The purpose of the gas collection system is to extract the maximum possible volume of gas but to leave the condensate behind. Landfill gas is warm and saturated with water so it produces a lot of condensate as it passes through the cooler ground, once it leaves the waste.
The proprietary make of well head used by many of the United Kingdom's landfill operators monitors and regulates the gas flow from each well, and also contains a barometric leg for draining condensate away. The gas flow regulation device used is best if it is specified to be sleeve valve type, which provides much finer adjustment than a butterfly valve. This is especially the case at low flows, and most importantly this more expensive valve choice does not restrict the condensate as can a butterfly valve.
The well heads are installed in line on the lateral pipe system of the landfill gas extraction pipework. Choosing the correct diameter of pipe for the various sections of the system is of prime importance, since the velocity of the gas flow against the gradient of the pipe run is a critical factor in successful de-watering. Experienced landfill gas extraction pipework designers choose their pipe diameters according to experience and on some occasions from modelling the gas flow rates.
Pipelines are carefully laid to falls from well to well, in order that the maximum amount of condensate is drained away via the well heads or other de-watering devices. The experienced landfill gas extraction pipework designer will assess the minimum fall according to the anticipated maximum settlement within the design life for the pipes. The fall will need to comfortably exceed the loss of height at the top of each pipe run after settlement and still provide a residual fall capable of allowing all condensate to continue to drain to the condensate collection points, also known as “knock-out pots”. if the designer fails to allow adequate falls in this manner, the landfill gas extraction pipework design will fail due to blockage of the horizontal interconnecting gas collection pipes when condensate water builds up.
It is a false economy to skimp on the number of individual pipelines used to transport the gas to the pumping station and generally better to have duplicate parallel pipe runs, rather than just one large diameter pipe collecting all of a field.
This gives flexibility which is necessary e.g. if repairs have to be carried out due to damage by machinery or settlement. It is also good practice to have the collection system divided in to sections. This is due to the fact that on an operational site it is sometimes necessary to remove and re-site sections of the collection system as filling of the site progresses, and if one pipe length of the landfill gas extraction pipework gets damaged at least part of the system will remain operational while repairs take place.
These are just a few of the considerations which a landfill gas extraction pipework designer must consider when designing the landfill gas extraction pipework for a landfill gas extraction system. It is recommended that all those seeking to install a landfill gas extraction and collection system engage an experienced designer, or a design and build contractor to do the design work.
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