The Effects of Flow Conditioning

Designing & operating an accurate flowmeter application requires understanding the fluid’s physical properties

The full cost of ownership consists of the initial capital, commissioning, training, spare parts, maintenance and calibration costs for the lifetime of the meter-related equipment.
The full cost is several times the initial capital investment and should be the deciding factor in equipment selection.
The technical selection—accuracy, reliability, drift, ease of calibration as well as reliability—indirectly affects the cost of ownership.
Proper installation and application of flowmeters are two of the most significant parameters in the measurement chain. These parameters influence the factors mentioned above and are neglected in most assessments. The misapplication of any device brings the wrath of field personnel on the operating company’s engineering staff—as it should. The selection, installation, operation and maintenance of quality equipment, if properly performed, are almost never discussed by operating personnel.
The role of flow conditioning is to ensure that the “real world” environment closely resembles the “laboratory” environment for proper performance of inferential flowmeters.

Flowmeters
The quest for custody transfer measurement performance over a wide range of operating conditions has long been the Holy Grail for the metrological community.
The future vision of primary flow devices is clear and defined. This vision consists of “smart” flowmeters, new flowmeter technology, in situ calibration and adoption of central calibration technologies.
Flowmeters are generally classified as either energy additive or energy extractive. Energy additive meters introduce energy into the flowing stream to determine flowrate. Common examples of energy additive meters are magnetic meters and ultrasonic meters. Energy extractive meters require energy from the flowing stream, usually in the form of pressure drop, to determine the fluid’s flowrate. Examples of energy extractive meters are PD meters, turbine meters, vortex meters and head meters.
Understanding the physical principles of the flowmetering technology is a key to success that is often overlooked by the designer and operator. If these principles are overlooked or not understood fully, the Law of Similarity may be violated resulting in inaccurate measurement and high maintenance costs.

Installation effects
All inferential flowmeters are subject to the effects of velocity profile, swirl and turbulence structure. The meter calibration factors are valid only of geometric and dynamic similarity exists between the metering and calibration conditions. In fluid mechanics, this is commonly referred to as the Law of Similarity.
In the municipal environment, multiple piping configurations are assembled that generate complex problems for standard writing organizations and flow metering engineers. The challenge is to minimize the difference between the actual or “real” flow conditions and the “fully developed” flow conditions in a pipe to maintain a minimum error associated with the selected metering device’s performance. One of the standard error minimization methods is to install a flow conditioner in combination with straight lengths of pipe to “isolate” the meter from upstream piping disturbances.
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” or “fully developed” flow depending on its radius of curvature. In addition, the disturbance generated by piping configurations is influenced by the conditions prior to these disturbances.
In general, upstream piping elements may be grouped into the following categories:

  • Those that distort the mean velocity profile but
    produce little swirl; and
  • Those that both distort and generate bulk swirl.
    As a result, the current focus of today’s measurement community is to lower uncertainty levels associated with “non-ideal” flow conditions.

Law of Similarity
The Law of Similarity is the underlying principle for present day theoretical and experimental fluid machines. With respect to calibration of flowmeters, the Law of Similarity is the foundation for flow measurement standards.
To satisfy the Law of Similarity, the central facility concept requires geometric and dynamic similarity between the laboratory meter and the installed conditions of this same meter over the entire custody transfer period.
This approach assumes that the selected technology does not exhibit any significant sensitivity to operating or mechanical variations between calibrations.
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 artifact.
A proper manufacturer’s experimental pattern locates sensitive regions to explore, measure and empirically adjust.
The manufacturer’s recommended correlation method is a rational basis for performance prediction provided the physics do not change. For instance, the physics are different between subsonic and sonic flow.
To satisfy the Law of Similarity, the in situ calibration concept requires geometric and dynamic similarity between the calibrated meter and the installed conditions of this same meter over the entire custody transfer period. This approach assumes that the selected technology does not exhibit any significant sensitivity to operating or mechanical variations between calibrations.
The meter factor determined at the time of calibration is valid if both dynamic and geometric similarity exists in the “field meter installation” over the entire custody transfer period.

Flow conditioners
As previously stated, all inferential flowmeters are subject to the effects of velocity profile, swirl and turbulence structure approaching the meter.
Many piping configurations and fittings generate disturbances with unknown characteristics. In reality, multiple piping configurations are assembled in series generating complex problems for standard writing organizations and flow metering engineers. The problem is to minimize the difference between “real” and “fully developed” flow conditions on the selected metering device thus maintaining the low uncertainty required for fiscal applications.
For clarity, this is referred to as “pseudo-fully developed” flow.
A method to circumvent the influence of the fluid dynamics on the meter’s performance is to install a flow conditioner in combination with straight lengths of pipe to “isolate” the meter from upstream piping disturbances. Of course, this isolation is never perfect. After all, the conditioner’s objective is to produce a “pseudo-fully developed” flow.
Pseudo-fully developed flow
From a practical standpoint, fully developed flow is generally referred to in terms of swirl-free, axisymmetric time average velocity profile in accordance with the Power Law of the Wall prediction. However, one must not forget that fully developed turbulent flow requires equilibrium of the forces to maintain the random “cyclic” motions of turbulent flow.
Unfortunately, fully developed pipe flow is only achievable after considerable effort in a research lab.
To truly “isolate” flowmeters, the optimal flow conditioner should achieve the following design objectives:

  • Low permanent pressure loss (low head ration);
  • Low fouling rate;
  • Rigorous mechanical design;
  • Moderate cost of construction;
  • Elimination of swirl (less than 2?);
  • Independent of tap sensing location (for orifice meters); and
  • Pseudo-fully developed flow.

When the swirl angle is less than or equal to 2? as conventionally measured using pilot tube devices, swirl is regarded as substantially eliminated.
For turbine and ultrasonic meters, when the empirical meter factors for both short and long piping lengths are approximately +/- 0.10% for water applications and shown to be independent of axial position, it is assumed to be at a “minimum” and pseudo-fully developed.
Flow conditioners may be grouped into three general classes based on their ability to correct the mean velocity profile, bulk swirl and turbulence structure.
The first class of conditioners is designed to primarily counteract swirl by splitting up the flow into a number of parallel conduits. This class of conditioners includes uniform tube bundles.
The second class of conditioners is designed to generate an axisymmetric velocity profile distribution by subjecting the flow to a single or a series of perforated grids or plates. Use of the blockage factor or porosity of the flow conditioner redistributes the profile.
The third class of conditioners is designed to generate a “pseudo-fully developed” velocity profile distribution through porosity of the conditioner and the generation of a turbulence structure. Varying the radial porosity distribution generates the turbulence structure.
All flow conditioners may be grouped into three general classes based on their mechanical design—tube bundles, vanes/screens and perforated plated.
A few years ago, Savant Measurement Corp., proposed a research project to the Southwest Research Institute aimed at developed in compact multi-tube orifice flow meter/header installation configuration.
At that time, a series of sliding flow conditioner tests performed in a 10-in. diameter orifice meter tube with an upstream length of A’=17D installed downstream of a tee had been completed and a report containing the test results had been published.
Ultrasonic meter technology is relatively new to fiscal applications. This technology shows tremendous potential for performance equal to or better than most “world class” calibrating laboratories.
The ISO standard requires passing a rigorous series of perturbation experiments to ensure compliance with the standard.

Final thoughts
Designing and operating an accurate flowmeter application requires understanding the fluid’s physical properties.
Understanding the physical principles upon which the selected inferential flowmeter is based and comprehending its sensitivities to physical and process conditions are critical. Designing and operating an accurate fiscal metering facility requires compliance with the Law of Similarity.
Isolating flow conditioners ensure the flow field the inferential meter “sees” closely the laboratory conditions. This minimizes the sensitivity to the dynamic similarity issues, that is part of the dynamic traceability.
The role of flow conditioning is to ensure that the “real world” environment closely resembles the “laboratory” environment for proper performance of inferential flowmeters.

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