Methane Flow Meter: Explosion-Proof Gas Mass Monitoring Tools for Biogas Plants

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Methane Flow Meter: Explosion-Proof Gas Mass Monitoring for Biogas Plants

Quick Answer: In biogas plants, methane flow measurement must be done with a mass flow meter, not a volumetric one. A thermal mass flow meter with ATEX Zone 1 certification gives you a direct reading in kg/h or Nm3/h, unaffected by changing gas composition and temperature. This is the standard choice for anaerobic digester gas, landfill gas, and wastewater biogas worldwide.

Why mass flow beats volumetric flow in biogas

Biogas is a mixed gas. The methane content can swing from 50% to 70% depending on feedstock, temperature, and how well the digester is running. A volumetric flow meter like a turbine meter or vortex meter sees only velocity. You then need separate pressure, temperature, and gas composition sensors to calculate mass flow. That adds cost, drift, and maintenance points.
Here is the thing. Plant operators and energy managers sell gas by energy content, not by cubic meters. Energy content is tied directly to methane mass. A thermal mass flow meter puts the mass flow number right on the screen. No external calculator needed.

How the explosion proof thermal mass meter works

Silver Automation Instruments supplies insertion and inline thermal mass flow meters for biogas service. The sensor has two PT100 platinum RTDs. One measures the gas temperature. The other is heated to a constant offset above that temperature. As gas flows past the heated sensor, it takes away heat. The meter measures the electrical power needed to keep that offset constant.
This power relates directly to the mass flow of the gas molecules hitting the sensor. So the reading is already compensated for temperature and pressure. You set the gas mixture once during commissioning and the meter handles the rest. Typical configurations we ship are DN50 or DN80 inline meters or insertion probes for pipe sizes from DN100 up to DN300.

Explosion proof certification and safety zones

Biogas contains methane, which is flammable. The measurement area is almost always classified as Zone 1 or Zone 2. Our thermal mass flow meters come with ATEX II 2G Ex db IIC T4 Gb certification for gas atmospheres. We also have IECEx versions for countries that accept that standard.
The transmitter housing is cast aluminum with a flameproof enclosure. The sensor probe is made of 316L stainless steel. For biogas with high H2S, we offer a Hastelloy C-22 sensor option. This matters a lot on landfill gas sites where H2S levels can exceed 1000 ppm. We have seen standard stainless probes fail in under a year there.

Real installation challenges in biogas plants

Most engineers skip this part. Biogas is wet. Raw biogas straight from the digester is saturated with water vapor. If the gas cools before it reaches the flow meter, water condenses. Liquid droplets hitting the heated sensor cause big spikes in the reading. The fix is a condensate trap and a filter upstream of the meter. We always recommend a stainless steel mesh filter, 40 micron, installed with a drain leg.
Another issue is gas pulsation from reciprocating gas compressors or from feeding cycles in the digester. A fast response meter will see these pulses. That is good, but you need to set the damping correctly in the transmitter. We usually start at 3 seconds and adjust from there.
In practice, a customer in Vietnam with a pig farm biogas system measured 60 kg/h average flow. During feeding time, the flow jumped to 90 kg/h for 15 minutes. Without damping, the totalizer was over-counting by nearly 8% each

Meter selection: insertion vs inline

For pipe sizes under DN80, we supply inline flow meters with flanged connections, usually PN16 or 150 lb. For larger pipes, an insertion probe with a ball valve retractor is more cost effective. You hot-tap the pipe, insert the probe, and fix the insertion depth at the pipe centerline.
Here is a detail that gets missed. The insertion depth calculation assumes fully developed turbulent flow. If you install the probe too close to a bend or a valve, the velocity profile is skewed and your mass flow reading can be off by 10% to 15%. We require 15D straight run upstream and 5D downstream. When that is not possible, we can supply a flow conditioner. Just tell us your pipe layout when you send the inquiry.

Typical outputs and integration

The meter gives you a 4-20 mA HART signal for mass flow, plus a pulse output for totalization. Modbus RTU over RS485 is standard in our electronics. You can connect it directly to a PLC, a biogas SCADA system, or to one of our paperless recorders for compliance reporting. Many carbon credit projects in Southeast Asia and Africa use this setup to log methane destruction data for verification bodies like Gold Standard or Verra.

FAQ

Can this meter measure raw biogas with H2S?

Yes. The standard wetted material is 316L, which handles H2S up to about 50 ppm in wet conditions. For higher concentrations, order the Hastelloy C-22 sensor. Also, specify the optional PTFE coating on the probe shaft. This prevents corrosion under the gasket area where crevice attack starts.

What flow range does the meter cover?

For a DN50 inline meter, the typical range is 0.5 to 150 kg/h for biogas at atmospheric pressure. An insertion probe in a DN150 pipe can measure from 5 kg/h up to 2000 kg/h. We need your minimum, normal, and maximum flow rates to select the right sensor power and bore size.

How do you calibrate a thermal mass meter for biogas?

We calibrate each meter on air at our factory, then apply a gas correction factor group stored in the transmitter. You tell us your expected methane, CO2, and N2 percentages. We set the factor. The meter then displays mass flow and total directly in methane equivalent. The accuracy is ±1.5% of reading plus ±0.5% of full scale under stable conditions.

Can I use this on landfill gas with trace siloxanes?

Siloxanes form abrasive SiO2 particles when burned, but they also coat the heated sensor over time. We have seen sensor drift after 12 to 18 months in heavy siloxane gas. Our recommendation is to install a siloxane filter upstream and schedule a sensor cleaning every 6 months. The insertion probe with retractor allows cleaning without shutting down the gas line.

What information do you need for a quote?

Send us the gas composition or at least the expected methane percentage, the pressure in bar or kPa, the temperature in °C, the pipe size in DN or inches, the minimum and maximum flow rate you expect, and the hazardous area zone if known. Also tell us if you need a remote display or any custody transfer approvals.

Silver Automation Instruments ships methane flow meters to biogas plants in Thailand, Vietnam, Indonesia, Australia, Nigeria, Kenya, Chile, and many other countries. For a fast quote and technical selection support, contact us directly.

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Tel: +86-25-68650347
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