Investing in a flare gas measurement system is becoming more and more important not only due to governmental legislation, but also improved control of emissions and leakage. A multitude of different flare gas measurement solutions are available. However, only one is regarded as the flare gas measurement technology of the future:- ultrasonic time of flight meters.

With our flow metering expertise and offshore platform knowledge and experience, we can offer both product and application advice on flare gas metering and information on current government legislation. Our service engineers, all offshore certified, are available for installation, commissioning and maintenance of all types of flare gas systems.

	FGM 160 As the exclusive UK channel partner for Fluenta AS, we are pleased to offer the FGM 160, an ultrasonic flare gas meter which has been developed to specifically measure flare gas in pipes where pressure, velocity and large pipe diameters represent a real challenge. The Fluenta flare gas meter is the most robust and accurate flare monitor on the market today. An essential tool for E&P operators. With over 500 flare metering systems in operation worldwide, Fluenta is a world leader in ultrasonic flare gas metering.
FGM 160 Literature	Ultrasonic Flare Gas Meter Flow Measurement 0.03 m/s to 100 m/s High Turndown (3320:1) Uncertainty: ± 2.5% - 5% Automatic Gain Control (AGC)for enhanced performance Digital Signal Processing (DSP) for optimized signal processing Software based user interface (Operator Console) Easy and fast to install Field mounted flow computer Low maintenance costs Serial interface based on Modbus protocol
Instrument Datasheet

Why Measure Flare Gas ?

Flare systems at offshore production platforms, refineries and chemical plants are primarily installed for safety purposes. The flare systems are mainly activated due to an unexpected shut-down or when it becomes necessary to suddenly dispose of large amounts of gas.

The effect of gas emission as both an environmental and an economical issue is increasing in importance amongst operators and oil companies. In addition to the obvious safety purposes of a flare, national legislation in more and more countries requires control of the emission, and in some countries operators have to pay taxes for their CO2 emission.

Oil production platforms are nowadays both designed and rebuilt for zero flare gas emission. This change in operation of the flare systems has also influenced on the requirements of the flare gas metering systems. From a continuous, more or less steady flowing amount of flare gas, today's picture is more with the gas flow either to be approximately zero, or at the specified maximum rate.

From an operator's point of view, there is no reason to measure the flare gas unless it is of economical benefit or it is required, for example, for tax payment purposes. In order to achieve economical benefit of a flare gas measurement, the purpose of the measurement could be to identify points of leakage, to obtain better control with emission rates or mass balancing. These application areas for ultrasonic gas flow meters have added metering requirements beyond the direct flare gas metering requirements. Also, this has opened a new market within refineries and onshore process plants.

The choice of technology when investing in a flare gas metering system is of utmost importance. An evaluation of cost versus benefit should be made, and the basis for the evaluation would be parameters as investment, installation and maintenance costs, measurement uncertainty, repeatability, measurement range, reliability and non-intrusiveness of the measurement technology. The ultimate flow meter would of course be the best sum of all these parameters.

As earlier stated, more focus have been put on the environmental and economical aspects of the gas flaring, and in some countries the operators have to pay taxes for their CO2 emissions. Accordingly, in order to fulfil the regularity requirements, the operators requirements regarding the flare gas metering systems have changed.

Flare Gas Metering

With flare systems being installed primarily for safety purposes, the flare gas metering systems must cope with dramatically changes in the flow velocity, gas composition and temperature over a very short time scale. Hence, the measurement challenges may vary a lot over a short time period.

Due to the nature of, for example, a process shut-down, when all of the process gas is flared, the flow velocity may exceed 100 m/s. As a result of this extremely high flow velocity, unwanted particles and components such as oil, water, salt and scale may be transported along the flare pipeline. Knowing this, it is quite evident that any instrumentation that intrudes into the flare pipeline might get influenced, or at the worst get damaged, during such a shut-down. Therefore, limitations of what metering systems that can be put in operation have arisen.

Flare Gas Measurement Methods

Traditionally, conventional metering systems used for flare gas measurements include insertion turbines, thermal mass meters and annubars.

A turbine meter utilises the principle that the gas is led through the meter rotor. The rotor is designed with a specific number of blades positioned at a precise angle to the flow stream. The gas impinges on the rotor blades causing the rotor to rotate, with the angular velocity of the rotor being directly proportionally to the gas velocity. Clean fluids are required to prevent contamination of the bearings unless sealed bearings are used. Insertion type turbine meters cause negligible pressure drop, but due to the local velocity measurement, the measurement uncertainty is higher than for conventional full-bore turbine meters. Typical flow range for such meters is up to 30 m/s.

Thermal mass meters are typically based on two Thermowell protected Resistance Temperature Detectors (RTDs). When placed in the process stream, one RTD is heated and the other is sensing the process temperature. The temperature difference between the two elements is related to the process flow as higher flow rates cause increased cooling of the heated RTD. Thus, the temperature difference between the two RTDs is reduced. As with the insertion turbine meter, the thermal mass meter causes negligible pressure drops. In addition, it has no mechanical parts, high temperature range and requires little installation space. Typical flow range for the thermal mass meters is 0.3 to 30 m/s.

Annubars have been used for years on flare applications. An annubar is a differential pressure device with the signal increasing proportional to the square of the flow. Annubars are good for high flow rate applications, but are not good for low flow applications due to the small pressure difference these flows represent. For mass flow applications, annubars require pressure and temperature compensation. The characteristics of the annubar are high measurement principle, it causes a pressure drop in the pipe as it intrudes with the process flow. This again implies potentially high maintenance costs. Typical turn down ratio is 10:1. Therefore, several annubars are required in order to cover a large flow range.

Other metering types, such as positive displacement meters, vortex meters, hot-wire anemometers, coriolis mass flow meters and sonic nozzles have too limited flow range to be considered for such metering applications. In addition, some of these metering types introduce an unwanted pressure drop in the pipe.

A metering technology that has gained more and more acceptance for flow measurements of flare gas is the ultrasonic time of flight meters.

Ultrasonic Time of Flight Flow Meters for Flare Gas Applications

The technique of transit time flow metering is well known. The ultrasonic time-of-flight gas flow meter is based on measurement of ultrasonic pulses, in which the transit time of the sonic signal is measured along one or more diagonal paths in both the upstream and downstream directions. The flow of gas causes the time for the pulse traveling in the downstream direction to be shorter than for the upstream direction, and this time difference is a measure for the rate of the gas volume.

As of today, the proven technology for flare gas measurement is ultrasonic time-of-flight meters. The high turn-down ratio, the fast dynamic response, the non-intrusive design and the low maintenance costs presented by this technology substantiate this. Operating in the market including the Norwegian continental shelf, operator and governmental requirements puts high demands on the metering systems in this respect.

Future trends point in the direction of more intelligent, "Smart" systems, that can communicate with a supervisory system through well-defined digital protocols. Further, remote diagnostics via e.g. internet or satellite communication are areas to be looked into. A future scenario might be that the manufacturer himself, given the permission by the operator, could sit at his desk and remotely monitor a single installation for status check, possibly saving a costly service trip for doing the same job. This feature could be possible with "intelligent" systems provided with self-diagnostics capabilities, given by the powerful hardware and sophisticated software available today.

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