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Flare Gas Flow Meter: Ultrasonic Heavy Industrial Monitors
Quick Answer: Ultrasonic flare gas flow meters measure gas sent to the flare stack with no moving parts, no pressure loss, and a turndown ratio that can reach 400:1. Silver Instruments supplies heavy‑industrial ultrasonic flare meters for pipe sizes from DN50 to DN1200, for gas compositions from 0 to 100% hydrogen, and for Zone 1 hazardous areas. Typical delivered accuracy is ±1% of reading across a flow range of 0.1 m/s to 80 m/s.
Why Ultrasonic Technology Fits Flare Gas Measurement
Flare lines are not normal process pipes. Flow can be near zero during normal operation and then swing to 60 m/s in minutes during an upset. Gas composition changes fast. Hydrogen, methane, ethylene, CO₂, and nitrogen can all show up in the same hour. A mechanical meter in that service will clog or drift within weeks. An ultrasonic meter has no mechanical parts in the pipe. It sends acoustic pulses diagonally across the gas stream and measures transit time difference. That raw signal works from 0.1 m/s up to 80 m/s. No recalibration needed when the gas mix changes, as long as the meter corrects for the speed of sound. Our Silver Automation Instruments ultrasonic flare meters calculate the actual speed of sound 20 times per second and compare it to the theoretical value for the gas mixture. This lets the meter detect composition shifts and flag bad data.
Heavy Industrial Build for Refinery and Offshore Flares
We build the transducers into 316L stainless steel housings with a titanium acoustic window. The body can be Hastelloy C276 if the flare gas carries high H₂S above 5% volume. Process connection flanges are typically ASME B16.5 Class 150 or 300, but we also supply PN16 and JIS 10K for projects in Southeast Asia and the Middle East. The electronics enclosure is a cast aluminium IP66 box rated for ATEX/IECEx Zone 1, Group IIC, T4. Junction boxes and cable glands are M20 or 3/4 inch NPT. Inside the enclosure, the transmitter reads the two acoustic paths, calculates flow velocity, and outputs a 4‑20 mA HART signal plus Modbus RTU for the DCS. Most engineers skip the integrator box and wire the meter straight into the control system. In practice, the signal cable can run 200 metres with no signal loss if you use a 2‑pair shielded twisted cable. Last year a refinery in Vietnam replaced eight thermal mass flare meters with our ultrasonic units on six flare headers, from DN150 to DN600. They sent us the spent thermal sensors. The thermal probes had a 3 mm crust of soot and the calibration had drifted by 15% in 10 months. The ultrasonic paths stay clean because the transducers are retractable. Operators pull them during a brief process window, wipe the face, and push them back in. No hot tapping needed.
Accuracy, Range, and Real‑World Numbers
A healthy flare line has a velocity range from 0.05 m/s during purging to maybe 75 m/s during an emergency release. That is a turndown of 1500:1 in velocity terms. Most flare specifications ask for a single meter to cover the whole range. Our two‑path ultrasonic flare meter delivers a turndown of 400:1 while keeping ±1% of reading. Between 0.1 m/s and 0.5 m/s the measurement error rises to ±3% of reading. Below 0.1 m/s the meter outputs the velocity value but marks it as low‑confidence. During a site acceptance test at a gas processing plant in Nigeria, the operator vented nitrogen at 12 bar through a DN300 spool. The Silver Instruments ultrasonic meter tracked the ramp from 0.2 m/s to 42 m/s and matched the reference venturi meter within 0.8%. The test report is public, and we can share it.
Flare Gas Composition and Speed‑of‑Sound Verification
Here is the thing. The hardest part of flare metering is not the flow velocity. It is the changing gas composition. When a cracker unit trips, the flare header fills with light hydrocarbons. Half an hour later the header may be 60% nitrogen from purge systems. A standard ultrasonic meter that relies on a fixed gas density assumption will report wrong mass flow. Our transmitter does not assume. It measures the actual speed of sound through the gas at operating temperature. The on‑board database holds sound‑speed curves for N₂, O₂, CO₂, H₂, CH₄, C₂H₆, C₃H₈, ethylene, propylene, and steam. The processor matches the measured SOS to the expected SOS for the operating pressure. If the operator sends a live gas chromatograph stream over Modbus, the meter uses that data to pin the composition. For flare lines without a GC, the meter itself provides a Good/Bad signal when the SOS differs from a user‑set baseline by more than 2%. We have seen this catch a nitrogen purge valve stuck open at a chemical plant in Indonesia. The flare flow was reading 15 t/h, but the SOS told the opera
Installation, Straight Runs, and Pipe Considerations
Flare lines do not come with nice long straight runs. Our two‑path meter needs 10D upstream and 5D downstream for a single bend. If you have a control valve or a tee upstream, the meter needs 20D. If the available straight run is shorter, we can add a flow conditioner plate in the spool. The conditioner is a perforated 316L disc that breaks swirls. It adds about 0.05 bar pressure loss at 50 m/s in a DN400 line. In one offshore platform project in the Persian Gulf, the flare header had a 6D straight run before the meter due to deck space. We put the conditioner 2D upstream and the meter met the accuracy spec during witness tests. Transducer ports are horizontal to avoid condensation. The body has a drain plug at the low point. For flare services with liquid droplets, we tilt the spool 5° towards the drain. That keeps the acoustic window dry. If you need to measure bidirectional flow because the flare system can pull vacuum during temperature drops, we supply the same meter with forward/reverse totalisers. The output can be absolute flow or signed flow. Configuration takes 15 minutes with the free USB configuration tool.
Outputs, Integration, and Emission Reporting
The transmitter provides one 4‑20 mA analogue output for flow rate, assignable to volumetric or mass flow. There are two additional 4‑20 mA channels for speed of sound and gas temperature. A pulse output gives totalised flow. Digital options are Modbus RTU and HART 7. For hydrocarbon accounting, the meter stores 90 days of minute‑by‑minute flow data with time stamps, temperature, and SOS. A built‑in web server lets an operator pull the logs with a browser, no special software. This matters because environmental agencies in countries like Thailand and Mexico require 15‑minute average flow logs for carbon tax reporting. Our meter can push the 15‑minute blocks directly to a plant historian over Modbus TCP if you add an Ethernet module. One pulp and paper plant in Brazil uses this feature to send flare flow data straight to the national emission platform. Their previous meter needed a separate flow computer. We eliminated that box.
What to Send Us for a Quote
If you are sizing a flare gas meter, send us the pipe inner diameter or DN, the normal and maximum flow velocity or mass flow, the typical gas composition including H₂ and H₂S, the design pressure and temperature, and the hazardous area classification. We size the meter body, pick the transducer material, and send you a dimensional drawing within two working days. Our email is [email protected], telephone +86-135-1213-1526, and WhatsApp 86-135-12131526. For WeChat users, our contact is +86 13512131526. We ship directly to refineries, offshore platforms, chemical parks, and LNG terminals in Southeast Asia, the Middle East, Africa, Latin America, and Oceania. Every flare meter comes with a calibration certificate traceable to NIST, a material certificate for wetted parts, and a pressure test report. No need to ask for them separately.
Frequently Asked Questions
Can the ultrasonic meter handle wet flare gas with liquid droplets?
Yes. The titanium window resists impact, and the spool geometry drains liquid. We specify a drain leg and a tilted installation. Accuracy degrades by about 1% to 2% at high liquid load, but the meter remains stable. If the gas carries more than 5% liquid by mass, we recommend a knockout drum upstream.
What is the minimum pressure for ultrasonic flare measurement?
The meter works from atmospheric pressure down to 0.3 bar abs. Below 0.3 bar the acoustic signal weakens. For vacuum flare lines, contact us for a low‑pressure transducer configuration. We have tested it down to 0.15 bar abs on a dry flare header.
How often do the transducers need cleaning?
Most sites clean them once a year during a turnaround. Retractable transducers let you pull, wipe, and reinsert in 20 minutes without breaking the pipe seal. Some gas plant operators in Malaysia clean them every two years.
Can I use the same meter on a hydrogen flare?
Yes. Hydrogen attenuates the acoustic signal, so we select a 100 kHz transducer pair instead of 200 kHz. The meter then reads from 0.3 m/s to 60 m/s at pure H₂ with ±1.5% accuracy. For hydrogen blends, the meter automatically switches SOS curves.
Does the meter require a permanent power supply on an offshore platform?
The transmitter accepts 24 V DC. Power consumption is 4 W typical, 6 W with the Ethernet module. A standard instrument loop supply is enough. No separate heater is needed because the electronics are rated for ‑40°C to +70°C ambient.