A Flow Measurement Technology for Gas Measurement

Dec. 21, 2013

Burning off waste gases through the use of a vent or flare system is common in the hydrocarbon industry like oil & gas, chemical, refining, natural gas plants, landfills, and even environmental waste treatment sites. Whether these vent or flare systems are for burning off excess waste gas, or exhausting a surplus of gas into the atmosphere, it’s now all a part of the environment and reducing these volatile organic carbons (VOC’s). In order to make sure that these venting systems are operating properly and according to the mandated guidelines for emissions as set forth by the Environmental Protection Agency (EPA), it is required to have a flow metering system monitoring and measuring that vented gaseous emission. The measurement of this bio-gas or VOC gas is not a straight forward measurement and typically presents problems for typical flow measurement technologies. Some of the adversities encountered in these flow measurement applications of gas are wide flow ranges with variation, varying gas composition, and area classifications requiring hazardous approvals on the instrumentation. One flow technology that presents itself though as a viable and befitting solution for this measurement of vented or flared gas is thermal mass flow.

Now even though thermal mass flow technology is extremely suitable for this measurement of vented bio-gas or VOC’s, the calibration of the specific meter for this application is absolutely required. It is not just the case of calibrating the thermal flow meter on air and then applying a correction K factor to complete the calibration process. The gas mixture must be used in the calibration because various gases have varying and different heat transfer properties, and since this technology operates on the premise of heat transfer which is correlated to the mass flow, it is important to do a proper gas mix calibration. Also, with VOC’s from vented flares, many of the oil production sites have gases that remain fairly consistent, but that can also vary depending upon the shale or gas formation where the gas is being drawn from, and also the season as to when the formation has extraction taking place. Either way, the calibration must be performed with an as close as possible component gas mixture that resembles the customers vent gas.

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Another requirement for the thermal mass flow meter that is vital to measurement success on the vented gas is the ability of the device to measure very low flow rates under normal and then extreme conditions of high flows during chaotic emergency conditions. It is during the upset conditions that require a high performing flow meter to respond to changes quickly. For accuracy throughout the entire range of the application and to meet EPA regulations, most meters must have at least a +5% or better accuracy statement, and this is found in the EPA’s 40 CFR Part 98. The calibration accuracy does change depending upon the measurement requirements as stated by the EPA, so make sure that you understand that requirement prior to reviewing the purchase of a mass flow meter.

In some applications like the vented flares at oil production pads or refineries, an agency approval rating of Class I, Division 2 must be certified for the equipment measuring the flow rate as the presence of combustible and flammable gases are present in the area. The approval should not rely on an enclosure-only classification, but rather the entire flow meter.

Keep in mind that the mounting requirements of some flow meter technologies require long straight runs of pipe with ideal flow profiles and no obstructions in the path of the flowing media. Many of these oil production pad sites have limited space, so long runs are not feasible. These restrictions can eliminate the use of many technologies and leave the availability for thermal mass flow meter technology even more. Always remember when looking at possible flow technology for this gas measurement that the size opening of your pipe may require that same size spool piece of a flow meter, which can be quite expensive in upfront cost and it may take a bit of extra mounting support to accommodate such a flow meter installation.

For these flow measurements on flare gas systems, the flare gas is typically a surplus gas or vapor which is typically burned through a gas flare, or more often known as a flare stack. Flare stacks and bio-gas venting is the process of burning away the combustible gas or flare gas. A production gas flare is a combustion stack used to burn off excess C1 through C6 hydrocarbons that cannot be processed. These unprocessed hydrocarbons must be burned off from a safety standpoint as excess long chain carbon molecules can pose a high risk of combustion in the atmosphere.

Historically, the process of vented flaring has been to burn off excess combustible gases which relieves high pressure building within a flare system. Going back to years when emissions venting was not a concern by the environmental regulatory agencies, it was common practice and widely accepted to perform vent flaring of gases into the atmosphere. Now with the proven studies being done and the indication that gaseous emissions into the air that we breathe is linked to dangerous health risks and attributed to changes in global warming, the EPA has cracked down and mandatory monitoring and reporting of these fugitive emissions must be done now. It is interesting to note that many companies are recycling their waste gases and capturing it for a renewable energy source instead of just destroying it with burning. Recycling of the waste gases has become a preferred method to adherence to EPA mandates, and it brings the world to a closer and better air space. Additionally, this recycling practice greatly reduces the carbon dioxide footprint and it benefits the energy source for those companies.

With the direction of many companies reusing their waste gas now and utilizing it as an onsite energy source, the state and local as well as federal environmental agencies are tightening up on the flare gas and vented gas usage. Restrictions from these environmental factions are becoming more and more stringent as requirements for operators at these companies to monitor the flare gas flow at several points throughout the process amongst a tangled array of piping that includes the stack itself. The thermal mass flow meter is a technology that provides the end user with an understanding of the gas flowing to the flare. This knowledge of mass flow rate of that gas source offers information of possible upset conditions within the flare off system.

Many operations and applications where waste gas is flared to atmosphere is still prevalent in the industrial and even environmental sector. Vent and flare stacks are seen at oil and gas wells, refineries, well drilling rigs, natural gas plants, wastewater treatment plants, chemical plants and landfills.

There are tight regulations with the reporting of Greenhouse Gas (GHG), which is mandated under EPA rule 40 CFR 98, that requires operations to measure, record and report the amount of flare gas emitted to the atmosphere. If a production facility like a landfill is seeking carbon credits, it is even more crucial and required to monitor and recording the flow of the vented gas. This carbon credit program requires each facility to measure and report their emissions each year, and give up one allowance for every metric ton of CO2, or the equivalent amount of nitrous oxide (N2O) and per fluorocarbons (PFCs) they emit. If the facility gives off less than their allowance, they can sell their credits, otherwise they may buy credits from other installations.

As it pertains to measuring flare gas with a known gas composition with no water vapor condensing, thermal mass flow technology is a great solution for flare gas metering. A thermal mass flow meter with a wide turndown ratio offers the ability to measure from very low velocities to extremely high velocities that accounts for wide flow swings during abnormal unpredictable application conditions. There is some tolerance of moisture content within the gas, but typically anything more than 3 to 4% cannot be tolerated. It is also a well-known fact that thermal mass flow meters provide a very quick response to flow changes, any low pressure losses, and can provide accuracies down to +0.5 of full scale. The cost for such a system is also quite attractive, and the operating cost is minimal. Many other technologies suitable for the measurement of flare gas are either much more costly, and can be unpredictable with changing flow profiles and other adversities in the media. It is not atypical to spend as much as $20,000 or more for other technology solutions.

Thermal Instrument Company’s line of mass flow meters are offered in either an in-line flow tube section or an insertion style probe for installation versatility. We are committed to high performance with our flow designs and all meters are calibrated prior to shipment to the meters maximum flow capability and then to the customer’s specific application conditions and media. Calibrating against a known NIST traceable high accuracy flow standard in our elaborate flow laboratory is the best method for calibrating any flow meter. Once the flow meter is installed in the customer application, Thermal Instrument Company can provide re-calibration services from a field service engineer at your site for a zero and full span flow calibration check with a NIST traceable calibration instrument pack. The other re-calibration alternative is to obtain one of our loaner devices, and return your meter to our calibration laboratory for a complete re-calibration and servicing that is returned with a NIST Certificate of Calibration.

Key Applications for Thermal Mass Flow Technology

Biogas and Landfill Gas

Biogas contains a mixture of methane, carbon dioxide, and minor quantities of other constituents. These gaseous emissions can be either vented to the atmosphere or they can be flared to exhaustion. Some companies will use this emission bio-gas as energy source to generate electricity, or they will earn money by selling the excess off to local industries for a fuel source. If the local or state agency allows it, these companies may choose to vent the emission bio-gas to a flare. With greenhouse gas emissions reduction programs in place, flaring the gas provides an opportunity to lower emissions. The bio-gas composition varies from industry to industry. The gas from wastewater treatment plant digesters is almost 65% methane concentration to 35% carbon dioxide, while landfills are a 50/50 mixture of methane to carbon dioxide. Flaring these gases off are what is referred to as methane deletion. There is carbon dioxide formed from this flaring process of methane, however, the destruction of methane is far more important as it is much more toxic and potent than carbon dioxide. This reduction of methane is an integral compliance action being taken by many companies.

Managing municipal solid waste is more than landfilling: publicity, education, engineering, long-term planning, and landfill gas waste-to-energy are specialties needed in today’s complex environment. We’ve created a handy infographic featuring 6 tips to improve landfill management and achieve excellence in operations. 6 Tips for Excellence in Landfill Operations. Download it now!  

Another important area where thermal mass flow meters can be applied are on tank venting where the local and state regulatory agencies require the emissions to be monitored and reported.

An accurate, reliable measurement of the vented vapors using thermal mass flow meter technology provides a good solid state method to quantify emissions. These flashing emissions can happen when a gas or a liquid has a sudden temperature increase or a pressure drop occurs. It is common for this flashing to happen at tank locations, pipelines, and gas transmission compressor stations.

Summary With vented and flared systems being used more and more to burn off waste gas from industrial production sites, the measurement of flow rate is vital to accommodating the regulatory agencies compliance rules. Flaring systems are used extensively in natural gas drilling, production and transmission, oil drilling and refineries, chemical processing plants as well as wastewater treatment, landfills and farming operations. The Thermal Instrument Company’s line of thermal mass flow meters offers a reliable solution to these required monitoring and reporting activities, and brings companies into compliance for their reporting requirements.
About the Author

Jerry Boisvert

Jerry Boisvert is with the Thermal Instrument Company.

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