In order to assist local authorities in conducting post-earthquake inspections and maintenance of biogas digesters, and to ensure the rapid resumption of their operation, this guide has been specifically developed. It is important to note that this method does not apply to rural households whose structures have completely collapsed or are deemed unsafe, nor to biogas systems that have been entirely destroyed and require rebuilding.
I. Main Types of Damage to Biogas Tanks After an Earthquake
Following an earthquake, biogas digesters can suffer from structural damage as well as damage to supporting equipment. The main structural issues include total collapse, cracking, misalignment, and subsidence. Additionally, damage to components such as pipelines, regulators, and stoves can occur due to building damage.
(A) Structural Damage to the Digester
1. Total Collapse: This occurs when the biogas tank becomes distorted, broken, or collapsed due to intense seismic activity.
2. Cracking: Seismic forces can cause cracks in the walls of the digester pool, the arch cover, or the water pressure tank, leading to gas leakage.
3. Misalignment: When seismic shear waves affect the structure, misalignment may occur between the feed pipe, water pressure system, arch cover, and the digester pool wall, resulting in leaks.
4. Subsidence: Longitudinal seismic waves can cause the entire digester pool to sink.
(B) Damage to Supporting Equipment
1. Pipeline Damage: Pipelines may be damaged due to cracks, breaks, displacement, bending, or deformation caused by the destruction of the surrounding buildings.
2. Regulator Damage: This includes damage to internal components, loosening of connections, or damage from falling objects. The desulfurizer may also be affected by exposure to air and heat.
3. Stove Damage: The stove may be damaged by falling objects, leading to loose or disconnected gas pipe joints, or corrosion from rainwater.
II. Steps and Methods for Post-Earthquake Biogas Inspection
The inspection process involves five key steps to determine whether a digester has been damaged:
1. Inspect the biogas supporting equipment, including pipes, fittings, regulators, and stoves.
2. Check the reading on the biogas pressure gauge.
3. Monitor the water level changes in the water pressure tank.
4. Examine the arch cover for cracks.
5. Empty the digester and inspect the internal structure.
Testing Methods:
1. Pipeline Inspection: Visually check for damage, deformation, or looseness. Apply soapy water to joints to detect leaks by observing bubbles.
2. Pressure Gauge Reading: If the reading is below 1, it suggests the digester is damaged.
3. Water Level Monitoring: A fluctuation of more than 20–30 cm in the water level indicates possible damage.
4. Arch Cover Inspection: Open the cover and look for cracks larger than 3 mm, which indicate structural damage.
III. Main Maintenance Methods for Damaged Digesters
Once the damage is identified, three primary maintenance approaches are used: structural repair, seepage and leak repair, and general maintenance.
(A) Structural Repair
1. Crack Repair: Cut the crack into a "V" shape, clean it, and fill with C20 fine concrete. Once set, apply pure cement slurry several times.
2. Misalignment Repair: Remove mud from the misaligned area, roughen the surface, and fill with C20 concrete. Then apply plaster and cement slurry.
3. Subsidence Repair: For fully submerged digesters without major structural damage, reconnect the pipe to the air supply.
(B) Leak and Seepage Repair
1. Seepage Repair: Use quick-setting water glass to block small holes. For large leaks, insert a soft plastic tube, then seal with cement mortar and water glass, followed by a cork plug and cement coating.
2. Leak Repair: Clean the interior, then apply pure cement slurry three times.
(C) General Maintenance
Remove any flaking material, rinse thoroughly, and perform the “three wipes and four brushes†technique to apply seven layers of coating for long-term protection.
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