Start-up and shutdown of reciprocating compressors

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Precautions for compression operations Question 1: What should be noted during normal startup and shutdown? 1. All necessary preparations before starting up should be completed beforehand. 2. Exhausting the air is necessary before starting, and a thorough inspection of the electrical components, equipment, and valves for proper operation must be carried out. 3. During pressurization, close attention must be paid to the pressure changes in each section, ensuring that the outlet pressure at the seventh section is not lower than the synthesis pressure. 4. Prevent faulty gases from the six-way and seven-valve systems from being sent to the synthesis process. 5. Operate the valves slowly, without force, and pay close attention to changes in current. 6. After air supply, conduct a comprehensive inspection to check whether the machinery is operating properly and the pressures in each section are normal. 7. Cranking is not allowed; check the valve switches, and then restore power after cranking. Parking precautions: 1. Do not accelerate too sharply when shifting from seventh to first gear; however, be careful not to allow pressure to build up too high after shifting out of seventh gear, and also avoid accelerating too much to prevent the backflow of copper cleaning gas. 2. When parking, the sequence of opening and closing the valves must not be reversed; care should also be taken to monitor changes in pressure across various sections. 3. When stopping the machine, pressure should be released from the high-pressure side to the low-pressure side; do so slowly, and open the valves gradually. 4. When parking, check the pressure readings on the pressure gauges at each section to see if any valves are not fully closed or if there are any leaks. Question 2: Why must pressurization start from the low-pressure stage, while depressurization should be carried out from the high-pressure stage? Because only by gradually increasing the pressure in the low-pressure section can gas be present in the subsequent sections, allowing the pressure to rise gradually. If only the valve for stage 7 is closed to build up pressure while the valve for stage 2 is not closed gradually, not only will the pressure in stage 4 rise very slowly, but it will also cause uneven pressures across the various stages of the compressor. When stopping the compressor, pressure relief should start from the high-pressure section; if it starts from the low-pressure section, the pressure difference between the rear and side sections and the front section will be greater. This imbalance in pressures across the different sections of the compressor can cause abnormal noises and vibrations, and the valve elements are also prone to damage. Additionally, high pressure may leak into the low-pressure area, leading to explosion accidents. Question 3: Why is it necessary to discharge oil and water in a timely manner? What should be noted when discharging oil and water? The oil and water in each section should be drained at regular intervals. If this is not done, it not only affects the pressure fluctuations in those sections, but the oil contamination that enters the cylinders also weakens the effectiveness of the lubricating oil there, and may even cause damage to it. It can also damage the cylinders and valve plates. If oil and water enter the copper washing tower, it affects or damages the composition of the copper melt. If oil and water enter the synthesis tower, it will affect the active life of the synthesis catalyst. Therefore, the oil and water in each section must be discharged regularly. When releasing oil and water, the action must be slow; the oil and water release valve must never be opened suddenly, as this could affect the pressure in various sections. When several compressors operate in parallel, it is advisable to stagger the timing of oil and water discharge from each compressor, so as to prevent excessive fluctuations in the gas load caused by all compressors discharging oil and water at the same time. During the first and second phases of summer, the frequency of discharge should be increased appropriately; the oil and water discharged must be clean, but care should also be taken to reduce air leakage. Question 4: What are the items included in the routine inspections for this position? Inspection items: 1. Pressure in each section: Under normal conditions, the pressure in each section remains relatively stable, but changes in the operating conditions of the production system (such as ； When there are changes in the air volume of the Roots pump, fluctuations in the desulfurization liquid level, fluctuations in the shift carbonization liquid level, or poor reaction performance in the synthesis tower), or when there are issues with certain parts of the compressor cylinder (such as ； When components such as piston rings, along with associated pipelines and equipment, fail, it can cause significant fluctuations in the pressure across various sections, leading to pressure levels that rise above normal. In addition to promptly identifying the cause and taking corrective action, it is also necessary to adjust the pressures in each section and keep them within the specified range. 2. Inspection of cooling water: The condition of the cooling water can be determined by examining the gas temperature at the inlet and outlet of each cylinder section. If the increase in the temperature of the inlet and outlet gases is not caused by a fault in the cylinder components, it indicates that the cylinder liners and the intermediate water cooler are not functioning properly. To maintain or reduce the gas temperatures at the inlet and outlet of each section, it is necessary to regularly monitor and adjust the cooling efficiency of the water jackets in each cylinder as well as the intermediate coolers, and to check the drainage conditions and drainage temperatures in all relevant areas. Factors such as discharge volume or the temperature of the discharged water will result in an increased amount of cooling water. If the inlet temperatures at various sections do not decrease even after increasing the water flow rate, it indicates that the dirt on the surfaces of the cooling tubes in the water cooler or on the walls of the cylinder liners is too thick, severely affecting the heat transfer efficiency; it is therefore necessary to schedule a inspection and cleaning. 3. Inspection of the temperature and lubrication conditions of various frictional parts: When the components of the compressor become damaged due to excessive friction and high temperatures, it will prevent the compressor from continuing to operate. Therefore, it is necessary to regularly monitor the heating of these frictional parts to ensure that their temperatures do not rise too high, as this is a crucial measure for maintaining the safe operation of the compressor. 4. Inspection of the oil filler system: Regularly check the oil storage capacity of the oil filler and the dripping condition of the drip holes, and maintain the specified dripping amount. When the oil dripping rate slows down or stops and adjustments prove ineffective, it is necessary to take timely action by cleaning the filter or replacing it with a spare oil injector. If the oil pipe is hot to the touch and gas is shooting out at an angle, it indicates that the oil injection check valve is damaged, and it should be cleaned or replaced promptly. Sometimes the oil injector motor trips or experiences other faults; in such cases, manual operation is used to inject oil for a short period of time to ensure normal mechanical operation, while efforts are made to repair the motor or address other faults so that everything can return to normal. 5. Inspection of the circulating oil pump system: During normal operation, it is necessary to monitor the oil pressure regularly; if any fluctuations in the oil pressure are detected, the cause should be identified promptly. The oil level in the crankcase should be maintained within the specified limits. If the oil in the crankcase becomes cloudy or foamy, it indicates the presence of water in the oil. Water usually enters due to moisture from stages 1 and 2 being carried in by the piston rods. If it’s not gas that carries the water, then there is a leak in the water supply lines, which requires immediate repair or replacement. 6. Inspection of moving parts: During operation, it is necessary to pay constant attention to the sound produced by the various moving parts. If abnormal knocking sounds or noises are detected, the cause should be investigated promptly and dealt with carefully. 7. Inspection of the motor: It is necessary to regularly check the current fluctuations of the motor. An increase in current when the voltage remains constant indicates an increased load on the motor, while a decrease in current indicates a reduced load. Each motor has a rated current value; if the operating current exceeds this value, it will degrade the motor’s performance, either causing the motor to trip or damaging its windings, circuits, or switches. Therefore, it is essential to prevent the operating current from exceeding this limit during operation. If an abnormally high current is detected, the load on the compressor should be reduced immediately, after which the cause of the increased motor load should be identified and addressed. The temperature of a motor generally refers to the temperature of the stator windings. When the compressor load increases, the motor has not been cleaned for an extended period, or when temperatures rise in summer, the temperature of the motor will increase. The motor’s temperature must not exceed the specified limits; otherwise, its insulation will be damaged. 8. Inspection of the oil discharge valves and bypass valves in each section: Leaks in these valves can reduce the compressor’s air delivery capacity, while leaks in the vent valves can affect the ammonia balance. Therefore, the oil discharge valves, vent valves, and bypass valves must be inspected regularly, and any leaks should be addressed promptly. The method of judgment is as follows: (1) Check whether there is leakage from a certain oil-water valve; if so, the pressure in that section will decrease. The extent of the pressure drop depends on the amount of leakage. By regularly monitoring changes in pressure, valve leaks can be detected in a timely manner, allowing for prompt correction. ⑵Listen: Use a listening rod to check the valve; if there is a leak, a hissing sound of air escaping can be heard. ⑴Feel the relief valve or oil drain valve, as well as the pipes downstream of these valves, with your hand. If there is a leak in the valve, the pipes will be cool due to reduced gas expansion and heat absorption by the compressor; sometimes water droplets may also be present. In the case of a severe leak, the pipes will be hot. ⑵If there is a leak in the first stage, the second stage, the third stage, the seventh stage, as well as in the oil discharge valves, the inlet pressure at the first stage of the compressor increases. Leaks in the oil discharge valves of stages 3 and beyond cause changes in the composition of the gas. In severe cases, the CO2 content in the converter drops significantly, resulting in a reduction in production. ⑶When it is impossible to determine which valve is leaking, the flange behind the valve is removed for inspection in order to make a decision. 9. Inspection and maintenance of safety valves in each section: The safety valves in each section serve as protective devices for the compressor; when the pressure becomes too high, they open automatically to reduce the pressure and prevent damage to the equipment. Under normal pressure conditions, they should be sealed, otherwise gas will leak out. If the safety valves are not sensitive enough, they cannot fulfill their protective function. During normal operation, it is necessary to pay attention to its performance; one can check by hand whether the pressure relief pipe behind the valve is hot, and listen for any signs of air leakage. Any leaks detected should be repaired promptly. To maintain the sensitivity of the safety valve, it should be calibrated and repaired regularly. 10. Inspection and maintenance of oil collectors and buffers: Generally, water should be drained regularly from the three-stage intake buffer pipes to prevent excessive water levels from reaching the first-stage and third-stage cylinders, which could cause liquid hammer and damage to the machinery, thereby affecting production. Question 5: What causes the increase in the pressure difference between the outlet of stage 1 and the inlet of stage 3 of the compressor? How to handle it. An excessive pressure difference between the second-stage outlet and the third-stage inlet can affect the compressor’s air delivery capacity, resulting in reduced production of synthetic ammonia and nitrogen fertilizers. It can also cause the pressure in the second stage to exceed the specified limits and lead to pressure imbalances across various stages of the machinery. 1. If the concentration of desulfurization ammonia water is too high, that is, if the amount of water used in the cleaning tower is insufficient, the ammonia content in the semi-water gas entering the compressor becomes too high. As pressure increases, ammonia reacts with CO2 in the semi-water gas to form ammonium carbonate crystals, which accumulate in the outlet pipes and valves of the second stage. This reduces the cross-sectional area of these pipes or causes blockages, resulting in an increased pressure difference between the second and third stages; in severe cases, it prevents pressure from being transmitted to the second stage. 2. It is caused by factors such as high conversion resistance, breakdown of the conversion catalyst, blockage of the heat exchanger tubes, excessively high liquid level in the hot water saturation tower, and excessive steam consumption. 3. These issues are caused by high resistance in the carbonization tower system, excessive liquid levels, water remaining in the tower during cleaning, ammonium carbonate crystals blocking the carbonization tower and its pipelines and valves, as well as incorrect valve settings when adjusting the tower. 4. Caused by the detachment of the valve core in a two-stage outlet or three-stage inlet valve, as well as issues such as inaccurate readings when the valve is opened. The second outlet and third inlet valves should be checked immediately; contact should be made promptly with the desulfurization, shift carbonization, and dispatch teams, and if necessary, the second return first and seventh connection third should be used to carry out pressure release and pressure restoration operations. Question 6: What are the reasons for valve backflow? How to determine the handling? What should be noted when replacing the valve? Reason: 1. The valve disc has been in use for a long time, or the quality of the accessories and the quality of maintenance are poor. 2. The coal oil and ammonia content is too high. 3. The valve disc spring is broken. 4. The valve disc is stuck by a foreign object. 5. Cracking of the valve gasket and loose set screw. Judgment: 1. From the perspective of temperature ； A faulty inlet valve can cause the temperature of the inlet cover to rise or fall, which can be determined by touching the inlet valve cover with your hand. The outlet valve is damaged, causing the outlet temperature to rise while the compression ratio remains unchanged. When the outlet valve is severely damaged, the pressure in the cylinder is almost equal to the pressure in the outlet pipe. As the temperature rises, the pressure drops rapidly. Due to the high outlet temperature, which is above 100°C and too hot to touch with bare hands, a thermometer can be used to measure it at this time. 2. From a pressure perspective ； When the inlet or outlet valve fails, the pressure in that section of the compressor as well as the pressure in the subsequent section decreases, due to a reduction in the amount of gas flowing into that section. Meanwhile, the pressure in the front section rises, as indicated by the pressure gauge; if this is detected late or not dealt with promptly, the pressure there will rise quickly enough to cause the safety valve to trip automatically. 3. Abnormal sounds ； The inlet and outlet valves are damaged, and abnormal noises are heard at the valves (cracking sounds, air leakage sounds, irregular clicking sounds), which can be detected using a listening rod. When a valve is damaged, abnormalities occur in terms of temperature, pressure, and sound. Operators must listen carefully, observe closely, and feel the valve frequently, accumulating experience over time to be able to make quick and accurate judgments. When a valve is damaged, the machine should be stopped immediately to replace the valve. 4. When replacing the valve, the following aspects should be taken into consideration ； ⑴When replacing the valve, it is first necessary to release all pressures in the system and check for any leaks, in order to prevent carbon monoxide poisoning during the valve replacement process. ⑵When removing the valve cover, it is not possible to take off all the nuts at once; instead, a few nuts should be loosened symmetrically, and then the valve cover can be pried open using a crowbar. Once it is confirmed that there is no pressure inside the cylinder, all the nuts can be removed, to prevent gas from pushing the valve cover open when it is opened, thereby avoiding accidents caused by the flying valve cover. ⑶After removing the valve, check for any damage to it; if the valve disc or spring is broken and the fragments are scattered, it is necessary to determine their whereabouts and try to find them to prevent them from falling into the cylinder and causing damage to it. ⑷During installation, the damaged gaskets on the valves should be completely removed, and be sure not to forget to use washers; also, make sure that the inlet and outlet valves are installed in the correct positions. ⑸When removing or installing the valve, do not use metal tools to strike it, as sparks generated by such striking could lead to an explosion or fire caused by the mixture of residual gas and air inside the cylinder. ⑹When installing the valve cover, tighten the nuts symmetrically to prevent air leakage caused by over-tightening on one side. Question 7: What causes abnormal knocking sounds inside the cylinder? How to handle it? ⑴The clearance between the piston and the cylinder head is too small, resulting in cylinder scoring. ⑵. Liquid present in the cylinder. ⑶The piston fixing nut is loose or the piston is damaged. ⑷The valve being loose or having too little packing causes over-pressurization of the cylinder. ⑸The valve is damaged or the set screw is loose. ⑹There are debris and other items inside the cylinder. Planful, normal reduction of the machine capacity or emergency shutdown is carried out depending on the severity of the accident. Question 8: What causes the slide to make noise? How to handle it? ⑴The clearance of the copper sleeve fit and wear is too large or the sleeve is damaged. ⑵The copper sleeve is lacking oil or has been damaged. ⑶. Excessive clearance between the crosshead and slideway, or damage caused by loss of oil. ⑷The crosshead pin nut is loose or worn. Based on the severity of the accident, carry out planned and normal reduction of machine operation or emergency shutdown to replace vulnerable components. Question 9: What causes knocking noises inside the crankcase? How to handle it? ⑴The clearance between the large-head tiles is too large. ⑵. The linkage screw is loose. ⑶The bearing is cracked or the clearance is too large. ⑷The large tiles lack oil and even have rough surfaces. ⑸. There is no oil in the crankcase. ⑹. Balanced looseness. Take planned and normal reduction of the machine speed or emergency shutdown actions based on the intensity of the noise. Question 10: The function of each circuit valve ； ⑴. One call, three times ； When heating the gas supply for the transformation process, since no gas is available from the carbonization workshop, it is necessary to open the one-to-three connection so that gas from stage one can flow into stage three, thereby ensuring gas circulation in the subsequent stages and maintaining pressure balance. However, during normal production, the one-to-three connection must be closed; otherwise, the gas from stage three will return to the gas holder via low pressure. ⑵. Second inning, one ； It serves to regulate the pressure in the second stage and to release pressure during startup and shutdown. During operation, if the pressure in the first stage is insufficient, it is possible to slightly open the connection between the second and first stages, allowing some of the gas from the second stage to return to the first stage for circulation, thereby preventing the first stage from experiencing negative pressure. It releases the pressure in the second stage during shutdown. ⑶. Six through seven ； While driving, when no air is supplied from the copper washer, it must be opened slightly so that a portion of the gas from stage six can enter stage seven for circulation, serving as a bypass adjustment mechanism. It must not be turned on during normal production; it should be kept completely closed to prevent internal leakage. Otherwise, the carbonized gas will enter the synthesis process without undergoing copper washing, leading to serious poisoning incidents. ⑷. Seven rounds, three ； When blockage occurs due to transformation or carbonization, draw negative pressure in three stages; immediately activate Seven-Return-Three for adjustment, but do not open it too much or too abruptly, and shut down the system urgently if necessary. Question 11: What are the hazards of liquid in the compressor? What are the reasons for liquid presence? What phenomena are there? How to deal with it? During the production process, if gas carries liquid into the compressor, the incompressibility of the liquid causes it to collide violently inside the cylinder under the force exerted by the piston. This results in intense vibrations of the compressor and loud noise. Compressed liquid in the equipment can cause serious accidents, such as the breaking of various compressor components like piston rods, connecting rods, bolts, and crankshafts, or damage to items such as the cylinder head and piston valves. In terms of operation, a small amount of liquid in the compressor can reduce the gas delivery volume. It has a significant impact on production. A section with liquid generally indicates low pressure in that section, low outlet temperature, water leaking out from the valve cover as well as from the contact between the cylinder block and the cylinder head, knocking sounds inside the cylinder, and an increase in current. In mild cases, drainage measures can be taken; first, open a section of the oil discharge system to allow drainage or to reduce the amount of water. In severe cases, an emergency shutdown must be initiated. However, once it is discovered, it is necessary to contact the relevant departments and dispatch team promptly for handling. Question 12: What are the reasons for high pressure during the three inflows and two outflows, and how to address it? ⑴. In the seventh stage, there are three instances of internal leakage or failure to seal properly; the gas leaking in enters the third-stage manifold. Since the pressure of the compressed gas is higher than that in the third-stage manifold, this results in an abnormally high pressure in that stage. ⑵The three-stage inlet and outlet valves allow some of the gas compressed in the three-stage cylinders to return to the main inlet pipe through these valves, resulting in a higher pressure at the main inlet. ⑶In the fourth and fifth sections, as well as the sixth and seventh sections, the piston rings may cause wear on the cylinder due to poor lubrication, which leads to some of the gas leaking into the balance plug and then returning to the inlet and outlet pipes of the third section via the return pipe, resulting in elevated pressure. ⑷When the pressure in the compressed third-stage manifold is higher than the pressure at the carbonization outlet, excessive resistance prevents the conversion and carbonized gases from being discharged; as a result, it is also difficult to discharge the gas from the compressed second stage, leading to an elevated pressure in that stage.

Valve positions in the compressor shutdown state: the inlet and outlet valves are closed, the return valve is fully open; if there are outlet valves in other sections, they are closed, and the vent valves are also closed； Cooling water for the cylinder liners and cylinder head; the inlet and outlet valves of the cooler are closed. Return water shut-off on cylinder packing ； Valve positions for the compressor when it is ready to start: slowly open the inlet valve fully, activate the crankshaft rotation device, supply cooling water to the cylinder liners and cylinder heads, as well as to the water inlet and outlet of the cooler; establish a circulation system for the water used to cool the cylinder gaskets. Activate the crankshaft rotation device, slightly open the blow-off valve of the cooler, and close it once no water or mechanical impurities are present. Check that the check valves at all levels are in the fully open position, the outlet valve is closed, and the vent valve is closed ； After verifying that the instrument control and electrical control interlocks are functioning properly, activate the interlocks ； Contact the electrician to check that the excitation device is functioning properly, and adjust it so that it can be activated ； There are no abnormal noises during cranking; the transmission mechanism is functioning properly. Stop cranking and get ready to start the engine ； Notify the electrician to restore power supply and start the compressor; run it under no-load conditions for 10–20 minutes. If the compressor operates normally, begin gradually increasing the load by using the backflow valves at each stage. During this process, ensure that the compressor runs without any abnormalities and that all process parameters remain within normal ranges ； After reaching the rated outlet pressure, slowly open the outlet valve to supply air to the user ； Used in vent valve failures, overpressure situations, and during purging ； When stopping the machine, first close the outlet valve; then open valve one to reduce pressure, and finally shut down the machine. The procedure is the reverse of that used to start it up ； Additionally, after system maintenance, it is necessary to purge with nitrogen and ensure that the analysis results are satisfactory before introducing process gas to start up the operation ； This post was last edited by Longsky Eagle on 2009-4-1 10:28.]

As a friend upstairs asked, why does the inlet valve need to be opened before starting a reciprocating compressor? I remember that reciprocating compressors usually start under no load. If the inlet valve is fully open, as you mentioned, that would mean starting the compressor under load – could this cause the compressor to operate beyond its rated capacity?

Fully opening the inlet valve and starting under no-load conditions are two different things. The no-load start of a reciprocating machine is equivalent to sucking the air that has been expelled back in. The opening degrees of the intake valve and exhaust valve are at full open, so the compressor hardly has to do any work. It has nothing to do with the opening degree of the inlet valve; this is different from how pumps work

Should the inlet and outlet valves be fully open in the case above? Do you mean that as long as each one is set to full open, it’s considered no-load? Could you explain it in more detail, please?

During no-load startup, the valve positions are as follows: the inlet valve is fully open, the outlet valve is fully closed, and all return valves are fully open. After startup, the positions of these return valves should be adjusted promptly, with the inlet valve position being adjusted according to the amount of air returning

During the start-up and shutdown of the content compressor, it is essential to prevent incidents such as liquid slugging as well as overheating and overpressure. Before starting up, all valves must be checked carefully: whether the inlet valve is open, whether the connection valves at various stages are open, whether the water in each container has been completely drained, whether the lubrication system is functioning properly, whether the shaft has been rotated, whether the water system is operational, and whether all vent valves are open, among other things! Although the work is simple, nothing should be taken lightly. As for no-load testing, the purging process must be thorough; otherwise, it will cause a lot of problems. I have been working with compressors for over 20 years, and ultimately, the answers to issues related to starting and stopping machines cannot be found here – they depend on daily experience and practice, which is especially crucial when it comes to identifying and dealing with accidents. I also hope that you will succeed on your first attempt to start the machine. These are compressors of the reciprocating type that we deal with. The camera on the phone isn’t good; it can’t take clear photos at night, haha