New Developments in Thermal Dispersion Mass Flow Meters: In-The-Field Compensation for Changes in Natural Gas Composition

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Courtesy of Sierra Instruments, Inc.

Introduction
The flow of natural gas in pipelines is often complicated by changes in the composition of the gas. The method described in this paper solves the problem of variable composition via a new four-temperature mass flow sensor. The new (low sensor feeds its measurements into the algorithms of an advanced mathematical model that includes gas properties in its calculations. The method described herein computes the properties of a specified pure gas or gas mixture to manage changes in gas composition, gas temperature, gas pressure, and outside temperature This advance in thermal dispersion mass flow meters provides an immediate solution for natural gas applications, including distribution systems, flare gas produced in refineries, and fiare gas produced in other applications, such as hydraulic fracturing.

Applications
Thermal dispersion mass flow meters measure the mass flow rate of pure gases and gas mixtures of known composition by sensing the heat that is convected from the surface of a heated velocity sensor immersed in the flow. Since it is the molecules of the gas, which bear its mass, that carry away the heat, thermal dispersion mass flow meters directly measure mass flow rate [1-6]. In addition to directly measuring mass flow rate, thermal dispersion mass flow meters have wide turn-down (more than 100:1), low pressure drop, and lower cost than most other technologies.

Since convective heat transfer depends on the properties of the gas (e.g., thermal conductivity, dynamic viscosity, and mass density), these properties must have known values. Traditionally, flow calibration of thermal mass flow meters is performed with the gas and the nominal temperature and pressure of the application. In cases where the specified gas mixture can not be used for flow calibration because of concerns for safety and expense, a surrogate gas mixture with similar heat transfer properties is used. Commonly, air or nitrogen is used as the surrogate gas and a correction factor based on the relationship between the properties of the surrogate gas and the gas properties of the specified pure gas or gas mixture is applied. Neither of these flow calibration options is optimal, particularly if temperature, pressure, or composition changes in the field.

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