hold. Since the clearance of the second-stage compressor is smaller, the possibility of jamming is higher, so the second-stage compressor is processed first. Processing method is to remove the two-stage compressor import and export pipeline, and it is found that the inner wall of the shell and the rotor are seriously rusted, disassemble the air end to manually coil the air end alone, and confirm that the first and second stage hosts are stuck by the rust.

The problem was dealt with by using kerosene flushing for the compressor, and at the same time, during the flushing process, the rotor of the yang rotor was constantly rotated manually, and then the rotor of both the first and second-stage compressors resumed their normal activities. After the start of the air compressor, according to the actual situation and changes in operating parameters to judge, the yin and yang screw sealing line coating has been peeling off, so that the gap between the yin and yang screw in the air end increased, resulting in the reduction of the compression efficiency of the second-stage air end, which can not compress the amount of air exported from the first-stage air end in a timely manner, resulting in the increased pressure between the first-stage and the second-stage air end, which then led to that the discharge temperature was significantly higher than the previous normal temperature.

1.2 Disconnection between aftercooler and inlet line connection

The inlet pipeline of the second-stage post-compression cooler is prone to cracking and air leakage. Due to the air compressor and cooler integrated into the skid, the inlet pipeline and cooler beam skeleton welded together, the cooler once cracked, with frequent loading and unloading of the load, the crack would continue to expand in the weld, and finally formed through the mouth. Repairing the cracked secondary compression aftercooler with aluminum welding requires high welding precision, which needs to be carried out under the guidance of the manufacturer and the later working life is difficult to be guaranteed.

1.3 Overload fault of main motor

Air compressor in normal operation appears repeatedly “main motor overload” fault shutdown. Open the electrical control cabinet of the air compressor, we found that the red alarm light of the thermal relay of the air compressor is on, after resetting the thermal relay of the air compressor, we reset the air compressor again, and the fault can be eliminated. The direct cause of the “main motor overload” of the air compressor is the protective action of the thermal relay.

2. Analysis of typical failure causes

2.1 Analysis of the causes of headstock corrosion

The most important factor affecting the coating of the rotor surface inside the air end is liquid water, which will cause serious damage to the coating of the rotor surface, the unit runs at a low loading rate for a long time, there is a significant pressure difference between the first and second stage of compression, and the exhaust pressure and temperature are large, which will lead to a large amount of condensate condensation in the air on the surface coating of the rotor of the air end, and then when the unit loads and unloads and switches to a critical state, the vibration of the machine will be larger than that of the unit at the time of unloading and loading, at the same time, frequent loading and unloading will also cause changes in the pressure ratio of the primary and secondary air ends. Therefore, if the unit is frequently loaded and unloaded for a long time, it will lead to large wear and tear of the air end and increase in temperature, which will lead to the shortening of the service life of the air end parts.

2.2 Analysis of the cause of fracture at the connection between the aftercooler and the inlet pipeline

After excluding the quality problem of the air compressor itself, the second stage compression bellow of the SM110 air compressor is connected with the inlet pipeline of the cooler in an “L” type, and there are certain design defects in the structural arrangement. When the air compressor is loaded and unloaded, the bellows frequently expands and contracts under the action of gas thrust, resulting in fatigue of the bellows, reduced strength, larger expansion and contraction, and the thrust is transmitted to the cooler, resulting in part of the weld of the cooler and the connection of the bundle of tubes to be subjected to stress and vibration and cracking.

2.3 Analysis of main motor overload fault causes

The air compressor adopts star-delta starter to make the peak current reduce greatly when the air compressor motor starts to accelerate. When the air compressor starts at reduced voltage, the main contactor and the sealed star point contactor are absorbed, the motor starts until the set time of the time relay, then the sealed star point contactor is disconnected, and the sealed delta contactor is absorbed to make the air compressor work normally, and when the air compressor is running normally, the sealed star point contactor is disconnected and does not participate in the work.

Disassemble the thermal relay, main contactor, sealed delta contactor and sealed star point contactor, we found that the contacts of the main contactor and sealed delta contactor have obvious erosion phenomenon, which is judged to be caused by heat and high temperature oxidation. Among them, the second phase of the main contactor connected with the thermal relay has the largest and most serious ablation area, and the moving contacts have been blackened, which is because the sealing star point contactor is only briefly involved in the buck start when the compressor is started, and it will not be absorbed after the motor startup is completed. The root cause of this is mainly due to the mismatch between the instrumented air supply and air consumption, the main motor of the air compressor will be frequently loaded and unloaded, and when the unloading state is changed to the loading state, there is a steep increase in current, which generates an inrush current that leads to contact burns, which in turn leads to contact failure and heating, and ultimately leads to thermal relay jumping, and the compressor stops with overloading faults.

3. Treatment measures

3.1 Optimization of operation methods

From the in-depth analysis of the cause of the failure, the air end rust, after-cooler bellows fracture, main motor overload failure, these are inseparable from the way the air compressor runs. To solve the above problems from the user’s point of view, the focus is to optimize the compressed air supply and consumption, and try to maintain a balance between the two. By reducing the frequency of loading and unloading of the air compressor, in order to reduce the number of air compressor electrical components impact current, which is conducive to the stability of air compressor mechanical and electrical equipment.

3.2 Improvement of air compressor drainage measures and bellows modification

Take effective measures to improve the unit drainage, which can effectively reduce the air compressor exhaust pressure and exhaust temperature, and prevent the head from rusting and destroying the rotor coating. Regular daily running test of the standby air compressor to more than 15 minutes is appropriate, the air compressor must also be manually coiled during power down or maintenance. Increase the frequency of drainage, daily timed to open the standby air compressor outlet manual valve drainage and sewage, you can discharge condensate through the electronically controlled solenoid valve, reasonably set up solenoid valve opening time, for the high temperature and high humidity of the environmental conditions, the opening time is appropriately shortened to 180s and the interval is appropriate; for the air end broken has been repaired, you need to put the manual drainage valve in a long time in the open position, avoiding the liquid water stored inside the equipment. In addition, open the manual drain valve of the air compressor to reduce the first stage exhaust pressure of the air compressor to prevent the exhaust temperature from being too high due to the high second stage inlet pressure.

Modify the structure of the equipment by machining 4 tie rods around the bellows to increase the strength of the bellows to a certain extent and allow the bellows to have a certain margin of expansion and contraction, and the bellows will frequently expand and contract under the action of the gas thrust, so as to keep the tension at the connection between the aftercooler and the inlet pipeline within the tolerable range.

3.3 Implementation of preventive replacement of spare parts

Contactors, thermal relays, solenoid valves and other spare parts play a key role in the operation of the air compressor, according to the frequency of failure of each spare part and its own service life, combined with the spare parts needed to make key equipment to replace the list, and implement the planned preventive replacement, for example, the air compressor needs to carry out cyclic maintenance for every 2000h of operation, the AC contactor contacts check, and found the hidden danger and replace preventively.

4. Conclusion

Our company analyzed in-depth the typical failures of Ingersoll Rand SM110 oil-free screw air compressor in a natural gas long-distance pipeline, and studied in-depth the reasons for frequent failures of the air compressor in daily operation according to the working principle of oil-free screw air compressor, and by optimizing the operating mode of the air compressor, the drainage measures of the air compressor and the reform of the bellows, and carrying out preventive replacements in a timely manner, we improved the typical failures such as corrosion of the air end, breakage of the connection between the secondary post-compressor cooler and the inlet pipeline, and overload of the main motor.   This not only improves the service life of the equipment and reduces equipment failures, but also prevents hidden dangers of air compressor operation in advance, providing a reference for the safe and smooth operation of air compressors in industrial production.