Gas Meter Types
In this post, we consider gas meter types often used for flow measurement. I have categorized them into four main groups, mass flow meters, the velocity flow meter, differential pressure, and PD meters.
What is a flow meter?
A flow meter is a precision instrument that measures the rate of gas flow or (liquid flow) in a pipe. There are four main meter styles for flow measurement.
- Positive displacement meters collect a fixed volume of fluid, then release and refill the gas or liquid. Then, tally how many times the capacity fills to determine the flow.
- Flow can also be established when measuring the rate of fluid over an area that is known.
- Other methods depend on forces produced by the flowing stream as it overpowers a known constriction, and indirectly calculates 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 deflection force of the fluid moving through a vibrating tube. These meters are highly accurate with high turndown capabilities and are independent of fluid properties. They are also extremely expensive to purchase and install and are not suitable for larger pipe sizes.
Thermal mass flow meters measure the mass flow based on heat transfer from a heated element. The measurement is in mass flow, and additional pressure and temperature correction are not required. They also provide excellent accuracy and repeatability and are easy to install.
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 passed 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, for gases, its use is limited to clean dry gases only, and pressure and temperature compensation are required.
Ultrasonic flow meters measure the difference in transit time of pulses that travel from a downstream transducer to the upstream sensor, compared to the time from the upstream transducer back to the downstream transducer. This style of a meter is extremely accurate but very expensive, and pressure and temperature measurements are required.
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.
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 tube.
The orifice plate is a differential pressure meter frequently used for natural gas measurement. It measures volumetric flow, not mass flow. Limitations of this meter include reduced low flow sensitivity, limited turndown, and it also creates a pressure drop, which impacts operating costs. Additionally, since it’s a volumetric flow meter, it requires the correction of temperature and pressure to achieve mass flow.
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, then acceptable signals may not be obtained.
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, gas cleanliness needs to be considered. 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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