A separate biogas holder can be a heavy plastic bag in a frame weighted on top, or an inflated truck or tractor inner tube, and its pressure comes from the weight on the gas over the area it presses: a bag with 1.5 m² of top area needs 150 kg of weights to deliver 10 cm of water column. A truck inner tube holds about 0.2 m³, so the parent’s figure of 1-3 m³ for one tube looks too high.
Part of Biogas Digesters
Gas holder design is choosing and sizing a vessel to store biogas at low, steady pressure: a bag, a tube, a drum or a dome.
The Parent’s Options
- Rubber inner tubes: large truck or tractor tubes inflated with biogas; simple, cheap, “hold 1-3 m³”.
- Plastic bag: heavy plastic (pond liner material) in a wooden frame, weighted on top to keep slight pressure.
- Floating drum or dome: see Floating-Drum and Fixed-Dome.
Check The Inner Tube’s Volume
An inflated tube is a torus with a ring radius R and a tube radius r, so its volume is V = 2π² R r². For a truck tube with R = 0.5 m and r = 0.15 m (assumed dimensions): V = 2 × 9.87 × 0.5 × 0.0225 = 0.222 m³, about 0.2 m³. For a smaller car tube (R = 0.35, r = 0.1): 0.069 m³. One tube is about a quarter of the 0.4-0.7 m³ evening store computed on Gas Collection & Storage, so several tubes are needed. The parent’s “1-3 m³” would need a ring of tubes. Measure your own tube.
Rubber tubes also leak slowly, perish in sunlight, and need a weight on top to push the gas out, since a tube’s own tension at low inflation gives little pressure.
Weight Gives Pressure
A bag or tube with a platform on top delivers p = W ÷ A (weight over area). With an assumed bag top area of 1.5 m² (1.5 m by 1.0 m):
- 5 cm of water column (490 Pa): W = 490 × 1.5 ÷ 9.81 = 75 kg.
- 10 cm (981 Pa): W = 981 × 1.5 ÷ 9.81 = 150 kg.
- 15 cm (1,472 Pa): 225 kg.
The weights must be spread so the bag fills evenly; stones in a plank tray work. If the bag is under a weight that is more than the burner or pipe expects, it will push the gas out too fast or rupture a joint.
What You Need
- Holder: heavy plastic bag or tubes, with a gas pipe fitting that does not pull out
- A frame and platform, sheltered from sun
- Weights adjustable in steps
- A water gauge to read the pressure (Pressure Regulation)
- Fencing, since a bag in the open can be damaged by animals and fire
Method
Step 1: Compute the weight for the pressure you need from the area.
Step 2: Build the frame so the bag can rise and fall freely, with a platform on top.
Step 3: Fit the gas pipe at the lowest point of the bag, and a drain for condensed water.
Step 4: Add weights in steps, reading the gauge, until the burner pressure (5-15 cm of water) is reached.
Step 5: Inspect for leaks and for sun damage.
Common Mistakes
- Trusting the parent’s 1-3 m³ per tube.
- Too little or too much weight.
- Placing the holder under a roof or against a building, where a leak can pool.
- Storing the holder next to a stove or lamp.
A holder is a store of explosive gas
A full 0.5 m³ holder of 60% methane contains 0.3 m³ of methane. Keep the holder in open air away from flames, ruptured plastic releases it at once, and its gas carries hydrogen sulfide. Loose weights falling from a frame can injure people; secure them.
What’s Next
- Learn next: Pressure Regulation
- Combine with: Piping & Valves