Effects Of Flow Conditioning On Gas Measurement

摘要

Gas flow rate mea sure-ment errors at field meter stations have many causes. For instance, errors can result from an improper installation configuration, unrealistic calibration of a meter or degradation of meter performance over time. Industry standards have been developed to help meter station designers and operators avoid situations that would produce gas metering errors. Typically, gas meter standards address meter design, construction, installation, operation and maintenance. Most of the standards focus only on the flow meter and the piping immediately upstream and downstream of the meter. Research has shown that many meter types, particularly inferential meters, are susceptible to errors when the flow field at the meter is distorted. The sources of flow field distortions are many. The piping geometry upstream of a flow meter can create flow distortions that may propagate several hundred pipe diameters downstream before completely dissipating. Sudden changes in the pipe diameter, either upstream or downstream of a meter, may also introduce flow field distortion. Branch flows, such as those produced by meter station headers, control valves, regulators and other flow restrictions or expansions, can also create distortions in the flow. Velocity profile asymmetry, swirl and combined profile asymmetry and swirl are examples of flow field distortions that can result in meter bias errors (i.e., measurement errors that are of a fixed magnitude and sign). Different meter types have different sensitivities to the various kinds of flow field distortions. The effect of flow field distortion on meter error is commonly referred to as an installation effect. Most industry standards for gas meters do not completely address installation effects, so it is often left to the meter station designer or operator to ensure that installation effects of this type are not significant.