An automated landfill gas well monitoring system samples each wellhead in turn, analyses the gas, and adjusts a valve to hold methane or suction where you want it. This post describes a distributed monitoring system specified for a landfill gas collection field of 150 extraction wells on 15 manifolds, and the practical limits that determine how many wells one analyser can serve.
The challenge: tuning many wells by hand
A big gas field changes constantly. Gas quality and yield vary from well to well and over time. Balancing the field by hand means visits, hand-held readings and delayed adjustments. The requirement here was automatic adjustment of each well to keep methane at the target level, with data sent to a master computer.
The approach: sample, analyse, adjust
How the system works
Each sample point is connected by 6 mm nylon tubing to a stainless steel IP68 enclosure housing the analysers. A switching mechanism draws samples in sequence, giving each enough time for a representative sample. Gas is dewatered with a coalescing filter and condensate trap, filtered, then analysed:
- Methane, 0 to 100%, by infra-red.
- Carbon dioxide, 0 to 60%, by infra-red.
- Oxygen, 0 to 25%, by electro-chemical cell.
- Hydrogen sulphide, optional.
Readings are logged at set intervals through a PLC and shown on a graphical interface at the central computer. The same PLC sends 4 to 20 mA signals to motorised valve positioners at the wells, so the loop closes automatically. Data travels on an electrically noise-resistant 485 data-highway.
Moisture protection
Wet gas is the main enemy of any sampling system. The design includes hydrophobic filters on each sensor head, sensor-head isolation valves, a line heater to vaporise residual moisture, an alarm for moisture ingress, and enough vacuum to overcome liquid in the sample lines.
The practical limits
| Item | Limit in this design |
|---|---|
| Sample tube length | Up to 500 m possible |
| One enclosure pair | Up to 64 sample points if each is within 150 m and only three sweeps per day are needed |
| Points per enclosure | More than 12 may need a second enclosure |
| This field | One analysis suite per manifold of 10 wells; sample points assumed within 5 m |
The limiting factor is sweep time. The more points and the longer the tubes, the longer each cycle takes, and the less often each well is read.
The valve side
The design assumed 150 low-pressure butterfly valves, 3 inch nominal bore, each with a 4 to 20 mA positioner, not hazardous-area rated by default (zoned actuators were an option). Calibration of each sensor on site and one day of commissioning per analysis suite, 15 days in total, were allowed for. Data cable, central PC, civil works, power and installation were excluded or priced separately.
Why it works
Automation converts periodic checks into continuous control. Holding methane steady at each well helps keep gas quality up for the flare or engine and helps prevent over-extraction drawing air into the field.
What to consider for your own project
- Decide how often you need each reading. This drives how many points one suite can serve.
- Check tube lengths and moisture. Both affect sample quality and time.
- Specify valves and positioners to suit zoning.
- Plan calibration. Sensors drift; certificates and a routine matter.
- Link it to your permit monitoring. The Environment Agency’s landfill gas guidance sets expectations.
FAQ
What gases are measured?
Methane, carbon dioxide and oxygen as standard, with hydrogen sulphide optional.
How far can samples travel?
Up to 500 m in nylon tubing, though shorter runs are quicker.
Can it adjust wells automatically?
Yes, via 4 to 20 mA signals to valve positioners.
Managing a large gas field?
Tell us the number of wells and layout and we will outline a monitoring system. Related: flare monitoring and SCADA.