Purging of chemical plants

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I. Water flushing 1) Principles and requirements for flushing a. Water flushing should be carried out at the maximum flow rate that can be achieved 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 material 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 supports, hangers, etc., can safely bear 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. j. After flushing the process pipelines, water should be drained from the system as completely as possible. During drainage, there should be a relatively large vent at the top; this prevents a vacuum from forming inside the equipment when the liquid level in the vessel drops, thereby avoiding any damage to the equipment. k. During winter flushing, care should 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 pipe containers lined with fire-resistant materials or other water-repelling substances. 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 conditions that must be met, 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 issues. 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 each tower equipment, it is necessary to enter the tower for inspection and remove any mechanical impurities. e. During the flushing process, equipment such as pipelines and valves generally needs to be flushed intermittently three times to ensure effective flushing. f. During the water flushing process, all standby pumps must be switched on and off once each. g. After the water flushing is successful, a flushing record 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 selecting air as the purging medium for gas pipelines, it is necessary to ensure an adequate volume of air 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. 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 purification 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 formulated 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 standards 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 plates 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 the control valves for purging. When it is necessary to control the purging air volume, a temporary purging valve should be selected. 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 only after the upstream system is qualified. k. For equipment with a pipe diameter greater than 500 mm and an inlet, manual cleaning should be carried out prior to purging, and any internal components that could interfere with the purging process must be removed. l. All containers such as tanks, towers, and reactors must be manually cleaned again after the system has been successfully purged. The respective internal components should be reinstalled, and sealing must be carried out in accordance with the procedures for concealed works sealing. 2) System purging gas source. The air used for purging pipes and systems in chemical plants requires a large volume of gas and a high flow rate through these pipes and systems; therefore, an adequate pressure drop along with sufficient air volume is necessary to meet the requirements for effective purging. To purge a 12# diameter process pipe, an air compressor capable of delivering purge air continuously is required; it must have a discharge pressure of 0.6–0.8 MPa and an air volume of approximately 7,000 m³/h (at standard conditions). 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. Domestic large-scale fertilizer and ethylene plants have provided valuable experience in this regard regarding project organization. Specifically, in the network schedule for equipment installation at these plants, priority is given to completing the installation of the compressors required for purging, along with their prime movers (usually industrial steam turbines), followed by individual commissioning tests and air-load operations. This ensures a reliable gas source for system purging, thereby accelerating the overall commissioning process of the entire project. For medium and small-scale 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 pipelines and equipment of the process system have been installed, and the strength pressure tests have passed. b. After the internal components such as orifice plates and rotameter in the purge pipes have been removed, they are reinstalled. The root valves of differential pressure gauges, level gauges, pressure gauges, etc. remain closed. 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 generally should not be insulated for the time being (when purging, a wooden hammer must be used to strike the outer wall of the pipes). f. The compressor that provides 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 assigned and are familiar with the purging plan. h. Draw a schematic 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 (Stage 32), and the commissioning department, as shown in Table 1-1-4-5, for filling out during the purging process. 4) Purging methods and key points a. Purge each system one by one in the order indicated in the purging flowchart. During purging, first purge the main trunk pipe; after it is qualified, then purge each branch pipe. During purging, it is also necessary to thoroughly clean the drain pipes, instrument pressure tapping pipes, analysis sampling pipes, etc., to prevent the formation of dead zones. b. Purging shall be carried out by continuously discharging at each vent point, while continuously tapping the pipes with a wooden hammer. Special attention shall be paid to tapping areas such as welds and dead corners; however, the pipes must not be damaged until the purging is deemed satisfactory. c. At the start of purging, air should be fed into the pipeline slowly. Only when air is observed escaping from the discharge outlet can the airflow rate be gradually increased to the required level for purging. This is done to prevent system overpressure or malfunctions in the air compressor system due to improper handling of valves, blind flanges, etc. 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 attention must be paid to preventing 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 activities must 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 took place. 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 the purge is completed and confirmed to be satisfactory, a full system reset should be performed to prepare the system for the subsequent comprehensive airtightness test. 5) Inspection methods and purge acceptance criteria. Whether the purging of each pipeline or system is satisfactory shall be jointly inspected by production and installation personnel. When the exhaust gas appears clean to the naked eye, with no colored impurities, a white cloth or a target plate coated with white lead paint should be placed at the exhaust outlet for inspection. If there is no rust, dust, moisture, other contaminants, or pitting on it within five minutes, then the purging is deemed successful. III. 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 tracing and heating; medium-pressure steam utilized for heating, direct process applications, and industrial steam turbines; and high-pressure steam used to drive large industrial steam turbines. In particular, when performing steam purging on the steam pipelines used to drive industrial steam turbines, it is necessary not only to remove any dirt and debris from the pipes, but also to eliminate any loose rust from the metal surfaces. This is because if such particles become entrained in the high-speed steam flow, they can cause significant damage to the rapidly rotating turbine blades. Therefore, it is very important to properly master the steam purging methods and strict quality requirements. Steam purging is generally carried out in two levels—high/medium pressure and low pressure—based on the operating parameter ranges of the pipelines (some methods also divide it into three levels: high, medium, and low pressure). The requirements for purging vary accordingly for each level. 1) Source and parameter selection of purge steam. To improve purging efficiency and reduce purging costs, pressure-reduction purging is commonly used for steam system purging. However, the steam consumption remains high; generally, it accounts for 50–70% of the steam flow rate inside the pipes at their rated load. High parameters (medium pressure, high temperature), long duration. Therefore, the purging of steam pipelines 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 gases are involved, external steam is generally used, or steam is supplied by a startup boiler built within the plant itself, in order to shorten the commissioning time before chemical feed is 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. 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 conjunction 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: for high-pressure steam pipes (4–12 MPa), ≥60 m/s; for medium-pressure steam pipes (1–4 MPa), ≥40 m/s; for low-pressure steam pipes (