JZ2LG20 screw compressor overhaul steps

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1 Composition and working principle of screw refrigeration compressor units. The JZ2LG20 type screw refrigeration compressor unit is an open-type twin-screw compressor equipped with an economizer; the compressing medium is ammonia. Under standard operating conditions of -15°C to 30°C, its volume ratio is 3.6, its pressure ratio is 5, and its cooling capacity is 628.2 KW. It includes: a compressor (comprising five parts: the casing, rotor, slide valve, shaft seal, and coupling), an air circuit system, an oil circuit system, a motor power system, and a control system. The screw refrigeration compressor is a rotary positive-displacement compressor that relies on the reduction in volume of the gas as it enters the machine, which leads to a sharp increase in gas density and thus an increase in the pressure of the gaseous refrigerant. Inside the body of a single-stage screw refrigeration compressor, there is a pair of male and female rotors: the male rotor (the driving rotor) has 4 teeth, while the female rotor (the driven rotor) has 6 teeth. With each rotation of the male rotor, the compressor completes four cycles of suction, compression, and exhaust. At the suction ends of the male and female rotors, there are two thrust ball bearings each to bear the radial forces. There is a main bearing at each of the exhaust and intake ends of the two rotors, and these bearings bear the axial thrust. The balance piston, located at the rear of the shaft journal at the suction end of the rotor, serves to balance axial forces and reduce the load on the main bearings. At the lower part of the male and female rotors, there is an energy regulation slide valve driven by a piston in an oil cylinder; it is controlled electromagnetically, allowing for stepless adjustment of the cooling capacity within the range of 10% to 100%. This setup also ensures that the compressor starts at a low speed, thereby resulting in a low starting torque. The operating position of the slide valve can be converted into an energy percentage by an energy sensing mechanism, and this value is displayed on the control panel of the unit. The major overhaul of a 2-screw compressor is generally carried out every three years. The specific steps for such an overhaul are as follows: 2.1 Preparatory work before the major overhaul of the screw compressor 2.1.1 Preparation of maintenance tools and materials (1) Maintenance tools: A set of specialized tools for major overhauls of screw compressors (usually provided with newly purchased compressors), a set of hex key plates, a set of dial indicators, a set of feelers, a 5T hydraulic puller, several adjustable wrenches, a 10-inch sliding wrench, an 8-inch flathead screwdriver, a 5P iron chisel, a 9” inner spring clip pliers, a vernier caliper, and a triangular scraper, among others. (2) Maintenance materials: maintenance tray (made of 304 stainless steel, with dimensions δ6X800X1200, and a Ф10 hole drilled at each corner of the tray; used for placing and removing the screw machine head as well as for draining the refrigerant oil that seeps out from the head), 20 KG of diesel, 10 KG of clean cotton rags, several thick cardboard sheets, 4 5L plastic or iron barrels, 1 2KW electric furnace, two brushes, several steel saw blades, one iron cylinder with an opening of Φ250X300, three clean empty barrels for crude oil (with a capacity of 170L each); 300 KG of L-ARD-46# refrigerant oil per unit; two 6316E motor bearings, two motor dust covers, and 10 KG of molybdenum disulfide lubricant. (3) Main components for the major overhaul of screw compressors: one set of rubber seals for the entire unit, 4 NSK angular contact thrust ball bearings, 4 main bearings, 4 inner adjustment shims, 4 outer adjustment shims, one balance piston, one balance piston sleeve, one set of oil pistons, one spiral duct, one set of spiral tube packing, one skeleton oil seal, one 1000 bearing, 4 anaerobic adhesives, one set of retaining rings for the entire unit, one set of bearings for the main unit, 2 sets of coupler diaphragms with an inner diameter of 120, one suction filter screen, one fine oil filter element, one coarse oil filter element, and 2 high-molecular oil filters, etc. 2.1.2 Preparatory work before disassembling the unit: (1) Operate the screw compressor until the oil temperature reaches around 48°C, then stop the main engine; run the oil pump for about one minute before cutting off the main power supply. Close the main inlet and outlet valves of the screw compressor, and use the pressure in the system to remove the oil accumulated in the oil cooler and oil separator; thereafter, remove the residual gases in the system, including those in the oil separator, until the system pressure reaches zero. (2) Remove the local insulation material from the screw compressor head, and ensure that all lifting equipment (including steel cables, lifting lugs, eye bolts, etc.) is in good condition and ready for use. (3) Unscrew the oil drain plug at the bottom of the screw compressor head, turn the coupling to drain any remaining oil from inside the head, and then reinsert the oil plug. 2.2 Disassembly and Installation of Screw Machines 2.2.1 Steps for Disassembling the Screw Machine Head (1) Remove the coupling guard, suction filter, suction check valve, oil pipes, coupling, and compressor foot bolts from the compressor unit. Then disconnect the flange that connects the main unit to the oil separator, and lift the compressor onto the maintenance tray prepared at the repair site. (2) Remove the cover of the energy level indicator and the electrical components, as well as the oil drain groove at the bottom of the cylinder. (3) After unscrewing the positioning pin, remove the suction end cover parallel to it in order to take out the oil piston; after removing the positioning pin, detach the suction end seat. (4) Remove the shaft seal cover and take out the stationary and rotating rings of the shaft seal. (5) After unscrewing the positioning pin, remove the exhaust end cover; loosen the round nut and take off the thrust bearing, being particularly careful to make assembly marks. (6) Remove the driving rotor using a special lifting ring screw, taking care to do so gently and evenly; at this point, the driven rotor is still in motion, so the driving rotor needs to be rotated; use the lifting ring screw to remove the driven rotor as well. (7) After unscrewing the positioning pin, remove the exhaust end seat and take out the slide valve. 2.2.2 Inspection and installation of the screw compressor head (1) Check whether the main bearings of the screw compressor are worn. There should be no rust, cracks, or other defects on the surface of the rotor shaft journals as well as on the surfaces of the shaft sealing components. The dimensions of the main shaft journal surfaces, after being polished, should be carefully measured in order to repair the main bearings accordingly; if the wear on the main bearings is severe enough to exceed the allowable clearance with the shaft, then new main bearings must be installed. The new main bearings do not have oil groove lines; these lines must be created using a triangular scraper. The bearings are then installed in their respective bearing holes at the suction and exhaust ends, and their inner diameter is measured to ensure that it meets the required clearance for fitting with the rotor shaft journal (0.0325–0.0475 mm). (2) Apply sealant on the surface where the suction end seat fits against the machine body; care should be taken to apply the sealant evenly. The suction end seat is placed at the intake end of the machine body, and after the positioning pin is pressed in, it is fixed using bolts. (3) Install the slide valve and its guide plate; the guide plate must first be positioned using positioning pins before it can be fixed with bolts. (4) After applying refrigerant oil of the same grade as that used during normal operation to the main bearing hole at the suction end and the internal holes of the machine, the male and female rotors are installed; the rotor that is installed last should be inserted slowly, without forcing it into the machine. The end faces of the two rotors should be in close contact with the suction end seat. At this time, the dimension between the rotor’s exhaust end face and the exhaust end seat should meet the specified assembly clearance for the unit (0.08–0.10 mm). (5) Apply sealant on the surface where the exhaust end seat fits against the engine body, making sure to apply it evenly; place the exhaust end seat at the exhaust end of the engine body, secure it in position with positioning pins, and then fix it with bolts. When installing the exhaust end cap, be careful of the inner bore of the main bearing and avoid scratching it. (6) Insert the adjustment shims and thrust bearings, and secure the inner ring of the thrust bearing to the rotor journal using a set screw; pay attention to the orientation of the thrust bearing. Install the bearing collar; once installed, gently rotate the driving rotor in the actual direction – the rotation should be smooth. Check and adjust the axial clearance between the main screw (male rotor) and the secondary screw (female rotor); use inner and outer adjustment washers to keep the axial clearance within the range of 0.08mm–0.10mm. The axial clearance of these screws directly affects the amount of work done by the screw mechanism. Adjustment method: The axial clearance between the male and female rotors is controlled primarily by increasing or decreasing the thickness of the internal and external adjustment washers. After installing the male and female screws and tightening the cap nuts, a dial indicator is used to measure the axial clearance until it meets the specified installation requirements. (7) After driving the exhaust end seat into the positioning pin, secure it with bolts; install components such as the shaft seal moving ring, and apply refrigerant oil to the friction surface of the moving ring; then install the shaft seal cover and the stationary ring. (8) Oil filling piston, suction end cover; install an energy indicator, making sure the pointer corresponds to the position of the slide valve. (9) Lift the assembled compressor into the unit and align it with the motor, then install the coupling after confirming coaxial alignment. 2.2.3 Replacement of various filters in screw compressors: (1) Replacement of coarse oil and fine oil filters: These filters should be cleaned regularly after the unit has been operating for a while, and replaced during major overhauls. (2) Replacement of the oil separator filter element: After the unit has been in operation for several years, the oil separation efficiency of the filter element declines, which leads to increased fuel consumption by the unit; therefore, it must be replaced. (3) Cleaning and replacement of the oil return filter and air intake filter: In most cases, there is no need for replacement, as there are no particularly dirty substances in the unit’s operating system; a slight cleaning is sufficient. 2.2.4 Maintenance of screw compressor motors (1) Replacing motor bearings: Open the end caps at both ends of the motor and use a hydraulic puller to remove the bearings. Then, use an iron oil drum to contain the used oil (enough to submerge the bearings), and heat it with a 2KW electric furnace to around 105°C. Insert it directly into the motor shaft (the bearings must not be forced into the shaft). Once it has cooled completely, fill the bearings and bearing caps with about 1/2 amount of molybdenum disulfide lubricant. (2) Cleaning of motor rotor and stator; Motor model: Y315M3-2/220KW-2 type. The motor is an open-type motor. Its advantage is that during operation, the airflow between the rotor and stator and the external environment facilitates rapid heat dissipation. The drawback is that it is prone to moisture absorption and contamination by environmental dust, which in turn affects its heat dissipation. Therefore, when a motor operates for extended periods in normal environments, its insulation is greatly affected by elements such as dust, moisture, and high temperatures, which leads to a decrease in its insulation resistance. When the insulation resistance drops to a certain level, it can lead to electrical accidents; therefore, regular shutdowns for inspection and cleaning are necessary. Previously, solvents such as carbon tetrachloride, gasoline, and dichloromethane were used as cleaning agents for manual scrubbing, which had certain drawbacks and those solvents were highly toxic. Use DQ-25 electrical equipment cleaner, a safe and economical cleaning agent that can be used on-site to remove oil, carbon deposits, and dust from the rotor, stator, air vents, cable connections, and contact points of electrical switches. It has good insulation properties, with a voltage resistance of over 25 KV; it evaporates quickly and leaves no residue after drying. Shake the tank for a moment so that the nozzle is 15–20 cm away from the motor rotor and stator to perform cleaning. Once the oil stain has been completely removed, it will dry naturally as it evaporates. (3) Precautions: The site where the motor is cleaned should have good ventilation, and it should be kept away from sources of fire. After cleaning the motor, it is heated from the outside using electric heating tubes; the motor can be connected to power and operated only when its insulation resistance is greater than 5 MΩ. 3 Commissioning and acceptance of screw compressors 3.1 Lubricating the screw compressor: Open the drain valve at the bottom of the coarse oil filter tank, and use a hand pump to fill the coarse oil tank with oil until it is about two-thirds full. Then open the exhaust port located at the end of the screw compressor’s head, near the diaphragm coupling. Supply the main power; start the oil pump to pump oil directly from the oil tank into the screw compressor (connect the drain valve circuit of the coarse filter tank to the oil tank using a 6-inch hose). If oil comes out of the exhaust port at the front end of the machine, tighten that exhaust port. Open the exhaust valves located at the top of the oil cooler and the oil separator, until two-thirds of the oil level gauge at the top of the oil separator is filled with oil; once filling is complete, close the relevant valves. Of course, in addition to this, the engine unit can also use two methods for refueling: an external oil pump and vacuum refueling. 3.2 Venting and leak testing of the screw compressor system: Open the main suction valve, wait until the system pressure rises to around 0.15 MPa, then close the main suction valve. Open the valve at the top of the oil separator to vent air; repeat this process about three times to remove all air from the system. Use soapy water or pH test strips to check for any leaks at various moving parts, temperature and pressure sensing points, as well as at pipe connections. 3.3 Pressure testing of the screw oil pump and testing of the spool valve operation: Start the oil pump to lubricate components such as the machine head; the oil pressure should be between 0.5–0.6 MPa. Press the buttons for increasing or decreasing load to activate the solenoid valves, and the spool valves should operate smoothly without any sticking. 3.4 Before adjusting, testing, accepting, and installing the coupling on the screw unit, check the direction of rotation of the motor. Connect the coupling and verify that the concentricity error of the center lines is not greater than Φ0.075 mm, and the axis tilt is not greater than 0.03 mm. The compressor should rotate smoothly without any obstruction when turned by hand after installation. Set the slide valve to position 0, start the compressor, and pay attention to factors such as vibration, oil temperature, oil pressure, and noise. Stop the compressor with the slide valve in position 0; it should also rotate smoothly when turned by hand after shutdown. For the vacuum test, the absolute pressure should be below 5.332 KPa. After the screw compressor has been running continuously for a period of time (usually 24 hours), record data such as the suction pressure, discharge pressure, suction temperature, discharge temperature, oil pressure, oil temperature, and oil pressure difference. Check whether there are any abnormal vibrations, noises, or localized heating in the unit. After the unit operates continuously for about a week, if there are no abnormalities, then the major maintenance of the screw compressor is successfully completed. In addition, detailed records of the usage and replacement of major repair parts and spare parts must be kept one by one and filed, to serve as a reference for the future maintenance management of screw compressors. 4 Summarize the precautions for major overhauls of screw mechanism refrigeration compressor units. 4.1 During shutdown for maintenance, the slide valve must be adjusted to position 0. 4.2 Precautions for assembly: Inspect each component and repair or replace any damaged parts. When disassembling or assembling, mark the positions of the parts to avoid mistakes in placement; components from different bearings must not be interchanged. Some parts on the rotor have similar shapes, so care must be taken to prevent confusion. All parts should be cleaned with diesel, then dried with compressed air or wiped clean with a clean cloth. Identify the male and female rotors as well as the intake and exhaust ends; positioning pins must be inserted before installing the fastening screws on the end caps. During reassembly, replace any damaged O-rings, retaining washers, and round nuts. When replacing the new O-rings and shaft seals, it is essential to apply oil; the existing sealant on both ends of the housing as well as on the intake and exhaust surfaces must be thoroughly cleaned off. 4.3 Main assembly clearances: rotor exhaust end face and exhaust end seat (0.08–0.10 mm); main journal and main bearing hole (0.0325–0.0475 mm); outer diameter of main bearing and housing bearing hole (–0.009–0.013 mm); balance piston and balance piston sleeve (0.036–0.0645 mm); slide valve and slide valve seat hole (0.0215–0.043 mm); balance piston sleeve and housing hole: 0.022–0.062 mm; at the meshing line of the male and female rotors (0.08–0.10 mm).