Thread Content
This post was last edited by yinkuilin6868 on 2015-11-23 at 13:36. Purging of chemical processing units is an essential step in chemical production; it serves to eliminate potential hazards and avoid unnecessary losses, as well as to check the quality of the cleaning process of the pipeline systems, ensuring that the systems can operate smoothly. I will introduce to you the three most common methods for purging chemical processing equipment, including their principles and requirements for purging, 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 start-up 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 ⑴ Pipes should be flushed with the maximum flow rate that can be achieved within them, or at a flow velocity of not less than 1.5 m/s, in a direction from high to low. ⑵The quality of the water used for washing 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. ⑶Flushing should be carried out in a sequential, segmented manner. The cross-sectional area of the discharge outlet must be no less than 60% of that of the pipe being flushed, 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. ⑷Since the pipes are connected to the equipment, blind flanges must be installed at the inlet and outlet of the equipment during flushing; the pipeline can be connected only after it has passed the flushing test. ⑸When flushing pipelines with water, it is necessary to ensure that pipe racks, hangers, etc. can safely bear the load when filled with water. ⑹If devices such as flow meters, valves, and filters are present 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. ⑺After flushing the process pipeline, drain all the 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. ⑻During 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 ⑴ Before flushing the system, a flushing plan must be prepared. ⑵The equipment and pipelines have been installed, pressure testing was successful, and verification according to the PID diagram showed no issues. ⑶As required by the flushing plan, the temporary flushing pipeline has been installed in place. ⑷The flushing personnel must be familiar with the flushing plan. ⑸Electrical and instrumentation equipment washed with water is now in normal operation. 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 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. 4 Fill in the pipe section and equipment flushing records as well as the acceptance records. Air (nitrogen) 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 residual debris from the process pipelines. 1 Principles and requirements for air (nitrogen) purging: ⑴ When using air (nitrogen) to purge 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. ⑵The pressure of the purging gas for process pipelines is generally controlled at 0.6 to 0.8 Mpa. The purge gas source pressure can be reduced or increased as appropriate. ⑶The 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 spool added. ⑷Before 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. ⑸During 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 purge air source: For system purging, a large amount of compressed air (nitrogen) is required, and the flow rate within 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 purge process. 3 Preparatory conditions for air (nitrogen) purging: ⑴ The pipelines and equipment in the process system have been installed, and the strength testing has been successful. ⑵All instruments in the purge pipeline have been protected. ⑶Blind plates have been installed on the equipment and pumps that are prohibited from being operated, as required. ⑷Temporary piping, valves, etc., for purging are already in place. ⑸The utility air compressor is running, ensuring a continuous supply of air. ⑹The 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. ⑺The purge record forms are fully prepared. See the table below: 4 Purging Methods and Key Points ⑴ 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. ⑵Purging is carried out by continuously releasing gas from each outlet, with a wooden hammer being used to strike the pipes repeatedly; special attention should be paid to areas such as welds and dead corners, but the pipes must not be damaged in the process. ⑶At 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 plates or valves, which could lead to malfunctions. ⑷When purging multiple systems, unified command must be followed to prevent damage to personnel and equipment. ⑸After 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 Acceptance Criteria: 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 there is no rust, dust, moisture, or any other debris or defects on the target within 5 minutes, then it is considered acceptable. 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 from 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 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 a) checking the steam pressure and temperature ； b) Opening degree of the purge valve ； c) Hydraulic characteristics of steam pipes. 1 Preparatory conditions for steam purging ⑴ Develop a comprehensive steam purging plan, which should include: the area to be purged, the purging pressure level, the purging methods, the sequence of purging, the criteria and methods for verification, as well as safety measures and precautions. ⑵Conduct a thorough inspection of steam pipes, fittings, pipe supports, pipe hangers, etc., to ensure they are secure and reliable. ⑶In accordance with the purging plan, all temporary piping, valves, vent pipes, etc. have been installed. ⑷Verify that all instrument components, control valves, steam traps, etc., on the purged pipeline have been removed, and that protective measures such as using splices have been taken. 3 Methods and key points for purging steam piping networks ⑴ Steam purging is usually carried out in the order of the pipeline network layout; 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 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. ⑵The purging method for steam pipelines should involve the process of heating the pipes → purging → cooling down → heating the pipes again → purging → cooling down, repeated over and over again, in order to remove the deposits adhering to the inner walls of the pipes and achieve an effective purging result. ⑶Steam 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. 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 essential; all parties must work together, based on the actual circumstances, to carry out the steam purging effectively.