In order to assist local authorities in conducting post-earthquake inspections and maintenance of biogas digesters, and to quickly restore their functionality, this guideline has been specifically developed. It is important to note that this plan does not apply to rural households whose structures have completely collapsed or become unsafe, nor to biogas digesters that are entirely destroyed and need to be rebuilt in the future.
**I. Main Types of Damaged Biogas Tanks After an Earthquake**
Due to intense seismic activity, biogas digesters can suffer from structural damage such as total collapse, cracking, misalignment, and subsidence. Additionally, damage may occur to the supporting equipment due to damage to the buildings they are housed in.
**(a) Structural Damage to the Digester**
1. **Total Collapse**: This occurs when the biogas tank becomes distorted, broken, or collapsed due to strong seismic forces.
2. **Cracking**: This refers to cracks forming in the walls of the digester pool, the arch cover, or the water pressure tank, leading to gas leakage.
3. **Misalignment**: This happens when the feed pipe, water pressure system, or the junction between the arch cover and the digester wall shifts due to shear waves, causing leaks.
4. **Subsidence**: This occurs when the digester pool sinks as a result of longitudinal seismic waves, affecting its overall stability.
**(b) Damage to Supporting Equipment**
1. **Pipeline Damage**: This includes breaks, disconnections, bends, or deformations in the biogas pipeline, often caused by damage to the surrounding structure.
2. **Regulator Damage**: This involves issues such as loose connections, internal damage, or failure due to falling objects or exposure to air and heat.
3. **Biogas Stove Damage**: This typically results from physical impact, loose or disconnected gas pipes, or corrosion from rainwater.
**II. Post-Earthquake Inspection Steps and Methods**
**(a) Inspection Procedure**
To determine if the digester has been damaged, follow these steps:
1. Inspect the biogas supporting equipment, including pipelines, fittings, regulators, and stoves.
2. Check the reading on the biogas pressure gauge.
3. Observe the water level changes in the water pressure system.
4. Examine the arch cover for cracks.
5. Empty the digester and inspect the internal structure.
**(b) Inspection Methods**
1. **Pipeline Inspection**: Visually check for damage, deformation, or looseness. Apply soapy water to joints to detect leaks by looking for bubbles.
2. **Pressure Gauge Reading**: Ensure the supporting equipment is intact, then check the pressure gauge. A reading below 1 indicates potential damage.
3. **Water Level Observation**: Monitor the fluctuation in the water pressure tank. A change exceeding 20–30 cm suggests damage.
4. **Arch Cover Inspection**: Open the arch cover and look for cracks. If a crack exceeds 3 mm, the digester is likely damaged.
**III. Main Maintenance Methods for Damaged Digesters**
Once the extent and location of the damage are identified, three main types of repairs can be carried out: structural repair, seepage and leak repair, and general maintenance.
**(a) Structural Damage Repair**
1. **Crack Repair**: Cut the crack into a "V" shape, clean it thoroughly, and fill with C20 fine stone concrete. Once set, apply pure cement slurry multiple times.
2. **Misalignment Repair**: Remove the mud from the misaligned area, roughen the surface, and fill with C20 concrete. After curing, apply plaster and pure cement slurry.
3. **Subsidence Repair**: For fully submerged but structurally sound digesters, reconnect the piping and air vent.
**(b) Seepage and Leak Repair**
1. **Seepage Repair**: Use quick-setting water glass to block small seepage holes. For larger leaks, insert a soft plastic tube, then seal with a 1:1 cement mortar and water glass mixture, followed by a cork plug.
2. **Leak Repair**: Clean the inside of the digester and apply pure cement slurry three times to seal any leaks.
**(c) General Maintenance**
For minor damage, remove any loose material, rinse the interior, and perform a thorough cleaning. Then, follow the "three wipes and four brushes" method to apply seven layers of coating for long-term protection.
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