Measuring the volume of flare gas expelled from oil and gas plants is one of the most challenging types of gas flow measurement. With increased scrutiny from environmental regulators and operational emphasis on plant balance and leak detection, accurate flare measurement has become increasingly critical in developing performance requirements for new and existing facilities.
Fluenta is committed to ultrasonic technology as by far the most effective means by which to measure flare gas, but what are the other alternatives and why is ultrasonic the best technology for precise measurement of flare gas.
Flare measurement is needed for both continuous and intermittent operations. At many oil production plants there is no means by which to capture gas that is expelled as part of the oil production process. Similarly intermittent testing can be required for testing wells or equipment maintenance as well as emergency shut downs. Because of the unpredictability of gas volumes, flare measurement needs to be able to manage wide fluctuations in the velocity of gas flow as well and different atmospheric conditions and changing composition of the gas. This is a significantly challenging environment, not to mention the volatility of natural gas!
This is why the turndown ratio of a meter is an important consideration. The turndown ratio indicates the range of flow that a meter is able to measure with acceptable accuracy. If, for example, gas flow varies from 100,000m3 per day to 1,000,000 m3 per day, a meter would require a turndown ratio of at least 10 to be accurate in such an environment.
Ultrasonic measurement is one of several different techniques that is used for flare measurement. Each is outlined below.
Differential pressure (DP) devices:
These devices were used in the earliest days of flare measurement and are often deployed for transmission and distribution of gas. DP devices have a limited turndown ratio and often need a long, straight line of piping to be effective. This limits the technology’s usefulness for flare measurement.
Thermal mass measurement:
These meters use two sensors to determine flow rate: one of the sensors is constantly heated flow is measured by monitoring the cooling effects of gas on the temperature of the sensor – the faster the flow, the cooler the sensor will become. With turndown ratios of up to 600, these meters are suitable for the unpredictable nature of flare measurement but they require constant correction when gas composition changes. For this reason, thermal mass meters are rarely used for flare measurement.
Photo/optical technology:
This method of measurement uses particles in the gas stream to reflect laser beams. The volume and velocity of gas is proportionate to the time it takes particles to travel between the laser beams. With a turndown ratio of 1500, this method of measurement is flexible enough for flare monitoring but it does not work with clean gas (where there are few or no particles) and is susceptible to degradation caused by moisture and condensation.
Ultrasonic:
The most widely used measurement tool for flare 
gas is ultrasonic. This method measures the time it takes for ultrasonic waves to travel across a pipe both upstream and downstream. The difference between these two values provides accurate measurement of flow. One of the major advantages of ultrasonic measurement is it is not impacted by the composition or cleanliness of the gas flowing. Furthermore there are no mechanical parts within the monitor, which limits the requirements for maintenance and support. Finally ultrasonic technology has the highest level of turndown ratios at 3000.
The combination of these factors makes ultrasonic by far the most effective technology for flare monitoring. This is why Fluenta has had such a firm commitment to this technology. For the important task of measuring flare gas, no other technology comes close.
