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Biogas collection systems

Biogas collection systems consist of biogas distribution piping system with stop valves, hot well, safety vent, compressor, receiver, gas holder and biogas consuming appliances (stoves, water heaters, engines etc.).

System is mounted after installing digester in the permanent position. The opening for biogas extraction should be located in the upper part of the digester. After installing hot well the stop valves and water traps that consist of a water-filled tank that allows biogas flow only in one direction.

Water traps
Due to temperature changes, the moisture-saturated biogas will inevitably form condensation water in the piping system. Ideally, the piping system should be laid out in a way that allows a free flow of condensation water back into the digester.

If depressions in the piping system can not be avoided, one or several water traps have to be installed at the lowest point of the depressions. If water traps can not be avoided it has to be decided if an ’automatic’ trap or a manually operated trap is more suitable. Manual traps are simple and easy to understand, but if they are not emptied regularly, the accumulated condensation water will eventually block the piping system.

Piping system
The piping system connects the biogas plant with the gas appliances. It has to be safe, economic and should allow the required gas-flow for the specific gas appliance. Galvanized steel pipes or PVC-pipes are most commonly used for this purpose. Most importantly, the piping system has to be reliably gas-tight during the life-span of the biogas unit.

Piping system should be protected from damage. Gas leaks can be checked with the help of liquid soap that is applied to pipe connection places. Piping system also has to include safety vent that will let the biogas out in the atmosphere if the pressure exceeds 0,5 bar. Excess biogas has to be burned in jumbo burners.

Gas pipes
At least 60% of all non-functional biogas units are attributable to defect gas piping. Utmost
care has to be taken, therefore, for proper installation. For the sake of standardization, it is
advisable to select a single size for all pipes, valves and accessories.
The requirements for biogas piping, valves and accessories are essentially the same as for
other gas installations. Flexible plastic gas pipes can be used but, laid in the open, they must be UV-resistant.

Steel pipes
Steel water supply pipes with diameter 1,2 – 1,8 cm and length of less than 30 meters are adequate for small-to-midsize plants of simple design. For larger plants, longer gas pipes or low system pressure, a detailed pressure-loss (pipe-sizing) calculation must be performed.
When installing a gas pipe, special attention must be paid to:
• gas-tight, friction-type joints
• line drainage, i.e. with a water trap at the lowest point of the sloping pipe in order to empty water accumulation
• protection against mechanical impact

Galvanized steel water supply pipes are reliable and durable alternative to plastic pipes. They can be dismantled and used again if necessary. They are impact-resistant, but also quite expensive and require specialized knowledge for installation, therefore they are only recommended to use when there is no alternative of installing plastic pipes.

Plastic pipes and fittings have a relatively low price and can be easily installed. They are
available in different qualities with adhesive joints or screw couplings (pressure water pipes).
PVC pipes are susceptible to UV radiation and can easily be damaged so they should be placed underground.

Pipe diameter
The necessary pipe diameter depends on the required flow-rate of biogas through the pipe and the distance between biogas digester and gas appliances. Long distances and high flow rates lead to a decrease of the gas pressure. The longer the distance and the higher the flow rate, the higher the pressure drops due to friction. Bends and fittings increase the pressure losses. Steel pipes show higher pressure losses than PVC pipes.

A pipe diameter of 1,8 cm is suitable for flow rates up to
1.5 m3/h and distances up to 100 m (PVC pipe). Another option is to select the diameter of 2,4 cm for the main gas pipe and 1,2 cm for all distribution pipes to the gas appliances.

Piping system placement
PVC pipes can be used for underground systems or systems protected from sun and mechanical damage. In all other cases it is recommended to use galvanized steel pipes with diameter of 2,4 sm.

Plastic pipes should be placed no less than 25 cm below the ground and be surrounded by sand or soft soil. After gas-tightness check installed piping system is covered with soil or sand. To check piping system for gas-tightness is normally conducted by pumping air into the piping system under the pressure 2,5 times more than the highest estimated biogas pressure. If after several hours the pressure in the system has become lower it indicates air losses and then all joints are checked by pouring on the liquid soap or soapy water (if joint is not tight than there will be bubbles).

Fittings and valves
The most reliable valves are chromium-plated ball valves. Valves normally used for water systems are not suitable for gas systems. Main gas valve should be located close to digester. Ball valves should be installed on all gas appliances. Correctly chosen and installed valves and fittings will allow regular maintenance works and repairs without closing the main gas valve.