An article to fully understand the principle, failure and maintenance of nitrogen compressors

An article to fully understand the principle, failure and maintenance of nitrogen compressors

Nitrogen compressors are mainly used in various industrial needs. Pipes are used to connect the original air inlet of the nitrogen compressors and the balance gas tank. When the sealing performance of the sealing box is not very good, the pipeline provides a certain pressure , Prevents air from leaking into the cylinder from the gap between the seal box and the piston rod, thereby ensuring the purity of the compressed nitrogen. The working conditions of the improved compressor have been greatly improved through the above improvements. Let’s explain the nitrogen compressor principle.

　　The principle of nitrogen compressor

In order to improve the purity of the compressed gas and improve the working conditions of the compressor, a balance gas tank is added between the front stage of the compressor and the fractionation tower, with a volume of about 0.5-1.0 cubic meters to meet the needs. The air pipe port is changed to perforate the first-stage valve cover, and the pipe is directly connected to the balance gas tank, and the diameter of the inlet pipe is thickened, which can reduce the resistance of the pipe to the gas, increase the gas flow rate, and improve the compressor The amount of intake air when the first-stage piston inhales.

Due to the non-water lubrication method, the temperature of each cylinder of the compressor has a certain degree of rise, of which the temperature rise of the three-stage cylinder is the largest. In order to improve the heat dissipation of the cylinder, avoid excessive temperature rise, the third-stage For the cylinder, the cylinder manufacturer is required to design its outer wall into a fin type, and the inner wall is inlaid with a copper sleeve with good thermal conductivity and wear resistance. The performance and life of the cylinder produced by the above method are greatly improved.

　　 The control system of the nitrogen compressor is generally relatively simple. Some compressors with an earlier age often use a wound motor, and the starting method is a frequency-sensitive rheostat. In recent years, they have basically changed to use squirrel-cage motors. Many of them use direct start mode, but the start current of this start mode is too large, there are great safety risks, and it brings many disadvantages in safety and operation. In order to improve these problems, we changed the motor control system to star-delta starting mode to reduce the starting current, and added interlock and alarm functions to the entire control system, mainly for the equipment’s oil supply system and water supply system. Interlock and alarm functions, and alarm functions are set for the three-stage exhaust temperature and exhaust pressure.

　　 Nitrogen compressor failure

Once the compressor is out of oil or water, the control system will give an alarm indication and delay the shutdown to protect the nitrogen compressor. When the third-stage exhaust temperature and pressure exceed the set point, the system will also sound an alarm. Remind the operator to eliminate the problem in time to avoid safety accidents; the expansion even reaches more than 3mm, and the right angle of the ring groove has been worn obliquely. Therefore, the piston body was sent to the lathe to reprocess the guide ring groove, and at the same time, contact the supplier to provide a widening non-standard guide ring. Due to the widening of the guide ring, the axial clearance is appropriately enlarged by 0.1-0.2mm than the standard, so that the piston body can be used continuously. It is recommended that one spare piston body at all levels should be used. When a guide ring problem is found during the routine inspection and a new ring needs to be reheated, the spare piston body can be used directly. Because according to the operating rules, the hot sleeve guide ring requires a longer furnace cooling During the process, replacing the piston body with a new one can save maintenance time and ensure that the machine is in good condition.

　　 After more than 6000 hours of operation of the nitrogen compressor, the inter-stage pressure was abnormal: the first-stage discharge pressure and temperature were high, and then dropped. Shut down and check, there is a stainless steel sheet in the secondary intake valve, and there are stainless steel fragments extruded on the upper and lower ends of the secondary piston body. There are stainless steel elastic rings in the ring groove of the piston ring of the piston body of this machine. At this time, the three second-stage piston rings only have 2.4-4.3mm (new ring 13mm, minimum radial thickness 7mm). Due to the serious wear of the piston ring, the elastic ring in the groove comes out and the cylinder wall wears out. The elastic ring is broken into pieces, causing internal leakage of the valve. After grinding the piston body and cylinder wall, replacing the new second and third stage piston rings, inspecting the air valves at all levels, removing the elastic rings at all levels, and returning to normal use, the elastic rings on the oxygen compressor of Hangyang are not removed. reuse.

　　 During the operation of the compressor, the third-stage exhaust temperature increased abnormally, and the first-stage piston rod did not move up and down, and the second and third-stage were normal. It was shut down to check that a cylinder collision accident occurred in the first stage: a fixing bolt of the connecting rod large-end bearing was broken, the piston stroke was increased beyond the limit, the aluminum piston body was smashed, the cylinder head was hit by the crack and the connecting rod was broken. Therefore, the machine was completely disassembled and overhauled, replacing the first-stage piston body, connecting rod, damaged main bearing bush, large head bush, connecting rod bolts, etc. The unit was re-aligned and finally resumed use. The gas valve is faulty. During daily inspections, you can touch the suction valves at all levels. If you find that it is hot or the temperature is obviously high, the valve has internal leakage, and it needs to be disassembled when shutting down. Generally, the valve is broken or the valve is not sealed. Strictly, a loose valve cover can also cause internal leakage and abnormal noise. Failure of this stage of suction and exhaust valve can cause the pressure and temperature of the previous stage to increase.

　　Nitrogen compressor operation

　　 After turning on the nitrogen compressor, the air supply operation was performed without careful inspection. After half an hour, the exhaust temperature on the DCS was over-temperature alarmed. On site inspection, it was found that the main inlet and return valve of the machine was not opened, and the cylinder backwater sight glass burst. Shut down immediately, and be careful not to force water supply at this time to prevent other accidents from sudden temperature drops.

After the nitrogen compressor was turned on and operated for more than two hours, the first-stage exhaust temperature exceeded 180°C and the interlocking shutdown (normal operating temperature was 155°C). After inspection, the two DN25 return valves of the first-stage and second- and third-stage cylinders were not opened. Cause. When the machine was overhauled the day before yesterday, the two valves were closed, and they were not restored after the overhaul, and they were not carefully checked before starting up, resulting in insufficient cylinder cooling and excessive exhaust temperature.

　　 When the nitrogen compressor is turned on, the hand gear oil pump supplies oil to the lubrication points of the crankcase. After the pressure gauge after the pump rises, the hand crank is stopped. At this time, the oil pressure on the DCS before entering the crankcase does not increase. After starting the machine, white smoke was emitted from the crankcase breathing hole and the oil scraper. The machine stopped hurriedly, opened the side cover of the crankcase, and checked the lubrication flow path with the hand crank oil pump. Read the trend chart on the DCS: the oil pressure appears on the DCS 30s after starting up, and it stops after running for 2 minutes. Because there was no oil entering on the tiles in the first 30s, the original residual oil vaporized.

The gear oil pump of the oxygen-nitrogen compressor is installed on the end of the fuselage, and the driving gear is connected to the crankshaft through a clutch and is driven by the crankshaft. During operation, the oil pressure after the pump and the oil pressure after filtration suddenly dropped and recovered, and sometimes even caused the oil pressure to be too low to directly interlock and shut down. After inspection, the clutch was worn, and the fault was eliminated after replacing the new clutch. The gear oil pump of the nitrogen compressor is directly connected to the crankshaft. Before starting the machine, the auxiliary gear oil pump (driven by a 1.5kW motor) outside the machine is turned on to supply oil to each lubrication point. When the main engine is running and the oil pressure is greater than 0.30MPa, the auxiliary pump will stop automatically and rely on its own gear oil pump to supply oil. Since the auxiliary pump is installed above the oil level of the crankcase, there is no oil pressure every time the pump is turned on. The oil pressure must be removed after the oil pressure gauge is exhausted. The oil pressure will increase by installing an exhaust valve on the oil pipe behind the pump. The auxiliary pump is exhausted before the second startup.

The inter-stage cooler of the oxygen-nitrogen compressor, the water pipe pass, the gas passes the shell pass, the heat exchange effect is poor due to the blockage of the copper heat exchange tube, the heat exchange tube is mechanically cleared, and this method is used for a long time, resulting in the heat exchange tube Blowby. During operation, the exhaust pressure of the second and third stages is greater than the water pressure, and bubbles appear in the backwater plexiglass sight glass of the second and third stage coolers, which can judge the leakage of the cooler. By disassembling the heads at both ends, installing self-made flanges, and inflating for leakage, the leakage of the heat exchange tube can be found, and the plugs are used to block it. At present, three heat exchangers have experienced copper pipe leakage for plugging. The seal between the tube sheet and the shell of this type of cooler adopts packing seal, and leakage can also occur if the seal is not tight. Replacement of new packing or proper tightening of head bolts can eliminate leakage.

　　Oxygen compressor secondary cooler tube sheet and heat exchange tube expansion joint sees water to the oxygen side, and eliminates the leakage point by re-expansion. The water of the nitrogen compressor cooler goes through the shell side, and the nitrogen goes through the tube side. After half a year of operation, the heat exchange effect of the cooler deteriorated, and the water pipe was disassembled to check that the baffle in the shell had fallen off. Because the aluminum baffle was used, it was corroded by circulating water (soft water), the water was short-circuited, and the heat exchange effect became poor. Therefore, the manufacturer was required to replace the new cooler core.

　　 Nitrogen compressor maintenance

1. Every time the machine is overhauled, after stopping the gas and water, hang warning signs on the relevant gas valve and waterway valve, make a written record, and do not operate to prevent unintentional operation from causing other machines to flow backwards or enter the water supply The inside of the crankcase must be inspected safely. After the overhaul is completed, the listing personnel will remove the warning signs and inform the operating personnel that the machine can be tested after the overhaul.

2. The oxygen compressor must handle the oil scraper ring. The piston rod must not be oily during operation. The oil scraper ring needs to be ground or replaced, and the piston rod needs to be replaced if necessary; the stuffing box leaks large amounts of air and replace the sealing ring in time, remember when assembling The three-lobe seal is on the side close to the cylinder. Otherwise, the three-six-lobe seal ring is installed reversely and air leakage will occur when the machine is turned on.

　　3. Make a statistical record of the start-up of the equipment, and establish a maintenance file for one machine and one account to facilitate finding problems and listing maintenance and repair plans. According to the quality of spare parts, routinely check the cylinder every 2,000 to 4,000 hours to check whether the guide ring gap is enlarged or worn, and whether the thickness of the piston ring is qualified; check the tightening of the valve and the bolts in the crankcase every 4,000 hours.

　　4. Lubricating oil is tested every six months, the oil circuit is cleaned once, and the oil cooler is core-pulled every year to remove the dirt on the oil side of the shell.

　　 5. Strengthen the inspection and management, find that the pressure between the upper level of the instrument control cabinet is abnormal, and shut down for inspection. Pay attention to whether each suction valve is overheated to judge whether there is internal leakage, carefully check whether there are bubbles in the cooler return mirror to judge whether there is leakage, whether the crankcase oil level is abnormally raised or dropped, and judge whether the oil cooler is bleeding Leakage: Water enters the oil side, the lubricating oil is emulsified, and the lubricating oil at the crankcase oil level sight glass is obviously discolored; when the oil enters the water side, the crankcase oil level drops and the lubricating oil enters the circulating pool with circulating water.

6. Perform an overhaul once after 16000～24000h operation, comprehensively inspect, disassemble and clean the unit, repair and replace worn parts, and re-align and level with the original recorded data and technical standards. The pressure vessel and safety valve shall be as specified Check.

　　7. When the oxygen compressor is overhauled and assembled, all parts in contact with oxygen should be thoroughly degreased and cleaned, and oil operation is absolutely avoided. In the process of starting the pressure increase and shutdown, the operating vent valve and the air supply valve must be opened or closed slowly, and must not be opened or closed quickly in an instant, otherwise the instantaneous local oxygen flow rate will rise excessively and safety accidents may occur.

　　 8. Make the start and stop reminder cards. List the startup and shutdown steps on the top, compare them one by one each time you start the machine, and proceed to the next step after verifying that it is correct to avoid equipment failure caused by improper operation.

The material used for the piston ring of a general gas compressor is polytetrafluoroethylene. Although it has good self-lubricating properties, it has poor thermal conductivity, corrosion resistance and morphological stability. In the compression process without water lubrication, The temperature in the cylinder rises significantly, which greatly reduces the service life of the piston ring. Therefore, to find a way to solve the above problems, you can use the method of adding fillers to the material of the piston ring to improve the shortcomings of PTFE , Called filled polytetrafluoroethylene, the piston ring made of filled polytetrafluoroethylene, its wear resistance and service life depend on the ratio of the material components and the structure of the piston ring, and the filled polytetrafluoroethylene filler component The selection of the ratio is mainly based on the comprehensive consideration of the compressed medium, the structure of the compressor, and the speed of the machine.