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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. Typically, there are several methods such as water flushing, air (nitrogen) purging, and oil cleaning. ◆◆Water flushing ◆◆ Water flushing is a method in which water is used as a medium; pumps are employed to pressurize and flush pipes and equipment. For example: flushing of the circulating water system. 01 Principles and requirements for flushing ❶ Water should be used to flush pipes at the maximum flow rate possible within those pipes, or at a flow velocity of not less than 1.5 m/s, with the flow direction being from high to low. ❷ The quality of the water used for flushing must meet the requirements of the materials used in the pipes and equipment. 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. ❸ Flushing must be carried out in sequence through segmented and continuous flushing. The cross-sectional area of the discharge outlet should not be less than 60% of the cross-sectional area of the pipe being flushed. It is necessary to ensure that the discharge pipeline remains unobstructed and safe. Only after the upstream flushing ports have been successfully flushed can the flushing of subsequent systems proceed. ❹ For pipelines connected to equipment, blind flanges must be installed at the equipment’s inlet and outlet during flushing. Only after the pipeline has been successfully flushed can it be connected to the equipment. ❺ When flushing the pipeline with water, ensure that pipe racks, hangers, etc. can safely bear the load when filled with water. ❻ When devices such as flow meters, valves, and filters are encountered along the pipeline, they must be removed or a temporary short circuit must be installed. They can only be reinstalled after the pipeline has been flushed. ❼ After flushing the process pipelines, drain all water from the system. During drainage, there should be a relatively large top vent to prevent a vacuum from forming and damaging the equipment. ❽ During winter flushing, pay attention to frost prevention. After rinsing, drain the water promptly; if necessary, use compressed air to dry it. 02 Preparatory conditions for water flushing ❶ Before flushing the system, a flushing plan must be prepared. ❷ Equipment and piping have been installed; pressure testing has been successful, and inspections against the PID diagrams have confirmed everything is correct. ❸ In accordance with the flushing plan, the temporary flushing pipelines have been installed. ❹ The rinsing personnel must be familiar with the rinsing procedure. ❺ Electric and instrumentation equipment washed with water is in normal operation. 03 Methods and key points for water flushing: Follow strictly the segmented flushing procedure outlined in the water flushing plan, which involves flushing the main lines first and then the branches; that is, only after each flushing point has been verified to be working properly can the flushing of subsequent sections of the system proceed. 04 Fill in the records of pipe section and equipment flushing, as well as acceptance records. ◆◆ Air (nitrogen) purging ◆◆ Air purging is a method in which air is used as the medium; after being pressurized to 0.6–0.8 Mpa by an air compressor, it is used to remove any residual impurities from the process pipelines. 01 Principles and requirements for air (nitrogen) purging ❶ When selecting air (nitrogen) purging for process pipelines, it is necessary to ensure an adequate volume of gas, so that the flow velocity of the purging gas exceeds the normal operating flow velocity; this minimum velocity should be no less than 20 m/s. ❷ The pressure of the purge gas for process pipelines is generally controlled at 0.6–0.8 Mpa. The purge gas source pressure may be lowered or raised as needed. ❸ The instruments on the purging pipeline should be removed to prevent damage to the instrument components. At the same time, appropriate protective measures should be taken for the control valve; if necessary, it must be removed and a temporary spool piece installed. ❹ Before purging, blind flanges must be installed at the inlet and outlet of equipment such as heat exchangers and columns. Only after the upstream section has been successfully purged can the purging of subsequent systems proceed. ❺ During cleaning, the safety valve should be disconnected from the pipeline, and a blind flange should be installed to prevent damage to the safety valve. 02 System purging air source: System purging requires a large amount of compressed air (nitrogen), which flows at high speed through pipes and the system. Therefore, it is essential to ensure sufficient pressure and airflow volume to guarantee the quality of the purging process. 03 Prerequisites for air (nitrogen) purging ❶ The piping and equipment of the process system have been installed, and the strength pressure test has been passed. ❷ All instruments in the purge pipeline have been protected. ❸ Blind flanges have been installed on equipment, pumps, etc. where blowing is prohibited, as required. ❹ Temporary piping, valves, etc. for purging have been prepared and are in place. ❺ The utility air compressor is in operation, meeting the conditions for continuous air supply. ❻ 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. ❼ The purging record sheet is fully prepared. See the table below: 04 Purging methods and key points. ❶ Purge each system one by one in accordance with the sequence shown in the purging flowchart. During purging, purge the main lines first, and then the branch lines. ❷ Purging is carried out by continuous discharge at each vent point. The pipes should be continuously tapped with a wooden hammer; particular attention should be paid to tapping areas such as welds and dead corners, but the pipes must not be damaged. ❸ At the start of purging, air should be introduced into the system slowly; only after gas begins to emerge from the inspection port can the flow rate of the purge air be increased, in order to prevent overpressure in the system due to issues such as incorrect installation of blind flanges or valves, which could lead to malfunctions. ❹ When purging multiple systems, unified command must be followed to prevent damage to personnel and equipment. ❺ After the purge is completed and confirmed to be satisfactory, a system reset should be performed to prepare for the subsequent airtightness test. 05 Inspection Methods and Acceptance Criteria for Purging: Whether the purging of each section of pipeline is successful should be jointly checked by production and installation personnel. When the exhaust gas appears clean to the naked eye, a white cloth target plate or a white lead paint target plate should be placed at the exhaust outlet for inspection. If there are no rust, dust, moisture, other impurities, or pitting on the target plate within 5 minutes, then the purging is considered successful. ◆◆Steam purging ◆◆ Steam purging involves using steam with various parameters as the medium for purging. It features a very high purging speed. The intermittent nature of this process allows the pipeline to undergo thermal contraction and expansion, thereby removing any deposits on its inner walls and achieving optimal purging results. Note: During steam purging, the temperature is high, the pressure is great, and the flow velocity is fast. As a result, pipes expand and shift when heated, then contract upon cooling. Therefore, the impact on the system structure must be fully considered to ensure the safety of personnel and equipment during the purging process. 01 Principles and requirements for steam purging: Steam purging is generally divided into three levels – high, medium, and low – based on the operating parameters of the pipeline. 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. 02 Preparation conditions for steam purging ❶ Develop a comprehensive steam purging plan, which should include: the area to be purged, the purging pressure level, the purging method, the sequence of purging, the criteria and methods for verification, as well as safety measures and precautions. ❷ Conduct a thorough inspection of steam pipes, fittings, pipe supports, pipe hangers, etc., to ensure they are secure and reliable. ❸ In accordance with the purging plan, all temporary piping, valves, vent pipes, etc. have been installed. ❹ Verify that all instrument components, control valves, steam traps, etc., on the purged pipeline have been removed, and that protective measures such as the use of spools have been taken. 03 Methods and key points for purging steam pipelines ❶ Steam purging is usually carried out in accordance with the configuration sequence of the pipeline network. Generally, high-pressure steam pipes are purged first, followed by medium-pressure pipes, and finally low-pressure steam pipes. For each level of piping, from near to far, the main lines should be purged first, followed by the branches. The steam traps for each pipe section should be installed after the pipeline purging is completed. ❷ The purging method for steam pipelines should involve heating the pipes → purging → cooling down → reheating the pipes → purging → cooling down, and repeating this process to remove the deposits adhering to the inner walls of the pipes, thereby achieving an effective purging result. ❸ 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 are sliding properly. 04 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.