Thermal Mass Flow Meters Monitor And Control Flow of Process and Laboratory Specialty Gases

The ability to measure accurately ensures reliable certification, as well as savings in process flows.

Thermal mass flow meters used in laboratories, gas fill plants, and process applications, offer many advantages over other types of flow measurement devices. Historically, production and process facilities have used orifice plates, venturis, averaging pitot tubes, turbines, and vortex meters to measure the flow rate or consumption of a gas. However, the thermal mass flow meter offers a number of advantages over these in determining mass flow directly. Typical applications include the measurement of air intake flow to the compressor at air separator plants, natural gas flow at hydrogen separator plants, and of course, the measurement of specialty gases, such as nitrogen or oxygen following their production. (See table of applications on p. 33.) Operation of the thermal mass flow meter is based on the principle of conductive and convective head transfer. In essence, it senses the movement of a gas as the molecules of that gas cool a heated sensor. This principle permits extremely accurate measurements (typically one percent) that have extraordinarily low-end slightest flow of a gas.

Theory of Operation
Thermal mass flow meters are temperature compensated, experience negligible pressure drop, are resistant to contamination, and (the more modern devices) can tolerate a wider range of gas temperature fluctuations. Reference grade platinum RTD windings are incorporated as the sensors; therefore, measurements are extremely stable over time. Typical thermal mass flow meters have a 100 to 1 turndown, so a very wide range of measurement is possible. A very important feature of these instruments is that they are easy to install. Most manufacturers provide 1/2-in. or 3/4-in. diameter probes that easily can be inserted into existing pipes from 2-in. to 24-in. (For smaller diameter pipes, in-line style with NPT fittings are available.) All that is required is for a hole to be tapped, a half-coupling welded in place, and a manufacturer-provided isolation valve assembly inserted. Even a simple compression fitting can be used to install the sensor. Most manufacturers provide guidelines that indicate how far into the pipe to locate the sensor. Some manufacturers offer a variety of strategies, even if the flow profile is not ideal. (The flow profile depends on a number of factors, the most important of which is the length of unobstructed straight run prior to the flow meter.) For larger pipes (up to 60 in.), accurate measurements require multiple probes (one in each quadrant). By averaging their outputs, excellent results can be achieved. Specifically, the indicator on a thermal gas flow meter shows gas mass flow rate in SCFM, SCFH, LBS/H or other convenient measurement units (without requiring external temperature or pressure devices to do so), and the devices will also totalize the on-going gas consumption (SCF or LBS). The meter's output signal, which is proportional to flow rate, can range from 4-20 ma. This measurement can be displayed (LCD or LED, depending on the manufacturer) on the instrument, and can also be transmitted to a laptop for readout. Some manufacturers offer a fitire that permits the totalizers to be reset externally by a simple contact closure, or by utilizing menuing systems accessed by a keypad, an RS232, or a laptop. Some models include relays that can be configured to transmit a pulsed output so that a remote counter can then be used as an independent method of recording consumption.

Many Features Offered
Thermal Mass Flow Meters are available with integral or remote enclosures that are powered from either 24-volt dc or 115-volt ac, and typicaly have a price tag of less than $3,000. Some recent entries in the marketplace offer (as a standard feature) remote enclosures that contain all of the electronics (as well as a display of flow rate, temperature, and total), eliminating the need for any electronics at the sensor location. In this type of configuration, the sensor has a convenient weather-proof and explosion-proof junction box for a six-conductor cable, but no electronics. This feature is highly desirable in very hot or cold climates where electronic components can be degraded or are subject to failure. The robust NEMA 4X remote electronics enclosure can be mounted up to 2,000 feet away from the sensor, and thus can be mouted indoors, or at least in a cabinet, where the ambient temperature is moderate and does not experience severe fluctuations. Neither cable length nor cable temperature affects the meter's accuracy since the circuitry in the remote enclosure has lead-length compemsation on the sense leads, which null out any deviation from the original factory calibration. The more advanced mamufacturers of this equipment offer models that calibrate for up to four different gas mixes, if necessary. These calibrations are stored as separate ranges, or channels. (This feature is particularly useful in hydrigen/CO plants, where many different gas mixtures need to be measured.) The ranges are selectable in the field through a touch display screen keypad or laptop, or can be remotely selectable through contact closures. The more advanced equipment includes routines that check the calibration of the meter by utilizing "zero flow" as a test point, and comparing it to the original factory calibration, which is furnished upon shipment. These routines also verify whether or not a sensor is claen, and also ensure that the meter has retained its calibration. In extreme situations, where moisture is present (although it is best to eliminate excess moisture with traps, dryers and filters), splash guards are available (from some manufacturers) to minimize the effects of moisture on the flow sensor. In summary, thermal mass flow meters provide an affordable and accurate means to measure industrial and specialty gas mass flow rates and consumption, and they do not require any external temperature or pressure devices. Furthermore, their extreme low-and sensitivity (velocities as low as 10 SFPM), negligible pressure drop. high accuracy, long-term stability, and ease of installation, are important features that benefit not only the producer, but the end-user as well. - SGR

 

Sage Metering, Inc. Landfill Gas Informational Flyer
Sage Metering, Inc. Natural Gas Informational Flyer
Sage Metering, Inc. Article on Industrial Gas Efficiency

 

Bob Steinberg is President of Sage Metering, Inc., Monterey, California. (www.sagemetering.com). Sage Metering produces, calibrates, and markets high performance thermal gas mass flow meters worldwide for process applications. Mr. Steinberg has founded three successful enterprises, and has more than 25 years of management, sales, and marketing experience in the process industries. In addition, he has had extensive experience managing, supporting, and training industrial rep organizations - specifically in the thermal mass flow meter industry. Mr. Steinberg can be reached at 866 - 677-7243 or by email: bob@sagemetering.com.

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