An Overview of Gas Flow Measurement - Chemical Engineering

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Gas-measurement technologies

There are two basic types of flowmeters: liquid and gas. Liquid is primarily measured in terms of volumetric flowrate, while gas is a mass-flow measurement because of the unique properties of gases when compared to liquids. While some volumetric technologies can measure gas flowrates, there can be problems with totalized flow. Generally, the best choice is mass-flow sensing technology when measuring air or other gases &#; especially in critical applications.

Coriolis. The principle of operation for Coriolis flowmeters relies on a vibrating tube where the flow of fluid causes changes in frequency, phase shift or amplitude, which is proportional to the mass flowrate. Coriolis meters are highly accurate and are frequently used in custody transfer applications, but they are on the expensive side and require labor-intensive inline applications.

Differential pressure. Differential pressure (DP) meters and sensors come in several designs, including orifice plates, pitot tubes and Venturis. The typical DP meter designs require the fluid to move through or past two points of reference, creating a differential pressure rate that is equivalent to the rate of flow using the Bernoulli equation with some modifications. If the gas is dirty, there can be orifice clogging issues that require frequent maintenance in order to maintain accuracy.

Ultrasonic. Flowmeters designed with ultrasonic flow-sensing technology rely on ultrasound and the Doppler effect to measure volumetric flowrate. In ultrasonic flowmeters, a transducer emits a beam of ultrasound to a receiving transducer. The transmitted frequency of the beam is altered linearly by particles or bubbles in the fluid stream. The shift in frequencies between the transmitter and receiver can be used to generate a signal proportional to the flowrate.

Optical. Flowmeters designed with optical sensing rely on laser technology and photo detectors. This technology requires the presence of particles in the gas stream. These particles scatter the light beam, and the time it takes for these particles to travel from one laser beam to the other laser beam can be used to calculate the gas velocity and volumetric flowrate. These meters have good accuracy and wide turndown, but are traditionally expensive.

Thermal dispersion. Flowmeters with thermal-dispersion sensors provide direct mass-flow measurement. Two thermowell-protected platinum resistance temperature detector (RTD) sensors are placed in the process stream. One RTD is heated while the other senses the actual process temperature. The temperature difference between these sensors generates a voltage output, which is proportional to the media cooling effect. This information can be used to measure the gas mass flowrate without the need for additional pressure or temperature transmitters.

In measuring flow accurately, second only to selecting the proper flow sensing technique is the method of calibration. There are two methods used in calibrating gas flowmeters, as follows:The direct method, where the meter is calibrated to a specific pure process gas or to the actual components of a mixed gas in use.

The air equivalency method, where the meter is calibrated using air, and then the calibration is adjusted with a pre-defined correction factor.

It is important to ask your supplier about the method of flowmeter calibration. Users should know if manufacturers contract out for calibration and if so, with whom, or if they operate their own calibration laboratory with direct-method calibration test stands and equipment that is traceable in accordance with NIST and ISO/IEC standards.

gas meter, device for measuring the quantity or rate of flow of a gas. Types of gas meters (by operating principles) include displacement, velocity, head, thermal, acoustic, and tracer.

An example of the displacement principle is the bellows-and-diaphragm gas meter (shown in the diagram). This type is widely used in commercial and domestic gas service to measure the quantity of gas delivered to a user. Bellows gas meters measure the quantity of gas passing through them by filling and emptying, in a regular sequence, one or more internal chambers of known capacity. Counting the times each chamber is filled and emptied gives the volume of gas delivered.

In velocity-type gas meters the gas flow moves impeller blades on a rotor. Rotation of the rotor is geared to a dial mechanism that records gas volume delivered. In the velocity vortex meter the rotor is mounted in an offset chamber in a short section of the flow pipe. Only a portion of the total gas flow is fed into this chamber, and total quantity measurements are based on recorded rotor movement. Velocity gas meters also include anemometer-type meters in which rotating cups or vanes either power a small generator or drive a series of recording registers.

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Head-type gas meters measure quantity of gas per unit time. The measurement is based on a deliberately produced pressure drop, or head, between two nearby points in a tube in the meter. This pressure differential can be converted to a flow rate. Devices used to produce the pressure head include orifice plates, venturi tubes, flow nozzles, and pitot tubes.

In thermal-type gas meters a heater (such as an electrical heating coil) is placed in the gas stream, and thermometers are installed on its upstream and downstream sides. Gas flow is measured in relation to either the temperature rise of the gas stream or the amount of electrical energy that is necessary to keep the heater at a constant temperature.

Acoustic gas meters measure the rate of gas flow by comparing the frequency shifts of two initially identical signals (one sent upstream, the other downstream) after they are reflected.

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A tracer-type gas meter measures flow rate by timing the passage of an injected radioactive material between two fixed detectors.

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