Effects of flow conditioning on gas measurement
Darin L. George
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.
[FIGURE 1 OMITTED]
Flow Conditioners
Inferential flow meters use meter calibration factors or empirical coefficients to account for the effects of parameters that are not measured. For example, the velocity profile at the meter is not typically measured. However, the velocity profile can have an affect on meter error. As an example, the discharge coefficient for orifice flow meters has been experimentally determined from tests in which there was typically a fully developed, swirl-free turbulent flow field upstream of the test meter. Thus, the readings from an orifice meter installed at a field site will only produce unbiased results if the flow field upstream of the meter also produces a fully developed, swirl-free, turbulent velocity profile.
An effective way to minimize or eliminate the adverse influence of the fluid dynamics (e.g., velocity profile asymmetry, swirl, etc.) on meter performance is to install a flow conditioner. Flow conditioners are devices that “condition” the flow field at or near the meter inlet. Flow conditioners attempt to “isolate” a flow meter from flow field distortions that may propagate from upstream. There is no flow conditioner design that can truly isolate a meter from all possible flow field distortions, although some designs are quite effective at “isolating” a fairly broad range of distortions propagating from upstream.
Flow conditioners can be grouped into three general classes based on their ability to correct velocity profile asymmetry, swirl and turbulence structure.
The first class of flow conditioners primarily corrects for swirl in the flow stream. These devices typically use honeycomb structures or tube bundles to split the flow into a number of smaller, parallel conduits. Examples include the uniform tube bundle (concentric and hexagonal 19-tube designs), International Standards Organization 5167 tube bundles, the AMCA (i.e., American Mechanical Contractors Association) honeycomb flow straightener and the Etoile.
The second class of flow conditioners is designed to generate an axisymmetric velocity profile by subjecting the flow field to a single perforated grid or plate, or a group of perforated grids or plates. The blockage factor, or porosity factor, of the grid or plate is chosen so as to redistribute the flow field (or velocity profile) downstream of the device. Examples include the Sprenkle and Mitsubishi plate designs.
The third class of conditioners produces a “pseudo”-fully developed velocity profile via the combination of a porous plate or grid and a turbulence generator. Radial variation in the grid or plate porosity helps generate the required turbulence structure. Examples of this class of flow conditioner include the Daniel Profiler [TM], Sens and Teule, Bosch and Hebrand, K-Lab and Laws, CPA 50E, Gallagher (also known as the GFC [TM]) and Spearman designs.
Since flow conditioners themselves create a disturbance in the flow stream, they typically require some separation length upstream of the flow meter in order to be optimally effective. The separation distance between the flow conditioner and flow meter varies as a function of the flow conditioner design.
The amount of permanent pressure loss across a flow conditioner is also a function of the design (see Table 1).
Orifice Meter Flow Conditioning
Orifice meters can be very sensitive to bulk swirl, velocity profile asymmetry and flow turbulence. The gas industry standard for orifice meter installations, API MPMS (American Petroleum Institute Manual of Petroleum Measurement Standards) Chapter 14, Section 3, Part 24 (also known as American Gas Association (AGA) Report No. 3, Part 2), was revised in 2000 to reflect new findings regarding flow conditioning. The meter installation configuration referenced in the standard is pictured in Figure 2.
[FIGURE 2 OMITTED]
The revised standard includes a recommendation for the minimum length of straight pipe required upstream of an orifice meter when no flow conditioner is used (up to 145 diameters of pipe may be required ahead of the meter). The minimum recommended lengths between the orifice meter and several common flow distorting configurations are also provided (see Table 2). A “catch-all” category is also provided for upstream piping configurations not referenced in the standard, such as complex headers found in many meter stations.
Good flow conditioners are able to reduce the upstream length requirement for orifice meters to as little as 10 pipe diameters. Figure 3 presents results of tests performed at the MRF with an orifice meter 10 D downstream of a compact header. (5) The axial location of a GFC TAS flow conditioner was varied in the meter tube and the resulting orifice discharge coefficient was compared to the baseline value for the case in which a fully developed velocity profile was present at the meter inlet with no flow conditioner included in the installation.
In Figure 3, the dashed lines shown at [+ or -] 0.23% deviation in the discharge coefficient denote the 95% confidence interval for the experiment. Test data points falling within those bounds are not considered significantly different, from a statistical standpoint, than the results for the baseline case. MRF tests found that, in this case, the GFC functioned as an effective flow conditioner in this configuration when placed as close as four to six diameters upstream of the orifice.
[FIGURE 3 OMITTED]
The latest edition of the orifice meter standard (API MPMS, Chapter 14.3) lists required lengths of straight upstream pipe when a 19tube, uniform tube bundle is placed between the distortion source and the orifice meter. Other isolating flow conditioners are not specifically discussed in the standard, but are required to pass performance tests for general use or specific applications. Figure 3 shows results from such a performance test.
Ultrasonic Meter Flow Conditioning
Over about the last 10 years, ultrasonic flow meters have gained relatively broad acceptance for natural gas custody transfer measurement in the United States and other countries. The installation requirements for ultrasonic flow meters in natural gas applications are found in AGA Report No. 9. (6) The newly revised Report No. 9 includes “default” meter installation configurations, which are shown in Figure 4.
These configurations may be used if there are no specific recommendations for installation configuration provided by the flow meter and flow conditioner manufacturers. It is very important to note that different ultrasonic flow meter manufacturers arrange their ultrasonic transducer arrays differently and use unique methods of computing flow rates from the measured gas velocities, so the effectiveness of a particular flow conditioner depends on the ultrasonic meter design.
[FIGURE 4 OMITTED]
Tests performed a few years ago at the MRF assessed ultrasonic meter performance in the presence of several flow disturbances and flow conditioners. The study suggested that by calibrating a meter and flow conditioner as a single assembly or system, accurate flow measurements might be obtained with as little as 10-20 diameters of straight pipe upstream of the meter. However, the flow rate measurement accuracy is still dependent on the upstream piping configuration, as well as the flow conditioner and meter design.
Figure 5 and Figure 6 present the measure ment errors for two different ultrasonic flow meter designs (denoted here as designs “A” and “B”) tested in an identical piping installation at the MRF HPL. The test piping configuration included 100 D of straight pipe immediately upstream of the meter–which is a piping geometry that generally yields fully developed turbulent flow at the meter inlet. Figure 5 and Figure 6 show the performance data for both meters with no flow conditioner installed upstream of the test meter and with a flow conditioner (denoted here as designs “A”, “B”, “C” and “D”) placed upstream (per the specifications of the flow conditioner manufacturer).
Note that even with a fully developed, turbulent flow field at the meter inlet; the addition of different types of flow conditioners changed the meter performance. Through proper flow calibration of the meter and flow conditioner assembly, the observed measurement biases seen in Figure 5 and Figure 6 could be removed by appropriately adjusting the meter calibration factor(s). The newly revised AGA Report No. 9 requires that all custody transfer metering packages be flow calibrated in a flow calibration facility or by a calibration traceable to a recognized national/international standard in order to optimize meter accuracy.
Ultrasonic flow meter experiments performed at the MRF also demonstrated that a single flow conditioner can produce very different results, depending on both the flow field distortion from upstream and the meter design. Thus, the interaction between a given upstream piping configuration, a given meter design and a given flow conditioner design must be considered carefully when choosing equipment for a given meter station.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Turbine Meter Flow Conditioning
Turbine flow meters used for natural gas custody transfer measurement usually have integral flow conditioners and, as a result, are relatively insensitive to upstream flow field distortions. Also, some meter designs incorporate dual rotors that allow the meter to detect and correct for certain types of flow field distortions or installation effects.
The gas industry standard for turbine meters, AGA Report No. 7, (7) references three different meter piping installation configurations: the “recommended” configuration, the “shortcoupled” configuration and the “close coupled” configuration. The standard also provides guidance on use of a flow conditioner. For instance, the “recommended” installation configuration referenced in AGA Report No. 7 requires 10 diameters of straight pipe upstream of the turbine meter, with a straightening vane (i.e., a concentric, 19-tube bundle) placed so that the outlet is fivepipe diameters upstream of the meter inlet.
In 2001, research (8) performed at the MRF concluded that the installation configurations referenced in AGA Report No. 7 produced measurement biases of less than [+ or -]0.25% of reading due to the effects of the installation configuration. In the cases of the “short-coupled” and “close-coupled” installation configurations, integral flow conditioning within the flow meter is required in order for the meter to achieve this measurement performance level.
Research performed at the MRF in 2002 found that turbine meters that have little or no integral flow conditioning may be sensitive to upstream flow field distortions. In general, it was found that as the amount of flow blockage at the meter cross section decreased, the sensitivity to upstream flow field distortion increased. Thus, meters having relatively low flow blockage at the meter cross section may need additional upstream flow conditioning to avoid any measurement bias due to upstream piping installation effects.
Flow Conditioner Maintenance
Improper flow conditioner installation or any deviation in the conditioner geometry due to mechanical damage, flow contaminant buildup, or other problem can result in a significant flow rate measurement error.
Conclusions
A critical factor in the use of inferential flow meters is to understand their sensitivity to distortions in the gas flow through the meter. Distortions such as velocity profile asymmetry and swirl are not directly measured by most inferential meters, but can produce deviations from the ideal flow in which the meters were calibrated. This situation can result in measurement bias errors. Many of the industry standards for gas meters do not completely address the piping installation effects that can lead to these flow distortions and subsequent measurement errors. Thus, in many case, these installation effects must be physically isolated from a flow meter to keep the meter within its performance specifications.
High-performance flow conditioners can improve the measurement accuracy of inferential flow meters by attempting to restore the flow field entering the meter to optimum conditions. Judicious design of a metering station with a flow conditioner must consider several factors, including:
* The ability of the flow conditioner to correct for velocity profile asymmetry, swirl and turbulence structure;
* The separation distance between the flow conditioner and the flow meter that yields optimum performance; and
* The flow field at the meter entrance for which the meter was designed and calibrated, or that is specified in the appropriate industry metering standard.
REFERENCES
(1) Mattingly, G. E. and T. T. Yeh, “Effects of Pipe Elbows and Tube Bundles on SelectedTypes of Flowmeters,” Journal of Flow Measurement and Instrumentation, Vol. 2, Butterworth-Heinemann, Ltd., January 1991.
(2) Grimley, T. A., “Performance Testing of 8-inch Ultrasonic Flow Meters for Natural Gas Measurement,” Topical Report to Gas Research Institute, GRI Contract No. 5097-270-3937, November 2000.
(3) API Manual of Petroleum Measurement Standards, Chapter 14.3, “Concentric, Square-Edge Orifice Meters,” API, Washington, D. C., April 2002.
(4) AGA Transmission Measurement Committee Report No. 3, “Concentric, Square-Edge Orifice Meters,” AGA, Arlington, VA, June 2000.
(5) Morrow, T. B. and E. Kelner, “Orifice Meter Installation Effects: Compact Orifice Meter Station Development,” Topical Report to Gas Research Institute Report No. GRI-99/0204, GRI Contract No. 5097-2703937, December 1999.
(6) AGA Transmission Measurement Committee Report No. 9, “Measurement of Gas by Multipath Ultrasonic Meters,” AGA, Arlington, VA, 2007.
(7) AGA Transmission Measurement Committee Report No. 7, “Measurement of Gas by Turbine Meters,” AGA, Arlington, VA, 2006.
(8) George, D. L. “Turbine Meter Research in Support of the Planned AGA-7 Revision,” Topical Report to Gas Research Institute, Report No. GR1-01/0226, GRI Contract No. 8346, Nov. 2001.
Authors: Darin L. George, Ph.D., is senior research engineer in the Mechanical and Materials Engineering Division at Southwest Research Institute. He is responsible for various aspects of research within the Flow Measurement Section at SwRI. His background as a nuclear and mechanical engineer spans 18 years of work on the measurement of liquid, gas and multiphase flows. He holds B.S. and M.S. degrees in nuclear engineering from Kansas State University and a Ph.D. degree in mechanical engineering from University of Michigan.
Edgar B. Bowles, Jr., is director of the fluids engineering department in the Mechanical and Materials Engineering Division at Southwest Research Institute. He focuses primarily on fluid dynamics and heat transfer. He has experience with both analytical and experimental engineering projects. During his 29 years at SwRI, Ed Bowles has been involved with a variety of projects for the oil and gas industry. His current duties include oversight of several large-scale flow test facilities, including the Metering Research Facility built in 1991 to advancing the state of the art of natural gas flow measurement in the US. and abroad, a rotating machinery group and a process piping design analysis group. He holds B.S. and M.S. degrees from Southern Methodist University.
Table 1: Flow Conditioner Pressure Loss.
Permanent
Pressure Loss
(No. of dynamic
Flow Conditioner heads)
AGA 19-tube bundle 1
ISO 5167 tube bundle 2
Sens & Teule bundle 10
Bosch & Hebrand bundle 10
PG&E tube bundle 1
Etoile 1
AMCA straightener 8
Vortab[TM] 0.8
Sprenkle plate 15
Bellinga plate 20
Daniel Profiler[TM] plate 6
Mitsubishi plate 2
Laws plate 0.8-2.5
CPA 50E plate 2.6
Spearman plate 3
Gallagher (GFC) 1.2
Table 2: Upstream Lengths Required for Orifice Meters Without a Flow
Conditioner (3).
Recommended Upstream Length
Distortion Source for Maximum Range
Single 90[??] elbow 44 D
Two 90[??] elbows in perpendicular 95 D
planes, < 5 D apart
Single 90[??] tee used as an elbow 44 D
Gate valve at least 50% open 44 D
Any other configuration 1145 D
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