Winter anti-freezing and anti-condensation work in chemical factory is the main means to ensure safe production. In order to prevent all freezing and condensation accidents, the anti-freezing and anti-condensation work according to the characteristics of the equipment should also be carried out in accordance with the principle of “prevention first, supplemented by elimination”. Strengthen the collection, delivery, use and management of easy-to-freeze and easy-to-coagulate items, take precautions in advance according to basic methods, conduct careful inspections in accordance with regulations, and reasonably handle freezing and blockage problems to achieve the goal of ensuring safe production.
1) Antifreeze method
Common anti-freezing and anti-condensation methods in chemical factory include emptying, heat preservation, heat tracing, and circulation. In winter, anti-freezing and anti-condensation should be based on the operating status of equipment and pipelines, combined with the problems and experience in the anti-freezing process in previous years, and one of them should be adopted. One or more methods.
- Emptying. Emptying antifreeze and anti-condensation methods can be roughly divided into three categories:
First, the equipment and pipelines that are intermittently operated on the ground or not used for a long time can be emptied according to the following steps:
(1) Close the relevant valve and add a blind plate if necessary to prevent internal leakage of the valve;
(2) Open the high-point exhaust valve and low-point exhaust valve to empty the materials in the equipment;
(3) Purge and replace residual materials with nitrogen;
(4) Normally open the low-point drain valve and check it regularly.
The second is the drainage of facilities and pipelines in underground valve wells, mainly fire-fighting facilities without heat (such as water monitors and fire hydrants). The root valve is closed from the valve well, and the water above the valve is drained through the drain valve and kept clean. The valve is normally open, and the head of the fire monitor is downward.
The third is the temporary use of equipment, pipelines or hoses. After use, the media in the pipes should be emptied in time and purged with nitrogen; if conditions permit, they can be placed in the insulation workshop.
- Insulation. From the perspective of anti-freezing, the thermal insulation of chemical plants mainly involves the following three aspects:
The first is equipment, pipelines and their accessories with thermal insulation wool. The problems that occurred in previous years should be combined before the winter. This work should be carefully investigated and improved. Pay special attention to equipment that does not have heat and only depends on the temperature of the material to prevent freezing. The heat preservation of the end of the pipeline, the blind end and the on-site pressure gauge ensures that the heat preservation is in place.
The second is to keep the indoor spaces such as workshops airtight to keep warm. Check whether the doors, windows and curtains of the workshops are intact, and put the heating in the workshops in time.
The third is to seal the underground valve wells for warmth. For the valve wells, use plastic cloth to seal well in advance, and take thermal insulation measures when necessary.
- Heat tracing. As far as winter antifreeze is concerned, the common heat tracing media include hot water tracing, steam tracing, and electric tracing. Hot water tracing is suitable for conditions where the operating temperature is not high or high temperature tracing medium cannot be used. Steam tracing is generally used for tracing where the operating temperature of the medium in the tube is less than 150°C.
Electric heat tracing is not only suitable for various situations of steam tracing, but also suitable for heat-sensitive medium pipelines. It can effectively control the temperature and prevent the pipeline from overheating. It is suitable for pipelines or equipment that are scattered or far away from the gas supply point and equipment with irregular shapes. Heat tracing.
The selection of heat tracing medium should be combined with material properties, index requirements, construction difficulty, operating cost, etc., and construction should be carried out in accordance with the specifications to avoid new problems while achieving the purpose of anti-freezing and anti-condensation.
- Loop. The cyclic antifreeze and anti-condensation methods are mainly used for equipment and pipelines that are intermittently operated or not used for a long time, and the above methods (emptying, heat preservation, heat tracing) are not used for antifreeze, or the above methods cannot meet the antifreeze requirements. The method is roughly divided into four ways:
One is to circulate by returning the material from the outlet of the operating equipment to the inlet of the standby equipment. This method is often used for antifreeze and anti-condensation of the standby equipment, such as by bypassing the pipeline of the single valve at the pump outlet, or opening the hole through the spool of the single valve.
The second is to bridge the equipment inlet pipeline to the equipment outlet pipeline. This method is often used to prevent freezing of the deactivated heat exchange inlet and outlet pipelines. Connect the smaller diameter pipelines before and after the inlet valve of the heat exchanger to maintain the inlet and outlet pipes. The pipeline medium is circulated.
The third is to circulate a small amount according to the normal operation mode, and generally try to close the outlet valve of the standby equipment as much as possible to reduce the amount of medium circulation and meet the antifreeze requirements.
Fourth, a small amount of medium is continuously or intermittently discharged. This method is mainly for antifreezing at the end of the pipeline that cannot be established, such as hose stations, storage tank dehydration pipelines and valves, steam pipeline low-point drain valves, eye washes, etc.
2) Antifreeze inspection
Anti-freezing inspection is an extremely important part of winter anti-freezing work. Regular anti-freezing inspections can promptly discover and deal with problems that occur during the anti-freeze process, effectively avoiding the further expansion of the freezing problem. Antifreeze inspection is mainly to check the effect of antifreeze and anticoagulation work, check for omissions, and further improve antifreeze and anticoagulation work. The main inspection contents and methods are as follows:
- Regularly check the drain condition of the steam trap for the steam tracing pipeline. If the drain is found to be abnormal, carry out the investigation and treatment in time. If the drain condition cannot be checked directly (such as concentrated to the drain recovery station), use a temperature measuring gun or touch the trap valve body with your hand. (Detection holes need to be reserved during heat preservation). Generally, the inspection requires that the temperature should be above 30℃ or it is normal to feel hot. Sometimes the warmness of the touch does not necessarily confirm that the drainage is normal, especially for the situation where it is concentrated to the drainage recycling station. The nearby normal drain pipeline conducts heat, which often misleads inspection judgments.
- For hot water tracing, check the water flow with a water flow indicator, or use a temperature gun or hand to touch the reserved inspection position of the electric tracing cable to check whether the temperature of the electric tracing cable is normal; when the tracing hot water pipeline is recycled , It should also be based on experience to avoid interference from the heat conduction of the normal hot water pipeline of the accessories, which will affect the inspection judgment.
- The electric heating with temperature indication should not only check the temperature indication value regularly, but also need to use a temperature measuring gun or hand to touch the electric heating cable to reserve the inspection position to prevent the temperature indication from being inaccurate and to ensure that the temperature of the electric heating cable is normal. Electric heating without temperature indication should detect the temperature.
- Airtight workshops should be equipped with thermometers to regularly check the sealing of doors and windows and the heating temperature, and adjust the amount of indoor heating according to the indoor temperature; check the insulation and sealing of valve wells, and monitor the internal temperature if necessary.
- Regularly check the emptied equipment and the low point of the pipeline for material outflow and freezing, or use a wrench to knock the pipeline to judge. If freezing is found, check the source of the material in time and take corresponding treatment.
- Regularly check the equipment and pipeline antifreeze cycle crossings. If there is a water flow indicator or sight glass, observe whether the indication is normal; if there is no water flow indicator or sight glass, use a temperature measuring gun or hand to touch the equipment body or related antifreeze cross line If it is difficult to judge by temperature, the listening stick can also be used to judge the flow of equipment or pipeline materials. For materials that take a small amount of external discharge cycle antifreeze, regularly check whether the external discharge volume is normal.
- Heat tracing of the instrument, regularly check the sealing condition of the heat tracing box of the instrument and the temperature inside it. At the same time, check the heat tracing of the instrument according to the heat tracing method according to the above method; in addition, you can compare the on-site and remote transmission instruments, and refer to the normal instrument. The indicated value is used to judge whether the meter is frozen or blocked.
- Regardless of whether it is long-term use or spare moving equipment, it should be cranked regularly to find out whether the equipment is frozen or blocked in advance; equipment that cannot be cranked (such as canned motor pumps), in addition to the antifreeze cycle check, when starting The point type should be carried out first, and the temperature of the easy-to-freeze parts (such as the back seat of the canned motor pump) should be measured, and then start after it is normal.
- The exhaust valve at the end of the gas (steam) pipeline, the U-shaped bend low-point exhaust valve, the hose station valve, etc., maintain a proper opening or perform regular drainage inspections to avoid accumulation of materials at low points and cause pipeline freezing and blockage.
- Regarding the meter gas, in addition to regularly draining the end and low points of the main line, the internal water content should also be tested to avoid accumulated freezing of the water in the meter, which will cause the meter to fail to adjust.
- Regularly check the outlets of draining consumables, whether there is water flowing out or freezing blockage at the low-point drain valve, dredge it in time if the problem is found, and confirm whether the root valve is tightly closed.
- Regularly check the system vent and breathing discharge ports, such as tank breathing valve and flame arrestor, emergency vent pressure relief port, air compressor vent port, and boiler safety valve vent port to ensure that the vent port is unblocked.
- Regularly check whether the outdoor fan inlet protection net, filter net, motor and other equipment cooling and heat dissipation fan inlet protection nets are blocked by freezing. If there is steam emission around the equipment or in snowy weather, it is prone to blockage.
- Regularly check whether the liquid in the outdoor water-sealed tank or liquid-sealed tank is blocked by freezing, and if any abnormalities are found, deal with it in time to ensure its proper function.
- In the event of a drastic drop in temperature or at night, the inspection should be strengthened, and the accompanying heat, the heating capacity of the plant, and the antifreeze cycle should be adjusted according to the temperature.
3) Freeze blockage treatment
In the process of thawing, it should be dealt with according to the specific conditions. Generally speaking, for the equipment that is blocked by freezing, short pipelines, meters, valves, etc., steam can be directly used to purge the blocked surface to dissolve, and heat preservation is required. Unlock the heat preservation in advance; if the frozen equipment is large and the pipeline is longer, the method of laying temporary heat tracing (such as using a metal hose) and heat preservation is often used to slowly defrost. If frozen equipment, pipelines, instruments, and valves are easy to remove, they can be removed and placed in a higher temperature room for thawing as appropriate.
In order to avoid safety accidents such as toxic and harmful gases, liquids spraying out, hurting people, or causing explosions in the process of handling freeze blockage, the following points need to be paid attention to during the process:
The first is to check whether the frozen equipment, pipelines and valves are cracked or damaged, and the valve switching status;
The second is to isolate relevant systems before processing;
The third is to operate slowly, use low-temperature steam to slowly dissolve, to prevent sudden heating from damaging frozen equipment, pipelines and valves;
The fourth is to consider the impact on the instrument and process conditions after the thawing process is suddenly dredged, and take countermeasures in advance.
4) Other matters
- Combining the anti-freezing experience in previous years, deal with the anti-freezing problems that occur, and prepare anti-freezing materials, such as insulation materials, traps, gold hoses, temporary joints, anti-freezing gloves, etc.
- Take precautions, put into use and check various anti-freezing measures before the beginning of winter, to provide enough time to deal with anti-freezing problems, and to avoid sudden drop in temperature leading to failure of anti-freezing measures.
- Adjust the accompanying heat in time to not only meet the needs of anti-freezing, but also avoid waste caused by excessive heat, excessive process indicators, and inaccurate instrument indications.
- The material pipeline with heat tracing should have an expansion direction to avoid the pressure of the equipment or pipeline caused by the heating and expansion of the material. If there is a pressure gauge, pay attention to monitoring its pressure.
- The material pipeline is running and flowing, and the heat tracing pipeline and the material pipeline it is tracing heat each other. If the material in the pipeline stops flowing, it should be checked and the heat tracing should be increased appropriately.
- The tracing temperature of the eyewash is appropriate, and it should be avoided that the tracing temperature is too high to cause new injuries in emergency use.
- If hot water or steam tracing needs to be temporarily stopped due to maintenance, attention should be paid to avoid freezing and blockage of the heat tracing pipe and the heated material pipeline.
- For equipment with heat tracing, if it needs to dissipate heat during operation, the heat preservation should be disengaged, adjusted down, or the heat tracing should be disabled to avoid over-temperature in the operation of the equipment.
- The electric heat tracing should pay attention to the heat tracing load to prevent overloading and tripping or catching fire; the electric heat tracing should be powered off first, and the anti-freezing measures of the heated materials should be considered during the maintenance.
- It is found that the steam tracing drainage is abnormal, such as water hammer, the drainage amount is too small or too large, and the tracing is not hot, and it must be dealt with in time.
- Adopt the method of cyclic anti-freezing, in the case of meeting the anti-freezing requirements, try to reduce the circulation volume to avoid unnecessary waste caused by excessive circulation.
- Similar to fire-fighting valve wells, when underground valves are closed, tools for opening the well covers and valves should be equipped to prepare for timely use in emergency situations.
- Eliminate the phenomenon of dripping and leakage on site in time, avoid the phenomenon of ice on the ground and ice hanging in the air. If it occurs, deal with it in time.
- Perform anti-freezing inspections regularly to ensure the quality of inspections. At the same time, focus on inspections based on different time periods, temperature conditions, and difficulty in anti-freezing.