Freezing mode is active – have you taken adequate measures to prevent freezing in the process pipelines of your chemical plant?

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In the pipeline design of chemical plants, insulation and heat tracing of pipelines are common practices; the design for preventing freezing and providing insulation for pipelines cannot be ignored, especially in chemical enterprises located in cold regions. The anti-freezing and insulation design for pipelines requires specific insulation measures to be determined based on the temperature of the medium and the process requirements. Otherwise, under harsh weather conditions, the liquid piping is prone to freezing and cracking; therefore, pipeline installation has special requirements and cannot be designed according to ordinary standards. Currently, none of the design standards in use in China provide a comprehensive set of guidelines for preventing condensation and freezing in equipment and pipelines. It is necessary to consider each individual pipeline and piece of equipment on site, and to provide detailed explanations on the design methods for preventing freezing and insulating process pipelines, along with relevant precautions, in order to ensure effective protection against freezing. Mechanism of medium freezing in process pipelines: As the medium flows within the pipes, due to heat loss, its temperature gradually decreases along the flow path. Within a limited pipe length, under normal heat loss conditions, the temperature of the medium generally does not drop to its freezing point or ice point ； However, when the fluid flowing within the pipe stops, it loses heat supply, and its temperature may drop to the freezing point or even lower, resulting in freezing. This can cause minor blockages in the pipes or, in severe cases, pipe rupture. Generally, when the medium inside a pipe freezes, it is the medium right against the inner wall of the pipe that starts to freeze first. In a circular pipe, the frozen medium forms a ring along the wall, and then the thickness of the frozen layer gradually increases until the entire center of the pipe is frozen, thereby blocking it. Based on the analysis of the freezing process of the medium mentioned above, it can be seen that the freezing of the medium inside the pipe is an unstable heat transfer process. Due to the complexity of calculating unstable heat transfer processes, simplified methods are generally used in engineering practice. That is, the thickness of the insulation layer required to prevent freezing is calculated using the heat balance method for stable heat transfer, with appropriate safety factors taken into account. Freezing protection design for process pipelines: For equipment and pipelines containing water in their fluid, freezing is prevented by using insulation and heat tracing, as well as by implementing slope arrangements for drainage and auxiliary pipelines. Anti-freezing design shall be applied to the following equipment and pipelines: ① Circulating absorbent pipeline ； ②Process circulating water pipeline ； ③Post-product treatment pipeline ； ④Desulfurization tower ； ⑤Pumps and tanks should be placed indoors as much as possible. The anti-freezing methods for pipelines include insulation and heat tracing. To prevent pipes or equipment from being damaged by freezing, the thickness of the insulation layer is calculated based on the duration of interruption in the flow of the medium. Heated insulation is a common type of insulation used in pipelines for transporting chemical materials; it compensates for the heat loss from the pipelines to the surrounding environment through heating, thereby maintaining the temperature of the materials inside the pipelines at a constant level or preventing them from freezing. Pipeline layout requirements and anti-freezing measures: All pipes that are prone to freezing must be installed in accordance with anti-freezing requirements. Compressors, water pumps, safety valves, pressure relief valves, and utility pipelines also have their own specific anti-freezing measures, which are outlined below. 1. Buried water pipelines: Industrial water pipelines generally have a relatively large diameter, and in cold regions, anti-freezing measures must be considered; therefore, they are mostly laid underground. In pipes with a relatively large diameter, ice formation merely results in a layer of ice forming on the inner walls. Although this does not cause blockages, it does restrict the flow rate. Given that underground pipes are difficult to clean, they are typically buried below the frost line to prevent the medium inside from freezing. 2. Water pipes above the freezing line ★Layout requirements: (1) When water pipes laid underground in cold regions emerge above the ground surface, cut-off valves, anti-freezing drainage valves, and measures to prevent continuous flow should be installed on the main pipes at the cooling water inlet and outlet, in accordance with the process requirements. The installation methods for anti-freezing pipes for water pipes above the freezing line in cold regions are shown in Figures 1 to 4. Among them, the anti-freezing measures shown in Figure 1 can be adopted when using circulating water ； Where there are no return water pipes near the fresh water supply, the anti-freezing measures shown in Figure 2 or Figure 3 can be adopted ； For pipelines transporting circulating water, fresh water, etc., in areas where the average temperature in the coldest month is 0°C or lower, the anti-freezing measures shown in Figure 4 can be adopted (heat tracing is required when the downtime is excessively long). Anti-freezing drain valves and anti-freezing circulation bypass pipes should be installed at the equipment, pipes, and valves, so that the water stored within the equipment and pipes can be drained during shutdowns or maintenance tasks, thereby preventing the equipment from cracking due to freezing. The antifreeze circulation bypass pipe and the antifreeze drain valve should be placed as close as possible to the valve, and they also need to be insulated to prevent freezing. (2) Piping for any medium shall not have any \"blind\" sections at any location, in order to prevent the medium inside the pipes from freezing, solidifying, suffering from local corrosion, or remaining unclean after purging. Overhead water pipes should be free of dead ends, dead legs, and pocket sections; the dead ends of pipes that are prone to freezing can be shortened as much as possible, or those dead leg sections can be converted into false pipe supports. For pocket sections that cannot be avoided, it is advisable to install drain valves at the lowest points. ★Anti-freezing measures: (1) To prevent outdoor above-ground pipes from freezing and cracking when not in use, drain valves should be installed. Pipes where no water flows under normal conditions, such as the instrument pipes of orifice plate flow meters, should be equipped with steam tracing to prevent freezing. (2) For equipment that may need to be temporarily shut down during operation of the plant, the isolation valves of its cooling water pipelines are protected from freezing by using valve insulation boxes or applying heat tracing to the valves. (3) The outlet discharge valve of the shut-off valve or check valve installed on a vertical pipe should be as close as possible to the shut-off valve or check valve; if possible, it can be mounted on the valve body itself. (4) The cooling water discharge pipes of pumps or compressors should be arranged to slope in the direction of the pipe, and connected along this slope to the return water or wastewater pipes. 3. Compressor inlet pipeline: Anti-freezing for the compressor inlet pipeline is typically achieved through hot water or steam heating, as well as the use of external insulation layers. Of course, heat tracing and insulation are also relative; whether the amount of heat provided meets the requirements depends on other factors as well. If the ambient temperature is too low, the compressor can be placed in a compressor room and heated uniformly. At this time, the compressor inlet pipe does not need to be frost-proof, but the inlet pipes exposed outside the building still require insulation to prevent freezing. 4. Water pumps: To prevent freezing of water pumps, a common method is to install bypass pipes on the inlet and outlet pipelines of the pump. To prevent the fluid inside the standby pump from freezing, an anti-freezing circulation pipeline equipped with a flow control orifice plate can be used, allowing the fluid to flow from the anti-freezing circulation pipeline of the standby pump back to the inlet pipe through the pump body. Generally, the anti-freezing circulation pipeline is installed between the pump outlet shut-off valve and the pump outlet; the flow-limiting orifice plate must be calculated appropriately, and the anti-freezing pipeline still requires insulation. The installation method of the anti-freezing pipeline for the water pump is shown in the figure. In the diagram, the pressure at the outlet ends (C and B) of the pumps in operation should be higher than the pressure at the inlet end (A), while the pressure at the outlet end (D) of the standby pump should be lower than the pressure at point B. Due to the presence of a check valve, fluid accumulates in the section above the check valve up to point B. Therefore, a bypass line was installed so that this accumulated fluid can flow from point B through the bypass line, back to point A via the pump, thereby preventing freezing. 5. Safety valves and pressure relief valves: For all media that are prone to freezing, or for those in which condensate (harmless) may form after steam is released from safety valves or steam relief valves, a φ8mm drainage hole should be installed near the bottom of the outlet pipe of the safety valve, in order to prevent pressure buildup and explosion due to freezing at the outlet of the safety valve. If the pressure relief valve discharges into a closed system, and it is possible for the gas in the main pipe to condense at the lowest ambient temperature, then heating should be provided at the parts of the discharge outlet of the pressure relief valve where liquid condensate may accumulate. The piping should slope from the outlet of the pressure relief valve toward the main pressure relief pipe; it is essential to avoid any \"pouch\" formation, as otherwise gas condensate will freeze inside the pipeline at the outlet of the pressure relief valve, causing blockages and leading to poor pressure relief and potential accidents. 6. Utility material pipelines: The anti-freezing measure for utility material pipelines is generally aimed at preventing the accumulation of condensate. In industrial installations, not only insulation measures should be employed to prevent freezing, but also discharge valves or steam traps should be installed in appropriate locations to allow the accumulated condensate to be removed promptly. ★A drain valve should be installed at the lowest point on the steam pipeline (1), which carries superheated steam; the location of this valve should be near the main steam pipe. (2) Steam traps should be installed at the low points or ends of saturated steam pipes where condensate may accumulate. If the steam distribution header and the condensate collection header are located near the main pipe, shut-off valves or check valves should be installed, and these valves must be positioned horizontally. At the end of each branch pipe equipped with steam tracing, a liquid separation tray and a drain valve should be installed. A discharge valve should also be placed at the lowest points where condensate may accumulate, so that the condensate can be removed promptly when the system stops supplying steam, thereby preventing it from freezing inside the pipes. (3) At the return condensate collection station, steam tracing pipes should have as few liquid pockets as possible (liquid pockets in steam tracing pipes are the main cause of freezing). In cases where liquid pockets are unavoidable, their height must not exceed 4 meters according to the regulations. A single accompanying tube can have at most 1 liquid bag; it is not permissible to have multiple liquid bags present simultaneously as permitted by the standards. ★Fire pipe (1): The safety spraying system of the equipment should be equipped with a drain valve on the downstream side of the water supply valve, so that the system can be drained completely when water supply is stopped; no liquid should remain in the pipes to prevent them from freezing and cracking in winter. Accident shower and eyewash facilities should be located indoors as much as possible; if they must be placed outdoors, they should be installed near the walls of the building, with the water supply pipes and valves installed inside the building. ★Air system pipes: The air system within the device (plant air and instrument air) is an area where freeze protection and insulation of pipes are often overlooked, as plant air and instrument air contain a small amount of water vapor. The anti-freezing method involves installing a steam trap behind the drain valve at the bottom of the buffer tank, or adding heat tracing to the bottom of the buffer tank; this prevents the water vapor present in the air from condensing and accumulating at the bottom of the equipment as it passes through the filter and separator. The buffer tank must be equipped with a drain valve to remove condensate. Drain valves should also be installed at those points in the air system piping where condensate water may accumulate. 7. Venting and drainage: In standard designs, the length of the short pipe located before the vent valve and drain valve is specified as 75–100 mm according to relevant codes. However, based on the author’s experience at the Shenhua Coal Gasification Project site and other projects in northern regions, freezing frequently occurs when this length is within the 75–100 mm range. Therefore, it is advisable to use a short pipe with a length of less than 50 mm. Additionally, effective cooling measures must be employed during welding. Summary: (1) There are specific methods for preventing freezing and insulating the process pipelines in chemical plants. Process pipelines are often insulated with steam or hot water to prevent freezing, and insulation for freeze protection is also one of the commonly used methods. (2) If the medium in the pipeline possesses sufficient sensible heat, an insulation layer can be used to reduce the loss of heat from the medium itself, thereby ensuring that the material does not experience flow stagnation due to excessive viscosity within the pipeline. (3) In the past, drainage devices were commonly used in utility pipelines and water pipes to prevent freezing. As chemical processing plants become larger, there is a greater demand for user-friendly operations; therefore, utility pipes that require long intermittent operation times should also be equipped with heating systems for insulation. Appendix: 30 Tips for Preventing Freezing in Chemical Plants 1. Ensure proper personal protection, wear warm clothing, and take precautions to avoid slipping when moving up and down the towers during inspections. 2. Strengthen dehydration efforts by promptly removing water from various dehydration points to prevent freezing and blockages. 3. Pay attention to the level gauges in use; if the angle valves at the top and bottom of these gauges freeze up, it will affect the accurate reading of the liquid level. If the transmitter associated with the level gauge freezes up, it will impact mechanical rotation and current flow, thereby affecting the accurate measurement of the liquid level. 4. The temperature of the circulating water should not be allowed to drop too low; it must remain above 8 degrees. If it falls below 8 degrees, freezing in the auxiliary lines of the cooler can occur, and in severe cases, the pipes may even crack. 5. Insulate the pipes in the stagnant water areas of the workshop’s circulating water system ； The deactivated circulating water pump was subjected to reverse flow measures and its contents were completely drained. 6. For equipment that is not in use in the workshop, such as make-up water pumps, circulation water pumps, coolers, condensers, etc., measures such as draining water and conducting regular inspections should be taken. The compressor cooling water should be checked regularly by the operator to ensure that the water valve remains open. 7. If a liquid hydrocarbon pipeline freezes and gets blocked, be careful of physical explosions when using steam to clear the pipeline; do not force the cleaning process, rather preheat it gradually. 8. On-site pressure gauges can also give false readings in cases of freezing and blockage; judgment should be based on experience. Any uncertainty should be reported promptly for handling. 9. When purging frozen and blocked pipelines, use steam to purge the elbows. 10. Pay attention to enhanced liquid removal from gas pipelines to prevent liquids from entering the boiler. Check the natural gas lines to prevent blockages caused by frozen crystal structures. 11. Strengthen inspections to monitor the temperature at various locations. 12. The water supply pipeline should be kept from being completely closed to maintain a continuous flow of water. 13. For decommissioned equipment, blind flanges should be installed at the connections to the production system, any accumulated water must be drained, and the area should be purged thoroughly. For equipment left outdoors and exposed to the elements, take measures to prevent water, snow, and ice from damaging it. For outdoor equipment, the frequency of routine inspections and gear turning must be increased. 14. For all equipment, steam and water pipelines, and control valves that are in use for production and daily operations or temporarily out of service, anti-freezing and thermal insulation measures must be taken. The water should be completely drained, or a method of maintaining a continuous low-volume flow of water and steam should be employed, so as to meet both anti-freezing and cost-saving requirements. After shutting off the water and steam supply, thorough purging must be carried out. 15. Strengthen routine inspections for dehydration; check for dehydration at the lowest points of various equipment and pipelines. The cooling water supply to pumps must not be interrupted, and heating systems must remain functional. Pressure gauges and level gauges should be checked regularly. Steam and water hose connections as well as other related components must ensure continuous steam output and steady flow of water. 16. Valves whose switches do not move should not be forced to be switched; vehicles whose control panels do not function should not be operated. 17. Frozen cast-iron valves should be slowly heated with warm water or a small amount of steam to prevent damage from sudden heating. 18. Construction and domestic water should be directed into gutters or areas that do not interfere with traffic, and ice slush must be removed at all times. 19. Strengthen management by establishing records for anti-freezing and anti-condensation measures (including accident logging, the condition of anti-freezing and anti-condensation equipment, and pipelines prone to freezing). 20. Valve chambers, fire hydrants, and pipe trenches in low-temperature areas should be inspected one by one to drain any accumulated water, and anti-freezing and insulation measures should be taken. 21. Heating with high-pressure steam is strictly prohibited; care must be taken to prevent high-pressure steam from entering the low-pressure system. The return pipes for high- and low-pressure steam must not be interconnected. A pressure reducing valve should be installed on the pipeline of the steam heating radiators; the pressure gauge connected to this valve must be calibrated properly. The radiators must be tested for leaks by introducing steam before they can be used. 22. Whenever working at heights, it is necessary to ensure that all water accumulation, snow, and ice in the work area have been removed before proceeding. 23. Therefore, for steam pipelines (except fire protection steam), close the root valve and the wall valve, open all drain valves to drain any remaining water, and purge them with nitrogen. 24. All drain points must be drained every hour. 25. After shutting down the circulating water system, the low-point drain must be opened to remove all water, and the area should be blown clean with air. 26. For the drain pipelines of the adsorption towers, desulfurization towers, water washing towers, and gas-liquid separators outside the workshop, electric trace heating is used for freeze prevention. Freezing is prevented by promptly draining the water separators and coolers in the workshop after they are shut down. 27. When indoor heating is shut off, electrical trace heating is used to prevent freezing of safety facilities in the workshop, such as tap water systems, eyewash stations, and fire extinguisher cabinets. 28. After water is sprayed on the workshop floors and platforms, they must be cleaned immediately to prevent icing. Workshop personnel should enhance their safety and prevention awareness when going onto platforms and climbing stairs. 29. When workshop staff perform maintenance work on frameworks and the work involves working at heights, all necessary safety measures must be taken: wear safety helmets and harnesses, and there should be a dedicated person to supervise the work. High-altitude work in strong winds with gusts of force 6 (wind speed of 10.8 M/S) or above is considered special high-altitude work, and such work can only be carried out upon approval by the general manager. In case of rain or snow, clean the stairs promptly. 30. During this period, workshop staff should increase the frequency of inspections; if any abnormalities are detected, they must report them to their superiors promptly.