4 Gas Flow Meter Types for Measurement

Natural Gas Flow Meter Types

There are four natural gas meter types often used for flow measurement. They are mass flow meters, velocity flow meters, differential pressure, and PD meters.

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What is a flow meter?

A flow meter is a precision instrument that measures a pipe&#;s gas flow rate (or liquid flow). While there are four main meter styles for flow measurement, here are three characteristics to determine flow:

1. Positive displacement meters collect a fixed fluid volume, then release and refill the gas or liquid. Then, tally how many times the capacity fills to determine the flow.
2. When measuring the rate of fluid over a known area, one can determine the flow.
3. Other methods depend on the flowing stream&#;s forces as it overpowers a known constriction and indirectly calculates flow.

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Actual Flow vs Standard Flow

Since gas is compressible accurate gas flow measurement is difficult. As the temperature increases, the gas molecules move further apart. Conversely, as the pressure increases, the gas molecules move closer together.

Most gas flow meters (differential pressure, turbine, positive displacement, vortex shedding) measure the gas flow at the actual operating conditions. This flow rate is ACFM (actual cubic feet per minute). However, it is more important to adjust or correct the flow rate for a particular pressure and temperature. This adjusted flow rate is often called STP (standard pressure and temperature) and is usually in units of SCFM (standard cubic feet per minute).

For this reason, most gas flow meters require pressure and temperature correction to convert the flow rate from operating conditions (ACFM) to standard conditions (SCFM).

Flowmeter Styles

1. Direct Mass Flow

Mass flow meters determine mass flow passing through the meter. Two types deserve mention here:

Coriolis flowmeters provide a direct mass flow measurement based upon the fluid&#;s deflection force moving through a vibrating tube. These meters are highly accurate with high turndown capabilities and are independent of fluid properties. They are also costly to purchase and install and unsuitable for larger pipe sizes.

Thermal mass flow meters measure the mass flow based on heat transfer from a heated element. The gas molecules create the heat transfer; the greater the number of gas molecules in contact with the heated surface, the greater the heat transfer. This flow measurement method depends only on the number of gas molecules and is independent of the gas pressure and temperature; therefore, additional pressure and temperature equipment are unnecessary. They also provide excellent accuracy and repeatability and are easy to install.

2. Velocity

In a velocity meter type, the rate of the medium passing through the meter determines the measurement.

Turbine flow meters measure volumetric flow based on fluid flowing past a free-spinning rotor, each revolution corresponding to a specific volume of gas or liquid. The meters have high turndown and accuracy. Unfortunately, because of the meter&#;s moving parts, its use is limited to clean dry gases only in gas applications, and pressure and temperature compensation are required.

Ultrasonic flow meters measure the difference in pulses transit time that travels from a downstream transducer to the upstream sensor, compared to the upstream transducer back to the downstream transducer. This meter style is highly accurate but very expensive, and pressure and temperature measurements are required.

The vortex gas flow meter has a shedder bar (an obstruction) in the flow path, causing the fluid to flow around the shedder bar and creating vortices on the backside of the bar. The frequency of vortex generation is a function of the gas velocity. Fluid velocity is determined based on the principle known as the Kármán effect. The frequency of vortex shedding is independent of the fluid composition. The meter requires temperature and pressure compensation and needs a minimum flow rate to produce vortices.

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3. Differential Pressure Meters

Differential pressure flow meters calculate flow by measuring pressure drop over an obstruction inserted in the flow path. Common types of flow elements are orifice plates, flow nozzles, venturi tubes, and averaging pitot tubes.

The orifice plate is a differential pressure meter frequently used for natural gas measurement. It measures volumetric flow, not mass flow. This meter&#;s limitations include reduced low flow sensitivity, limited turndown, and a pressure drop, impacting operating costs. Additionally, it requires temperature and pressure correction to achieve mass flow since it&#;s a volumetric flow meter.

An averaging pitot tube is a differential-pressure flow measurement device commonly used for combustion air measurement. The device has limitations with measuring gas flow, especially low-flow sensitivity and turndown. The measure is contingent upon achieving velocity pressure, and if the current is too low, the user may not obtain adequate signals.

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4. Positive Displacement Gas Flow Meter

Positive displacement meters require fluid to displace components mechanically and measure volumetric flow at the operating temperature and pressure. While they have sufficient accuracy, pressure and temperature compensation are needed to achieve mass flow, and since they have moving parts, the user must consider gas cleanliness. A PD meter may be called a PD flow meter or a volumetric flow meter. An example of a PD meter is the diaphragm meter.

Why is measuring mass flow important?

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Flow measurement is a critical aspect of operations involving pumps, valves and other industrial systems. Users choosing flow meters to measure the flow of liquids or gases must consider key factors to make the right decision. There are significant differences between meter designs, with each type of device having its own pros and cons.

Evaluating Meter Technologies

Flow meters are excellent tools to measure, monitor and control the distribution of a host of fluids. There is the question of which technology to use because a wide variety of meter configurations are available and each must be properly deployed to achieve optimal performance. Coriolis: Coriolis flow meters directly measure fluid mass over a wide range of temperatures with very high accuracy. Their unobstructed, open flow design is suitable for viscous, non-conductive fluids. With no internal moving parts, Coriolis meters require minimal attention once installed (see Image 1). Image 1. Coriolis flow meters are designed to directly measure fluid mass over a wide range of temperatures with very high accuracy. (Images courtesy or Badger Meter)

Image 1. Coriolis flow meters are designed to directly measure fluid mass over a wide range of temperatures with very high accuracy. (Images courtesy or Badger Meter)

Differential pressure: Differential pressure (DP) meters employ a proven, well-understood measuring technology that does not require moving parts in the flow stream. DP meters are not greatly affected by viscosity changes. Electromagnetic: Electromagnetic meters measure virtually any conductive fluid or slurry. They provide low pressure drop, high accuracy, high turndown ratio and excellent repeatability. The meters have no moving parts or flow obstructions, and are relatively unaffected by viscosity, temperature and pressure when correctly specified. Positive displacement: Positive displacement meters are highly accurate (especially at low flows) and have large turndown ratios. These devices have only one or two moving parts, making them easy to maintain. There is no need for straight pipe lengths as with other metering approaches. Thermal mass: Thermal mass meters carry a relatively low purchase price. They are designed to work with clean gases of known heat capacity, as well as some low-pressure gases not dense enough for Coriolis meters to measure. Turbine: Turbine meters are known for high accuracy, wide turndown and repeatable measurements. They produce a high-resolution pulse rate output signal proportional to fluid velocity and, hence, to volumetric flow rate. Turbine meters are limited to use with only clean fluids, and they require periodic recalibration and service. Impeller: Impeller meters are frequently used in large diameter water distribution systems. Their attributes include direct volumetric flow measurement (often with visual indication); universal mounting; fast response with good repeatability; and relatively low cost. Their performance suffers in applications with low fluid velocity. Ultrasonic: Ultrasonic flow meters have no moving or wetted parts, suffer no pressure loss, offer a large turndown ratio, and provide maintenance-free operation. Clamp-on ultrasonic meters can be used for troubleshooting a wide range of flow issues (see Image 2). Image 2. Clamp-on ultrasonic flow meters can be used for troubleshooting a wide range of flow issues, from verifying the reading of another meter to monitoring flow systems over an extended time period.

Image 2. Clamp-on ultrasonic flow meters can be used for troubleshooting a wide range of flow issues, from verifying the reading of another meter to monitoring flow systems over an extended time period.

Variable area: Simple, inexpensive and reliable, variable area meters provide practical flow measurement solutions for many applications. Be advised most of these meters must be mounted perfectly vertical. They also need to be calibrated for viscous liquids and compressed gases. Vortex: Vortex meters have no moving parts that are subject to wear, so regular maintenance is not necessary. They can only measure clean liquids. Vortex meters may introduce pressure drop due to obstructions in the flow path. Oval gear: The latest breed of oval gear meters directly measures actual volume. It features a wide flow range, minimal pressure drop and extended viscosity range. This design offers easy installation and high accuracy, and measures high temperature, viscous and caustic liquids with simple calibration. Nutating disc: Nutating disc meters have a reputation for high accuracy and repeatability, but viscosities below their designated threshold adversely affect performance. Meters made with aluminum or bronze discs can meter hot oil and chemicals.

Identifying Selection Criteria

In a typical industrial facility, fluid characteristics, flow profile, flow range and accuracy requirements are important for determining the best flow meter for a particular measurement task. Additional considerations such as mechanical restrictions and output-connectivity options impact the user&#;s choice. For plant operations, the key factors in meter selection include: Process media: Fluids are conventionally classified as either liquids or gases. The most important difference between these two types of fluid lies in their relative compressibility (i.e., gases can be compressed much more easily than liquids). Consequently, any change that involves significant pressure variations is generally accompanied by much larger changes in mass density in the case of a gas than a liquid. Type of measurement: Industrial flow measurements fall under one of two categories: mass or volumetric. Volumetric flow rate is the volume of fluid passing through a given volume per unit time. Mass flow rate is the movement of mass per time. It can be calculated from the density of the liquid (or gas), its velocity and the cross-sectional area of flow. Flow rate information: A crucial aspect of flow meter selection is determining whether flow rate data should be continuous or totalized. A flow rate has to do with the quantity of a gas or liquid moving through a pipe or channel within a given or standard period of time. Desired accuracy: Flow meter accuracy is specified in percentage of actual reading (AR), percentage of calibrated span (CS) or percentage of full-scale (FS) units. It is normally stated at minimum, normal and maximum flow rates. A clear understanding of these requirements is needed for a meter&#;s performance to be acceptable over its full range. Application environment: Users must decide whether the low- or high-flow range is most important. This information will help in sizing the correct instrument for the job. Pressure, temperature, density and viscosity conditions are equally important parameters. Fluid characteristics: Users should be cautious that the selected flow meter is compatible with the applicable fluid and conditions. Thick and coarse materials can clog or damage internal meter components, hindering accuracy and resulting in frequent downtime and repair. Installation requirements: Planning a flow meter installation starts with knowing the line size, pipe direction, material of construction and flange-pressure rating. Complications due to equipment accessibility, valves, regulators and available straight-pipe run lengths should also be identified. Power availability: Today&#;s installations normally call for intrinsically safe instruments, which are &#;current limited&#; by safety barriers to eliminate a potential spark. Another option is to employ fiber optics. Necessary approvals: Approvals for the use of flow measurement equipment in hazardous plant locations include FM Class 1 Division 1, Groups A, B, C and D; and FM Class 1, Zone 1 AEx d (ia) ia/IIC/T3-T6. Standards such as the Measuring Instruments Directive (MID) in the European Union (EU) apply to fiscal and custody transfer metering for liquids and gases. Output/indication: Flow meter users must decide whether measurement data is needed locally or remotely. For remote indication, the transmission can be analog, digital, or shared. The choice of a digital communications protocol such as HART, FOUNDATION Fieldbus or Modbus also figures into this decision. Choosing the right flow measurement solution can have a major impact on operational and business performance. Companies anticipating a flow meter purchase should consult with a knowledgeable instrumentation supplier in the early stages of a project. This will ensure a successful application once the equipment is installed.

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Flow meters are excellent tools to measure, monitor and control the distribution of a host of fluids. There is the question of which technology to use because a wide variety of meter configurations are available and each must be properly deployed to achieve optimal performance.Coriolis flow meters directly measure fluid mass over a wide range of temperatures with very high accuracy. Their unobstructed, open flow design is suitable for viscous, non-conductive fluids. With no internal moving parts, Coriolis meters require minimal attention once installed (see Image 1).Differential pressure (DP) meters employ a proven, well-understood measuring technology that does not require moving parts in the flow stream. DP meters are not greatly affected by viscosity changes.Electromagnetic meters measure virtually any conductive fluid or slurry. They provide low pressure drop, high accuracy, high turndown ratio and excellent repeatability. The meters have no moving parts or flow obstructions, and are relatively unaffected by viscosity, temperature and pressure when correctly specified.Positive displacement meters are highly accurate (especially at low flows) and have large turndown ratios. These devices have only one or two moving parts, making them easy to maintain. There is no need for straight pipe lengths as with other metering approaches.Thermal mass meters carry a relatively low purchase price. They are designed to work with clean gases of known heat capacity, as well as some low-pressure gases not dense enough for Coriolis meters to measure.Turbine meters are known for high accuracy, wide turndown and repeatable measurements. They produce a high-resolution pulse rate output signal proportional to fluid velocity and, hence, to volumetric flow rate. Turbine meters are limited to use with only clean fluids, and they require periodic recalibration and service.Impeller meters are frequently used in large diameter water distribution systems. Their attributes include direct volumetric flow measurement (often with visual indication); universal mounting; fast response with good repeatability; and relatively low cost. Their performance suffers in applications with low fluid velocity.Ultrasonic flow meters have no moving or wetted parts, suffer no pressure loss, offer a large turndown ratio, and provide maintenance-free operation. Clamp-on ultrasonic meters can be used for troubleshooting a wide range of flow issues (see Image 2).Simple, inexpensive and reliable, variable area meters provide practical flow measurement solutions for many applications. Be advised most of these meters must be mounted perfectly vertical. They also need to be calibrated for viscous liquids and compressed gases.Vortex meters have no moving parts that are subject to wear, so regular maintenance is not necessary. They can only measure clean liquids. Vortex meters may introduce pressure drop due to obstructions in the flow path.The latest breed of oval gear meters directly measures actual volume. It features a wide flow range, minimal pressure drop and extended viscosity range. This design offers easy installation and high accuracy, and measures high temperature, viscous and caustic liquids with simple calibration.Nutating disc meters have a reputation for high accuracy and repeatability, but viscosities below their designated threshold adversely affect performance. Meters made with aluminum or bronze discs can meter hot oil and chemicals.In a typical industrial facility, fluid characteristics, flow profile, flow range and accuracy requirements are important for determining the best flow meter for a particular measurement task. Additional considerations such as mechanical restrictions and output-connectivity options impact the user&#;s choice. For plant operations, the key factors in meter selection include:Fluids are conventionally classified as either liquids or gases. The most important difference between these two types of fluid lies in their relative compressibility (i.e., gases can be compressed much more easily than liquids). Consequently, any change that involves significant pressure variations is generally accompanied by much larger changes in mass density in the case of a gas than a liquid.Industrial flow measurements fall under one of two categories: mass or volumetric. Volumetric flow rate is the volume of fluid passing through a given volume per unit time. Mass flow rate is the movement of mass per time. It can be calculated from the density of the liquid (or gas), its velocity and the cross-sectional area of flow.A crucial aspect of flow meter selection is determining whether flow rate data should be continuous or totalized. A flow rate has to do with the quantity of a gas or liquid moving through a pipe or channel within a given or standard period of time.Flow meter accuracy is specified in percentage of actual reading (AR), percentage of calibrated span (CS) or percentage of full-scale (FS) units. It is normally stated at minimum, normal and maximum flow rates. A clear understanding of these requirements is needed for a meter&#;s performance to be acceptable over its full range.Users must decide whether the low- or high-flow range is most important. This information will help in sizing the correct instrument for the job. Pressure, temperature, density and viscosity conditions are equally important parameters.Users should be cautious that the selected flow meter is compatible with the applicable fluid and conditions. Thick and coarse materials can clog or damage internal meter components, hindering accuracy and resulting in frequent downtime and repair.Planning a flow meter installation starts with knowing the line size, pipe direction, material of construction and flange-pressure rating. Complications due to equipment accessibility, valves, regulators and available straight-pipe run lengths should also be identified.Today&#;s installations normally call for intrinsically safe instruments, which are &#;current limited&#; by safety barriers to eliminate a potential spark. Another option is to employ fiber optics.Approvals for the use of flow measurement equipment in hazardous plant locations include FM Class 1 Division 1, Groups A, B, C and D; and FM Class 1, Zone 1 AEx d (ia) ia/IIC/T3-T6. Standards such as the Measuring Instruments Directive (MID) in the European Union (EU) apply to fiscal and custody transfer metering for liquids and gases.Flow meter users must decide whether measurement data is needed locally or remotely. For remote indication, the transmission can be analog, digital, or shared. The choice of a digital communications protocol such as HART, FOUNDATION Fieldbus or Modbus also figures into this decision. Choosing the right flow measurement solution can have a major impact on operational and business performance. Companies anticipating a flow meter purchase should consult with a knowledgeable instrumentation supplier in the early stages of a project. This will ensure a successful application once the equipment is installed.

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