Flow Conditioning Why, What, When and Where. Gas Flow Conditioning Why, What, When and Where

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1 Gas Flow Conditioning Why, What, When and Where James E. Gallagher, Chief Executive Officer Savant Measurement Corporation BACKGROUND Measurement is the basis of commerce between producers, royalty owners, transporters, process plants, marketers, state and federal governmental authorities, and the general public. In fact, accurate measurement of hydrocarbon fluids has a significant impact on the Gross National Product of exporting and importing countries, the financial performance and asset base of global companies, and the perceived efficiency of operating facilities. The need for accurate fiscal measurement is obvious. Given the present or future levels of the cost of these critical resource materials one can quickly quantify the material and economic value unaccounted for that is associated with each ± 0 01 per cent systematic uncertainty that might unknowingly exist in the measurement systems for these materials. For reasons such as these, it is essential that fluid quantity and flowrate measurements are precise and accurate with minimal bias errors. Furthermore, it is incumbent upon those involved in fluid custody transfer to establish and maintain the traceability chains that link their measurements to appropriate domestic and international standards. In this manner, fiscal transfer of fluids can be done equitably with the confidence of both seller and buyer alike. Accurate flow measurement is defined as measurement with a low uncertainty. Stated another way, accurate flow measurement requires maximum absolute accuracy and high precision. An important objective is to minimize the bias error associated with the measurements. These errors are best minimized through the use of primary or secondary calibration systems. WHY - Pseudo-Fully Developed Flow and Law Of Similarity Two concepts need to be fully understood pseudo-fully developed flow and the Law of Similarity. Pseudo-Fully Developed Flow From a practical standpoint, we generally refer to fully developed flow in terms of swirl-free, symmetric, time average, velocity profile in accordance with the Power Law or Law of the Wall prediction. To bridge the gap between research and industrial applications, the term pseudo-fully developed flow will be defined as follows: The slope of the orifice meter s discharge coefficient deviation or meter factor deviation that asymptotically approaches zero as the axial distance from the flowmeter to the upstream High Performance Flow Conditioner increases. Inferential flowmeters (orifice, turbine, vortex, ultrasonic and subsonic venturis) are designed under laboratory conditions for optimum performance. In addition, the flowmeters signature response curves to swirl, nonsymmetry and turbulence vary between the aforementioned technologies and sometimes within the line size of a specific flowmeter design. Law of Similarity The Law of Similarity is the underlying principle for present day theoretical and experimental fluid mechanics. With respect to calibration of flowmeters, the Law of Similarity is the foundation for flow measurement standards. The following subsections describe the unique role of the Law of Similarity for the calibration concepts used for both liquid and gas Artefact compliance Central calibration In situ calibration Artefact Compliance To satisfy the Law of Similarity, the artefact compliance concept requires geometric and dynamic similarity between the experimental database and the installed operating meter over the entire life of the facility. This approach assumes that the technology does not exhibit any significant sensitivity to operating or mechanical variations between mechanical inspections and verifications. This concept has been adopted only for orifice flowmeters, and relies heavily on mechanical compliance. The artefacts are the orifice meters comprising the empirical database. As a result, A.G.A. Report No. 3 contains voluminous specifications for the mechanical Copyright of 6

2 tolerances and conditions of the orifice meter. The sensitivity of the technology to geometric variations has been explored extensively resulting in the numerous specifications. This ensures confidence in the application of this technology within the standard s uncertainty tolerance. For orifice meters, the empirical discharge coefficients determined from the experiments are valid if geometric and dynamic similarity exists between the metering installation and the experimental database. Geometric similarity requires that the experimental flow system be a scale model of the field installations. The experimental pattern's design locates sensitive dimensional regions to explore, measure and empirically fit. A proper experimental pattern for orifice meters allows the user to extrapolate to larger meter tube diameters without increasing the uncertainty. Dynamic similarity implies a correspondence of fluid forces between the two metering systems. For orifice meters, the velocity profile and turbulence level of the flow in the laboratory must be similar to the velocity profile and turbulence level of the flow in the field installation. The amount of deviation allowed increases with β ratios below At β ratios greater than 0 40, the amount of deviation allowed decreases with increasing β ratios. For the orifice meter, the inertial and viscous forces are the ones considered significant within the application limitations of this standard. As a result, the Reynolds number, which measures the ratio of inertial to viscous forces, correlates dynamic similarity in all empirical coefficients of discharge for orifice meters. The experimental pattern was designed to explore sensitive Reynolds number regions. A proper experimental pattern for orifice meters allows the user to extrapolate to higher Reynolds numbers without increasing the uncertainty. Central Facility To satisfy the Law of Similarity, the central facility concept requires geometric and dynamic similarity between the laboratory artefact meter run and the installed conditions of this same artefact meter run over the entire custody transfer period. The entire custody transfer period is the time between re-calibration of the artefact meter run. This approach assumes that the selected technology does not exhibit any significant sensitivity to operating or mechanical variations between calibrations. This concept has been adopted for turbine, displacement and ultrasonic meters in gas applications. The meter factor determined at the time of calibration is valid if both dynamic and geometric similarity exists between the field installation and the laboratory installation of the artefact. For turbine, ultrasonic and displacement meters, the meter factors determined at the time of calibration are valid if geometric and dynamic similarity exists between the field installation and the laboratory installation of the artefact meter run. Geometric similarity requires that the artefact meter run be calibrated in the laboratory. No geometric extrapolation of the data is possible. Even if voluminous specifications for the mechanical tolerances and conditions of the flowmeter existed, the flowmeter designs do not scale geometrically. For example, the stator and rotor blockage for a 150mm turbine is significantly different than the blockage for larger turbine meters. Dynamic similarity implies a correspondence of fluid forces for the metering system. For inferential flowmeters, the velocity profile and turbulence level of the flow in the laboratory must be identical to the velocity profile and turbulence level of the flow in the field. For turbine meters, the inertial and viscous forces are the ones considered significant within the application limitations. As a result, the Reynolds number, which measures of the ratio of the inertial to viscous forces, correlates dynamic similarity in all empirical meter factors for turbine meters. For rotary displacement meters, the differential pressure versus actual volumetric flowrate or meter run velocity is considered significant within the application limitations. Several designs require filtration upstream of the meter to protect the mechanical integrity of the flowmeter. For ultrasonic meters, the actual volumetric flowrate or flowmeter average pipe velocity is considered significant within the application limitations. An influence quantity for ultrasonic meters is the line size. Since the ultrasonic wand s diameter is independent of line size, the designs are geometrically dissimilar due to blockage effects (intrusive probes) or re-circulation zone effects (nonintrusive probes). A proper manufacturer s experimental pattern locates sensitive regions to explore, measure and empirically adjust. A proper meter design should allow the user to correlate the performance based on one of the following Copyright of 6

3 Reynolds number, Strouhal number, or meter piping velocity. In Situ To satisfy the Law of Similarity, the in situ calibration concept requires geometric and dynamic similarity between the calibrated artefact meter run and the installed conditions of this same artefact meter run over the entire custody transfer period. The calibrated artefact is the installed meter run in the field. The entire custody transfer period is the time between re-calibration of the artefact meter run. This approach assumes that the selected technology does not exhibit any significant sensitivity to operating or mechanical variations between calibrations. This concept has been adopted for turbine, ultrasonic and orifice meters in liquid applications and some limited gas applications. The meter factor determined at the time of calibration is valid if both dynamic and geometric similarity exists in the field artefact installation over the entire custody transfer period. Geometric similarity requires that the field artefact meter run be calibrated under field operating conditions. No geometric extrapolation of the data is possible. Also, no mechanical changes to the artefact are allowed without recalibration. Dynamic similarity implies a correspondence of fluid forces for the metering system. For inferential flowmeters, the velocity profile and turbulence level at the time of calibration must be identical to the velocity profile and turbulence level of the flow over the entire custody transfer period. The manufacturer s experimental pattern locates sensitive Reynolds number s regions to explore, measure and empirically adjust. A design may allow the user to extrapolate based on Reynolds number, actual volumetric flowrate or flowmeter average pipe velocity. WHAT - Are Isolating Flow Conditioners? Accurate flow measurement is best achieved with an optimized flow profile. All those involved in the design, implementation and operation of a custody transfer station benefit from flow profile standards of accuracy that far exceed those of the past 40 years. Flow conditioners can be grouped into three types Those that eliminate swirl only (tube bundles) Those that eliminate swirl and nonsymmetry, but do not produce pseudo fully developed flow (static mixers) Those that eliminate swirl, nonsymmetry and pseudo fully developed flow (high performance flow conditioners) To truly isolate flowmeters, a GFC or pfc is placed in series before the flowmeter achieving the following design objectives: Low permanent pressure loss (low head ratio). Low fouling rate. Rigorous mechanical design. Moderate cost of construction. Elimination of swirl (less than 2 - when the swirl angle is less than or equal to two (2) degrees, as conventionally measured using pitot tube devices, swirl is regarded as virtually eliminated). Independence of tap sensing location (for orifice meters), or symmetrical profile. Pseudo-fully developed flow for both short and long straight lengths of pipe. For orifice flowmeters, the term Cd deviation (%) refers to the percent deviation of the empirical coefficient of discharge or meter calibration factor from fully developed flow to the disturbed test conditions. Desirably, this deviation should be as near to zero as possible. As explained above, a minimal deviation is regarded as approximately ± 0.25 % for gas applications. For liquid turbine, vortex, subsonic venturis and ultrasonic flowmeters, when the empirical meter factors for both short and long piping lengths are approximately ± 0.15 %, then it is assumed to be at a minimum and to be pseudo-fully developed. For gas turbine, vortex, subsonic venturis and ultrasonic flowmeters, when the empirical meter factors for both short and long piping lengths are approximately ± 0.25 %, then it is assumed to be at a minimum and to be pseudofully developed. As previously stated, inferential flowmeters are sensitive to swirl, symmetry, and turbulence. Advances in High Performance Flow Conditioner technology and pragmatic demands of the industry have Copyright of 6

4 lead to the development of a matrix of products that satisfy the requirements of the many varied applications of this technology. Solutions (GFC and pfc) are now available with the following mechanical features: Two Piece High Performance Flow Conditioners for Orifice meter applications Single Plate designs (flange mount, pin mount) for Orifice meter applications Single Plate designs (in-line, pin mount) for Orifice meter applications Two Piece High Performance Flow Conditioners for USM applications Single Plate designs (flange mount) for USM applications Two Piece High Performance Flow Conditioners for Turbine Meter applications Single Plate designs (flange mount) for Turbine Meter applications This added product flexibility allows the user to better select that design that meets the needs of installation complexity, performance requirements and budget constraints. When considering the use of any High Performance Flow Conditioner technology the following factors should be considered:. Permanent Pressure Drop allowed Complexity of installation upstream of the meter Allowable space for the total station footprint Allowable weight for the meter station design (offshore) Whether the meter run may be deployed as an asset into a different installation in the future. WHEN The Real World The problem in the real world is to minimize the difference between real and distorted flow conditions on the selected flowmetering device, thus maintaining the low uncertainty required for fiscal applications. For clarity, this requires generating pseudo-fully developed flow in the field installations. A method to circumvent the influence of the fluid dynamics on the meter s performance is to install a GFC or pfc in combination with straight lengths of pipe to isolate the flowmeter from piping induced perturbations or disturbances. By including high performance flow conditioners in their latest metering station design standards, the American Petroleum Institute (API), American gas Association (AGA) and ISO have recognized a technology that insures an unparalleled degree of flow accuracy. This technology, a High Performance Flow Conditioner (GFC or pfc ) is placed upstream of inferential flowmeters, conditions the flow such that it enters the flowmeter with a uniform, non-swirling, fully developed profile. This happens regardless of the pipe configuration prior to the isolating flow conditioner. All inferential flowmeters (orifice, ultrasonic, turbine and vortex flowmeters) are subject to the effects of velocity profile, swirl, and turbulence structure. The meter calibration factors or empirical discharge coefficients are valid only if geometric and dynamic similarity exists between the installed and calibration conditions or between the installed and empirical database conditions - in other words, under fully developed flow conditions commonly referred to as the Law of Similarity. In the industrial environment, multiple piping configurations are assembled in series, generating complex problems for standards-writing organizations and flow metering engineers. The challenge is to minimize the difference between the actual flow conditions and the fully developed flow conditions in a pipe, in order to maintain minimum uncertainty associated with the selected metering device s performance. Research programs in both Western Europe and North America have confirmed that many piping configurations and fittings generate disturbances with unknown characteristics. Even a single elbow can generate very different flow conditions - from ideal to fully developed flow - depending on its radius of curvature (that is, mitered or swept). In addition, the disturbance that piping configurations generate is further influenced by the conditions prior to these disturbances. In general, real world upstream piping elements may be grouped accordingly - Copyright of 6

5 Those that distort the mean velocity profile (non-symmetry) but produce little swirl (single elbow) Those that both distort the mean velocity profile (non-symmetry) and generate moderate bulk swirl (DEOPs) up to 20 Those that both distort the mean velocity profile and generate significant bulk swirl (headers) greater than 20 As a result, today s measurement industry is increasingly focused on lowering uncertainty levels associated with these distorted flow conditions. WHERE Do I Install a GFC and pfc? So where does one install a GFC or pfc to isolate pipinginduced perturbations, minimize bias errors and optimize performance? Well it depends on the flowmeter technology and the applicable industry standards and technical reports. Orifice Flowmeters For orifice flowmeters, the A.G.A. Report No. 3 and ISO 5167 require that the isolating flow conditioner pass a series of rigorous perturbations (Beta ratio, piping disturbance, piping length and pipe Reynolds number) to ensure compliance with the standard. For the GFC, this isolating flow conditioner has passed all of the perturbations requirements for 17D, 29D and 45D upstream piping lengths. The optimum GFC location is 7D upstream of the orifice plate. The allowable location is 5D to 13D upstream of the orifice plate. Turbine, Vortex, Subsonic Venturi and Ultrasonic Flowmeters For gas turbine flowmeter applications, A.G.A. Report No. 7 and ISO 9951 cover the requirements for their installation and performance. The A.G.A. Report No. 7 allows traditional, short-coupled and close-coupled piping runs. The GFC and pfc can be installed anywhere provided that there is a 3D settling chamber upstream of the profile device and a 2D settling chamber prior to the turbine inlet. The ISO 9951 standard requires passing a rigorous perturbation series of experiments to ensure compliance with the standard. The effects of piping-induced disturbances is presumed to be dependent on the turbine manufacturer and line size due to the varying blockage (70 80%) within a given manufacturer and between manufacturers of gas turbine flowmeters. The optimum GFC location is 5D upstream of the gas turbine or vortex flowmeter. The allowable location is 3D to 7D upstream of the turbine or vortex flowmeter. For gas ultrasonic flowmeters, the optimum GFC location is 5D or more upstream from the inlet of the flowmeter body. For liquid turbine, vortex, subsonic venturi and ultrasonic flowmeters, the optimum GFC location is 5D or more upstream from the inlet of the flowmeter body. SUMMARY During the design phase of a fiscal metering facility, the capital cost of equipment is one important point that needs to be front and center in the decision making process of equipment selection. The designer should consider not just the capital cost but the total cost of ownership of the facility capital, maintenance and performance. The net effect of the cost of performance i.e. uncertainty, will have a drastic effect on the bottom line of the meter station, the inclusion of High performance High Performance Flow Conditioners in the design fits well into equation of cost vs. benefit in terms of overall performance and equipment justification. All inferential flowmeters (orifice, ultrasonic, turbine, vortex, subsonic venturis and ultrasonic) are sensitive to the presence of swirl, non-symmetry of the velocity profile and turbulence in the stream. The sensitivity to these three parameters varies with flowmeter technology and the specific design within a flowmeter technology. The major economic drivers that effect the selection of High Performance Flow Conditioners are - Reduced Meter Station size Improved flowmeter performance (linearity, repeatability, reproducibility) Standardized meter station designs Reduced LUAF By deploying a GFC or pfc in combination with inferential flowmeters (orifice, ultrasonic, turbine, subsonic venturis and vortex) the operator is assured of achieving the best possible flow environment for the flowmeter in real world configurations. Copyright of 6

6 In summary, to minimize bias error and optimize flowmeter performance, a High Performance flow Conditioner, the GFC or pfc, should be installed in combination with straight lengths of pipe for every field and laboratory application. Copyright of 6