There are four commonly used types of natural gas meters for measuring flow: mass flow meters, velocity flow meters, differential pressure meters, and PD meters.
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:
- 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.
- Measuring fluid flow involves determining the rate of fluid passing through a known area.
- Other methods rely on the forces exerted by the flowing stream as it overcomes a known restriction, indirectly calculating flow.
Due to the compressibility of gas, accurately measuring gas flow poses challenges. As temperature rises, gas molecules spread farther apart, while increasing pressure causes them to come closer together.
Many gas flow meters (including differential pressure, turbine, positive displacement, and vortex shedding meters) measure gas flow at the actual operating conditions, denoted as ACFM (actual cubic feet per minute). However, it's crucial to adjust or correct the flow rate for specific pressure and temperature conditions. This adjusted flow rate is often referred to as STP (standard pressure and temperature) and is typically measured in units of SCFM (standard cubic feet per minute).
Hence, most gas flow meters require pressure and temperature correction to convert the flow rate from operating conditions (ACFM) to standard conditions (SCFM).
Mass flow meters determine the mass flow passing through the meter. Two notable types are:
Coriolis flowmeters: These provide direct mass flow measurement by detecting the deflection force of the fluid as it moves through a vibrating tube. They offer high accuracy and turndown capabilities, and are unaffected by fluid properties. However, they are expensive to purchase and install and are not suitable for larger pipe sizes.
Thermal mass flow meters: These measure mass flow based on heat transfer from a heated element. The heat transfer is influenced by the number of gas molecules in contact with the heated surface. This method of flow measurement relies solely on the gas molecule count and is independent of gas pressure and temperature, eliminating the need for additional equipment. They offer excellent accuracy, repeatability, and ease of installation.
In velocity meter types, the measurement is determined by the rate of the medium passing through the meter.
Turbine flow meters measure volumetric flow by observing the fluid flowing past a free-spinning rotor, with each revolution representing a specific volume of gas or liquid. These meters offer high turndown and accuracy. However, due to the presence of moving parts, their application is restricted to clean, dry gases in gas applications, necessitating pressure and temperature compensation.
Ultrasonic flow meters measure the difference in transit time of pulses traveling from a downstream transducer to an upstream sensor, compared to the transit time from an upstream transducer back to the downstream transducer. While highly accurate, this meter style is also very expensive and requires pressure and temperature measurements.
The vortex gas flow meter features a shedder bar, an obstruction in the flow path, which causes the fluid to flow around it, creating vortices on the backside of the bar. The frequency of vortex generation is proportional to the gas velocity, determined by the Kármán effect. The frequency of vortex shedding remains constant regardless of fluid composition. However, the meter necessitates temperature and pressure compensation and requires a minimum flow rate to generate vortices.
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.
Positive displacement meters operate by mechanically displacing fluid components and measure volumetric flow at the operating temperature and pressure. Despite their adequate accuracy, achieving mass flow requires pressure and temperature compensation. Additionally, due to their moving parts, gas cleanliness must be considered. Positive displacement meters are also referred to as PD flow meters or volumetric flow meters. An example of a PD meter is the diaphragm meter.
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