Blasting purge

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 is often not achieved because the flow rate of the purging medium does not meet the specified requirements. As pipeline fittings 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 explosive cleaning method is replacing other methods that were used in the past. Blowout purging involves sealing one end of the pipeline to be purged with a rupture 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; once it exceeds the maximum pressure that the rupture disc can withstand, the disc ruptures suddenly, 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 that are not allowed to be involved in the purging process. Flowmeters, valves, etc. have been isolated, and this purging pipeline has also been separated from other adjacent pipelines and equipment ； (4) The selection of the blasting system is very important, as it directly affects the cleanliness of the system cleaning process as well as the efficiency of that process. Generally, the division should be based on the pipeline construction drawings and in conjunction 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 gas 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 advisable to choose the lowest point of the pipeline, preferably at one of its ends in a location that is easy to protect. The blasting direction is usually horizontal or downward, and a sturdy support should be in place at the blasting site ； (6) Ordinary asbestos sheets can be used for blast discs, with a thickness of generally 1–3 mm. A “cross” or “plus sign” pattern can be marked on the asbestos sheet. A blast pressure of around 0.3–0.4 MPa is sufficient; too low a pressure results in more blasting attempts and makes it difficult for the blast disc to break, while too high a pressure is unsafe and can lead to injuries. Process characteristics and procedures for purging: The blasting purging technique is used for purging large-diameter steel pipes, and it has the following features: (1) The flow rate of the purging medium is high (not less than 15 m/s), giving it strong carrying capacity; this allows it to remove soil, welding slag, residual water in low-lying areas of the pipes, as well as other debris from within them ； (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 located in hard-to-reach areas of the pipelines is also carried away by the rapidly expanding airflow due to the vibrations caused by detonation, thereby significantly improving the cleaning efficiency ； (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 cleaning is particularly suitable for large-diameter pipes ； (5) Compared with other flushing and purging methods, blasting purging requires less equipment and materials (the main equipment is an air compressor, with atmospheric air as the gas source), saving labor and time while reducing costs; it also has low technical requirements and is easy to implement on a wider scale ； (6) Medium and large-diameter steel industrial pipes with a diameter ranging from DN100 to 600 mm can all 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 testing; if results are unsatisfactory, repeat the process starting from the step of installing rupture discs → Acceptance