Effects of flow conditioning on gas measurement

Pipeline & Gas Journal, Feb, 2008 by Darin L. George, Edgar B. Bowles, Jr.

Gas flow Grate measurement 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.

Gas Flow Meter Types

Since different flow meter designs exhibit different sensitivities to installation effect errors, it is important to understand how each meter design or type is influenced by the various flow field distortions that may occur at a meter station. There are essentially two types of gas flow meters--discrete and inferential devices. Discrete meters determine the volumetric flow rate of a gas stream by continuously separating the flow stream into discrete segments and then counting the number of segments measured per unit of time. An example of a discrete meter is the positive displacement meter.

Inferential flow meters infer volumetric flow rate by measuring one or more dynamic properties of the flowing gas stream. Examples include the orifice meter, turbine meter, ultrasonic meter and Coriolis meter. Inferential flow meters are generally more susceptible than discrete meters to measurement errors caused by installation effects or flow field distortions. However, since each meter type and design has unique sensitivities to installation effects, a meter station designer or operator must be aware of these sensitivities in order to eliminate or at least minimize measurement errors caused by installation effects.

Installation Effects

Research performed in North America and western Europe has clearly demonstrated that typical meter station piping configurations and fittings generate a variety of flow distortions.

Even simple piping elements, such as a 90 [degrees] elbow, which produces two counter-rotating vortices (commonly referred to as Type 2 swirll and velocity profile asymmetry at its outlet plane, can create flow distortions that may result in meter bias errors. Research has also shown that a flow distortion may propagate through a series of piping elements (e.g., pipe bends, valves, contractions/expansions, etc.) without dissipating. In some instances, a flow distortion may actually increase in severity as it passes through a series of piping elements. This finding requires that a meter station designer or operator always be concerned about both nearby and distant piping elements from which flow distortions may propagate.

Figure 1 illustrates how a pipe flow distortion dissipates as it propagates downstream. In this example, pipe centerline velocity profiles were measured at several axial locations along a straight pipe downstream of a single 90[degrees] longradius elbow. A fully developed turbulent flow profile (denoted by the solid line on Figure 1) was provided at the inlet to the elbow. In this case, the test piping was 12-inch diameter, schedule 40 carbon steel pipe. The axial locations where measurements were made downstream of the elbow are denoted in Figure 1 in terms of nominal pipe diameter, D.

The velocity profiles were measured by traversing the centerline of the pipe (in the plane of the elbow) at 10, 40, 59, 78 and 97 pipe diameters downstream of the elbow. The test medium was distribution-grade natural gas (about 95% methane) at a line pressure of approximately 600 psia. Note that the velocity profile has nearly recovered to the fully-developed condition at 97 pipe diameters downstream. The measurements used in this [example.sup.2] were made in the High Pressure Loop (HPL) at the Metering Research Facility (MRF) located at Southwest Research Institute (SwRI) in San Antonio, TX.