Hey everyone, does anyone have a question bank on reciprocating compressors?

Reciprocating piston air compressors play an extremely important role in the flexible packaging plastics industry. For example: it is used on a punching machine for pneumatic punching during bag manufacturing ； When printing plastics, it is also used on cylinders to lift and lower the offset printing rollers. Reciprocating piston compressors offer excellent performance, featuring advantages such as low noise, low fuel consumption, and low specific power. However, in operation they are prone to faults such as reduced displacement, high exhaust temperature, high oil temperature, cylinder knocking, and piston jamming or seizure. I. Working principle of reciprocating piston air compressors: A reciprocating piston air compressor is typically driven by an electric motor, which rotates a shaft through a V-belt; the piston then moves back and forth as a result of this rotation, causing changes in the volume of the piston chamber formed by the air valves, piston, and cylinder, thereby achieving the compression of air. Compressed air is compressed again through an intercooler, and finally enters the air storage tank. The air storage tank is mainly composed of a cylinder, pressure gauge, safety valve, etc., and it can stabilize the pressure in the air pipeline, reducing the pressure fluctuations caused by the operation of the compressor. If, for some reason, the compressed air inside exceeds the rated pressure, the safety valve will open automatically to release the compressed air into the atmosphere, ensuring safe operation. Our factory uses V-0.3/10 and W-0.6/10 type air compressors produced by Fujian Compressor General Factory, with the opening pressure of the safety valves being 1.05 to 1.08 times the rated exhaust pressure. Of course, the opening pressure of the safety valve varies for air compressors of different specifications. II. Pre-production inspection: Before starting the air compressor, it is necessary to conduct some inspections to rule out potential faults. The checks before startup mainly include checking the power supply voltage and the oil level. Power supply voltage: The allowable variation range for the power supply voltage is approximately ±5%. Oil level check: The oil level should generally be between 1/3 and 2/3 of the oil gauge. When the oil level is 1/3 below the mark, filtered lubricant of the same grade should be added promptly; 13# compressor oil is suitable for winter use, while 19# compressor oil is appropriate for summer use. Use SAE30 when the temperature is above 10°C, and SAE10 when it is below 10°C. III. Reduced displacement 1. Oil shortage – A reduction in displacement is directly related to a lack of oil. Once during production, the air compressor developed a problem of reduced exhaust volume; it was unable to raise or lower the pressure rollers of the printing machine, and there was also a certain knocking sound coming from the cylinder. We first inspected the air silencer and cleaned it, then checked the oil level; finding it below 1/3 of the mark, we immediately added oil of the same grade. After turning on the power switch and attempting to start the device, knocking noises were still heard. Later, the components of the motion mechanism—the crankshaft, connecting rods, pistons, and cylinders—were taken apart one by one for inspection, and it was found that the crankshaft had cracks; it was clear that it was on the verge of breaking, likely due to a lack of oil for some time already. Due to a lack of oil, the moving parts experience dry friction, and excessive loading causes varying degrees of damage to these parts. We cleaned and ground the various moving parts that were damaged, replaced those that were severely damaged, then reinstalled them and tested the machine. The knocking sound disappeared, and the displacement returned to normal. It is clear that engine oil is absolutely essential; otherwise, there will be serious consequences. 2. Poor tightness of the air silencing filter and air valves: A decrease in exhaust volume is also related to the tightness of the air silencing filter and air valves. The air silencing filter must be cleaned regularly. Cleaning the dirt on the valve plates and valve discs helps the air compressor maintain a normal exhaust volume. Under normal conditions, the filter should be cleaned every 200 hours, and the air valve should be cleaned every 500–800 hours. 3. Poor quality of lubricating oil: Low-quality lubricating oil can cause the piston rings to get stuck, thereby reducing the exhaust volume. Therefore, high-quality lubricating oil should be selected. After long-term use, the lubricant will contain impurities and dust, so it also needs to be filtered. Generally, the engine oil should be changed every 500 to 800 hours, and the oil used previously should be filtered. 4. Excessively high exhaust temperature: An excessively high exhaust temperature can also cause the piston rings to be stuck, resulting in a reduced exhaust volume. Lowering the temperature will solve the problem. Two points need attention here: (1) The ambient temperature should not be too high, generally not exceeding 40°C. (2) If the gas valve leaks, the high-temperature gas discharged will return to the cylinder. At this point, we should carefully inspect the air valve, grind the valve plate, or replace the valve disc in order to eliminate air leaks; only then can the problem of excessively high temperatures be resolved. IV. Causes of freezing and countermeasures Air compressors can also experience the phenomenon of “freezing” during operation. The reasons that cause air compressors to \"freeze\" can be attributed to both electrical and mechanical factors. 1. Electrical reasons: The electrical part of an air compressor mainly consists of a motor, an air pressure automatic switch, etc. When the air compressor is operating normally, the pressure switch is closed. When the pressure in the air reservoir reaches the rated discharge pressure, the compressed air pushes against the rubber element, causing the spring to move via the bridge mechanism; this in turn separates the moving contact from the stationary contact inside the bakelite housing, breaking the circuit and stopping the motor from rotating. Conversely, when the pressure in the air reservoir drops to a certain level, the spring rebounds, the circuit is closed, and the motor starts to operate (see Figure 1). If the coil of contactor C is not powered, the pneumatic switch cannot function, resulting in a \"freeze\" condition. For example, when a phase is missing, the power switch K cannot close, and coil C does not receive power, resulting in a \"freeze\" condition. 2. Mechanical reasons that cause a “freeze”: These include broken connecting rods, pistons getting stuck in the cylinders, as well as excessive or insufficient clearance between the piston and the cylinder. Rod breakage is mostly related to overloading, so overloading should be avoided to prevent rod breakage. At the same time, attention should be paid to the assembly clearance of the cylinder and piston as well as their lubrication condition. Additionally, when the air compressor is shut down and not in use, the drain valve at the bottom of the air tank should be opened to release the wastewater. If not used for an extended period, care should be taken to prevent moisture from entering, keep the air valves and oil seals clean, apply oil to the open surfaces of the cylinder, piston, etc., and seal them with oil-paper. In short, to ensure the proper operation of the compressor and extend its lifespan, regular maintenance is extremely important. At the same time, it is required that maintenance workers have a strong sense of responsibility to detect and resolve faults as early as possible. Malfunctions of paper cutters and the sharpening of blades – Some minor issues that arise during paper cutting due to the blades often leave many operators at a loss as to where the problem lies. The following points are provided for reference by colleagues. Fault 1: When a newly sharpened blade is put into use, its edge suddenly breaks off a little while cutting, even without coming into contact with any hard objects. The main reason for this problem is an improper control of the feed rate during grinding (the feed rate being too high), which leads to excessively high temperatures during grinding (that is, high temperatures at the point where the blade contacts the grinder wheel), thereby \"damaging\" the blade. Fault 2: The cutting blade does not reach the bottom during cutting. This is mainly due to the out-of-tolerance straightness of the blade edge. Fault 3: Shortly after the blade has been sharpened and is used for cutting, the cut surface of the paper shows blade marks, or poor cutting quality occurs during the cutting process. This is mainly due to overheating during sharpening, which makes the blade too soft.

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Unit Review Questions 1. What is the interlock shutdown temperature for the exhaust gases at the first and second stages of the feed gas compressor? Answer: The exhaust temperature of 125°C triggers an automatic shutdown. 2. What are the alarm and automatic shutdown values for the exhaust temperatures at the first and second stages of the screw compressor? Answer: The screw compressor triggers an alarm at an exhaust temperature of (105)°C for the first and second stages, and shuts down automatically at (125)°C. 3. What are the recommended lubricant type, pressure, and temperature for normal operation at PSA gas stations? Answer: (1) The hydraulic oil recommended for normal use is HS32 low-viscosity hydraulic oil. (2) The normal operating pressure is around 4.8 MPa. (3) The normal operating temperature is around 35°C. 4. Conditions for starting the feed gas compressor: Answer: (1) The pressure in the lubricating oil main line of the feed gas compressor is greater than 0.35 MPa; (2) The temperature of the lubricating oil main line is greater than 27°C; (3) The barring device is disengaged. 5. What are the reasons for insufficient or interrupted supply of lubricating oil? Answer: (1) The main oil pump is damaged and the auxiliary oil pump fails to start supplying oil in a timely manner ； ⑵Leakage or rupture of the oil supply system pipes and connection flanges, blockage of the oil pipes ； ⑶An excessively low oil level in the tank results in the oil pump drawing in a small amount of oil or failing to draw any oil at all. 6. Why is it necessary to stop the compressor when the lubricating oil pressure drops suddenly by a large amount? Answer: When the lubricating oil pressure drops sharply, insufficient oil supply is provided to the various friction points; in severe cases, the oil supply is completely interrupted. This leads to increased friction between the components, a rapid rise in temperature, and can result in bearing burnout or shaft seizure. 7. What are the reasons for a decrease in compressor lubricating oil pressure? Answer: (1) Low oil level in the tank, (2) Clogged filter, (3) Failed oil relief valve, (4) Reduced capacity of the oil pump, (5) Leaks in the oil pipelines, (6) Degraded oil quality, (7) Faulty pressure gauge. 8. What issues should be considered when using nitrogen to purge screw compressors? Answer: The pressure at the first-stage outlet of the screw compressor is 0.2 MPa; the set pressure value for the first-stage safety valve is 0.22 MPa, while the pressure of the nitrogen used for purging is 0.4 MPa. Therefore, before introducing nitrogen, open the secondary safety valve bypass valve; after the purging is complete, close the nitrogen valve first, and then close the secondary safety valve bypass valve. 9. What are the steps for emergency shutdown of a screw compressor? Answer: (1) Open the bypass valve. (2) Completely close the discharge and suction valves. (3) Rotate the compressor using a special tool. (4) Check the faulty area. 10. What are the outlet pressure, temperature, and model of the lubricating oil for the screw compressor? Answer: (1) The lubricating oil pressure is around 0.4 MPa. (2) The normal operating temperature of the lubricating oil is 35°C. (3) TSA46. 11. What are the pressure, temperature, and model of the lubricating oil outlet for the feed gas compressor? Answer: (1) The lubricating oil pressure is around 0.4 MPa. (2) The normal operating temperature of the lubricating oil is 35°C. (3) LDAB-150. 12. Why does the standby pump sometimes start after the fuel tank is refueled? Answer: The reason is that filling the tank stirred up the dirt deposited at the bottom of it. The suction filter of the operating pump became clogged, causing the oil pressure at the pump’s outlet to drop, which triggered the automatic start-up of the standby pump. 13. What is the function of the outlet return control valve PV-1205 for K-1201? Answer: Control the compressor inlet pressure. 14. Screw compressor outlet pressure control? Answer: The outlet pressure at stage 1 is not greater than 0.22 MPa, and the outlet pressure at stage 2 is not greater than 0.66 MPa. 15. What are the steps for switching the feed gas compressor? Answer: (1) Put the standby machine into idle operation according to the normal startup requirements, and check that all systems are functioning properly. (2) Fully open the bypass manual valve on the secondary standby unit, and slightly open the valve on the safety valve bypass line of the secondary separator. (3) Then open the intake valve of the standby unit, and slowly close the bypass manual valve on the secondary circuit. (4) Raise the secondary pressure of the standby unit to 2.0 MPa, and reduce the load on the unit to be switched to 50%. (5) Open the outlet valve of the standby unit, and close the bypass valve of its secondary separator safety valve. (6) Immediately fully open the bypass manual valve on the secondary side of the machine being switched, and close the outlet valve of the machine being switched. (7) Load it to 100% capacity, adjust the manual valves on its secondary bypass to raise the pressure to the normal outlet pressure, and unload the switchable unit to 0% capacity. (8) Open the bypass valve of the safety valve of the secondary separator of the switched machine to relieve pressure. Close its inlet valve. Once the system pressure is stable, PV1205 can be set to automatic mode, and the bypass valve for the second standby unit should be completely closed. (9) Stop the main motor of the switched machine, handle it as a normal shutdown, and complete the switching.