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Section 1: Purging and Cleaning Objectives and Methods Before a chemical plant is put into operation, all the process pipelines and equipment that have passed installation inspections must be purged and cleaned (hereinafter referred to as purging). The purpose of this is to use fluid media such as air, steam, water, and various chemical solutions, along with purification and flushing processes as well as physical and chemical reactions, to remove debris such as sediment, grease, welding slag, and rust that remain within these components or are attached to their inner walls from the construction and installation phase. This is done to prevent blockages in pipelines and equipment during the commissioning phase of the plant； Damage to machines, valves, and instruments ； Contaminating catalysts and chemical solutions, affecting product quality, and preventing combustion and **accidents ； It is an important commissioning procedure to ensure the smooth commissioning of the device and long-term safe operation. In chemical plants, there are a wide variety of pipes and equipment, each with different operating conditions as well as variations in materials and structure; consequently, the cleaning methods suitable for them also differ. But it usually includes the following methods: water flushing, air blowing, acid cleaning and passivation, oil cleaning, and degreasing, etc. Their main features and areas of application are outlined below. I. Water flushing: Water flushing is a method that uses water as a medium, pressurized by a pump, to clean pipes and equipment. It is widely used to remove debris remaining inside pipelines for transporting liquid media, as well as equipment such as towers and tanks. Water flushing of pipes should be carried out at the maximum flow rate that can be achieved within the pipes, or at a flow velocity of not less than 1.5 m/s (this does not include high-pressure or ultra-high-pressure water jet cleaning equipment, nor methods for removing scale from the inner and outer surfaces of pipe bundles). For general chemical processing equipment and pipelines, clear water with a turbidity of less than 10*106 and a chloride ion content of less than 100*106 is commonly used for flushing. However, for equipment and pipelines made of austenitic stainless steel, such as those in urea production facilities, deionized water must be used for flushing in order to prevent stress corrosion cracking (SCC) of the equipment and pipelines due to the accumulation of chloride ions (Cl-). Water flushing features ease of operation and no noise. II. Air purging: Air purging is a method that uses air as a medium; after being pressurized by a compressor (usually to 0.6–0.8 MPa), this air is used to remove any residual debris from the pipes through which the gas is transported. When using air for purging, there must be an adequate volume of air such that the flow velocity of the purging gas is greater than that of the gas used in normal operation; generally, this velocity should be at least 20 m/s. This ensures that the gas has sufficient energy (or momentum) to remove any residual deposits in the pipes and equipment, thereby facilitating smooth commissioning of the installation and ensuring safe operation. During air purging, the amount of air consumed is generally high, and a certain purging time is required. Therefore, air purging is usually provided by using the largest air compressor in the facility, or a large compressor capable of handling compressed air in the facility (such as the pyrolysis gas compressor in ethylene plants). For small and medium-sized chemical plants that lack the capability to supply large amounts of continuous purge air, a segmented purging method can also be used. This involves dividing the system’s pipelines into multiple sections, each of which is further divided into several sub-sections; purging is then carried out section by section, with each section being isolated from the system after it has been purged. In this way, the purging quality can be ensured even when the gas supply volume is low. For large-diameter pipes or those in which dirt is difficult to remove, the blasting purge method can also be used. Oil-free air should be used for purging the oil-free pipeline and instrument air pipeline. Due to reasons such as source and cost, nitrogen is generally not used as a purging gas for ordinary pipelines and equipment; instead, it is commonly used for air purging, and for the protective displacement of pipelines and equipment after the system’s air has been dried to the required standard. III. Steam purging: Steam purging is a type of purging that uses steam with varying parameters as the medium, with the steam source provided by a steam generator. Steam purging has a very high purging speed, and therefore possesses a great deal of energy (or momentum). The intermittent steam purging method causes the pipeline to contract and expand due to temperature changes, which facilitates the removal of deposits adhering to the inner wall of the pipeline; thus, the best purging effect can be achieved. Steam pipelines should be purged with steam, and this is especially necessary for power steam pipelines. When purging power steam pipes, it is necessary not only to thoroughly remove any dirt and debris attached to the inside of the pipes, but also to eliminate the rust on the metal surface; otherwise, if this rust is carried along by the high-speed steam flow, it can cause severe damage to the rapidly rotating turbine blades, nozzles, and other components. Steam purging involves high temperatures, high pressures, and fast flow rates; as a result, the pipes expand when heated and contract when cooled. Therefore, steam pipes are equipped with compensators, steam traps, pipe supports, hangers, slides, etc., all of which are designed to account for such expansion and contraction. When air purging is not sufficient to clean non-steam pipelines, steam purging can also be used; however, it is necessary to consider whether the pipeline’s structure can withstand high temperatures and thermal expansion and contraction, and take appropriate measures to ensure the safety of personnel and equipment during the purging process. Section 2: Purging and Cleaning Operations I. Water Flushing (1) Principles and requirements for flushing a. Water flushing should be carried out at the maximum flow rate possible within the pipe, or at a flow velocity of not less than 1.5 m/s; the direction of flushing should preferably be from higher to lower levels. b. The quality of the water used for flushing must meet the requirements specified for the materials of the pipes and equipment to be flushed. c. Flushing should be carried out in a sequential, segmented manner; the cross-sectional area of the discharge outlet should be no less than 60% of that of the pipe being flushed. It is also necessary to ensure that the discharge pipes remain unobstructed and safe. Only after the upstream flushing outlets have been successfully flushed can the process proceed to flush the subsequent sections of the system. d. The pump’s outlet pipeline can be flushed in accordance with the procedures only after the pump’s inlet pipeline has been successfully flushed. e. When the pipeline is connected to the tower, a blind flange must be installed on the inlet side of the tower during flushing; the connection can be made only after the pipeline has been flushed successfully. f. When flushing gas pipelines with water, ensure that pipe racks, hangers, etc., can safely withstand the load when filled with water. g. Where there are devices such as orifice plates, flow meters, valves, traps, filters, etc., on the pipeline, they must be removed or temporary short-circuiting facilities must be installed; only after the previous section of the pipeline has been cleaned can these devices be reinstalled, after which the cleaning process can proceed for the next section of the pipeline. h. Large-diameter pipes with a diameter of 600 mm or more, as well as containers with inlets, etc., must first be manually cleaned thoroughly. j. After flushing the process pipelines, water should be removed from the system as completely as possible. A large vent at the top should be provided during drainage, so that when the liquid level in the container drops, a vacuum does not form inside the equipment and cause damage to it. k. During winter flushing, care must be taken to prevent freezing; after flushing, the water should be drained completely, and compressed air can be used to dry it out if necessary. Water must not be introduced into equipment and pipeline containers lined with fire-resistant or other water-repellent materials. (2) Conditions required for water flushing: a. Before flushing the system’s pipelines and equipment, it is necessary to prepare a flushing plan. This plan usually includes seven sections: the basis for preparation, the scope of flushing, the required conditions, the preparatory work before flushing, the flushing methods and requirements, the flushing procedure, and inspection and acceptance. b. The equipment and pipelines have been installed, pressure testing has been successful, and verification according to the PID diagram shows no errors. c. Installation of the temporary flushing piping required by the flushing procedure is complete. d. All instruments in this system have been properly calibrated, and the electrical equipment is operating normally. e. Each pump motor passes individual testing and is connected. f. The flushing water has been delivered to the plant area. g. Arrangements have been made for the personnel responsible for flushing and for installation and maintenance workers; the flushing personnel must be familiar with the flushing procedure. (3) Methods and requirements for water flushing: a. Water flushing is carried out in stages according to the flushing procedure specified in the plan; that is, once each flushing port has passed the inspection, the process is resumed to flush the subsequent systems. b. After the inlet pipelines of each pump have been flushed successfully, start the pumps to flush the outlet pipelines in accordance with the procedures; once those are also successful, proceed to flush the towers and other equipment. c. During flushing, blind plates must be installed on the inlet side of the heat exchangers and towers; access to the equipment is permitted only after the upstream sections have been flushed successfully. d. After flushing the equipment in each tower, it is necessary to enter the tower to inspect and remove mechanical impurities. e. During the flushing process, various pipelines, valves, and other equipment generally need to be flushed intermittently 3 times to ensure an effective flushing result. f. During the water flushing process, all standby pumps must be switched on and off once each. g. After successful water flushing, the flushing records for the pipe sections and equipment should be filled out. (4) Inspection and acceptance criteria for water flushing. According to the national standard “GBJ235-82”, it is considered qualified if the color and transparency of the water at the outlet are consistent with those observed at the inlet, unless otherwise specified in the design. II. Air Purging (1) Principles and Requirements for Purging a. When using air as the process gas medium for purging pipelines, it is necessary to ensure an adequate flow rate of air, so that the flow velocity of the purging gas is greater than that during normal operation, or at least 20 m/s. b. The pressure of the air used for purging process pipelines is generally required to be 0.6–0.8 MPa; for applications where high purity in purging is required, the pressure can be increased slightly, but it should not exceed the operating pressure of the pipelines. Low-pressure and vacuum pipelines can be purged using a gas pressure of 0.15–0.20 MPa, as appropriate. c. For the purging of pipelines and systems, a purging plan should be prepared first. It usually includes: the basis for preparation, the scope of purging, the source of purge gas, the conditions required for purging, temporary piping, the methods and requirements for purging, operating procedures, inspection and acceptance criteria for purging, safety precautions during purging, as well as the preparation of equipment such as purging tools and test plates. d. All instrument measuring components installed on the purging pipeline (such as flow meters, orifice plates, etc.) should be removed to prevent dirty debris flowing during purging from damaging these instrument components. At the same time, appropriate protective measures should also be taken for the control valve (in principle, it should only be used after purging in front of the valve is completed; if necessary, it should be removed and connected with a temporary short pipe). e. Before purging, blind flanges must be installed on the inlet side of equipment such as heat exchangers and towers; access to the equipment is permitted only after upstream purging is completed successfully. Under normal circumstances, the heat exchanger itself is not involved in the air purging process. f. During purging, in principle, the control valves in the system shall not be used as purge control valves. If it is necessary to control the purge air volume of the system, a temporary purge valve should be used. g. During purging, the connection between the safety valve and the pipeline should be disconnected, and a blind plate or baffle should be installed to prevent dirt and debris from reaching the bottom of the valve and causing wear to its sealing surface. h. During system purging, all instrument pressure lead lines should be opened for purging, and they should be purged again during the system’s comprehensive airtightness test. i. All vent flare lines and drain lines should be purged after the main pipe to which they are connected; the drains on the equipment housings, as well as the pipes leading from level gauges and flow meters and the valves associated with them, must all be purged as well. j. During the purging process, the purge air can enter the downstream system through the normal flow path only after the upstream system is qualified. k. For pipelines with a diameter greater than 500 mm and equipment having inlets, manual cleaning must be performed prior to purging, and any internal components that may impede the purging process must be removed. l. All containers such as tanks, towers, reactors, etc., should be manually cleaned again after the system has been purged successfully, and the corresponding internal components should be reinstalled. When sealing them, the procedures specified for sealing work must be followed. (2) System purge gas source. The air used for purging pipelines and systems in chemical plants requires a large volume of gas and a high flow rate through these pipelines and systems; therefore, an adequate pressure head along with sufficient air volume is necessary to meet the requirements for effective purging. When purging a process pipeline with a diameter of 12#, an air compressor with a discharge pressure of 0.6–0.8 MPa and an air flow rate of around 7000 m3/h (standard) is required to continuously supply the purge air. Therefore, obtaining a purge gas source by utilizing an existing one in the device or a large compressor that provides compressed air within the device is the best way to complete system purging quickly. In large and medium-sized ammonia synthesis plants, air purging is carried out using the process air compressors or the raw material air compressors of air separation units (referring to ammonia plants that use residue oil or coal as raw materials). In large and medium-sized ethylene plants, air purging is generally provided by using the air from the plant’s pyrolysis gas compressor. In such devices, these compressors have a high air volume, and multiple purge ports can be used simultaneously during purging, which helps to reduce the system purging time. Large domestic fertilizer plants and ethylene plants all possess experience in engineering organization in this regard: that is, in the schedule for the equipment installation work of the plant, priority is given to completing the construction of the compressors and their prime movers (usually steam turbines), carrying out individual tests and air-load operations on them, so as to provide a gas source for system purging; this helps to accelerate the overall timeline for bringing the plant into operation. For medium and small chemical plants that do not have large-capacity air compressors, methods such as section-by-section purging and blast purging are generally used for purging operations. (3) Conditions required for air purging: a. The piping and equipment of the process system have been installed, and the strength pressure tests have passed. b. The orifice plates, rotameters, etc. in the pipeline shall be reinstalled after the internal components have been removed; the root valves for differential pressure gauges, level gauges, pressure gauges, etc. shall be in a closed state. c. Blind plates have been installed on the equipment, pipelines, pumps, valves, etc. that are not to be operated. d. The construction and installation of temporary piping, valves, etc., for purging have been completed. e. Process pipes that need to be purged are generally not insulated temporarily (a wooden hammer is used to strike the outer wall of the pipe during purging). f. The compressor that supplies the purge air source is running, and the utility services ensure a continuous air supply for the compressor. g. The purging operators and installation/maintenance personnel have been arranged, and are familiar with the purging plan. h. Draw a schematic flow diagram of the purging process, indicating the purging procedure, flow direction, exhaust outlets, temporary pipelines, temporary valves, etc., as well as the matters that need to be addressed in advance. j. Prepare the purging record form to be signed by representatives from the user, the construction unit (Phase 32), and the commissioning department, as shown in Table 1-1-4-5, for filling out during the purging process. Table 1-1-4-5 Pipeline Purging Record: Project Name ____________ Management Number ___________ Number of Purges, Start and End Dates, Purging Medium, Condition of Target Plate, Remarks, Pressure, Flow Rate, Space Velocity. Conclusion: Condition of target plate, reset status, component protection status. User’s project (construction) _______ Signature of the implementing unit representative ____________ (4) Purging methods and key points: a. Purge each system one by one in accordance with the sequence shown in the purging flowchart. During purging, purge the main pipes first; once the main pipes are satisfactory, then purge the individual branch pipes. During purging, it is also necessary to thoroughly clean the drain pipes, instrument pressure lead pipes, analysis sampling pipes, etc., to prevent the formation of dead zones. b. Purging is carried out by continuously discharging gas from each outlet, while the pipes are continuously struck with a wooden hammer; special attention should be paid to areas such as welds and dead corners, but the pipes must not be damaged, until purging is successful. c. At the start of purging, air should be introduced into the pipeline slowly; only once it is confirmed that air is being discharged from the outlet can the flow rate be increased gradually to the required level for purging. This is to prevent overpressure in the system due to issues such as faulty valves or blind flanges, as well as to avoid malfunctions in the air compressor system. d. When using high-flow compressors for purging, multiple systems should be purged simultaneously to shorten the purging cycle. However, when switching between systems, this process must be carried out slowly, in close coordination with the compressor operators, and under unified command. Special care must be taken to prevent surging caused by a decrease in the flow rate at the compressor outlet. e. To ensure that the purging process is carried out in an orderly manner without any omissions, another flowchart of the purging process needs to be drawn. Colored markers should be used to indicate whether the preparations before purging have been completed, what stage of the purging process has been reached, and the date on which it was carried out. This allows all personnel involved in the purging process to clearly understand the progress and helps to prevent any areas from being overlooked during the system purification. This diagram should be archived for reference. f. During the system purging process, temporary reset should be carried out as required by the flowchart. After confirming that the purging is complete and satisfactory, the entire system should be reset in order to prepare it for the subsequent comprehensive airtightness test. (5) Inspection methods and purge qualification criteria. Whether the purging of each pipeline or system is successful should be checked jointly by production and installation personnel. When the exhaust gas appears clean to the eye and free of any colored particles or impurities, a white cloth or a target plate coated with white lead oil is used at the exhaust outlet for inspection; if no rust, dust, moisture, or other contaminants or blemishes are detected on it within five minutes, then the purging is considered successful. Image 3: Steam Purging. The steam systems in chemical plants typically have multiple pressure level parameters to meet the requirements of different equipment and process conditions. For example, there is low-pressure steam used for mixing and heating, medium-pressure steam used for heating, in direct process applications, and in industrial steam turbines, as well as high-pressure steam used to drive large industrial steam turbines. Especially with regard to the steam pipes used to drive industrial steam turbines, during steam purging it is necessary not only to remove any dirt and debris from the pipes but also to eliminate the rust on the metal surfaces, as such substances, if carried along by the high-speed steam flow, can cause serious damage to the rapidly rotating turbine blades. Therefore, it is very important to properly master the steam purging method and adhere to strict quality requirements. Steam purging is usually carried out in two categories – high-pressure, medium-pressure, and low-pressure – based on the operating parameter range of the pipelines (there are also three categories: high, medium, and low pressure), and the requirements for purging vary among these categories. (1) Source and parameter selection of purge steam. To improve purging efficiency and reduce purification costs, steam system purging is usually carried out using reduced-pressure purging, but the steam consumption remains high, typically requiring 50-70% of the steam flow rate within the pipes at their rated load. High parameters (medium pressure, high temperature), long duration. Therefore, the purging of steam networks is mostly carried out simultaneously with the startup of their feed steam boilers. In the steam generators (waste heat boilers) of chemical plants, such as those for ethylene and synthetic ammonia production, where high-temperature process gas is used, external steam is generally employed or steam is supplied by the plant’s own start-up boiler in order to shorten the commissioning time, before any chemical materials are introduced into the plant. The requirements for the steam source in steam purging are that its steam parameters (pressure, temperature) and steam volume must be sufficient to meet the purging needs of the steam pipelines at various pressure levels. Steam parameters are the main factors that directly affect the purging effect. Steam purging utilizes the energy (also known as momentum) generated by the flow of steam within the pipe to wash away rust and debris inside it; the greater the energy, the better the effect. The factors that affect the energy of the steam medium during purging include: the steam parameters during purging (pressure, temperature) ； Hydraulic characteristics of steam pipelines ； The degree of opening of the main valve during purging, etc. In fact, these three factors are interrelated, and it is necessary to select appropriate purging parameters based on the specific circumstances. The principle for selecting steam purging parameters is to ensure that the steam momentum inside the pipe during purging (also known as blowing) is greater than that under rated load; in other words, the purging coefficient at any point in the system to be cleaned must be greater than 1. That is, the purging coefficient k = (flow rate of purging steam)² * specific heat capacity of steam at the area to be purged / [（flow rate of steam at rated load）² * specific heat capacity of steam under rated conditions]. Typically, when using steam for purging, the steam flow rate inside the pipes is set at 50–70% of the rated value; therefore, the pressure of the steam source and the superheating temperature of the steam during purging can be calculated by using the reference calculation methods based on the parameters of the purging steam, in combination with the hydraulic characteristics of the pipe section being purged. Generally, the flow velocity of the purge steam in pipes at different pressure levels is set as follows: High-pressure steam pipes (4–12 MPa) ≥ 60 m/s; Medium-pressure steam pipes (1–4 MPa) ≥ 40 m/s; Low-pressure steam pipes (