Explosive purging for pipeline flushing and cleaning
Thread Content
After passing the strength test and before the pneumatic tightness test, steel pipes shall be flushed and purged in accordance with the requirements of GB50235-2010 \"Code for Construction and Acceptance of Industrial Metal Piping Projects\". For many years, techniques such as water flushing, air purging, and steam purging have been primarily used for flushing and purging. This is quite effective for small-diameter pipes; however, for pipes with larger diameters, the ideal purging effect cannot be achieved because the flow rate of the purging medium does not meet the specified requirements. As pipeline accessories and instruments become more advanced, the technical requirements for production processes keep rising, and some construction techniques can no longer meet these requirements. In the cleaning and flushing of large-diameter steel pipes, the blasting cleaning method is replacing other methods that were used in the past. Blasting purging involves sealing one end of the pipeline to be purged with a blast disc whose strength is much lower than that of the pipeline, while compressed air is introduced at the other end. The air pressure within the system gradually increases; when it exceeds the maximum pressure that the rupture disc can withstand, the disc suddenly breaks, and the compressed air in the system expands rapidly and is ejected at extremely high speeds through the rupture opening. At the same time, the slag and other debris inside the pipeline are also expelled as a result of the vibrations caused by the blasting and the impact of high-speed air currents. Repeating this process several times will meet the purging requirements. Preparatory work and construction requirements for blasting purging: (1) The pressure test of the pipeline has been successful, and the quality of the work meets the requirements specified in the construction standards and design documents ； (2) The purging plan has been approved by the project owner and the supervision unit, and is available for those involved in the purging process to refer to ； (3) Orifice plates, flow meters, valves, etc. that must not be subjected to purging have been isolated; this purge pipeline has also been isolated from other adjacent pipelines and equipment ； (4) The selection of the blasting system is very important; it directly affects the degree of cleanliness achieved during system purging and the efficiency of the purging process. Generally, the division should be based on the pipeline construction drawings along with the actual process flow. Full consideration should be given to the effects of forces caused by blasting. Temporary reinforcement points should be minimized as much as possible to facilitate the connection of intake pipes within the system, the installation of pressure gauges, and the replacement of blast valves. Moreover, there should be sufficient air capacity upstream of the system to ensure the quality of purging in that area ； (5) Once the system is determined, the blasting site should be selected based on the actual installation location of the pipeline. The principle is that there must be sufficient space for blasting. There should generally be at least 10 meters of open area at the blasting site; it is preferable to choose the lowest point of the pipeline, as well as a location at the end of the pipeline that is easy to protect. The blasting direction is usually horizontal or downward, and there must be sturdy supports in place at the blasting site ； (6) Ordinary asbestos sheets can be used for burst discs, with a thickness of generally 1–3 mm. A “+” or “X” pattern can be marked on the asbestos sheet. A burst pressure of around 0.3–0.4 MPa is sufficient; too low a pressure leads to an increased number of explosions and makes it difficult for the burst disc to break, while too high a pressure is unsafe and can result in injuries. Process characteristics and procedure for purging: The blasting purging technique is used for purging large-diameter steel pipelines, and it has the following characteristics: (1) The flow rate of the purging medium is high (not less than 15 m/s), giving it strong carrying capacity; this enables it to remove soil, slag from within the pipes, residual water in low-lying areas of the pipes, as well as other debris ； (2) High purging efficiency. For pipelines of the same specification and length, blasting and purging 4 to 6 times is generally sufficient to meet the cleanliness requirements ； (3) Due to the high speed of the blasting purge airflow, not only rust and debris are blown out. Debris in the dead corners of the pipelines is also carried away by the rapidly expanding gas flow due to the vibrations during blasting, resulting in a significant improvement in the cleaning quality ； (4) For large-diameter pipes, it is difficult to find a suitable air source of sufficient capacity, and the quality of such air sources cannot be guaranteed; blasting for cleaning is particularly suitable for large-diameter pipes ； (5) Compared to other flushing and purging methods, blast purging requires less equipment and materials (the main equipment is an air compressor, with atmospheric air serving as the gas source). It saves labor and time, has low costs, and does not demand high technical expertise, making it easy to be widely adopted ； (6) Medium and large-diameter steel industrial pipes with a diameter in the range of DN100~600 mm can be used. When this purging technique is used for steel pipelines with a diameter of less than DN200 mm, an electric disc valve can also be used in place of a rupture disc; by quickly opening and closing the valve, the same effect as purge blasting can be achieved. Purging process flow: Determine the purging system → Install temporary blind flanges (to isolate components and systems that cannot be purged) → Install pressure gauges (for temporary reinforcement of local pipelines and pipe racks) → Open all valves in the system → Install rupture discs → Inflate → Cause an explosion → Conduct quality inspection and target testing (if not satisfactory, repeat from the step of installing rupture discs) → Acceptance
### Detailed Explanation of Blasting Purging **Definition** Blasting purging is a process that uses rapidly released high-pressure gas (such as compressed air or nitrogen) to generate shock waves, thereby removing impurities such as weld slag and rust from within pipes. Compared to conventional purging, it is characterized by **high-pressure transient release**, making it suitable for handling complex piping systems. --- #### **Implementation steps ** 1. **Preliminary preparation** - **System isolation**: Disconnect from unrelated devices and install blind flanges to prevent backflow. - **Installing blasting devices**: Install a burst disc at the end of the pipeline (the rated bursting pressure must be calculated accurately, usually 1.1–1.3 times the design pressure) or a quick-opening valve. - **Safety measures**: Set up a safety zone, and personnel must wear PPE (goggles, noise-canceling earplugs). 2. **Operation procedure** - **Segmented processing**: Long pipelines need to be purged in segments, with each segment having a length of ≤300 meters (as recommended by ASME B31.3). - **Pressurization and release**: - Pressurize to a preset value (such as 0.6 MPa), and release pressure instantly by quickly opening a valve or causing a rupture of the burst disc. - Repeat 3-5 times until the target plate passes the inspection. 3. **Quality inspection** - **Target plate testing**: Aluminum target plates (thickness 0.5–1 mm) are used and placed at the emission outlet. The acceptance criterion is: – No visibly visible particles ； - The diameter of the impact crater ≤ 0.8 mm (refer to SH/T 3518 standard). - **Endoscopic assistance**: For areas such as bends and valves where access is difficult. --- #### **Applicable scenarios** - **Large-diameter pipes**: DN≥300mm, where conventional purging is inefficient. - **Complex structures**: multi-branch, long-distance (such as gas supply pipelines in chemical plant reactors). - **High cleanliness requirements**: Pharmaceutical sterile pipelines, food-grade nitrogen pipelines. - **Dry-sensitive systems**: Water-sensitive pipelines (such as liquid oxygen delivery pipes). --- #### **Safety key points** - **Pressure control**: Operating above the design pressure is strictly prohibited; safety valves must be installed (for redundant protection). - **Emission management**: The exhaust outlet should be directed 45° upward, away from personnel and equipment; silencers should be installed if necessary. - **Gas selection**: Nitrogen is used for flammable pipelines (for explosion prevention), while oxygen pipelines require degreasing. - **Environmental monitoring**: In confined spaces, the oxygen level must be checked (≥19.5%). --- #### **Advantages and Disadvantages Comparison** | **Advantages** | **Disadvantages** | |---------------------------|---------------------------| | High efficiency (processing time per cycle ≤5 minutes) | High equipment cost (requires air compressor unit) | | Thorough cleaning (reaches SA2.5 level) | High noise level (≥85dB; noise reduction required) | | Suitable for complex structures | Not suitable for pipes made of fragile materials | --- #### **Industry Application Examples** - **Petrochemical industry**: Reactor pipelines in catalytic cracking units, for removing catalyst powder. - **Electricity **: Remove scale from the boiler steam pipes before putting them into use. - **Pharmaceutical**: Piping for purified water systems, ensuring no microbial residues. - **LNG storage and transportation**: Purging of cryogenic pipelines to prevent moisture freezing. --- **Precautions ** - **Compliance**: Adhere to standards such as API 570 and GB 50235. - **Residue treatment**: Collect the blown-out impurities to prevent environmental pollution. - **Record keeping**: Save process data such as pressure curves and photos of the target plate. By properly designing pressure parameters and following strict operating procedures, blasting cleaning can significantly improve the efficiency of pipeline cleaning and ensure the safe operation of the system. -
After steel large-diameter pipes pass the strength tests, they need to be cleaned using blast cleaning technology before undergoing pressure integrity tests. This method involves sealing one end of the pipe and then injecting compressed air. When the pressure exceeds the maximum pressure tolerance of the rupture disc, the disc ruptures, allowing the compressed air to be discharged rapidly. This process expels any welding slag and debris from inside the pipe. Repeating this procedure multiple times ensures that the pipe is thoroughly cleaned. This method is suitable for pipes with larger diameters, as it can provide a purging medium with high flow rates and high carrying capacity; it is easy to operate, costs less, and can effectively improve work efficiency and the quality of purging. Before use, ensure the safety of pipeline isolation, select an appropriate blast opening, and install the burst disc correctly. After the blasting and purging are completed, quality inspection and acceptance are also required. .