Going with the Flow
Learning more about the effects of flow conditioning on water measurement can enhance water and wastewater treatment performance and cut costs
The full cost of ownership related to operating a water or wastewater treatment facility consists of the initial capital, commissioning, training, spare parts, maintenance, and calibration costs for the lifetime of the equipment. The full cost is several times the initial capital investment and should be the deciding factor in selection of flow measurement equipment. The technical selection -- accuracy, repeatability, drift, ease of calibration, and 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.
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 flow rate. 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 flow rate. Examples of energy extractive meters are PD meters, turbine meters, vortex meters, and head meters (orifice, pitot, venturi, etc.).
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.
All inferential flowmeters are subject to the effects of velocity profile, swirl, and turbulence structure. The meter calibration factors are valid only if 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 flowmetering engineers and organizations that write equipment standards. 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 rate 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
- 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 mechanics. With respect to calibration of flowmeters, the Law of Similarity is the foundation for flow measurement standards.
To satisfy the Law of Similarity, the meter compliance concept requires geometric and dynamic similarity between the experimental database and the installed operating meter over the entire life of the facility.
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 does 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.
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 flowmetering 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, we will refer to this as pseudo-fully developed flow.
A method to circumvent the influence of the fluid dynamics (swirl, profile, and turbulence) 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, we generally refer to fully developed flow in terms of swirl-free, axisymmetric time averaged velocity profile in accordance with the Power Law or 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 only is achievable after considerable effort in a research laboratory.
Optimal Flow Conditioner
To truly isolate flowmeters, the optimal flow conditioner should achieve 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 degrees)
- Independent of tap sensing location (for orifice meters)
- Pseudo-fully developed flow
When the swirl angle is less than or equal to 2 degrees, as conventionally measured using pitot tube devices, swirl is regarded as substantially eliminated.
For turbine and ultrasonic meters, they are assumed to be at a minimum and pseudo-fully developed when the empirical meter factors for both short and long piping lengths are the following:
- Approximately plus or minus one-tenth of one percent (+/- 0.10 percent) for water applications; and
- Shown to be independent of axial position.
Classification of Flow Conditioners
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 plates.
In the fall of 1996, Savant Measurement Corporation proposed a research project to the Southwest Research Institute aimed at developing a compact, multi-tube orifice flowmeter/header installation configuration. At that time, a series of sliding flow conditioner tests performed in a 10-inch 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 Meters and Turbine Meters
Ultrasonic meter technology is relatively new to fiscal applications. In the author's opinion, this technology shows tremendous potential for performance equal to or better than most "world class" calibration laboratories.
The ISO standard for turbine meters requires passing a rigorous series of perturbation experiments to ensure compliance with the standard.
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 ensures the flow field the inferential meter sees closely approximates the laboratory conditions. This minimizes the sensitivity to the dynamic similarity issues, which is part of the dynamic traceability.
As stated earlier, the role of flow conditioning is to ensure that the real world environment closely resembles the laboratory environment for proper performance of inferential flowmeters.
CLASSIFICATION OF FLOWMETERS
- Sonic Nozzle
- Subsonic Nozzle
This article originally appeared in the 09/01/2004 issue of Environmental Protection.