Preparation and operation process of blind flange pumping and blocking operation for large gas pipeline

Large blind flange pumping and blocking operations above DN1400 occur less frequently in the daily maintenance and project expansion of the gas system of iron and steel enterprises. Such operations are relatively complex, requiring high preparation and cooperation of personnel and machines. The operation with gas takes a relatively long time and is dangerous. By sorting out the preparation process and operation process, the whole process of pumping and blocking operation will be standardized and institutionalized to improve the operation efficiency and ensure the smooth and safe process of blind flange pumping and blocking operation.

As the main carrier of gas transmission, the gas pipeline will reserve branch line interface in the pipeline design stage to facilitate the user contact in the later stage. Effective gas isolation (GB6222-2005 Industrial Enterprise Gas Safety Code Sealed butterfly valve cannot be used as a reliable isolation device, but only when it is used together with water seal, plug plate, spectacle valve, etc.). At present, the more common combination is butterfly valve + blind flange. Later, after the user contacts are completed, the blind flange can be extracted and the valve can be opened for air supply after replacement; At the same time, in order to cooperate with large-scale maintenance and expansion projects, blind flanges need to be installed behind the butterfly valve to meet the requirements of safe operation when gas is stopped at positions without effective partition combination.

Gas leakage is inevitable during blind flange extraction and blocking operation, which belongs to operation with gas. Considering the toxic, flammable and explosive characteristics of gas, how to ensure the safety of operation, take emergency measures, and complete the pumping and blocking operation efficiently and qualified is a problem that the maintenance department should focus on. In recent years, an iron and steel company has successfully completed more than 10 times of large gas blind flange pumping and blocking operations with DN1400-DN3600 caliber, relying on comprehensive preliminary preparation and scientific and reasonable construction organization, and has accumulated a lot of experience.

1. Preparation before operation

“Three points for preparation and seven points for maintenance”. Safety, construction and personnel preparation are the key points for smooth operation.

1.1 Safety and emergency preparedness

  • 1) The maintenance unit shall prepare the operation plan, safety plan and emergency plan one week in advance according to the requirements of the Owner, and the Owner shall approve them; Personnel safety disclosure shall be made in advance to ensure that the gas work permit, work permit at heights and other special work certificates of construction personnel are complete; Apply for gas protection work permit, high-altitude work permit, blind flange drawing and blocking safety work permit and other work permits to the gas protection department in advance.
  • 2) The gas pipeline is generally erected on the ground. In order to meet the requirements of operation and emergency evacuation, the location of the blind flange needs to be set up according to the actual situation of the site (for pipelines larger than DN1600, it is recommended to set up another layer of operating platform in the middle of the pipeline to facilitate the disassembly and assembly of bolts) and two-way runway inclined ladder (the erection standard refers to: GB 51210-2016 Unified Standard for Safety Technology of Construction Scaffolds).
  • 3) The blind flange side of gas pipeline shall be provided with nitrogen replacement conditions. If not, it is necessary to add purging pipe, blow off pipe and other devices in advance. At the same time, the wind direction sign shall be set at the high point of the operation position.
  • 4) With gas extraction and blocking blind flanges, operators shall wear respirators or ventilated gas masks (GB 6222-2005 Gas Safety Regulations for Industrial Enterprises 10.2.6). As the operation time is long and the working surface is limited, it is recommended to use a long tube respirator, and the air source is from the clean compressed air pipe network in the plant area. If an air compressor is used, the air compressor shall be placed 10m away from the air outlet at the operating position. One set of respirator for each worker. Another 2 sets shall be prepared, connected with air source and placed on the operation platform for standby.
  • 5) In case of insufficient illumination at the operation location, explosion-proof lighting fixtures shall be installed in advance. The projector can only be installed 10 m away from the operation point (GB6222-2005 Gas Safety Regulations for Industrial Enterprises 10.2.6).
  • 6) The blind flange pumping and blocking operation belongs to the operation with gas hazard, which should not be carried out in thunderstorm days, and should not be carried out at night (GB6222-2005 Gas Safety Regulations for Industrial Enterprises 10.2.6); Operators shall wear anti-static work clothes and shoes; It is not allowed to pull and block 2 or more blind flanges on the same pipe at the same time.

1.2 Preparation for on-site confirmation

  • 1) To ensure the smooth removal of bolts during operation, all bolts shall be derusted and oiled one week in advance. If the bolts are seriously rusted, it is recommended to replace them one by one on the premise of taking gas protection measures.
  • 2) During the operation, it is necessary to use a screw jack to jack up the two flanges to a certain distance, and it is necessary to weld the jack support iron on both sides and directly below the pipeline in advance. The support iron spacing matches the length of the existing equipment. Support the iron belt bracket to prevent the jack from falling when loosening.
  • 3) If the pipe brackets on both sides of the valve are fixed brackets, cut the brackets and the fixing points of the brackets in advance to ensure that the pipes have enough displacement when the flanges are stretched; If there are elbows near the pipes on both sides of the valve or the distance between the brackets is far, the pipes shall be fixed in advance to prevent the flange plate from being staggered or opened after the bolts are removed.

1.3 Preparation of blind flange and gasket

The blind flange shall be made of steel plate, without sand hole, smooth on both sides and burr on the edge. The size of blind flange shall be correctly matched with the flange, and the thickness of blind flange shall be determined after calculation according to the purpose of use (GB6222-2005 Gas Safety Regulations for Industrial Enterprises, The blind flange should be a whole steel plate structure. If assembly is required, 100% NDT should be carried out on the butt weld (Article 6 of 8.4.2 of GB 51128-2015 Code for Design of Gas Storage, Transmission and Distribution System in Iron and Steel Enterprises).
It is required that the thickness of blind flange shall meet the requirements of flange spacing, and at the same time, it shall be ensured that no deformation will prevent the blind flange from being drawn out during use. Refer to Table 1.

Table.1 Diameter and Thickness of Blind Flange

Diameter of blind flange/mm ≤500 600~1000 1100~1500 1600~1800 1900~2400 ≥2500
Thickness of blind flange/mm 6-8 8-10 10-12 12-14 16-18 ≥20

If the dead weight of the blind flange is larger than DN1600, double lifting lugs can be installed (note that the location of the lifting lugs should not coincide with the location of the bolts). In order to fine tune accurately when hoisting and positioning, 3 chain blocks can be used for combination. See Figure 1.

20230108093532 15942 - Preparation and operation process of blind flange pumping and blocking operation for large gas pipeline

Fig.1 Installation of Double Lifting Lugs
It is recommended to select finished silicone gasket for better sealing effect. The size is correctly matched with the flange, and the thickness is 4-6 mm. The gasket and blind flange shall be bonded with Loctite 407 glue (or the same type), and the bonding position of blind flange shall be smooth and oil-free before bonding. Considering that the butterfly valve is not sealed tightly, the air flow washes the gasket bonding surface, and the blind flange installation process may cause gasket bonding failure, a circle of silicone rod can be stuck inside the gasket for protection. The diameter of the silicone rod is equal to the thickness of the gasket. Apply oil proof sealant on both sides of the rubber rod. See Figure 2.
Considering the curing time of glue, the fabrication of blind flange and gasket shall be completed 2 days in advance. It is recommended that the finished product be hoisted vertically to avoid blind flange deformation and gasket damage. If there is a blind flange ring, the gasket shall be installed and placed in the same way.

1.4 Preparation of operating machines and tools

  • 1) The crane shall enter the site in advance to determine the station position to ensure that the hoisting radius is effective, and the hoisting range is not affected by other buildings and structures.
  • 2) Tools that do not produce sparks, such as copper tools or iron tools coated with a thick layer of lubricating grease (GB 6222-2005 Gas Safety Regulations for Industrial Enterprises 10.2.6), shall be used. Grease the non copper wrenches, crowbars, scrapers and blind flanges in advance and place them on the working surface for easy access and location.
  • 3) The on-site commander, crane operator and crane driver are equipped with explosion-proof walkie talkies with sufficient power.

2. Blind flange pulling and blocking operation

2.1 Confirmation of on-site personnel

  • 1) The gas protection station personnel, construction workers and other personnel shall evacuate to 10m away from the windward side and 40m away from the downwind side of the operation site according to the wind direction sign (GB 6222-2005 Industrial Enterprise Gas Safety Regulations 10.2.6). If there is no fire source within the determined range, the warning line shall be set up and the safety manager shall be responsible for monitoring.
  • 2) Operators shall wear air respirators to ensure that the masks are free of air leakage and air pressure. Note that the operators need to assign special personnel to take care of the compressed air bag and air compressor.

2.2 Confirmation of pipeline operation mode

If there is gas in the pipeline behind the valve, confirm in advance with the production discipline and gas protection station personnel that the pipeline end is effectively isolated, and the nitrogen replacement in the pipeline is qualified.

2.3 Operation process

  • 1) The operation commander orders the operation to start.
  • 2) The operator shall remove the bolts according to the division of labor, and put the removed bolts into the hopper prepared in advance to prevent them from falling and missing. Keep 5-8 bolts at the bottom of the flange and do not remove them after loosening.
  • 3) Three jacks shall be operated at the same time, the flange opening shall be extended 4-5cm, the blind flange or blind flange ring shall be shaken left and right to ensure the effective spacing, and the crane shall be guided to lift. After the spreading operation, the operator shall try to avoid standing directly against the flange opening, and try to keep the side or back facing. If the respirator is abnormal or uncomfortable, evacuate to a safe place as soon as possible.
  • 4) After lifting out, clean the flange port quickly with a scraper.
  • 5) The fabricated blind flange or blind flange ring shall be hoisted in, and the commander shall command to fine tune it with a chain block to ensure that the gasket position is appropriate and effective.
  • 6) Loosen the jack, insert the bolt and tighten it diagonally. The commander shall observe the compression of the gasket at any time to avoid damaging the silicone gasket due to over tightening on one side.
  • 7) After fastening, the gas protection station personnel conduct on-site inspection to confirm that there is no leakage point; The equipment discipline and spot inspection personnel confirm that the bolts are tightened effectively.
  • 8) Operators and tools shall be removed from the site and handed over to the production and operation personnel for operation.

3. Conclusion

Large blind flange pulling and blocking operation is a low probability and high-risk operation in general iron and steel enterprises. Before operation, the maintenance and safety disciplines must pay high attention to it. Supervise the construction unit to conduct personnel education and training in advance, organize operation drills and emergency drills in advance, and ensure that all operators cooperate with each other tactfully, and are skilled in professional skills and emergency escape skills, so as to successfully and efficiently complete blind flange pulling and blocking operation.

Author: Wang Wei

Source: China Blind Flanges Manufacturer – Yaang Pipe Industry (

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