Definitions

orifice meter

Gas meter

A gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consumes fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly affected by temperature and pressure. Gas meters measure a defined volume, regardless of the pressurized quantity or quality of the gas flowing through the meter. Temperature, pressure and heating value compensation must be made to measure actual amount and value of gas moving through a meter.

Several different designs of gas meters are in common use, depending on the volumetric flow rate of gas to be measured, the range of flows anticipated, the type of gas being measured and other factors.

Types of gas meters

Diaphragm/bellows meters

These are the most common type of gas meter, seen in almost all residential and small commercial installations. Within the meter there are two or more chambers formed by movable diaphragms. With the gas flow directed by internal valves, the chambers alternately fill and expel gas, producing a near continuous flow through the meter. As the diaphragms expand and contract, levers connected to cranks convert the linear motion of the diaphragms into rotary motion of a crank shaft which serves as the primary flow element. This shaft can drive an odometer-like counter mechanism or it can produce electrical pulses for a flow computer.

Diaphragm gas meters are positive displacement meters.

Major manufacturers of diaphragm meters in the US include Actaris, Elster American Meter, and Sensus.

Rotary meters

Rotary meters are highly machined precision instruments capable of handling higher volumes and pressures than diaphragm meters. Within the meter, two figure "8" shaped lobes, the rotors (also known as impellers or pistons), spin in precise alignment. With each turn, they move a specific quantity of gas through the meter. The operating principle is similar to that of a Roots blower. The rotational movement of the crank shaft serves as a primary flow element and may produce electrical pulses for a flow computer or may drive an odometer-like counter.

Major manufactures include Elster American Meter, Dresser Roots, and Romet Limited.

Turbine meters

Turbine gas meters infer gas volume by determining the speed of the gas moving through the meter. Because the volume of gas is inferred by the flow, it is important that flow conditions are good. A small internal turbine measures the speed of the gas, which is transmitted mechanically to a mechanical or electronic counter. These meters do not impede the flow of gas, but are limited at lower flow rates.

Major Manufacturers include: Elster American Meter, Elster Instromet, and Sensus

Orifice meters

An orifice gas meter consists of a straight length of pipe inside which an orifice plate has been installed. An orifice plate is simply a thin round metal sheet with a hole of a precisely known diameter bored into its centre. Orifice meters are a type of differential meter, all of which infer the rate of gas flow by measuring the pressure difference across a deliberately designed and installed flow disturbance. The gas static pressure and its temperature must be measured in addition to the differential pressure. Orifice meters are less accurate than other measurement methods and they do not handle a large range of flow rates. They are however well accepted and understood in industrial applications since they are easy to field-service and have no moving parts.

Ultrasonic flow meters

Ultrasonic flow meters are much more complex than meters that are purely mechanical, as they require significant signal processing and computation capabilities. Ultrasonic meters measure the speed of gas movement by measuring the speed at which sound travels in the gaseous medium within the pipe. AGA Report No. 9 covers the proper usage and installation of these meters, and it specifies a standardised speed-of-sound calculation which predicts the speed of sound in a gas with a known pressure, temperature and gas composition.

The most elaborate types of ultrasonic flow meters average speed of sound over a total of four paths in the pipe. The length of each path is precisely measured in the factory. Each path consists of an ultrasonic transducer at one end and a sensor at the other. The meter creates a 'ping' with the transducer and measures the time elapsed before the sensor receives the sonic pulse. Two of these paths point upstream so that the sum of the times of flight of the sonic pulses can be divided by the sum of the flight lengths to provide an average speed of sound in the upstream direction. This speed differs by the speed of sound in the gas by the velocity at which the gas is moving in the pipe. The other two paths are identical except that the sound pulses travel downstream. The meter then halves the difference between the upstream and downstream speeds to calculate the velocity of gas flow.

Ultrasonic meters are high-cost and must have no liquids present at all in the measured gas, so they are primarily used in high-flow, high-pressure applications such as utility pipeline meter stations, where the gas is always dry and lean, and where small proportional inaccuracies are intolerable due to the large amount of money at stake. The turndown ratio of an ultrasonic meter is probably the largest of any natural gas meter type, and the accuracy and rangeability of a high-quality ultrasonic meter is actually greater than that of the turbine meters against which they are proven.

Inexpensive varieties of ultrasonic meters are available as clamp-on flow meters, which can be used to measure flow in any diameter of pipe without intrusive modification. Such devices are based on two types of technology: (1) time of flight or transit time; and (2) cross correlation. Both technologies involve transducers that are simply clamped on to the pipe and programmed with the pipe size and schedule and can be used to calculate flow. Such meters can be used to measure almost any dry gas including natural gas, nitrogen, compressed air and also steam. Clamp-on meters are available for measuring liquid flow as well.

Coriolis meters

A coriolis meter is a pipe with a laterally displaced section that is excited to vibrate at its resonant frequency. Coriolis meters are often used with liquids but can be used with natural gas as well. When the fluid within the displaced section is at rest, both the upstream and downstream elbows of the laterally displaced section will vibrate in phase with each other. The frequency of this vibration is determined by the overall density of the pipe (including its contents). This allows the meter to measure the flowing density of the gas in real time. Once the gas begins to flow, however, the coriolis effect comes into play. This effect implies a relationship between the phase difference in the vibration of the upstream and downstream pipe elbows and the mass flow rate of the fluid contained by the pipe.

Again, owing to the amount of inference, analog control and calculation intrinsic to a coriolis meter, the meter is not complete with just its physical components. There are actuation, sensing, electronic and computational elements that must be present for the meter to function.

Coriolis meters can handle a wide range of flow rates and have the unique ability to output mass flow. Since they measure flowing density, coriolis meters can also infer gas flow rate at flowing conditions.

AGA Report No. 11 provides guidelines for obtaining good results when measuring natural gas with a coriolis meter.

Heating value

The volume of gas flow provide by a gas meter is just that, a reading of volume. Gas volume does not take into account the quality of the gas, the amount of heat available when burned. Utility customers are billed according to the heat available in the gas. The quality of the gas is measured and adjust for in each billing cycle. This is known by several names as the calorific value, heating value, or therm value.

The calorific value of natural gas can be obtained using a process gas chromatograph, which measures the amount of each constituent of the gas, namely:

Additionally, to convert from volume to thermal energy, the pressure and temperature of the gas must be taken into consideration. Pressure is generally not a problem; the meter is simply installed immediately downstream of a pressure regulator and is calibrated to read accurately at that pressure. Pressure compensation then occurs in the utility's billing system. Varying temperature cannot be handled as easily, but some meters are designed with built-in temperature compensation to keep them reasonably accurate over their designed temperature range. Others are corrected for temperature electronically.

Indicating devices

Any type of gas meter can be obtained with a wide variety of indicators. The most common are indicators that use multiple clock hands (pointer style) or digital readouts similar to an odometer, but remote readouts of various types are also becoming popular - see automatic meter reading and smart meter.

Remote Readouts

Remote reading is becoming popular for gas meters. It is often done through an electronic pulse output mounted on the meter. There are different styles available but most common is a contact closure switch.

Flow measurement calculations

Turbine, rotary and diaphragm meters can be compensated using a calculation specified in AGA Report No. 7. This standardised calculation compensates the quantity of volume as measured to quantity of volume at a set of base conditions. The AGA 7 calculation itself is a simple ratio and is, in essence, a density correction approach to translating the volume or rate of gas at flowing conditions to a volume or rate at base conditions.

Orifice meters are a very commonly used type of meter, and because of their widespread use, the characteristics of gas flow through an orifice meter have been closely studied. AGA Report No. 3 deals with a broad range of issues relating to orifice metering of natural gas, and it specifies an algorithm for calculating natural gas flow rates based on the differential pressure, static pressure and temperature of a gas with a known composition.

These calculations depend in part on the ideal gas law and also require a gas compressibility calculation in order to account for the fact that real gases are not ideal. A very commonly used gas compressibility calculation is AGA Report No. 8, detail characterization.

See also

References

Sizing Chart for Roots Rotary Meters

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