A high-strength and high-transparency borehole supporting material for soft coal seam extraction, a preparation method thereof and a hole cleaning process

Abstract

The application discloses a high-strength and high-permeability borehole supporting material for soft coal seam extraction, a preparation method thereof and a hole cleaning process, and belongs to the technical field of soil layer or rock drilling. In order to consider the support and extraction efficiency of the gas extraction hole in the soft coal seam, the borehole supporting material is prepared by adding non-alkali accelerating material according to the slurry material mix proportion designed based on the close packing principle of fine particles and combined with single-grade aggregate design, and has the advantages of high strength, high gas permeability and short setting time. According to the hole cleaning process of the application, the extraction pipeline is pre-buried in the hole, and then the borehole supporting material is injected into the hole to support the hole. The process uses the pre-buried porous pipeline and the supporting material for gas extraction and regular cleaning, thereby realizing long-term high gas permeability of the extraction hole. The application reduces the risk of hole collapse, and after the hole collapse, there is still enough hole to communicate with the outside world, so that the extraction efficiency is maintained. In addition, the gas permeable hole can be regularly cleaned, and the extraction efficiency of the porous supporting material is maintained.

Description

Technical Field

[0001] This invention relates to the field of drilling technology for soil or rock, and more particularly to a high-strength, high-permeability borehole support material for extraction from soft coal seams, its preparation method, and borehole cleaning process. Background Technology

[0002] Gas drainage is the primary means of coalbed methane extraction and the most effective way to prevent gas disasters. Due to the low hardness of the coal seam, gas drainage boreholes in soft coal seams are prone to deformation and collapse, which greatly reduces the efficiency of gas drainage and seriously restricts the progress of mine gas drainage to meet standards.

[0003] Currently, the main technology for improving the stability of gas extraction boreholes is to insert gas extraction screen pipes (hole protectors) into the already drilled boreholes to support the borehole walls. Even if the borehole collapses, the strength of the screen pipes can provide a channel for gas flow. For example, Chinese invention patent CN104863625A discloses a method for extracting gas from coal seams using internal support pipes in underground coal seams. This method involves drilling gas extraction boreholes along the dip or strike of the coal seam in underground coal mines. After drilling, compressed air is used to blow away coal dust from the borehole, and an internally supported PVC pipe is lowered to the designed position and then sealed. The gas is then extracted through a network, with the internally supported PVC pipes providing a smooth channel for gas flow and extraction. However, engineering practice has revealed significant drawbacks to using screen pipes for borehole protection. On the one hand, the screen pipes, mostly made of plastic, have limited support capacity for the borehole walls. On the other hand, the screen holes on the surface of the screen pipes are easily clogged by broken coal dust, leading to a sharp drop in gas concentration in the later stages of extraction.

[0004] Chinese invention patent CN111287710A discloses a protective sphere for gas drainage boreholes in soft coal seams. This protective sphere comprises a sphere and a hollow spherical structure. The hollow spherical structure is located in the center of the sphere, and a tetrahedral support structure is located inside the hollow spherical structure. Multiple small holes are formed on the surface of the sphere and radiate evenly throughout the sphere, centered on the sphere's center. The hollow spherical structure communicates with the outside of the sphere through these holes. This invention provides support to the borehole after filling it, reducing the possibility of borehole collapse or deformation. Even after borehole collapse, it maintains the smooth flow of the formed gas drainage channel. The diameter of the sphere is 0.5 to 0.85 times the diameter of the required gas drainage borehole. In the event of borehole collapse, the collapsed coal fragments can only block some of the small holes, not all of them. This design only addresses the blockage caused by small coal chunks during collapse, but its small-aperture gaps cannot prevent low-particle-size coal dust from clogging the holes during extraction. When coal dust blockage occurs, the required pressure to flush the blockage with high-pressure water is difficult to achieve under actual engineering conditions, and the internal support structure cannot effectively clean the inside of the protective spheres, making its application unsustainable.

[0005] Based on the above problems, using large-pore concrete as the support material for gas drainage boreholes can avoid the low drainage efficiency caused by surface pore blockage. However, traditional large-pore concrete suffers from low strength, poor durability, and long setting time. Furthermore, there are currently no relevant borehole cleaning processes available for reference, making it difficult to directly apply to the support of gas drainage boreholes. If a high-strength, highly permeable, and short-setting-time large-pore concrete could be prepared as a borehole support material, and a corresponding borehole cleaning process for gas drainage borehole support could be developed, the technical problems in gas drainage would be effectively solved. Summary of the Invention

[0006] In view of the above-mentioned deficiencies of the prior art, in a first aspect of the present invention, a high-strength, highly permeable, and short-setting-time borehole support material is provided, the raw materials of which include the following components in parts by weight:

[0007] 3.8–4.5 parts cement, 0.45–0.65 parts limestone powder, 0.75–0.95 parts silica fume, 0.9–1.2 parts quartz powder, 16–19 parts quartz sand, 0.8–1.2 parts water, 0.05–0.065 parts water-reducing agent, and 0.3–0.45 parts quick-setting agent.

[0008] Preferably, the cement grade is 52.5R and the median particle size (D50) of the cement particles is 20 μm.

[0009] Preferably, the limestone powder has a particle size distribution range of 0.3 to 200 μm and a median particle size (D50) of 10 μm.

[0010] Preferably, the particle size distribution range of the silica fume is 0.02 to 100 μm, and the median particle size (D50) is 0.2 μm.

[0011] Preferably, the particle size distribution range of the quartz powder is 0.125 to 0.2 mm; and the particle size distribution range of the quartz sand is 10 to 20 mesh.

[0012] Preferably, the water-reducing agent is a polycarboxylate-type high-efficiency liquid water-reducing agent with a water reduction efficiency of ≥30%.

[0013] Preferably, the accelerator is an alkali-free liquid accelerator with a pH of 2.5 to 3.0, an initial setting time of <5 min, and a final setting time of <15 min.

[0014] The selection of components and optimal parameters for the above-mentioned borehole propping material are determined based on the engineering practice requirements for gas extraction from soft coal seams. This material, after setting, exhibits high strength, high permeability, and a short setting time. High strength enhances borehole stability and reduces the risk of borehole collapse. The borehole propping material has a large-pore and multi-pore structure, ensuring sufficient connections to the outside environment even after collapse, maintaining extraction efficiency. In particular, it enables the periodic flushing of coal dust blocking the borehole with high-pressure water to clear the extraction channel, improving the toughness and sustainability of the extraction borehole. The borehole propping material has excellent fluidity, allowing for continuous pumping when combined with downhole high-pressure grouting equipment, thus improving the construction efficiency of the borehole propping process.

[0015] In a second aspect of the present invention, a method for preparing a borehole support material is provided, comprising the following steps:

[0016] (1) Prepare the raw materials of the corresponding components according to the proportion, mix cement, limestone powder, silica fume, quartz powder and quartz sand evenly, and ensure that the powder material is fully wrapped around the quartz sand.

[0017] (2) Add 20-50% of the total water usage to the powder and quartz sand system obtained by mixing, and mix evenly;

[0018] (3) Continue to add water and water-reducing agent accounting for 30-40% of the total water usage, mix until the slurry is completely fluid, then add the remaining water and continue mixing until the components are uniform;

[0019] (4) Finally, add quick-setting agent to the obtained mortar, mix evenly, and obtain the borehole support material.

[0020] Based on the compositional properties of the borehole support material of this invention, the key to obtaining the target structure is to ensure that powder materials such as cement, limestone powder, silica fume, and quartz powder are fully coated around the quartz sand during preparation. This step fully utilizes the lubricating and binding effects of the powders to achieve high strength and high flowability while maintaining porosity.

[0021] In a third aspect of the invention, a method for cleaning boreholes in soft coal seam extraction is provided using a convenient and easy-to-maintain borehole support material, comprising the following steps:

[0022] S1. Drill holes in the soft coal seam to form extraction boreholes, and place the extraction pipes in the center of the extraction boreholes.

[0023] S2. Pump the prefabricated borehole support material slurry to the extraction borehole and fill it around the extraction pipeline. After the slurry solidifies, the gas extraction operation can be carried out.

[0024] S3. To address the blockage caused by pulverized coal, pressurize water and introduce it into the extraction pipeline during the maintenance cycle of the extraction process. This will flush out the pulverized coal blocking the extraction pipeline and the borehole support material, thus clearing the blockage.

[0025] Preferably, the extraction pipeline includes a pipe and a support plate disposed on the outer wall along the extension direction of the pipe; the pipe is composed of a pipe fitting having a plurality of vent holes.

[0026] During construction, operators can adjust the grouting pressure according to the actual borehole depth. For gas boreholes in soft coal seams, the typical borehole depth is 70–90 m. At depths above this, a pressure of 0.5–3.0 MPa is sufficient to meet grouting requirements. After the grout has solidified normally for 0.5–3.0 hours, a drainage device can be installed for extraction, which is simple, efficient, and saves construction time. The curing period for gas drainage can be adjusted according to the geological conditions at the drilling site. After 6–12 months of normal drainage, high-pressure water flushing can clear any blockages.

[0027] Based on the above technical solutions, the inventive concept of this invention lies in balancing support for gas extraction holes in soft coal seams with extraction efficiency. It designs the slurry material mix ratio based on the principle of aggregate accumulation, combines a single-graded aggregate design, and incorporates alkali-free accelerators to prepare a high-strength, high-permeability borehole support material. This material possesses advantages such as high strength, high permeability, and short setting time, which can improve the stability of the extraction borehole and reduce the risk of borehole collapse. The borehole support material and the extraction pipeline together constitute a complete gas extraction pathway, and the number of vent holes in the extraction pipeline can be adjusted according to actual construction needs, ensuring high gas flow even in the event of borehole collapse. Simultaneously, by pre-embedding a pipeline with pre-drilled holes containing support plates within the extraction borehole, the function of periodically cleaning the vent holes can be achieved, solving the problem of clogging caused by crushed coal dust. This invention not only solves the problem of limited support capacity of traditional gas extraction screens for the borehole wall but also has a self-cleaning function, preventing the screen holes on the screen surface from being blocked by crushed coal dust and avoiding the drawback of a sharp drop in gas concentration in the later stages of extraction.

[0028] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0029] This invention provides a borehole support material that is designed for gas extraction and maintenance needs. It uses raw materials with specific components and features high strength, high permeability, and short setting time. It has good construction performance and excellent support effect.

[0030] This invention provides a method for preparing a borehole support material. The steps are simple, and the borehole support material with the required structure can be easily prepared according to this method.

[0031] This invention also provides a method for cleaning boreholes in soft coal seam extraction using borehole support material. By combining the borehole support material with the extraction pipeline, the cleaning process is convenient and easy to maintain, solving the technical problem of a sharp drop in gas concentration in the later stages of extraction. Attached Figure Description

[0032] Figure 1 These are actual photos of the cross-section of the borehole support material;

[0033] Figure 2 This is a schematic cross-sectional view of a gas extraction hole obtained by the hole cleaning method of the present invention;

[0034] Figure 3 This is a schematic diagram of the cross-section of the extraction pipeline;

[0035] In the diagram, 1 represents the coal seam from which gas is extracted; 2 represents the borehole support material; 3 represents the pipeline; 4 represents the support plate; 5 represents the pipe fitting; and 6 represents the ventilation hole. Detailed Implementation

[0036] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0037] In the following embodiments:

[0038] The cement is Conch brand, grade 52.5R, and the cement particle size D50 is 20μm;

[0039] The particle size of limestone powder is 0.3–200 μm, and D50 = 10 μm;

[0040] The particle size distribution of silica fume is 0.02–100 μm, and D50 = 0.2 μm;

[0041] The particle size distribution of the quartz powder is 0.125–0.2 mm;

[0042] The particle size distribution of quartz sand is 10–20 mesh;

[0043] The water-reducing agent used is a polycarboxylate-type high-efficiency liquid water-reducing agent with a water reduction efficiency of ≥30% from the brand Sicar.

[0044] The accelerator used is an alkali-free aluminum sulfate type liquid accelerator with pH=2.5~3.0, solid content of 50%, initial setting time <5min, and final setting time <15min;

[0045] like Figure 2 , 3 As shown, the extraction pipeline used in the construction consists of a pipe and a support plate installed on the outer wall along the extension direction of the pipe. The pipe is a fitting with several vent holes.

[0046] Example 1

[0047] The drilling support material in this embodiment is made using the following method:

[0048] (1) Weigh out 4.24 parts cement, 0.56 parts limestone powder, 0.84 parts silica fume, 1.1 parts quartz powder, 17 parts quartz sand, 1 part water, 0.06 parts water-reducing agent, and 0.4 parts quick-setting agent by weight; add cement, limestone powder, silica fume, quartz powder, and quartz sand to a mixer and mix thoroughly. Use slow mixing for 3 to 5 minutes to ensure that the powder is fully coated around the quartz sand.

[0049] (2) Add water accounting for 30% of the total water usage to the well-mixed powder and quartz sand system, and stir slowly for 1.5 minutes until the mixture is uniform.

[0050] (3) Add 40% of the total water usage and water-reducing agent to the mixer and stir quickly for 2-4 minutes. After the slurry is completely fluid, pour the remaining 30% of water into the mixer and stir at medium speed until the components are evenly dispersed.

[0051] (4) Finally, add quick-setting agent to the well-mixed mortar and stir slowly for 1 minute to obtain the borehole support material.

[0052] The method for cleaning boreholes using the obtained borehole support material in the extraction of soft coal seams is as follows:

[0053] S1. Use a 100mm diameter drilling rig to drill holes in the soft coal seam to be extracted, forming an extraction borehole with a diameter of 100mm and a length of 100m; place an extraction pipe with a diameter of 10mm and a length of 100m in the center of the extraction borehole.

[0054] S2. The well-mixed slurry of the borehole support material is pumped into the extraction borehole containing the extraction pipeline using a high-pressure pumping device. The grouting is completed around the extraction pipeline. The grouting pressure is 1MPa. The injection is stopped when the borehole support material has been filled to 90m. After the slurry solidifies for 1 hour, the extraction device is installed to extract the gas.

[0055] S3. Six months after extraction, high-pressure water is introduced into the pipe fittings to flush out the coal dust blocking the extraction pipeline and borehole support materials, thus clearing the extraction channel.

[0056] Example 2

[0057] The drilling support material in this embodiment is made using the following method:

[0058] (1) Weigh out 4.24 parts cement, 0.56 parts limestone powder, 0.84 parts silica fume, 1.1 parts quartz powder, 18.5 parts quartz sand, 1 part water, 0.06 parts water-reducing agent, and 0.4 parts quick-setting agent by weight; add cement, limestone powder, silica fume, quartz powder, and quartz sand to a mixer and mix thoroughly. Use slow mixing for 3 to 5 minutes to ensure that the powder is fully coated around the quartz sand.

[0059] (2) Add water accounting for 30% of the total water usage to the well-mixed powder and quartz sand system, and stir slowly for 1.5 minutes until the mixture is uniform.

[0060] (3) Add 40% of the total water usage and water-reducing agent to the mixer and stir quickly for 2-4 minutes. After the slurry is completely fluid, pour the remaining 30% of water into the mixer and stir at medium speed until the components are evenly dispersed.

[0061] (4) Finally, add quick-setting agent to the well-mixed mortar and stir slowly for 1 minute to obtain the borehole support material.

[0062] The method for cleaning boreholes using the obtained borehole support material in the extraction of soft coal seams is as follows:

[0063] S1. Use a 100mm diameter drilling rig to drill holes in the soft coal seam to be extracted, forming an extraction borehole with a diameter of 100mm and a length of 100m; place an extraction pipe with a diameter of 10mm and a length of 100m in the center of the extraction borehole.

[0064] S2. The well-mixed slurry of the borehole support material is pumped into the extraction borehole containing the extraction pipeline using a high-pressure pumping device. The grouting is completed around the extraction pipeline. The grouting pressure is 1MPa. The injection is stopped when the borehole support material has been filled to 90m. After the slurry solidifies for 1 hour, the extraction device is installed to extract the gas.

[0065] S3. Six months after extraction, high-pressure water is introduced into the pipe fittings to flush out the coal dust blocking the extraction pipeline and borehole support materials, thus clearing the extraction channel.

[0066] Example 3

[0067] The drilling support material in this embodiment is made using the following method:

[0068] (1) Weigh out 4.24 parts cement, 0.56 parts limestone powder, 0.84 parts silica fume, 1.1 parts quartz powder, 17 parts quartz sand, 1 part water, 0.06 parts water-reducing agent, and 0.45 parts quick-setting agent by weight; add cement, limestone powder, silica fume, quartz powder, and quartz sand to a mixer and mix thoroughly. Use slow mixing for 3 to 5 minutes to ensure that the powder is fully coated around the quartz sand.

[0069] (2) Add water accounting for 30% of the total water usage to the well-mixed powder and quartz sand system, and stir slowly for 1.5 minutes until the mixture is uniform.

[0070] (3) Add 40% of the total water usage and water-reducing agent to the mixer and stir quickly for 2-4 minutes. After the slurry is completely fluid, pour the remaining 30% of water into the mixer and stir at medium speed until the components are evenly dispersed.

[0071] (4) Finally, add quick-setting agent to the well-mixed mortar and stir slowly for 1 minute to obtain the borehole support material.

[0072] The method for cleaning boreholes using the obtained borehole support material in the extraction of soft coal seams is as follows:

[0073] S1. Use a 100mm diameter drilling rig to drill holes in the soft coal seam to be extracted, forming an extraction borehole with a diameter of 100mm and a length of 100m; place an extraction pipe with a diameter of 10mm and a length of 100m in the center of the extraction borehole.

[0074] S2. The well-mixed slurry of the borehole support material is pumped into the extraction borehole containing the extraction pipeline using a high-pressure pumping device. The grouting is completed around the extraction pipeline. The grouting pressure is 1MPa. The injection is stopped when the borehole support material has been filled to 90m. After the slurry solidifies for 1 hour, the extraction device is installed to extract the gas.

[0075] S3. Six months after extraction, high-pressure water is introduced into the pipe fittings to flush out the coal dust blocking the extraction pipeline and borehole support materials, thus clearing the extraction channel.

[0076] Example 4

[0077] The drilling support material in this embodiment is made using the following method:

[0078] (1) Weigh out 4.24 parts cement, 0.56 parts limestone powder, 0.84 parts silica fume, 1.1 parts quartz powder, 18.5 parts quartz sand, 1 part water, 0.06 parts water-reducing agent, and 0.45 parts quick-setting agent by weight; add cement, limestone powder, silica fume, quartz powder, and quartz sand to a mixer and mix thoroughly. Use slow mixing for 3 to 5 minutes to ensure that the powder is fully coated around the quartz sand.

[0079] (2) Add water accounting for 30% of the total water usage to the well-mixed powder and quartz sand system, and stir slowly for 1.5 minutes until the mixture is uniform.

[0080] (3) Add 40% of the total water usage and water-reducing agent to the mixer and stir quickly for 2-4 minutes. After the slurry is completely fluid, pour the remaining 30% of water into the mixer and stir at medium speed until the components are evenly dispersed.

[0081] (4) Finally, add quick-setting agent to the well-mixed mortar and stir slowly for 1 minute to obtain the borehole support material.

[0082] The method for cleaning boreholes using the obtained borehole support material in the extraction of soft coal seams is as follows:

[0083] S1. Use a 100mm diameter drilling rig to drill holes in the soft coal seam to be extracted, forming an extraction borehole with a diameter of 100mm and a length of 100m; place an extraction pipe with a diameter of 10mm and a length of 100m in the center of the extraction borehole.

[0084] S2. The well-mixed slurry of the borehole support material is pumped into the extraction borehole containing the extraction pipeline using a high-pressure pumping device. The grouting is completed around the extraction pipeline. The grouting pressure is 1MPa. The injection is stopped when the borehole support material has been filled to 85m. After the slurry solidifies for 1 hour, the extraction device is installed to extract the gas.

[0085] S3. Six months after extraction, high-pressure water is introduced into the pipe fittings to flush out the coal dust blocking the extraction pipeline and borehole support materials, thus clearing the extraction channel.

[0086] The performance of the borehole support material in the embodiment was tested, and a photograph of the actual borehole support material is shown below. Figure 1 As shown in Table 1, the flexural strength and compressive strength of the material were tested using the three-point bending method. The initial and final setting times were measured using a Vicat apparatus, following the procedures specified in GB / T 1346-2011. Porosity was tested according to the procedures specified in DBJ / T13-274-2017. The corresponding test results are shown in Table 1.

[0087] Table 1:

[0088]

[0089] The above results demonstrate that the borehole support material of the present invention possesses the advantages of high strength, high permeability, and short setting time. Combined with... Figure 1 As can be seen, the borehole support materials are connected by an adhesive material surrounding the quartz sand, forming large and porous structures. Compared with existing large-pore concrete, this invention has better mechanical properties. In the embodiment, the compressive strength of the borehole support material reaches up to 36.2 MPa, and the flexural strength reaches up to 7.8 MPa, providing excellent support. Compared with ordinary filling materials, this borehole support material has high porosity, with visible pores and large pore diameters, making it very suitable for gas extraction and high-pressure water flushing in soft coal seams. From a compositional perspective, the lower the proportion of quartz sand, the higher the compressive and flexural strength of the support material, and the lower the porosity. Furthermore, the higher the amount of accelerator, the shorter the initial and final setting times. These proportions can be flexibly adjusted according to actual application conditions. This invention improves the stability of the extraction borehole, prevents borehole collapse, increases extraction efficiency, and solves the problem of borehole blockage caused by crushed coal dust, demonstrating good application results in gas extraction from soft coal seams.

[0090] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A borehole support material for extraction of soft coal seams, characterised in that, The raw materials for the borehole support material include the following components in parts by weight: 3.8-4.5 parts cement, 0.45-0.65 parts limestone powder, 0.75-0.95 parts silica fume, 0.9-1.2 parts quartz powder, 16-19 parts quartz sand, 0.8-1.2 parts water, 0.05-0.065 parts water-reducing agent, and 0.3-0.45 parts quick-setting agent; the cement grade is 52.5R, and the median particle size of the cement particles is 20 µm; the limestone powder has a particle size distribution range of 0.3-200 µm, with a median particle size of 10 µm; the silica fume has a particle size distribution range of 0.02-100 µm, with a median particle size of 0.2 µm; the quartz powder has a particle size distribution range of 0.125-0.2 mm; and the quartz sand has a particle size distribution range of 10-20 mesh.

2. The drilling support material according to claim 1, characterized in that: The water-reducing agent is a polycarboxylate-type high-efficiency liquid water-reducing agent with a water reduction efficiency of ≥30%.

3. The borehole support material of claim 1, wherein: The accelerator is an alkali-free liquid accelerator with a pH of 2.5-3.0, an initial setting time of <5 min, and a final setting time of <15 min.

4. A method for preparing a borehole support material as described in any one of claims 1 to 3, characterized in that, Includes the following steps: (1) Prepare the raw materials of the corresponding components according to the proportion, mix cement, limestone powder, silica fume, quartz powder and quartz sand evenly, and ensure that the powder material is fully wrapped around the quartz sand. (2) Add water accounting for 20-50% of the total water usage to the powder and quartz sand system obtained by mixing, and mix evenly; (3) Continue to add water and water-reducing agent accounting for 30-40% of the total water usage, mix until the slurry is completely fluid, then add the remaining water and continue mixing until the components are uniform; (4) Finally, add quick-setting agent to the obtained mortar, mix evenly, and obtain the borehole support material.

5. A method for hole cleaning for extraction of soft coal seams using a borehole support material as claimed in any one of claims 1 to 3 or a borehole support material produced by the method of claim 4, characterised in that, Includes the following steps: S1. Drill holes in the soft coal seam to form extraction boreholes, and place the extraction pipes in the center of the extraction boreholes. S2. Pump the prefabricated borehole support material slurry to the extraction borehole and fill it around the extraction pipeline. After the slurry solidifies, the gas extraction operation can be carried out. S3. To address the blockage caused by pulverized coal, pressurize water and introduce it into the extraction pipeline during the maintenance cycle of the extraction process. This will flush out the pulverized coal blocking the extraction pipeline and the borehole support material, thus clearing the blockage.

6. The method of claim 5, wherein: The extraction pipeline includes a pipe and a support plate installed on the outer wall along the extension direction of the pipe; the pipe is composed of a pipe fitting with several vent holes.

Images