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Purging of chemical plants is an essential step in chemical production; it is carried out to eliminate potential hazards and avoid unnecessary losses, to check the quality of the pipeline system’s cleanliness, and to ensure smooth operation of the system. Today, the editor will introduce to you three common methods for purging chemical processing equipment, including their purification principles and requirements, preparation conditions, key operation points, acceptance criteria, and precautions! Purpose of purging: Before the plant is put into operation, it is necessary to purge and clean the pipes and equipment that have passed installation inspections. This is done by using fluids such as air (nitrogen), steam, water, and chemical solutions to remove any residual impurities from the construction process, thereby ensuring the smooth operation of the plant. Common methods include water flushing, air (nitrogen) purging, and oil cleaning. Water flushing is a method that uses water as a medium, with pumps being used to apply pressure in order to flush pipes and equipment. For example: flushing of the circulating water system. 1. Principles and requirements for flushing?? Pipes should be flushed with the maximum flow rate possible within them, or at a flow velocity of not less than 1.5 m/s, in a direction from high to low. ??The quality of the water used for washing must meet the requirements of the materials from which the pipes and equipment are made. For the flushing of ordinary equipment and pipelines, a turbidity of less than 10 mg/L and a chloride content of less than 100 ppm are commonly used. ??Flushing should be carried out in a sequential, segmented and continuous manner; the cross-sectional area of the discharge outlet must be no less than 60% of that of the pipe being flushed. It is essential to ensure that the discharge pipeline remains unobstructed and safe. Only after the upstream flushing outlets have been flushed successfully can the flushing of subsequent sections of the system proceed. ??For pipelines connected to equipment, blind flanges must be installed at the equipment’s inlet and outlet during flushing; the pipeline can be connected only after it has passed the flushing test. When flushing the pipeline with water, it is necessary to ensure that pipe racks, hangers, etc. can safely withstand the load when filled with water. ??If there are devices such as flow meters, valves, filters, etc., on the pipeline, they must be removed or temporary short circuits must be installed; only after the pipeline has been flushed can these devices be reinstalled in their original positions. ??After flushing the process pipelines, drain all water from the system. When draining, there should be a large vent hole at the top to prevent a vacuum from forming and damaging the equipment. ??During winter flushing, care must be taken to prevent freezing. After flushing, drain the water promptly; if necessary, use compressed air to dry it. 2 Preparation conditions for water flushing?? Before flushing the system, a flushing plan must be prepared. ??The equipment and pipelines have been installed, pressure testing has passed, and verification according to the PID diagram shows no issues. ??In accordance with the flushing plan requirements, the temporary flushing pipeline has been installed in place. ??The flushing personnel must be familiar with the flushing plan. Electrical and instrumentation equipment washed with water is operational normally. 3 Methods and key points for water flushing: Follow strictly the segmented flushing procedure outlined in the water flushing plan, which involves flushing the main components first and then the branches; that is, only after each flushing point has been verified to be functioning properly can the flushing of subsequent systems proceed. 4 Fill in the records for flushing and acceptance of pipeline sections and equipment. Air (nitrogen) purging: Air purging is a method in which air is used as the medium; it is pressurized to 0.6–0.8 Mpa by an air compressor, and then used to remove any remaining debris from the process pipelines. 1 Principles and requirements for air (nitrogen) purging?? When using air (nitrogen) for purging process pipelines, it is necessary to ensure an adequate flow rate of gas, so that the flow velocity of the purging gas is greater than that during normal operation, or at least 20 m/s. The pressure of the purge gas for process pipelines is generally controlled at 0.6–0.8 Mpa. The purge gas source pressure can be reduced or increased as appropriate. ??The instruments on the purging pipeline should be removed to prevent damage to the instrument components. At the same time, appropriate protective measures should also be taken for the control valve; if necessary, it must be removed and a temporary coupling installed. ??Before purging, blind flanges must be installed at the inlets and outlets of equipment such as heat exchangers and tower bottoms; only after upstream purging is completed successfully can the purging of the subsequent systems proceed. ??During purging, the safety valve should be disconnected from the pipeline, and a blind flange should be installed to prevent damage to the safety valve. 2. System purging air source: For system purging, a large amount of compressed air (or nitrogen) is required, and the flow rate of this air through the pipes and the system is high; therefore, it is essential to ensure an adequate pressure and flow rate in order to maintain the quality of the purging process. 3 Preparation conditions for air (nitrogen) purging: The pipelines and equipment in the process system have been installed, and the strength pressure tests have passed. ??All instruments in the purge pipeline have been protected. ??Blind plates have been installed on the equipment and pumps that are prohibited from being operated, as required. ??Temporary piping, valves, etc. for purging have been prepared and are in place. ??The utility air compressor is operating, ensuring a continuous supply of air. ??The purging personnel and operators are familiar with the purging plan. Following the purge flowchart, I have a clear understanding of the purge procedure, flow direction, exhaust outlets, etc. ??The purge record forms are fully prepared. 4 Purging methods and key points ?? Purge each system one by one in the order indicated by the purging flowchart. During purging, purge the main lines first, and then the branch lines. ?? Purging is carried out by continuously releasing gas from various discharge ports, while the pipes are struck continuously with a wooden hammer; special attention should be paid to areas such as welds and dead corners, but the pipes must not be damaged. At the start of purging, air should be introduced into the system slowly; only after gas begins to flow out from the inspection port can the amount of purge air be increased. This is done to prevent overpressure in the system due to issues such as incorrect installation of blind plates or valves, which could lead to malfunctions. When purging multiple systems, it is necessary to follow unified command to avoid damage to personnel and equipment. ??After confirming that the purging is complete and satisfactory, the system should be reset to prepare for the next phase of the airtightness test. 5 Inspection Methods and Purging Qualification Standards: Whether the purging of each section of pipeline is successful should be checked jointly by production and installation personnel. When the exhaust gas appears clean to the eye, a white cloth target or a white lead oil target should be used at the exhaust outlet for inspection; if no rust, dust, moisture, other debris, or imperfections are present on the target within 5 minutes, then it is considered qualified. Steam purging involves the use of steam with varying parameters as the medium for purging. It offers a high purging speed, and the intermittent nature of steam purging allows the pipes to contract and expand due to temperature changes, thereby removing deposits adhering to the inner walls of the pipes and achieving optimal purging results. Note: Steam purging involves high temperatures, high pressures, and fast flow rates; the pipes expand due to heating and contract when cooled. Therefore, it is necessary to fully consider the impact on the system’s structure to ensure the safety of personnel and equipment during purging. 1 Principles and requirements for steam purging: Steam purging is generally classified into three levels – high, medium, and low – based on the operating parameters of the pipelines. The steam system in distillation operates at low pressure; therefore, steam with a pressure of <1 Mpa is used, and the flow rate must be ≥30 m/s. During steam purging, it is necessary to pay attention to checking the steam pressure and temperature ； Opening degree of the purge valve ； Hydraulic characteristics of steam pipes. 2 Preparatory conditions for steam purging ?? A comprehensive steam purging plan should be developed, which should include details such as the area to be purged, the purging pressure levels, the methods of purging, the sequence of purging, the criteria and methods for verification, as well as safety measures and precautions. ??Conduct a thorough inspection of steam pipes, fittings, pipe supports, pipe hangers, etc., to ensure they are secure and reliable. ??In accordance with the purging plan, all temporary piping, valves, vent pipes, etc. have been installed. ??Verify that all instrument components, control valves, steam traps, etc., on the pipeline to be purged have been removed, and that protective measures such as using splices have been taken. 3 Methods and key points for purging steam pipelines?? Steam purging is usually carried out in the order of the pipeline configuration; generally, high-pressure steam pipelines are purged first, followed by medium-pressure pipelines, and finally low-pressure steam pipelines. For each level of piping, from near to far, the main pipeline should be purged first, followed by the branches. The steam traps for each pipe section should be installed after the pipeline cleaning is completed. ?? The purging method for steam pipelines should involve heating the pipes → purging → cooling down → heating the pipes again → purging → cooling down, and this process should be repeated to remove the deposits adhering to the inner walls of the pipes, thereby achieving an effective purging result. ?? Steam purging must first fully warm the pipes and remove condensate to prevent water hammer. During the first purge cycle, special attention should be paid to checking the thermal expansion of the pipelines and whether the pipes slide properly. 4 Safety precautions for steam purging: Set up warning barriers to prevent people from entering the area around the purging ports, thereby avoiding accidents. During the purging process, proper coordination and planning are necessary; all parties should work together in accordance with the actual circumstances to carry out the steam purging effectively.