Biogas burns best at low steady pressure, 5-15 cm of water column (0.49-1.47 kPa), set by a floating drum or a weighted bag and watched on a U-tube water gauge; a relief pipe dipping 20 cm into water vents any pressure above 20 cm (1.96 kPa). The relief must discharge outdoors, high and away from flame, because the gas it releases is explosive and carries lethal hydrogen sulfide, and a separate water seal in the line stops a flame running back.
Part of Biogas Digesters
Pressure regulation keeps gas at the pressure appliances need, vents any excess safely and prevents air or flame entering the holder.
Units: A Centimetre Of Water Is 98 Pascals
A column of water h metres high exerts ρ g h = 1,000 × 9.81 × h pascals, so 1 cm is 98.1 Pa. The numbers in this article:
| Quantity | Water column | Pressure |
|---|---|---|
| Burner working range (parent) | 5-15 cm | 0.49-1.47 kPa |
| Relief setting (parent) | 20 cm | 1.96 kPa |
| 1 m of displaced slurry in a fixed dome | about 100 cm | 9.81 kPa |
The weight of a floating drum or weighted bag naturally provides 5-15 cm (parent), by p = W ÷ A (Floating-Drum, Gas Holder Design). A fixed dome has no such regulation: its pressure rises with every cubic metre stored, up to several kilopascals (Fixed-Dome), and needs a small floating-bell regulator between the dome and the appliances, a floating drum on a small scale that holds its output at its own W ÷ A whatever the dome does.
The Gauge
A clear tube bent into a U, half filled with water and connected to the gas line at one end: the difference in water levels, in centimetres, is the pressure in centimetres of water column. A reading of 12 cm is 12 × 98.1 = 1.18 kPa. Fix it to a board with a ruler, protect it from frost, and keep the water topped up.
The Relief
A pipe from the gas line ends under h cm of water in a container; if gas pressure exceeds h cm of water, bubbles escape and the pressure cannot rise further. The parent’s 20 cm relief protects a plant working at up to 15 cm: 20 ÷ 15 gives 33% headroom. Pick h a little above the highest working pressure, and no higher than the weakest part of the system (dome, bag seams) can bear.
Where the gas goes matters. It is 55-70% methane and carries hydrogen sulfide:
- Put the relief container outdoors, with the vent pipe up and away from doors, windows, lamps and stoves.
- Use a relief above ground, where escaping gas will not collect in a pit (hydrogen sulfide is 1.18 times as heavy as air, carbon dioxide 1.52).
- Keep the water at the right level: evaporation lowers it (relief lifts earlier), freezing locks it solid (no relief at all).
Stopping Air And Flame
- Flashback seal. A water seal in the line to each appliance, with the gas passing through the water, stops a flame travelling back through the pipe into the holder. Its depth must exceed the working pressure so the water is not blown out.
- No vacuum. If the holder empties and the appliance keeps drawing, air can be pulled in through leaks or a relief pot, producing an explosive air-gas mixture. Close the gas valve before the holder empties.
What You Need
- A clear tube and board for the gauge
- A container and pipes for relief and the flashback seal, made of materials that resist biogas (not copper or brass: hydrogen sulfide attacks them)
- Water, kept topped up, and frost protection
Method
Step 1: Fit the gauge near the holder and read it at different fill levels.
Step 2: Add weight or reduce it until the pressure at a mid-fill reading is in the 5-15 cm range.
Step 3: Install the relief on a tee in the line near the holder, outdoors, set to about a third above the maximum working pressure.
Step 4: Fit a flashback water seal before the first appliance.
Step 5: Check water levels weekly and before cold nights.
Common Mistakes
- Relief indoors. The vented gas fills the room.
- Relief set too deep, so the digester over-pressures before it opens.
- A dry relief or seal.
- No flashback seal.
Vented biogas explodes and poisons
Relieved gas is explosive at 5-15% methane in air and carries hydrogen sulfide, which is lethal and dulls the sense of smell; vent only to open air, high, away from people and ignition, and never check a relief by flame. A pit or low corner near the vent can hold heavier gases. See Anaerobic Digestion Science.
What’s Next
- Learn next: Piping & Valves
- Combine with: Cooking Burners