Common Fault Diagnosis and Maintenance Points Exploration of Oil-free Screw Air Compressor

The compressed air system is one of the most important public systems in a power plant, and we need to make sure that the quality requirements are very high. When it is used as the vapor source of the air pressure nitrogen system, we should have higher requirements on the compressed air level. Dry oil-free screw air compressor are increasingly used in power plants because of their own characteristics.

Air compressor is the core equipment of compressed air system. According to the different structure and working principle, it can be divided into two categories: volumetric and power type, in which volumetric air compressor can be subdivided into reciprocating  and rotary  air compressor. Oil-free screw air compressor is a kind of volumetric gas compression machinery with rotary movement of working volume. Gas compression depends on the volume change to realize, and the volume change is realized with the help of a pair of rotors in the casing for rotary movement. Compared with reciprocating air compressor, it has the advantages of simple and compact structure, less wearing parts, simple maintenance and high reliability. Compared with centrifugal air compressor, it has the characteristics of forced air delivery, unaffected volume of air delivery by exhaust pressure, and excellent dynamic balancing characteristics, etc. However, the disadvantage of screw air compressor is that its rotating parts have higher repair and replacement costs.

For the specialization of the air compressor in the field of maintenance, excessive use of equipment will rely too much on professional maintenance personnel outside the plant, resulting in the consequences of high cost of equipment operation and maintenance and untimely discovery and treatment. In order to fundamentally avoid the accident of air compressor operation, the production unit needs to train its own operation and maintenance personnel with a certain degree of experience in air compressor maintenance. This paper takes the DS-250WC dry oil-free twin-screw compressor of Shouli Company of the United States as an example, and introduces its composition, working principle, common fault diagnosis and precautions for preventive maintenance. This plays a positive role in reducing power plant operation and maintenance costs and improving the reliability of key equipment.

1.1 Composition of oil-free screw compressor

Shouli DS-250WC is a two-stage compression, dry oil-free, water-cooled, low-noise fixed screw compressor driven by frequency conversion motor. It consists of air end assembly, air intake system, cooling and muffling system, lubrication system and vacuum ejection system.

The main equipment is enclosed in an acoustic enclosure and consists of components such as compression air end, electric motor, intercooler, oil cooler, after-cooler, oil pump, oil tank and unit base.

1.1.1 Air end assembly

The core of the oil-free screw air compressor is the compression air end. In order to improve the compression efficiency of the whole machine and reduce the temperature of the compression process, the compression air end adopts a two-stage compression method in its design. Take the high-pressure stage as an example to introduce the high-pressure part of the composition.

(1) What is rotor?

There parallelly configures a pair of mutually meshing spiral rotor in the air end shell, usually with convex teeth outside the pitch circle of the rotor  known as the male rotor or male screw. Rotor with concave teeth in the pitch circle, called negative rotor or negative screw. The cross-section of each rotor has a different shape and number of “teeth”, “groove”, respectively, 4, 6 teeth. Driven by synchronized gears, the positive rotor makes 6 rotations and the negative rotor makes 4 rotations per rotation cycle.

In the two ends of the air end, respectively, open a certain shape and size of the orifice. One is for suction, called the intake port; the other is for exhaust, called the exhaust port. The working area is the interleaf volume between the positive rotor and the negative rotor. It is larger at the intake port and decreases along the axial length of the rotor until the exhaust port, where this volume change drives the air to be compressed. The rotors are finely machined and assembled through more than two dozen processes to achieve extreme accuracy and durability of the rotor line shape and the clearance between the rotors. The surface of the rotor is coated with Teflon , which serves to prevent corrosion of the metal rotor.

(2) What are Bearings and gears?

The air end is equipped with anti-friction rolling bearings and precision gears, which are lubricated by the lubricating oil inside the air end. Among them, the radial bearings keep the rotor precisely positioned in radial direction and withstand radial force in the process of doing work to ensure the coaxiality and clearance of the yin and yang rotors. At the non-driving end of the air end, the yin rotor is driven by the yang rotor through the timing gear. At the driving end of the air end, the male rotor is connected to the main shaft of the air compressor through the speed increasing gear. The last pair of bearings on the yin and yang rotors can realize the function of axial positioning and bear the axial force in the process of doing work. It is shown in Fig. 1.

(3) What is seal?

Use four air seals and labyrinth seals  between the compression chamber and bearings to minimize air leakage from the head and to ensure that there is no crosstalk between the lubricant and the compression chamber.

1.1.2 Intake system and functional description

The air intake system consists of an air filter (hereinafter referred to as “air filter”), an air intake regulating valve, and an air venting muffler. After the ambient air is filtered by the air filter, clean air is fed into the air end of the compressor through the air inlet control valve. The regulating valve is a compact butterfly valve, which controls the inlet air to the compressor and at the same time allows the compressor to empty the pressure inside the compressor during unloading.

The inlet regulating valve is controlled by a two-position three-way solenoid valve. When starting the compressor, the solenoid valve is not energized and the butterfly valve is in the closed position, thus realizing the compressor’s light load starting. When the speed of the motor is close to its rated speed (about a few seconds later), the controller controls the solenoid valve to get power, the inlet butterfly valve opens, and at the same time, shut off the bleeder line, the compressor runs normally.

When the compressed air demand is lower than its rated discharge volume, the pressure in the discharge line gradually rises, when the pressure reaches the unloading pressure set by the controller, the controller controls the solenoid valve to de-energize, and the inlet valve butterfly valve closes under the action of the spring, while the bleeder valve opens, and the compressor unloads and runs.

The compressor reloads until full load operation. In case of shutdown, when the shutdown button is pressed, the controller will immediately control the solenoid valve to be de-energized, the inlet valve will be closed and the bleeder valve will be opened at the same time, and the internal compressed air will be bled off.

1.1.3 Cooling and anechoic system and functional description

The cooling and muffling system mainly consists of intercooler, exhaust muffler, exhaust check valve, aftercooler and condensate separator.

The air in the air end chamber generates a large amount of heat in the process of being compressed. The high-temperature compressed air from the primary air end passes through the intermediate cooler to cool and separate the condensate therein, and then is sent to the secondary air end. The high-temperature compressed air from the secondary air end goes through the exhaust silencer and check valve to enter the after-cooler for cooling, and then passes through the water separator to separate the liquid condensate, and then enters into the compressed air pipeline network.

Screw air compressor rotates at a high speed, the operation noise is large, this time the muffler plays a role in reducing the noise. Outlet exhaust check valve serves to isolate the compressor from the compressed air piping system during startup to ensure that the compressor starts with no load. When stopping, it prevents the compressed air in the compressed air pipeline network from flowing back into the compressor and realizes the unloading control of the compressor.

1.1.4 Lubrication system and functional description

The lubrication system mainly consists of independent oil pump, oil tank, oil cooler, oil filter, oil breather and so on. There is no contact between compressed air and lubricating oil in the air end, but its high-speed gears and bearings are lubricated by lubricant. Before the compressor starts, firstly start the oil pump, in a certain period of time, the system automatically checks the oil pressure, after it reaches the requirement, the controller then starts the main motor. Ensure that before the air end starts, the lubricant required for each part has been supplied in advance. After the air end stops, the oil pump will still continue to run for a period of time, continue to play the role of cooling, to extend the service life of parts. When the oil pressure is too high, the built-in pressure relief valve of the oil pump will introduce the lubricant directly to the bypass up to the inlet side of the oil pump.

The oil pump is flanked by a pre-filter to ensure that larger particles are separated before the lubricant is drawn into the pump. The lubricant from the pump is cooled by a cooler, filtered by an oil filter and directed to the various lubrication points of the air end of the compressor, and then returns to the oil tank.

1.1.5 Vacuum priming system and functional description

Vacuum ejector has 3 ports, a small amount of compressed air after cooling and condensate separation, is introduced from the exhaust pipe, filtered through the control line filter, and then goes through the pressure reducing valve to reduce the pressure as a primary fluid into the ejector inlet, the gas at the top of the tank (containing a small amount of lubricating oil) is filtered through the oil filter and then accessed into the ejector as a secondary fluid, the clean and oil-free air is directly discharged to atmosphere through the ejector outlet. The primary fluid inlet pressure is adjusted to ensure that the tank generates a slight negative pressure.

1.2 Specifications and performance parameters

The main specification parameters are shown in Table 1.

1.3 How does an oil-free air compressor work?

The Yin Yang rotor and the inner surface of the casing form a closed compression chamber, the volume of the compression chamber decreases from the suction port along the axial direction, when the screw compressor operates normally, the pressure of the air enclosed in the chamber rises continuously, and finally connects with the exhaust port of the casing, and the compressed air is discharged. Its working principle consists of suction, compression and exhaust processes.

1.3.1 Intake process

When the rotor rotates, the tooth groove space of the yin and yang rotor is the largest space when it turns to the opening of the air inlet end wall, at this time, the tooth groove space of the rotor is connected with the air inlet, because the gas in the tooth groove is completely discharged when the exhaust is completed, the tooth groove is in a vacuum state when it is turned to the air inlet, the outside world gas is sucked in, and enters into the tooth groove of the yin and yang rotor in the axial direction. When the gas fills the whole tooth groove, the inlet side of the rotor turns away from the casing air inlet, the air intake process is over.

1.3.2 Compression process

At the end of suction, the tips of its yin and yang rotor teeth will close with the housing, at which time the gas will no longer flow outward in the tooth grooves. The engagement surface gradually moves along the rotor axis toward the exhaust end. The space in the tooth groove between the engagement surface and the exhaust port gradually decreases, and the gas in the tooth groove is compressed and the pressure is increased, which continues until the inter-tooth volume pair is connected to the exhaust port.

1.3.3 Exhaust process

When the rotor’s meshing surface turns to connect with the exhaust port, the gas begins to be discharged until the tooth tip and the tooth groove of the meshing surface moves to the exhaust surface, at this time, the meshing surface of the yin and yang rotor and the casing of the exhaust port of the tooth groove space is 0, that is, the exhaust process is completed, at the same time the rotor’s meshing surface and the casing of the inlet port of the tooth groove between the length reached the maximum length and the inlet process is carried out once again.

2. Diagnosis and prevention of common faults: Oil-free Screw Air Compressor

2.1 Alarm for over temperature of air compressor

There are three external cooling water users, namely, the air cooler at the outlet of the air end, the head cooling of the air compressor, and the lubricant oil cooler. Find the cause when the temperature is abnormal alarm, after ruling out the abnormal thermal measurement point, it is necessary to analyze from the perspective of the cold source and the heat source at the same time. As a cold source, when the external cooling water is abnormal, it will probably lead to the following fault alarm, monitoring points of temperature alarm and the corresponding location as shown in Figure 2.

2.1.1 Outlet temperature of primary and secondary air end T1, T2

The value of one-stage (low-pressure stage) air end temperature T1 is determined by the temperature of air intake before air filter and the working condition of one-stage main engine. In the engineering design stage of equipment selection, the air filter before the air temperature range has been determined, when the temperature of the air sucked into the air compressor inlet is within the range of normal values, T1 is subject to the working state of the first-stage air end. The working state of the primary air end mainly includes the actual speed of the air compressor and its compression work heat production, and the actual cooling effect of the cooling water jacket. Further analyzed, when the speed of the air compressor is within the rated range, T1 is affected by the cooling capacity of the cooling water jacket in the first-stage air end.

The outlet temperature of the second-stage (high-pressure stage) air end T2, is determined by the inlet temperature of the second-stage air end T4 and the working condition of the second-stage air end. When the T4 temperature is within the normal requirements of the design and the air compressor speed is within the rated range, the T2 temperature is affected by the cooling capacity of the cooling water jacket in the second-stage air end.

2.1.2 Outlet temperature of the intercooler T4, outlet temperature of the aftercooler T5

The outlet temperature of the intercooler T4 is determined jointly by the inlet temperature of the intercooler T1 and the cooling effect of the intercooler. The outlet temperature of the after-cooling period T5 is jointly determined by the inlet temperature of the after-cooler T2 and the cooling effect of the after-cooler. When either the intercooler or the aftercooler inlet temperature is within the normal design range, the outlet temperature of the cooler is determined by the heat exchange effect of the cooler itself.

2.1.3 Outlet temperature of lubricating oil cooler T3

The outlet temperature of lubricant oil cooler T3 is determined by the temperature of inlet oil of lubricant oil cooler and the actual heat exchange effect of the oil cooler. When T3 exceeds the temperature alarm, first check the inlet temperature of the oil cooler, i.e. the temperature inside the lubricant return tank. When the inlet oil temperature of the oil cooler is within the normal design range, the flow rate of cooling water through the heat exchanger and the heat transfer coefficient will determine the actual outlet temperature.

Through the above analysis, it can be seen that when the parameters of the external cooling water system supplied to the air compressor are abnormal, it will inevitably cause the cooling function of each cooling user of the air compressor to be abnormal, which in turn will cause the T1, T2, T3 and T4 to exceed the temperature and alert.

Check and verify the actual water supply flow: by reading the pressure measurement point of cooling water supply pipeline in front of each air compressor and comparing it with the design range specified by the equipment manufacturer, and at the same time, it can be verified by checking the temperature rise of the cooling water inlet and outlet pipelines of the air compressor, e.g., the temperature rise of the cooling water in the DR-250CW model is less than 15°C.

The temperature of the actual water supply should be less than the upper limit of the design temperature specified by the air compressor manufacturer, which can be verified by reading the parameters of the cooling water system measurement point. For example, DR-250CW specifies that the cooling water inlet temperature should not exceed 32℃.

The influence of actual water supply quality on the heat exchange effect of each cooling user of the air compressor is reflected in the heat exchange coefficient. Unqualified cooling water quality will cause scaling on the water side surface of each heat exchanger, increasing the heat transfer thermal resistance, and the heat exchange capacity slowly decreases with the development of time and the actual scaling degree. Therefore, during the operation stage of the air compressor, we should regularly take samples of cooling water quality for laboratory tests. DR-250CW model cooling water requirements are shown in Table 2 below.

2.2 Stuck air compressor head: Oil-free Screw Air Compressor

The head clutches dead is the malignant accident of air compressor operation, which consumes a lot of time and material cost, such as replacing part of spare parts in light cases and scrapping the whole machine for replacement in serious cases. In order to ensure high working efficiency, the air end is designed with precise fitting clearance between internal parts. The structure is designed to configure radial support bearings, axial thrust bearings, gears, which can play a role in maintaining the dynamic and static clearance, i.e., between the yin / yang rotor, between the rotor and sealing chamber, end sealing dynamic and static clearance, transmission gear mesh clearance. Maintaining a reasonable gap between the dynamic and static parts is not only a guarantee of work efficiency, but also a guarantee of safety. Therefore, when the bearings or gears have abnormal faults and lose the precise positioning function, the dynamic and static clearance will inevitably change, which in turn will cause touching and grinding. Deduced from the structure, the causes of abnormal bearing or gear work can be found from the shaft, bearing, lubricant 3 aspects.

Small original unbalance force of the rotor is the screw compressor’s own characteristics, and the rotor surface coating has the function of preventing rotor corrosion. Therefore, only when the surface coating of the rotating screw damages and fails, it can cause the rotor deflection to increase, which in turn causes the rotor to rub directly against each other or the rotor vibration to increase. After the air compressor is shut down, the temperature of the work chamber drops and condensation dew is formed. If effective measures are not taken in time, the condensate will erode the surface of the rotor, resulting in the coating falling off. Therefore, introducing and correctly putting into the shutdown blowback device to prevent condensate from accumulating on the surface of the rotor, reducing condensate corrosion of the rotor coating, can extend the service life of the coating and prevent the rotor from being further corroded resulting in an increase in rotor unbalance force, especially in the coastal power plant, the air is relatively humid, the salt in the sea breeze is very easy to cause corrosion of the coating fall off.

The air filter is the first barrier to foreign objects. When the filter is thin or broken, the foreign matter will enter the air compression channel and cause the head to be locked. In daily production, we should include the air filter inspection as a regular task, and check the cleanliness of the air filter every 1000h. If the local air quality is poor, we should increase the maintenance frequency according to the situation. When maintaining the air filter, use less than 5kg of compressed air from the inside out along the direction of the oblique angle blowing, blowing mouth from the inner surface of the filter element is about 10mm, from top to bottom along the circumference, blowing point is that the whole process of blowing force should not be too small or too large. The number of times the air filter cleaned should not be more than 3 times, so at most every 4000h to replace the air filter.

Bearing performance is affected by normal mechanical wear and aging. It is necessary to regularly test the lubricant back to the tank oil quality, record and track the content of mechanical impurities, when exceeding the standard, use an external oil filter to filter out the mechanical impurities, to prevent the bearing components from increasing wear and tear, and ultimately cause the bearing fit clearance to exceed the tolerance standard. In addition, abnormal vibration conduction is the main cause of bearing or gear failure failure, and its typical feature is that the fault needs to go through the time to develop the process, i.e., the initial vibration exceeds the standard, then gradually increase, after developing to a certain extent the yin and yang rotor wear, end of the air seals wear, oil seals wear, and even the working chamber of high temperature compressed air run into the return pipeline, resulting in the lubricant exceeds the temperature of the boil, the oil tank respiratory bubble gas and other signs of abnormality, and finally the rotor hold dead. Whether in the unit after major and minor repairs or in the daily production stage, vibration monitoring should be implemented to detect anomalies in a timely manner and take measures to eliminate vibration. On-site vibration measurement should be selected at the two ends of the main shaft, primary/secondary air end/animal screw ends in a total of 10 positions, measuring, recording and comparing the horizontal, vertical and axial directions, as shown in Figure 3.

Air compressor with motor, selects the driving end and non-driving, measured in 3 directions at 2 places. Air end and supporting motor vibration standards are detailed in Table 3.

4. Conclusion: Oil-free Screw Air Compressor

(1) The first and second stage compression air ends are the core components of the dry oil-free screw air compressor, and the work of air inlet filtration, cooling and silencing, lubrication and vacuum pumping are the auxiliary processes. Understanding and mastering the process is the basis for analyzing and preventing faults.

(2) Abnormal external cooling water parameters are the main external factors of air compressor alarming for over-temperature, and the loss of precise positioning function of bearings or gears is the direct cause of stuck air compressor head.

(3) Dry oil-free screw air compressors are easy and convenient to maintain in the process of use, but the maintenance cost of air end parts is high. In addition to good regular inspection, we should strictly control the parameters of the external cooling water of the air compressor every day as well as strengthen the supervision of the level of daily vibration of the air compressor. Click here.