Reasons to measure flare gas flow in-clude compliance with environmen-tal regulations; identifying leaks, for

instance from pressure relief valves thathave failed to re-seat properly; reconcilingplant mass balances; and flare burner

control, which is especially importantwhen the flare gas contains a lot of inerts.

A typical flare gas system is a kind of“sewer” used to burn off waste gases (Fig-ure 1). Headers from individual processunits range in diameter from 50 to 200mm, while main flare lines leading to theactual flare are anywhere from 300 mm to1.8 m in diameter. A typical installation in-cludes at least one flowmeter on the mainflare line upstream of the knock-out drum.Smaller headers from individual processunits may have their own flowmeters.

A challenging applicationFlare gas flow measurement is a chal-

lenging application. It requires the abilityto measure over a wide range of velocities:from 0.03 m/s under lowest flow condi-tions, through 0.15–0.5 m/s for most nor-mal operation, up to 80 m/s or more in themain header during emergency flaring.Temperatures range from –20 °C to 80 °Cfor typical flare systems and from –200 °Cto 100 °C for liquefied natural gas (LNG)flares. Pressures range from 5–15 kPa innormal operation up to 7 bar under emer-gency conditions.

Flare header flowmeters must with-stand condensation and corrosion fromwater, liquid hydrocarbons and acid gas-es. They must be mechanically strong, to

resist the high drag forces that occur un-der emergency conditions. And they mustoperate with high levels of hydrogen andcarbon dioxide, which are challenging forultrasonic flowmeters because of theirhigh attenuation factors.

Flowmeters based on differential pres-sure, vortex shedding and insertion ther-mal-mass types have all been tried on flaregas duty, but none has performed partic-ularly well. Their failings include limitedturndown, inability to follow unsteadyflows, corrosion, intolerance of liquid car-ry-over, and sensitivity to changes in gascomposition.

Ultrasonic flowmeters are better suitedto this duty. The measurement of flare gasflow with ultrasound began in the early1980s with development work by Pana-metrics and Exxon in Baytown, Texas,USA. Today ultrasonic flowmeters are theaccepted technology for flare gas mea-surement, with more than 3,000 installa-tions worldwide.

Ultrasonic flowmetersAn ultrasonic “transit time” (or “time of

flight”) flowmeter for low-pressure gas ap-plications uses a pair of ultrasonic trans-ducers in direct contact with the gas. Ul-trasound pulses are transmitted alter-nately from one transducer to the other;the mounting arrangement of the trans-ducers means that the pulses in one di-rection travel with the gas flow, while in

How much do you flare?How to measure flowrates of flare gas accurately and reliably

Daryl Belock is Director of Strategic Marketing at GE Sensingin Billerica, MA, USA.

Instrumentation

PROCESS · worldwide 3-200618

Figure 1 (left): Flares are used to burn off unwanted gases underboth routine and emergency conditions. The wide range offlowrates makes this a challenging application for flowmeters.

Figure 4: Isolating valves allow the transduc-ers to be changed while the flare header is inservice.

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Figure 2: This diagonal trans-ducer arrangement is typicallyused for ultrasonic flowmeters

in headers of less than350 mm diameter.

Measuring the flow of flare gasis important in chemical, petro-chemical, refining, and otherplants that vent hydrocarbonsand other waste gases. Ultra-sonic flowmeters help cus-tomers comply with environ-mental emission requirements,reduce leaks and understandtheir process losses.

DARYL BELOCK

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the other direction they travel against theflow (Figure 2). Measuring the differencebetween the downstream and upstreamtransit times allows the gas velocity, andhence the volumetric flowrate, to be cal-culated.

Knowing the distance between thetransducers also makes it possible to cal-culate the speed of sound in the gas, andhence the average molecular weight. Thisnot only yields the mass flowrate, which isimportant for reporting purposes, but canalso help to identify the nature and sourceof the gas being flared.

Most ultrasonic flowmeters in flareheaders of less than 350 mm diameter usethe diagonal mounting arrangement ofFigure 2. Larger pipes typically use the“mid-radius bias-90” configuration of Fig-ure 3, which is easier to maintain and isless sensitive to velocity profile effects.Other arrangements are used where spaceis tight. Isolating valves and insertionglands allow the transducers to bechanged without shutting off the gas flow(Figure 4).

Location and installationFinding the right location for a flowme-

ter can be tricky. Ultrasonic flowmeters,like many other flow measurement tech-nologies, do not measure velocity acrossthe full cross-section of the pipe. Thismeans that for accurate results they de-pend on a fully-developed flow profile ofknown shape. For low-pressure gas appli-cations such as flare headers, the stan-dard advice is to allow a straight lengthequivalent to 20 pipe diameters upstreamof the flowmeter, and 10 diameters down-stream. This may be difficult to achieve ina flare header with many joins and bends,but it is important for accurate results.

Avoid locating an ultrasonic meterdownstream of any source of gas-borne ul-trasonic noise, such as flow-control andpressure-regulating valves. Despite ad-vanced digital signal processing technolo-gy, these ultrasonic noise will degrade thesignal to noise ratio, and in extreme casescould affect flowmeter performance.

The transducer nozzles need to be posi-tioned to a tolerance of ±1.5 mm and ±1°.This is a difficult task that is best carriedout in a workshop rather than in the field.For new flare systems, or plants that canbe shut down for maintenance, the pre-ferred approach is to use a “flow cell”—apipe spool that carries all the mountinghardware needed for the transducers.

This approach allows machining andwelding to be carried out under controlledconditions, and final dimensions to bemeasured accurately. The path length andthe axial distance between the transduc-ers, for instance, are typically measured to

±0.1 mm. The complete flow cell is cali-brated using air at known conditions oftemperature and humidity.

On-site installationThe flow cell approach is a luxury that

many plants cannot afford. Instead, three-quarters of all GE flare gas ultrasonicflowmeter installations worldwide are in-stalled directly into the pipe in the field.This is known as a “hot tap” or a “cold tap”,depending on whether flare remains inservice during installation. Properly done,this procedure results in a flowmeter in-stallation that is accurate enough for mostpurposes and causes little or no loss ofproduction.

Jigs and clamps are used to locate thetwo nozzles accurately on the flare headerand hold them securely while they arewelded in place. After welding, inspectionand pressure testing, isolating valves arefitted to the nozzles. The final step is topenetrate the pipe, using first a pilot drilland then a hole saw manipulated throughthe bores of the isolating valves. Furthertaps are made to accommodate pressureand temperature transmitters down-stream of the ultrasonic transducers.

Modern ultrasonic flowmeters offer arange of features to aid calibration andmaintenance. For instance, diagnostic da-ta can be sent to service engineers by e-mail or even in real time, via a web server.

Operation and maintenanceOnce installed, flare header flowmeters

have several useful functions. For envi-ronmental reporting and mass balancing,the average molecular weight and mass

flowrate derived from the flowmeter sig-nals are typically recorded by the plant’scontrol system.

If a flowmeter indicates gas flow in theflare header, but this is not expected fromthe plant’s operating conditions, the in-stallation point of the flowmeter and themolecular weight can both help in track-ing down the unit responsible. The actualsource of the flow is often a pressure reliefvalve that has failed to re-seat properly.

Operators also use flare gas flowmetersto control steam injection to the flare, as away to prevent smoke formation. �

Further reading[1] Industrial Flares, Section 13.5. Sept. 1991.

http://www.epa.gov/ttn/chief/ap42/ch13/fi-nal/c13s05.pdf.

[2] Flare Gas Ultrasonic Flow Meter. Proceedingsfrom the Thirty-Ninth Annual Symposium onInstrumentation for the Process Industries.1984. By Smalling, Lynnworth, Wallace.

[3] Flare Gas Ultrasonic Flowmeters — OptimisingPerformance & Verification of New and Exist-ing Installations. Steve Milford. GE Panamet-rics. June 2002. NEL Flare Gas Seminar.

Instrumentation

worldwide 3-2006 · PROCESS 19

Figure 3: The Panametrics DigitalFlow GF868Flare Gas Flowmeter from GE Sensing shows a

typical installation for large flare headers.

www.process-worldwide.com178477

• The homepage ofGE Sensing

• Futher information about the flaregas flowmeter GF868

• E-mail contact with theauthor

Phone: +1 (0)9 78 / 4 37 - 10 00

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