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FEATURE ARTICLE
THERMAL MASS FLOW METERS MONITOR NATURAL GAS
February 2006

THERMAL MASS FLOW METERS MONITOR NATURAL GAS
AT THE PLANT, AT THE DEPARTMENT AND AT THE BURNER

By Bob Steinberg

With rising gas prices, accurately monitoring gas flow rate and consumption in industry is critically important. It is no longer sufficient to depend solely on the billing meter. By also providing a NIST traceable thermal mass flow meter at the entry of the plant in addition to the billing meter, you can track the gas usage within your facility each minute, each hour and each day, and gain critical information about your natural gas demand. Are you maximizing operating efficiencies? Are you wasting energy heating a building with all the doors open? Are you adjusting for peak usage? Are you providing your Gas Broker with an accurate assessment for your next gas allocation?

In addition, to get an even better handle on plant efficiency, thermal mass flow meters can also monitor the gas lines going into different building locations or departments. By sub-metering, you will be able to assess departmental inefficiencies, and assign costs to different operating areas. You will also have an opportunity to institute conservation measures as appropriate.

Finally, by monitoring the natural gas (or backup fuel of propane) into a furnace or burner with a thermal mass flow meter (along with the air or oxygen line), you can optimize your combustion process. Proper control of the air/fuel ratio can improve efficiency, lower fuel consumption, improve product quality and increase product yields.

Thermal mass flow meters are ideal instruments to measure natural gas at the plant for a number of reasons. They are affordable (typically $1500 to $3000), and they measure mass flow directly (SCFM, SCFH, LBS/Min, etc.) without the need for temperature and pressure corrections. This eliminates ancillary equipment (temperature and pressure gages and their associated wiring and installation). Furthermore, they are rangeable over a 100:1 turndown (i.e., their operating range is from 1% of specified full scale to 100% of full scale). In contrast, orifice plates and venturis generally have a 3:1 turndown, and at best a 5:1 turndown. Turbine meters and vortex meters typically have a 10:1 turndown. The wide turndown is useful in natural gas applications due to the large swings in demand throughout the day, and especially at off-hours when production is at its lowest capacity. Also, seasonal variations can be quite severe. On burner applications, the low-end sensitivity inherent in a thermal mass flow meter is particularly pronounced. In fact, many manufacturers can resolve flow rates as low as .1 of an SCFM in a 1” pipe (approximately 15 SFPM).

There are many other important features that are characteristic of thermal mass flow meters. They have negligible pressure drop, particularly important in the case of monitoring natural gas, so precious energy is not wasted. They always have a linear 4-20 ma output (proportional to mass flow of the gas) that is suitable for a DCS, PLC or Data Logger. They are offered with a flow rate display – many manufacturers also provide a display of total flow, and some provide a temperature readout as well.

In addition, thermal mass flow meters have no moving parts, are dirt insensitive (and cleanable if necessary), and have high accuracy and repeatability that is typically %. Furthermore they are easy to install – all manufacturers offer both in-line styles with NPT fittings or flanges for the smaller lines, and insertion styles for medium to large lines. The insertion style only requires installing a half coupling on to an existing pipe, and adding some convenient mounting hardware (that the manufacturers provide) such as a compression fitting or valve assembly. The latter permits insertion or retraction of the flow meter without interrupting the gas supply. Most manufacturers offer both integral and remote styles in either general-purpose enclosures (typically NEMA 4X) or explosion proof enclosures (Class I, Div 1, Gps B, C, and D).

Finally, some of the recent entries in the market place offer (as standard features) a convenient keypad or RS232 user-interface to access an easy-to-use menuing system. The user can reconfigure settings in the field, view average, peak or minimum flow rates, conduct diagnostics, or even select from a choice of up to four different (totally independent) calibration ranges.

Fundamentally, thermal mass flow meters operate on the principal of heat transfer. In the case of the “constant temperature method”, there are always two reference grade platinum sensors (sheathed in 316SS). One of the sensors is self-heated and the other is the reference, and measures the gas temperature. As gas flows by the heated sensor, the gas molecules carry heat away from the surface of the sensor, and the sensor cools down as it loses energy. The sensor drive circuit replenishes the lost energy by heating the flow sensor up until it is a constant temperature differential above the reference sensor. The electrical power required to maintain a constant temperature differential is directly proportional to the gas mass flow rate. Some manufactures incorporate the “constant power method” where the resulting temperature difference is measured.

In summary, thermal mass flow meters provide an affordable means to monitor natural gas at the plant, at the department or at the burner, providing the opportunity to realize improved operating efficiencies, promote conservation, and monitor and improve plant processes.

About the Author

Bob Steinberg is President of Sage Metering, Inc., headquartered in Monterey, CA. Sage Metering produces, calibrates and markets high performance thermal gas mass flow meters worldwide for Industrial and Municipal applications. Over the course of his career, Mr. Steinberg has started three successful enterprises and has over 20 years of management, sales and marketing experience in the Process Industry. In addition, he has had extensive experience managing, supporting and training industrial manufacturers’ representative organizations – specifically in the Thermal Mass Flow Meter Industry. Mr. Steinberg can be reached at 866-677-7243 or via email at bob@sagemetering.com, or visit www.sagemetering.com.


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