36 Q&A on atmospheric and vacuum distillation techniques – guaranteed to turn beginners into experts in no time

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36 Q&A on atmospheric and vacuum distillation techniques – guaranteed to turn you from a beginner into an expert in no time! 1. What is visbreaking? 　　Viscosity reduction cracking is a form of mild thermal cracking aimed at converting heavy, high-viscosity oils—such as atmospheric residue, vacuum residue, full-range heavy oil, and topped heavy crude oil—into fuel oil with low viscosity and a low pour point. The viscosity reduction reaction generally takes place at 470–490°C and a pressure of 0.4 Mpa. The allowable conversion rate for feedstocks with high levels of alkanes and asphaltenes is relatively low, at around 4% ； For feedstocks with a high aromatic content, the permissible conversion rate is slightly higher, at around 7%. Although viscosity-reducing cracking is a processing method that was developed in the early days, it remains an important method for producing fuel oil from high-viscosity residue today, thanks to its mature technology, simple operation, and low investment requirements. 　　2. What is the cause of the “black water” coming out of the constant-pressure condenser? How to prevent it? 　　The blackening of condensate water on the roof is caused by the presence of many iron sulfide particles. To neutralize hydrogen chloride in the oil and gas, an ammonia injection process is employed at the tower top in order to prevent the severe corrosion caused by hydrogen chloride dissolving in water to form high-concentration acids; therefore, it is necessary to keep the pH value as stable as possible. \"Black water\" appears when the pH value fluctuates significantly, and a high concentration of Fe+ is detected during the monitoring of condensate water. 　　The preventive measure is to stabilize the pH value of the condensate at the top of the stabilizer; the use of an online pH analyzer can help maintain a stable pH level there. 　　3. What is a flooded tower? 　　In a flooded tower, an increase in the flow rates of the gas and liquid phases leads to an increase in the pressure drop across the tray layers. As the pressure above the lower tray layers rises, in order for overflow to occur properly, the liquid level in the inlet overflow pipe also has to increase. When the liquid level rises to the same level as the outlet of the upper tray, the liquid cannot flow downward, resulting in a flooding of the tower. Tower flooding generally occurs at the lower part of the tower, that is, when the color of the oil in the lowest extraction side stream turns black. It is related to factors such as excessively high processing volume, water in the crude oil, and excessive amount of stripping steam. 　　4. What are the three balances that should be maintained during the operation of a distillation column? 　　The operation of a distillation column requires an understanding of material balance, gas-liquid phase equilibrium, and heat balance. 　　Material balance means that the amount of material entering the tower per unit time should be equal to the sum of the amounts of material leaving the tower. Material balance reflects the production capacity of the tower; it is primarily regulated by the feed rate and the outlet rates at the top and bottom of the tower. During operation, changes in material balance are specifically reflected in the liquid level at the bottom of the tower. When the operation of a tower does not conform to the overall material balance, it can be reflected in changes in the tower pressure difference. For example, if more material enters than exits, the pressure difference across the tower increases. For a fixed distillation column, the column pressure difference should be within a certain range. The tower pressure difference is excessive; the velocity of the rising steam within the tower is too high, resulting in severe entrainment of droplets. In severe cases, flooding occurs, disrupting normal operation ； If the pressure difference across the tower is too low, the speed of the rising steam inside the tower is too slow, which reduces the efficiency of gas-liquid mass transfer on the tray surfaces; in some cases, liquid leakage occurs, further **reducing the efficiency of the trays. Poor control of material balance can lead to chaos in the operation of the entire tower; mastering material balance is a key aspect of tower operation. If the normal material balance is disrupted, it will affect the other two balances: the gas-liquid equilibrium will fail to achieve the desired results, and the thermal equilibrium will also be disturbed and need to be readjusted. 　　Gas-liquid phase equilibrium mainly reflects the quality and loss of the product. This is achieved by adjusting the operating conditions of the tower (temperature, pressure) as well as the gas-liquid contact on the tray. Only when temperature and pressure are fixed is there a definite vapor-liquid equilibrium composition. When temperature and pressure change, the composition determined by the gas-liquid phase equilibrium changes, which in turn affects the quality of the product and the amount of loss. Vapor-liquid equilibrium is closely related to material balance. When material balance is properly maintained, the upward vapor velocity within the tower is appropriate and vapor-liquid contact is satisfactory; as a result, heat and mass transfer efficiencies are high, and tray efficiency is also high. Of course, temperature and pressure also change as the material balance changes. 　　Heat balance refers to the equilibrium between the heat entering the tower and the heat leaving it, which is reflected in the temperature at the top of the tower. Heat balance is the foundation upon which material balance and gas-liquid phase balance are achieved, and in turn it depends on them. Without a hot vapor phase and a cold reflux, the entire distillation process cannot take place ； Changes in the operating pressure and temperature of the tower (i.e., changes in the composition of the gas-liquid phase equilibrium) result in corresponding changes in the heat released due to vapor condensation on each tray, as well as the heat absorbed during liquid vaporization; this is reflected in changes in the heat supplied to the feed and the heat removed from the tower top. 　　Mastering material balance, gas-liquid equilibrium, and heat balance is crucial for distillation operations; these three equilibria influence and restrict one another. In operation, control of material balance is usually the main focus, with heat balance adjusted accordingly, in order to ultimately achieve gas-liquid phase equilibrium. 　　To maintain a stable equilibrium in the liquid level at the bottom of the tower, it is necessary to keep stable: ① the feed rate and feed temperature ； ②Top of tower, side stream, and bottom draw rate ； ③Column top pressure. 　　To maintain a stable top temperature, it is necessary to stabilize: ① feed rate and feed temperature ； ②Backflow at the top, flow rates and temperatures at various sections of the circulating backflow ； ③Top pressure ； ④Stirring steam volume ； ⑤The raw materials and reflux are free of water. By closely monitoring the top temperature of the tower and the liquid level at the bottom, analyzing the causes of fluctuations, and making timely adjustments, it is possible to maintain the three balances of the tower and ensure its proper operation. 　　5. What is the impact of changes in crude oil properties on the operation of distillation units? How to handle it? 　　Changes in the properties of crude oil usually refer to a shift in the composition of the crude being processed, either towards lighter or heavier components, and this is evident when the crude oil being processed is transferred from one tank to another. 　　As the properties of the crude oil become lighter, the pressure at the top of the fractionating tower increases, the amount of non-condensable gas at the tower top rises, the cooling load on the tower top increases, the temperature after cooling goes up, the liquid level in the fractionating tower drops, the pressure in the crude oil side of the heat exchanger increases, the yield of gasoline from the top fraction increases, and the dry point of the top fraction product decreases. 　　As the properties of crude oil become heavier, the pressure at the top of the fractionator decreases, the temperature after cooling drops, the liquid level at the bottom of the fractionator rises, gasoline production decreases, and the dry point increases. 　　With changes in the properties of crude oil, during balanced operation, the most noticeable changes are those in the pressure at the top of the initial distillation tower and the liquid level at its bottom. In units without a initial distillation tower, with the outlet temperature of the heating furnace remaining constant, changes occur in the top pressure of the atmospheric pressure tower and the liquid level at the bottom of the tower. 　　When the properties of crude oil become lighter, the top temperature of the tower should be increased appropriately due to the rise in the vapor pressure of oil and gas; when it becomes heavier, the top temperature should be decreased appropriately to ensure that the dry point of the gasoline at the tower top meets the requirements. 　　Once the crude oil becomes lighter, it is necessary to increase the fuel supply to the burners and raise the load on the heating furnace to ensure that the lighter fractions of the oil vaporize. Crude oil becomes heavier, reducing the load on heating furnaces. As the crude oil becomes lighter, its vaporization rate increases, which raises the heat and hydrocarbon vapor pressures above the vaporization section of the distillation tower. As a result, the distillation temperature of the side-stream products also increases. To address this, it is necessary to increase the amount of product taken out, as well as the reflux flow at the middle section of the tower, while reducing the steam supply to the bottom of the tower in order to lower the load on the top of the tower. The steam supply to the side-stream stripping tower cannot be reduced, as this could affect the flash point of the products. If necessary, measures such as reducing the crude oil processing volume and lowering the outlet temperature of the heating furnace can be taken to ensure stable operation and safe production. 　　As the crude oil becomes heavier, the vaporization rate in the tower decreases, and the temperature at the side-stream distillation outlet drops as well. At this point, it is necessary to reduce the amount of product taken out to ensure that the product quality remains satisfactory. 　　6. What are the factors that affect the temperature and pressure changes at the tops of the initial distillation tower and the atmospheric pressure tower? 　　The top temperature and pressure of the initial distillation tower and the atmospheric pressure tower are important parameters for controlling product quality; if no adjustments are made when these temperatures and pressures change, it will lead to variations in the quality of the product at the tower top. 　　①When the outlet temperature of the crude oil furnace rises, the increased amount of vaporization within the tower leads to changes in the top-of-tower pressure and temperature. In such cases, the reflux rate should be increased, and the top-of-tower temperature should be appropriately raised to ensure product quality. 　　②When the properties of crude oil and vacuum residue become lighter, the pressure at the top of the tower increases; therefore, the temperature at the top of the tower should be increased appropriately ； When the crude oil becomes heavier, the pressure at the top of the tower decreases, and the temperature at the top of the tower should be reduced accordingly. 　　③As the water content in the crude oil entering the primary distillation column increases, the amount of stripping steam in the atmospheric distillation column also rises, leading to an increase in the temperature and pressure at the top of the column. The top temperature should be appropriately lowered; however, care must be taken to prevent any further rise in the top pressure. If the pressure continues to rise, the reflux rate must not be increased any further. 　　④Water is carried back to the tower top, causing the temperature at the tower top to drop significantly while the pressure at the tower top increases. 　　⑤An increase in the feed flow rate to the tower leads to an increase in the top pressure and a rise in the liquid level; if overfilling occurs, the top pressure and temperature will rise sharply. 　　⑥Check whether the gas discharge pipeline at the top of the tower is unobstructed. If low-pressure gas is used as fuel, the burner should be inspected; if the burner is blocked and gas discharge is hindered, the pressure at the top of the tower will increase. If the product tank fails to discharge the gasoline inside it in a timely manner due to faulty instrument readings or problems with the oil extraction pump, the tank will fill up, which can also lead to an increase in the pressure at the top of the tower. It is necessary to take immediate action to extract the oil from the tank so as to restore normal pressure at the top of the tower. 　　⑦An increase or decrease in the gasification volume within the tower, an increase in the steam injection rate, and an increase in the pressure at the top of the tower ； The steam flow decreases, and the pressure at the top of the tower drops. The change in steam injection pressure is consistent with the change in steam injection flow rate; an increase in steam injection pressure leads to an increase in the pressure at the top of the tower. 　　7. What are the reasons for the darkening of the color of the side-stream oil from the initial distillation tower? How to handle it? 　　When the amount of oil drawn from the side stream of the initial distillation tower is too high, that is, when a large volume of heavy fractions are extracted, heavier hydrocarbon fractions will be distilled out, which can cause the color of the side stream oil to deteriorate; in such cases, the flow rate of oil drawn from the side stream should be reduced. 　　The oil level at the bottom of the initial distillation tower is too high, and the gaseous stream carrying heavy fractions causes the color of the side-stream oil to darken. If the draw-off volume remains unchanged, check whether the liquid level at the bottom of the tower is maintained at a normal level. Lowering an excessively high oil level can restore the oil’s color to normal. 　　When the feed to the distillation tower contained a high amount of water, the vapor volume increased, the gas-phase load changed, and this could also carry heavier hydrocarbon fractions that caused the color of the side-stream oil to darken. 　　An increase in crude oil processing volume or abnormal operations that lead to tower surges can cause the color of the side-stream oil to darken, and in severe cases, black oil may appear. The side-stream oil from the initial distillation column often enters the atmospheric distillation column via mid-stage reflux; careful operation is required, as otherwise the discolored oil can contaminate the side-stream oil of the atmospheric distillation column and affect the quality of its products. If the processing volume is excessive, it should be reduced. When abnormal operation results in flooding of the column, the side draw from the primary distillation column must be immediately shut off; the contaminated oil should then be redirected to a reject oil tank. 　　8. How does a distillation column operate stably? 　　The operation of the distillation column is the most important aspect of a vacuum and atmospheric distillation unit; it plays a crucial role in ensuring the stable operation of the entire facility and is key to controlling the quality of the main products. The distillation column should operate smoothly to minimize fluctuations, and to achieve this it is necessary to: ① stabilize the feed and discharge flow rates of each column. In addition to the flow rate pumped out by the bottom pump, attention should be paid to changes in the oil extraction rates from each side stream; these flow rates should be adjusted in small increments, based on the quality and yield of the respective products. 　　②The temperature of each material should be maintained stable; when there are changes in temperature, adjustments should be made promptly and appropriately based on product quality. 　　③The initial distillation column, the atmospheric pressure column, and the vacuum column are distillation columns operated in series one after another; if the operation of the preceding column is unstable and the properties of the oil at its bottom change, this will inevitably affect the operation of the subsequent columns, so close attention must be paid during operation. When the properties of crude oil change, adjustments should first be made to the initial distillation column, followed by necessary adjustments to the atmospheric distillation column and the vacuum distillation column in sequence. 　　④In the operation of towers, a stable liquid level at the bottom of the tower and a stable amount of stripping steam are important conditions for the stable operation of each tower. 　　⑤When the operating pressure of the tower changes, the extraction temperatures at various stages also change accordingly; adjustments should be made promptly to ensure product quality. The vacuum level at the top of the vacuum distillation tower has a significant impact on its extraction efficiency and the separation of fractions, so it is essential to maintain stable pressure. 　　⑥Changes in the water content of crude oil have a significant impact on the initial distillation tower; when there are substantial changes in water content, operational adjustments should be made promptly. 　　9. What issues should be considered when using bottom steam injection in production operations? 　　In production, a certain amount of superheated steam is introduced into the bottom of the atmospheric pressure tower as well as that of the wet vacuum tower, with the aim of reducing the vapor pressure of oil and gas within the distillation column and increasing the vaporization rate of the oil. To prevent condensate vapor from entering the tower, the injected steam is heated in a heater to become superheated steam at a temperature of 380–450°C. 　　The pressure of superheated steam is generally kept below 0.3 Mpa, as this pressure is slightly higher than the operating pressure in atmospheric-pressure towers; the pressure difference between the two is small, making it easy to adjust the stripping flow rate. 　　Before turning on the stripping steam, all condensed water should be drained. The steam valve should be opened slowly, and attention should be paid to changes in pressure inside the tower as well as to changes in the liquid level at the bottom of the tower. When determining the amount of steam to be blown in, the vacuum distillation tower takes into account, on the one hand, what is favorable for the vaporization of the oil. On the other hand, it must be considered that if an excessive amount of steam is used, it will lead to a decrease in vacuum level, which in turn reduces the vaporization rate of the oil; in such a case, it is no longer possible to increase the amount of steam introduced. Sometimes, the vacuum level drops for other reasons and cannot be restored; when the steam supply to the bottom of the tower is high, it is possible to maintain a higher vacuum level by appropriately reducing the amount of steam supplied there. 　　It is necessary to maintain stable superheated steam pressure, with fluctuations within 0.015 Mpa. 　　10. What are the symptoms of water in the tower top reflux oil, and how to deal with it? 　　The tower top reflux oil is drawn from the tower top reflux oil tank. If the water level in this tank is not properly controlled or fails to function correctly, and the water level rises above the normal range, exceeding the level of the reflux gasoline extraction pipe, then the reflux oil will contain water ； Alternatively, corrosion and perforation of the tube bundles in the tower top water cooler can allow most of the cooling water to leak into the return oil tank; without time for dehydration, this can also result in the return oil containing water. High water content in crude oil, improper operation of the electrodesalination and dehydration tanks, and electrical system failures can also lead to an excessive amount of water in the reflux oil tank at the top of the distillation tower; inadequate dehydration results in the reflux oil containing water. The oil returning with water enters the top of the tower. Since the heat of vaporization of water is more than 4 times greater than that of the oil, the volume of water vapor is 10 times larger than that of oil vapor. This results in an increase in pressure at the top of the tower and a decrease in its temperature; subsequently, the temperature of the main product stream also drops, causing supercooling in the upper part of the tower. No oil flows out from the side streams, leading to pump cavitation, or the oil in the main stream contains water. If this is not addressed promptly, the pressure at the top of the tower can rise sharply, potentially causing the tower to fail, and the safety valve may open. 　　When it is observed that the temperature at the top of the tower drops significantly, along with a decrease in the distillation temperature of the first column and the pump operating under vacuum conditions, it can be preliminarily concluded that the reflux oil contains water. It is necessary to promptly check whether the control level for the oil-water interface in the reflux tank is set too high; by opening an exhaust valve below the control valve, it is possible to directly observe whether the reflux oil contains water, thus allowing for an accurate determination. 　　The water level in the return oil tank is too high, resulting in the return oil containing water. The following measures should be taken: ① Remove any obstacles to instrument control, open the dehydration valve or the auxiliary line valve to increase the water removal rate, thereby rapidly lowering the water level. 　　②If there is a leak in the cooler tube bundle, stop using it and carry out repairs promptly. 　　③Appropriately increase the top temperature of the tower to accelerate the evaporation of water inside it. 　　④An increase in the pressure at the top of the tower can trigger the operation of the air-cooling fan, while reducing the valve controlling the steam supply at the bottom of the tower in order to decrease the amount of steam introduced into the tower. 　　If the electrical operation of the electrodesalination tank is abnormal and the crude oil contains too much water, which enters the distillation column and causes water to be carried in the reflux stream from the top of the column, water injection into the crude oil desalination tank should be stopped. The electrical fault in the electrodesalination tank must be resolved as soon as possible by restoring power supply. The amount of demulsifier added to the crude oil can be increased, depending on the degree of water removal, to facilitate the operation of the electrodesalination tank. 　　When oil returning contains water, it is necessary to make an early judgment and take prompt action to remove the water from the oil; this will allow normal operations to be resumed quickly. Delaying detection or taking slow action poses a serious threat to safe production. 　　11. What are the symptoms when the oil-water separation tank at the top of the tower is filled with gasoline? How to handle it? 　　When the liquid level control of the oil-water separation tank at the top of the tower fails, or when the pipelines leading out of the unit become blocked and gasoline cannot be delivered, or when the temperature at the top of the tower is too high resulting in an excessive amount of distillate, or when the motor of the pump used to transfer the oil from the top of the tower trips and this is not detected in time, these situations can cause the oil-water separation tank to fill up with gasoline. This leads to a sudden sharp increase in pressure at the top of the tower. The low-pressure gases from the top of the tank are then sent through pipelines to the heater for combustion. Once the tank is filled with gasoline, it will enter the heater and burn, causing the temperature inside the heater to rise rapidly. Black smoke is emitted from the heater’s chimney, and gasoline leaking beneath the burner can catch fire, leading to an explosion. 　　Upon discovering that the oil-water separation tank at the top of the tower is filled with gasoline, first close the low-pressure gas valve leading to the heating furnace and switch to direct venting; immediately increase the flow rate of gasoline leaving the facility. If there is no alternative route, modify the defective tank to reduce the gasoline level inside it as soon as possible. In the event of a pump failure, activate a backup pump promptly to lower the temperature at the top of the tower and reduce the amount of gasoline distilled. 　　Once operations return to normal, drain all the oil remaining in the low-pressure gas tank, and the heating furnace can be used again with low-pressure gas as fuel. 　　12. How to determine whether the liquid level indicated by the glass plate level gauge is correct during normal production? 　　The glass plate level gauge utilizes the principle of a fluid \"U-tube\"; the liquid levels in the two tubes remain at the same level, so the liquid level inside the tower is consistent with the level indicated by the glass plate. Incorrect readings from glass plate level gauges cause problems in the operation of distillation towers or storage tanks. 　　To use a glass plate level gauge correctly, it is essential that the connections at the upper and lower ends of the glass plate to the tank be unobstructed. If either of these connections becomes blocked, it will affect the proper functioning of the level gauge. In winter, when temperatures are low, poor insulation can cause heavy oils to solidify, while in the case of light oils, impurities such as rust can cause blockages, all of which may lead to inaccurate readings from the level gauge and result in false indications. 　　When using a glass plate level gauge, it is necessary to compare the level indicated by this gauge with the level controlled by the instrument. If any abnormalities are detected in the level indicated by the glass plate level gauge, an inspection should be carried out to check whether the heating is adequate and whether the gauge is sensitive enough. The level or boundary level can be increased or decreased to determine whether the indication provided by the glass plate level gauge is accurate. 　　13. What is the impact of changes in the pressure at the top of the atmospheric pressure tower on product quality? 　　As the pressure at the top of the tower increases, the amount of oil that vaporizes decreases; the products from the top of the tower and its side streams become lighter. When the pressure at the top of the tower decreases, the amount of oil that vaporizes increases, and the products from the top of the tower and its side streams become heavier. There are few means to adjust the top pressure of a tower; it can be regulated using the top temperature. For example, if the top pressure increases, the reflux at the top can be reduced appropriately, the top temperature as well as the distillation temperatures of the various side streams can be increased. Improving the cooling conditions at the top can help reduce the top pressure. 　　With the top temperature unchanged, an increase in pressure will result in a decrease in the yields of the various side streams. 　　14. How to maintain a stable outlet temperature of the furnace? To keep the outlet temperature of the furnace stable, it is necessary to continuously monitor changes in the temperature, flow rate, and pressure of the crude oil fed into the furnace, pay close attention to temperature variations at various points within the furnace, and make adjustments as needed. Among them, the inlet temperature of the radiant tube and the furnace temperature are particularly important; fluctuations in these two temperatures indicate changes in the outlet temperature of the furnace. By making timely adjustments based on these two temperature changes, a stable operation of the furnace outlet temperature can be achieved. To ensure that the outlet temperature fluctuations remain within the specified range for the process (±1°C), the main measures for adjustment include: (1) First and foremost, it is necessary to be diligent in four aspects: frequent monitoring, thorough analysis, regular inspections, and timely adjustments, while striving to ensure consistency in operations across different teams. 　　(2) Adjust the “three doors and one plate” in a timely, strict, and accurate manner to ensure good combustion conditions inside the furnace. 　　(3) The number of burners to be ignited is determined based on the furnace load and combustion conditions; the total flame height should not exceed two-thirds of the furnace height, and the heating in all parts of the furnace must be uniform. 　　(4) Ensure stable pressure of fuel oil, steam, and gas, with strict requirements for the stability of the properties of the fuel oil. 　　(5) When the processing volume and climate remain unchanged, under normal circumstances, adjust and fix the burner dampers and flue dampers of the furnace carefully; the extent of adjustment should be small, and no drastic changes should be made. 　　(6) When the furnace outlet temperature is well controlled under automatic control, interference caused by excessive manual adjustments should be minimized as much as possible. 　　(7) When the feed temperature changes, it can be adjusted according to the magnitude of the change in feed flow rate. When the feed rate is changed by 1% at a time, it is generally necessary to adjust the outlet temperature simultaneously or 1–2 minutes in advance. 　　(8) When switching burners in the furnace, the burners can be switched at intervals based on the calorific value of the fuel, the length of the original flame, and the number of burners originally lit. Be sure not to focus on switching in one direction only. The method of switching is as follows: first shorten the original flame, open the valve of the switching nozzle; once the switching nozzle has ignited, then close the valve of the original nozzle. 　　15. How can one determine whether a heating furnace is operating properly by observing the smoke emitted from its chimney? Generally, it is possible to assess the normal operation of a heating furnace by looking at the color of the smoke coming out of its chimney. The criteria for judgment are as follows: (1) It is normal for the smoke emitted from the furnace’s chimney to be colorless or light blue in color. 　　(2) Intermittent emission of small amounts of black smoke indicates insufficient steam supply, poor atomization, incomplete combustion, improper adjustment of the oil-to-vapor ratio in certain burners, or an excessive load on the heating furnace. 　　(3) Excessive black smoke is caused by a sudden increase in fuel, instrument failure, a sudden drop in vapor pressure, or severe burn-through of the furnace tubes. 　　(4) Gray smoke indicates increased gas pressure or the presence of oil. White smoke indicates an excessive amount of vaporized steam, a cracked superheated steam pipe, or superheated steam being discharged into the flue. 　　(5) Yellow smoke indicates chaotic operation and improper adjustment, resulting in intermittent engine shutdowns and incomplete combustion. 　　16. Why is atomized steam used when burning oil? What is the impact of the amount? 　　The purpose of using atomized steam is to utilize the impact and stirring action of the steam to spray the fuel oil in a mist form, allowing it to mix thoroughly with air for complete combustion. 　　The amount of vaporized steam must be appropriate. When too low, atomization is poor, the fuel oil does not burn completely, and the flame tip becomes soft and dark red in color ； When in excess, the flame turns white; although the atomization is good, it tends to cause flame collapse, disrupting normal operation. The atomized steam must be free of water. Otherwise, the flame emits sparks, stutters, and even goes out. 　　17. What is the impact of the vapor pressure at atomization on the operation of the heating furnace? 　　If the vaporization pressure is too low, the fuel oil cannot be properly atomized; as a result, the fuel oil does not burn completely. The flame becomes weak and pale red in color, black smoke is emitted from the chimney, and coking easily occurs in the combustion chamber and burner nozzles. When the vaporization pressure is too high, the flame turns white; it becomes harder, shorter in length, and tends to flicker, making it easy to go out. The furnace temperature drops, and the air pressure output by the instruments increases accordingly. The opening degree of the fuel control valve increases as well, which results in slow responses when trying to raise the temperature. This also leads to waste of steam and fuel. 　　Fluctuations in the vapor pressure of the mist cause the flame to fluctuate as well, with varying durations; the condition of the fuel improves or worsens, or black smoke is emitted from the chimney, leading to fluctuations in the temperature inside the furnace and at its outlet. It is generally advisable for the vapor pressure to be 0.07–0.12 Mpa higher than that of the fuel oil. 　　18. What are the effects of changes in the properties of fuel oil and variations in pressure on the operation of heating furnaces? 　　(1) If the fuel oil is heavy and has high viscosity, it fails to atomize properly, resulting in incomplete combustion, sparks at the burner, excessive smoke inside the furnace, and even a situation where no oil can be sprayed from the nozzle, causing the furnace to shut down. This also leads to an increase in pressure in the fuel oil pump, black smoke coming from the chimney, and coking at the burner. 　　(2) In the case of fuel oil, a too low viscosity can lead to a decrease in pressure in the fuel oil pump, resulting in insufficient fuel supply. This in turn causes the furnace temperature to drop or the furnace to shut down, as well as condensation in the return lines, disrupting the stable operation of the system. 　　(3) When the fuel oil contains water, it causes fluctuations in the fuel oil pressure, sparks appear in the furnace flame, and it becomes easy to extinguish the fire. When the water content is high, phenomena such as fuel oil pump cavitation, furnace shutdown, and fuel oil overflowing from the tank can occur. 　　(4) Excessively high pressure of the fuel oil results in a red or black flame that is long and weak; combustion is incomplete, and this can lead to black smoke or flame out, especially when adjusting the temperature and flame, while the fuel oil pump motor is prone to tripping ； If the fuel oil pressure is too low, there is an insufficient supply of fuel oil, the furnace temperature drops, the flame shortens, and some nozzles go out. 　　In short, fluctuations in fuel oil pressure lead to unstable furnace flames, and the temperatures in the furnace and at the outlet fluctuate accordingly. 　　19. What are the causes and symptoms of tempering in heating furnaces? How to prevent it? 　　Phenomenon: A positive pressure is generated inside the furnace, causing the explosion-proof door to fly open; flames are ejected from the furnace, which can injure people due to backfire, or an explosion occurs inside the furnace, resulting in damage to the equipment. 　　Reason: (1) A large amount of fuel oil is injected into the furnace, or a large amount of oil is carried in the gas ； (2) The opening degree of the flue baffle is too small, reducing the furnace’s suction force and preventing the smoke from being expelled ； (3) The furnace is operating under overload, preventing the flue gas from being discharged in time ； (4) Backfire occurs during ignition at startup, mainly due to a faulty gas valve that allows gas to enter the furnace, or because the furnace is not thoroughly purged before attempting another ignition in case the initial attempt fails, resulting in an explosion and backfire inside the furnace. 　　Prevention: (1) It is strictly prohibited to allow large amounts of fuel oil and gas to enter the furnace before ignition; it is also strictly prohibited to have gas mixed with oil ； 　　(2) Determine the actual position of the flue dampers, and be careful not to close them completely or too tightly when adjusting them ； 　　(3) It must not be operated under overload conditions; a negative pressure should always be maintained inside the furnace ； 　　(4) Strengthen equipment management; gas valves that are not functioning properly should be replaced or repaired promptly, and flashback arrestors should also be checked regularly, with any faulty units replaced as soon as possible ； 　　(5) Before starting up and igniting, it is necessary to check whether the valves for gas and fuel oil are airtight; before each ignition, the combustible gases in the furnace must be purged with steam. 　　20. What are the signs of a cracked furnace tube? What is the reason? How to handle it? 　　(1) Phenomenon: In mild cases, a small amount of oil sprays out from the cracked section of the furnace tube; both the furnace temperature and the flue gas temperature rise. In severe cases, a large amount of oil sprays out from within the furnace tube and burns, with flue gas escaping from areas such as the elbow boxes, tube sheets, and access holes. The chimney emits large amounts of black smoke, and the furnace temperature rises sharply and suddenly.
(2) Cause: Local overheating and coking of the furnace tube; in areas where coking is severe, bulging occurs leading to cracks ； High-temperature oxidation peeling or inappropriate furnace tube material ； Poor maintenance quality, corrosion, erosion, etc. 　　(3) Handling method: When the furnace tube is slightly cracked, reduce the temperature and feed rate, and handle it as a normal shutdown. When the furnace tube overheats severely, the heating furnace shuts down immediately, feeding is stopped, and a large amount of steam is blown into the furnace chamber; steam is introduced at the furnace inlet to clean the interior of the tower (care must be taken regarding the fire situation inside the furnace chamber during this process) ； If the vacuum furnace catches fire, immediately restore the vacuum system to atmospheric pressure ； The others are handled as emergency shutdowns. 　　21. What are the causes, symptoms, and preventive measures for furnace tube coking? 　　(1) Causes of furnace tube coking 　　① Uneven heating of the furnace tube, with the flame hitting the furnace tube, resulting in localized overheating of the tube ； 　　②Fluctuations in feed volume and flow deviation cause the oil temperature to rise and fall erratically, or the flow rate to be too low; as a result, the oil stays in the system for too long and breaks down ； 　　③The raw materials are polycyclic compounds that undergo polymerization or decomposition, or they contain impurities. 　　④During maintenance, the coking was not removed thoroughly; as the furnace started operating again, the existing coke in the furnace tubes acted as a catalyst, promoting the formation of new coke. 　　(2) Judgment of tube coking phenomenon ① In a brightly lit furnace, if dark spots are seen on the tubes, it indicates that coking has occurred in those areas of the tubes ； 　　②The processing capacity remains unchanged, while both the furnace temperature and the pressure inside the furnace increase ； 　　③The slow response of the furnace exit temperature indicates coking at the thermocouple sheath. 　　(3) Measures to prevent coking 　　① Maintain uniform temperature in the furnace chamber to avoid local overheating of the furnace tubes; a combustion method using multiple burners with even flame distribution and a bright furnace interior should be employed ； 　　②During operation, close monitoring, analysis, and adjustment of parameters such as furnace feed rate, pressure, and furnace temperature are required ； 　　③Carry out thorough cleaning during shutdowns ； 　　④Strictly prevent material deviation. 　　22. How to detect and handle pump cavitation? 　　Pump cavitation may occur under the following conditions: (1) The instrumented flow rate indication fluctuates significantly or shows zero. 　　(2) The pressure and current indicators experience large fluctuations or show no reading. 　　(3) The pump vibrates significantly and produces noise. 　　(4) Abnormal noises are heard inside the pipeline. 　　The main reasons for pump evacuation are: (1) The density of the sealing oil is too low, and the sealing oil contains water ； 　　(2) Excessive sealing oil volume ； 　　(3) The liquid level in the tower and container is too low ； 　　(4) The filler gland is too loose, allowing cooling water to flow into the pump body ； 　　(5) The medium temperature is too high, resulting in a high saturated vapor pressure and thus gas blocking ； 　　(6) The steam valve for cleaning the pump inlet is not closed properly or there is a leakage ； 　　(7) There is air in the pump or non-condensable gases are present in the medium being pumped (air leakage at the inlet); (8) The inlet pipeline is blocked, the inlet valve is not open or is opened too little, or the valve element has fallen off. 　　Methods for dealing with pump evacuation: (1) Strictly control the quality of the sealing oil, and ensure that the sealing oil tank is thoroughly dehydrated ； 　　(2) Appropriately adjust the oil sealing injection volume ； 　　(3) Strictly control the liquid levels in towers and vessels ； 　　(4) Tighten the packing gland or apply external cooling water spray ； 　　(5) Lower the medium temperature and drive all the vaporized gas in the pump into the vent line ； 　　(6) Close the steam valve tightly or replace it ； 　　(7) Expel the air from the pump, locate the source of the leak, and find a way to fix it ； 　　(8) Cleaning of inlet pipelines, clearing of pipelines, opening of inlet valves, inspection and replacement of valves. 　　23. Why does a hot oil pump need to be preheated? How to preheat? If the pump is not preheated, the cold oil or condensed water inside the pump will mix with the hot oil at temperatures of 200–350°C, resulting in vaporization and causing vacuum formation in the pump. 　　When hot oil enters the pump body, uneven heating of various parts of the pump leads to uneven expansion, resulting in leaks and cracks. It can also cause the shaft arch waist phenomenon, leading to vibrations. 　　The medium transported by hot oil pumps has a high viscosity; it has poor flow properties at normal and low temperatures, and may even solidify, which prevents the pump from starting or results in a long start-up time that leads to tripping. 　　Preheating steps: (1) First, use steam to blow out all the oil or water remaining in the pump. 　　(2) Open the outlet valve to introduce hot oil into the pump, discharge it continuously through the vent, and keep turning the pump; close the outlet valve once the pump becomes hot. 　　(3) Slowly open the inlet valve (this is when evacuation is most likely to occur); keep rotating the shaft and continuously discharge gas through the vent. 　　(4) Gradually open the outlet valve to allow circulation between the inlet and outlet. 　　24. Why can’t a cold oil pump be used to pump hot oil? 　　(1) The materials of the components in cold and hot oil pumps are different; for example, the pump body, impeller, and sealing rings of a cold oil pump are made of cast iron, while those of a hot oil pump are made of cast steel. The seal for the pump body is made of 40Cr alloy steel. Generally, cast iron cannot operate at high temperatures. 　　(2) Cold oil pumps have a small temperature difference during operation, resulting in minimal thermal expansion and small gaps between components (such as the seal gap between the inlet and outlet of the impeller). Hot oil pumps, on the other hand, have larger gaps; when a cold oil pump is used to handle hot oil, wear can occur between the impeller and the pump casing, or even catastrophic failure may happen. 　　(3) Cold oil pumps usually do not have seal oil or flushing oil; since the components of the mechanical seal that operate at high temperatures are also exposed to high temperatures, they wear out rapidly, or even get stuck. 　　25. What abnormal noises do pumps often make during operation? 　　(1) Abnormal noises in rolling bearings 　　① In newly installed rolling bearings, excessive radial tension during assembly can make it difficult for the rolling elements to rotate, resulting in a slight humming sound; at this time, the temperature of the bearing rises. 　　②If the oil level in the bearing housing is insufficient, the rolling bearings will emit a steady whistling sound during operation. 　　③If the gap between the rolling elements and the spacer frame is too large, a loud swishing sound can be heard during operation. 　　④When peeling occurs on the raceway surfaces of the inner and outer rings or on the surface of the rolling elements in a rolling bearing, intermittent impacts and vibrations occur during operation. 　　⑤If a rolling bearing is damaged (including a broken separator, cracked rolling elements, cracks in the inner or outer rings, etc.), there will be cracking noises during operation. 　　(2) Loosening noise: A sound produced by the loosening of rotor components on the shaft, often with a periodic pattern. If components such as impellers and shaft sleeves become loose on the shaft, they will produce a knocking sound. If the pump shaft is bent, the knocking sound will be louder. Operating in such an environment is very dangerous; it can cause cracks in the impeller and breakage of the pump shaft. 　　26. What conditions must the facility meet before it can begin operation? 　　The plant can proceed to the commissioning phase only if the following conditions are met: (1) The plant has been repaired, and all associated equipment, pipelines, instruments, etc. have passed inspection and meet the quality requirements ； 　　(2) Flanges, gaskets, nuts, plugs, access holes, thermometer sleeves, thermocouple sleeves, etc. shall all be installed and tightened as required ； 　　(3) Complete the joint review and approval process for the plant startup plan and process cards ； 　　(4) Detailed instructions on the equipment modification and maintenance work were provided to all personnel involved in the facility, and a meeting was organized for them to study and discuss the start-up plan ； 　　(5) The safety facilities of the equipment are flexible and easy to use, and the hygiene conditions meet the requirements for operation. 　　27. What are the conditions before oil is introduced into the device? 　　The unit can be fed with oil only when the following conditions are met: (1) The equipment and pipelines associated with the unit are connected, pressure testing is complete, and all identified issues have been resolved ； 　　(2) Remove all the blind plates that were installed, replace the gaskets on the corresponding flanges, and tighten them ； 　　(3) Prepare an adequate amount of lubricating oil and various chemical raw materials, and have them ready for use ； 　　(4) Ensure an adequate amount of sealing oil and top-reduction reflux oil is available, and remove all water ； 　　(5) Water, electricity, steam, fuel, and air for instruments have all been supplied to the device, and it has been confirmed that the motor rotation direction is correct ； 　　(6) Seal all processes, and have them inspected at three levels—by the operator, the team leader, and the workshop supervisor—to confirm there are no issues ； 　　(7) Contact the production scheduler to understand the arrangements regarding the tanks used for crude oil and various products, and contact the quality inspection department to obtain the existing analysis results. 　　28. What issues should be noted during the oil cooling circulation process? 　　During the oil transfer and cooling cycle, it is necessary to strictly follow the operating procedures; the speed at which oil is transferred must be carefully controlled. In addition, the following points must be taken into account: (1) Before allowing oil to enter, contact the production scheduler and relevant departments to arrange for the process of returning the oil and to prepare the appropriate tanks for this purpose, as well as to purge these tanks thoroughly. 　　(2) During the oil inlet process, drainage can be carried out at various low points depending on the actual conditions of each unit, in order to remove as much water as possible from the equipment and prevent a large amount of water from being pushed into the oil return tank. However, special attention must be paid to the venting at each low point; once oil is detected in the water, that vent valve must be closed immediately to prevent oil leakage. 　　(3) After oil is introduced into the vacuum furnace, 1–2 burners can be ignited in the heating furnace first; the temperature at the furnace outlet should be ≤80°C. The ignition of the heating furnace must be carried out in accordance with its operating procedures. 　　(4) Once oil is detected at the residue oil sampling port, sampling begins to determine the water content; sampling and analysis are carried out every 20–30 minutes. Once the water content is below 3% (in some cases, it is required to be below 1%), the internal cooling circulation in the unit can be switched on. 　　(5) After changing the internal cooling cycle in the unit, it is necessary to purge the oil return line promptly, and use steam to warm up the line in preparation for switching to crude oil. 　　(6) During the cold cycle, the instruments should be activated, and the performance of these instruments should be assessed based on the discrepancy between their readings during the cold cycle and those during normal production. 　　(7) During the cold cycle, it is necessary to ensure even distribution of the feed to each heating furnace branch; there should be no short circuits, and if any occur, they must be resolved. 　　(8) Contact the relevant units to find out the oil inflow volume and verify whether it matches the actual allowable oil inflow volume of the plant. 　　(9) In a cold cycle, if the bottom pump fails, the amount of crude oil should be reduced immediately, and the liquid levels at the bottom of each tower must be controlled to prevent them from rising too high. If the cycle is stopped, the sequence for shutting down the pumps is to first stop the crude oil pump, then the bottom pump of the initial distillation tower, followed by the bottom pump of the atmospheric distillation tower, and finally the residue oil pump. To restart the pump, the sequence is reversed. 　　(10) During the cold cycle, the electrodialysis system can be put into use as early as possible, depending on the circumstances, so that it can fully perform its dehydration function. (11) When activating the electrodialysis system, in installations equipped with crude oil transfer pumps, it is necessary to first start those pumps and open their outlet valves; the outlet valve of the crude oil pump should then be controlled based on the pressure at the inlet of the transfer pump, after which the electrodialysis system is gradually integrated into the process. 　　29. What are the principles and precautions for shutting down the system for cleaning? 　　(1) Before shutting down operations, proper organization for pipeline cleaning must be carried out, with each pipeline assigned to a specific person. 　　(2) Ensure proper wiring connections to prevent cross-wiring, which could cause injuries or equipment failures. 　　(3) Unified command should be exercised during line cleaning to ensure that the heavy oil pipeline has sufficient vapor pressure, thereby guaranteeing the effectiveness of the cleaning process. 　　(4) Before introducing steam for line cleaning, the steam condensate must be completely drained, and steam should be introduced slowly to prevent water hammer. 　　(5) In the sweeping process, heavy oils and oils that are prone to solidification are processed first, followed by light oils and oils that are not prone to solidification. 　　(6) Pressure must be held before line cleaning; for heavy crude oils, pressure needs to be held repeatedly in order to achieve a better cleaning effect. 　　(7) Before cleaning the lines, all meters must be bypassed and switched to the auxiliary line; steam must not pass through the meters. 　　(8) During line cleaning, all connected lines, main and auxiliary lines, spare lines, dead ends, and control valves must be cleaned thoroughly; no dead corners are allowed to remain. 　　(9) It is absolutely not allowed to vent oil vapor from the various low points during the line cleaning process; such vents can only be used to check the progress of cleaning, and must be closed promptly afterward. 　　(10) After the line cleaning is completed, the cleaning valve should be closed promptly, and the steam and condensate inside the equipment and pipelines must be completely drained. 　　(11) Records should be kept for the shutdown and line cleaning. Details such as the steam supply time, the steam cut-off time, and the operator’s name must be carefully recorded to ensure accountability. 　　30. What is the purpose of a vaporization tower and what are the precautions? 　　After the unit is shut down, although the lines on each side have been drained outside the unit, all the intermediate reflux lines and tower feed lines still direct fluid into the tower; these residual oils end up inside the tower. In addition, there is a large amount of oil remaining on the tray levels, as well as plenty of oil vapor in the top of the tower and within it. If these residual oils and vapors are not properly removed, the entry of air can result in explosive gases, thereby compromising safety during maintenance. To ensure safe maintenance, the tower is usually vented through steam to remove the oil and vapor from inside it, which in turn creates the conditions necessary for cleaning the tower. 　　When operating the distillation tower, pay attention to the following points: (1) The flow pattern at the top of the tower must be the same as that during normal operation, and no condensing coolers must be omitted. 　　(2) Open the vent at the manhole of the overhead oil-water separator, and open all the drain branch line valves. (3) All side draw valves as well as the stripper tower draw valves must be closed to prevent steam from entering the side lines. But the valve connecting the main tower to the stripping tower must be opened. 　　(4) All the stripping towers connected to the main tower shall be steamed together in sequence, and the bottoms of the stripping towers shall be vented. 　　(5) The vent valve of the bottom liquid level gauge should also be opened. 　　(6) It is necessary to ensure a sufficient amount of steam and adequate steam blowing time; the time during which the medium is not in contact with the steam tower varies, and this should be carried out in accordance with safety regulations. 　　(7) When steaming the tower, steam is generally supplied mainly from the bottom of the tower and the feed stream, with steam at the middle reflux section serving as a supplementary source. 　　31. Under what conditions must the device be shut down urgently? 　　(1) In the event of a major accident in this unit, and despite efforts to address it, the problem cannot be resolved and continues to worsen; or in the event of a fire or explosion in other related units that poses a serious threat to the safe operation of this unit, operations must be halted immediately. 　　(2) In the event of a burn-through of the furnace tubes, severe oil leakage and fire in the distillation tower, or explosions or fires involving other heat exchangers and pump equipment, operations must be stopped immediately. 　　(3) If the main pumps, crude oil pumps, and bottom-receiver pumps fail and cannot be repaired, and the standby pumps cannot be started either, an emergency shutdown can be carried out. 　　(4) If the supply of raw materials, power, steam, or water is interrupted for an extended period and cannot be restored, emergency shutdown can be carried out. 　　32. What are the main principles for handling emergency shutdowns? 　　The main handling principles are: (1) Notify the fire department and report to production control. 　　(2) Immediately shut down the heating furnace, blow an appropriate amount of steam into the furnace chamber, reduce the flue dampers as well as the primary and secondary air valves, and try to prevent the furnace temperature from dropping too rapidly; instead, direct the low-pressure gas from the top of the three towers either to direct venting or to the flare. 　　(3) Immediately stop the crude oil pump, the pumps at the bottom of each tower, and the pumps on the side lines; finally, stop the seal oil pump and the reflux pumps at the top of each tower. In the event of a power outage, close all the outlet valves of the pumps. 　　(4) Shut off the valves connected to the faulty equipment. In the event of a fire, it is necessary to identify the location of the fire, close all valves connected to the pipelines and equipment involved in the fire, in order to cut off the source of the fire. 　　(5) Shut off all stripping steam; direct the superheated steam to vent. The vacuum pump of the vacuum distillation column is shut down to restore normal pressure; when normal pressure is re-established, the vent valve of the final vacuum pump must be closed to prevent air from entering the vacuum distillation column. 　　(6) Special oils should be transferred to the oil tank. 　　(7) If resuming production is not possible for the time being, contact the cleaning team for fuel oil and heavy oil products. 　　(8) The memory oil in the equipment should be removed as soon as possible. 　　(9) Make every effort to maintain local circulation; handle other matters as per normal shutdown procedures to prevent overheating and overpressure. 　　(10) Problems should be addressed in a proactive, timely, and decisive manner. 　　33. How to handle a cracked furnace tube? 　　Phenomenon: The furnace temperature and flue gas temperature rise suddenly, black smoke emerges from the chimney, and it becomes impossible to see inside the furnace. However, in recent years, as improvements have been made to the quality of maintenance of the underlying equipment and stricter procedures have been implemented for equipment inspection and acceptance, such accidents occur very rarely. Therefore, it is important to make a clear judgment and not assume that tube rupture is the cause of black smoke from the chimney, poor visibility inside the furnace, or rising temperatures, as this could lead to incorrect handling. Generally, tube rupture occurs due to prolonged lack of maintenance; signs such as swelling, bubbling, peeling, and darkening of the tube surface can indicate that a rupture is about to occur. Do not mistake phenomena such as automatic control failure, large amounts of fuel oil being injected into the furnace, low steam pressure, poor atomization by the nozzles, and a large amount of fuel oil entering the furnace for tube rupture. 　　Causes:
(1) Local overheating of the furnace tubes: for example, when fuel oil or gas containing oil is sprayed onto the furnace tubes and burned ； The burner is not aligned, causing the flame to shoot straight at the furnace tubes ； 　　(2) Unbalanced flow in several channels of the radiant furnace tube, resulting in overheating ； 　　(3) The furnace tubes have been neglected for a long time; defects have been detected over time ； The furnace tube material is of poor quality, resulting in sand holes or cracks due to high-temperature oxidation and erosion by the oil ； 　　(4) Defects in construction quality remaining from the furnace tube maintenance. 　　Handling principles: (1) If the furnace tube ruptures, the fuel valve should be closed immediately to cut off the gas; the low-pressure gas produced by the device should either be vented or directed away from the flame ； The feed to the heating furnace must be cut off, and the production scheduler must be informed promptly, as well as the fire department and relevant agencies. 　　(2) Immediately open the furnace fire-fighting steam valve. 　　(3) Listen to the blower, and appropriately reduce the flue dampers to decrease the amount of air in the furnace; however, do not close them too much to prevent explosion in the furnace chamber. 　　(4) If a fire breaks out in the vacuum distillation furnace, immediate action should be taken to restore the vacuum system to normal pressure; steam should be introduced into the vacuum distillation tower promptly, the steam valves of the primary and secondary vacuum ejectors should be closed, and the vent valve of the secondary evacuation condenser should be shut off. Be careful of this valve; do not open it to prevent air from entering and causing an explosion in the vacuum tower. Steam should be blown into the vacuum tower to reach normal pressure without exceeding it. 　　(5) Others shall be handled as emergency shutdowns. 　　34. What are the safety measures for working with fire in equipment? 　　(1) The safety measures for hot work on the equipment are all aimed at eliminating the conditions that can lead to combustion and explosion. It mainly involves releasing combustible substances at the area where welding is to take place, in order to reduce their concentration to a level that is safe for welding. Common methods include: ① Displacement method: Using steam, nitrogen, or other inert gases to displace flammable and explosive gases from pipelines and equipment; welding is only permitted after analysis shows that the conditions are safe. In distillation units, welding is allowed only after the pipelines have been cleaned with steam. 　　②Cleaning method: Wash the equipment and pipelines involved in welding operations with water; if necessary, remove any sediment as well. Welding can only proceed after the analysis shows that everything is satisfactory. To improve the cleaning effect, hot water is often used for rinsing. 　　③Isolation method: Isolate flammable and explosive materials to minimize the scope of displacement. Methods such as disconnecting the pipelines connected to domestic equipment and installing blind flanges when working with fire on the equipment, removing flammable materials from the floor, and covering sewer openings with asbestos cloth all fall under this category. 　　(2) Hot work analysis must be conducted promptly and accurately; it is not allowed to arbitrarily expand the area designated for safe hot work or extend the duration of such work. 　　(3) Smoking is prohibited on site, and welding tools must not be left unattended or handled carelessly. 　　(4) For hot work under positive pressure, it must be approved by the chief equipment engineer and experienced technicians; if necessary, approval from the plant manager in charge of safety or the chief engineer must be sought to obtain a hot work permit. As it constitutes special hot work, it should be carried out by experienced welders. 　　(5) At the location where welding is to be carried out, foam and steam fire extinguishing equipment must be available for use as a means of protection. The principle of \"no welding without three conditions\" must be strictly followed: no welding without a valid welding permit, no welding without proper fire prevention measures in place, and no welding without an observer present or if the observer is not on site. 　　35. What are the characteristics of pneumatic and electric instruments? 　　Features of pneumatic instruments: (1) Simple structure and reliable operation. It has strong resistance to interference from environmental temperature, humidity, and electromagnetic fields. Since there are no components such as semiconductors or contacts, sudden failures occur rarely, and the average time between failures is longer than that of electric meters. 　　(2) Easy to maintain. 　　(3) It inherently possesses intrinsic safety and explosion-proof characteristics. 　　(4) It is suitable for use with pneumatic actuators, but is not suitable for long-distance transmission; it has a slow response and low speed. 　　(5) Low price. Pneumatic instruments are widely used in small and medium-sized enterprises as well as in situations where on-site adjustment is required. 　　Features of electric instruments: (1) Thanks to the use of integrated circuits, they are compact in size, respond quickly, have high precision, and are capable of performing complex information processing, calculations, and advanced control functions. 　　(2) The signals are easy to transmit over long distances and facilitate centralized management. 　　(3) Easy to use in conjunction with computers. 　　36. What precautions should be taken when installing and using control valves? 　　(1) Inspection before installation: When the control valve arrives at the site, it should be inspected immediately. The inspection includes aspects such as installation dimensions, material, and accessories; in addition to visual inspection, hydraulic testing and calibration are also required. 　　(2) When the pipe diameter is larger than the valve diameter, a reducer filter should be installed ； The support position of the control valve must be considered to avoid creating installation stress on the valve. 　　(3) Facilitates maintenance. Maintenance. Valves equipped with attachments should be designed for easy observation, adjustment, and operation. 　　(4) Before installing the control valve, the process pipeline must be purged thoroughly before the valve is installed. A vent valve (or discharge valve) is provided after the control valve. (5) The flow direction must be taken into consideration during installation. The valve body must be installed vertically; if an inclined installation is necessary in special cases, supports should be used.