Flow conditioning
Encyclopedia
Flow conditioning ensures that the “real world” environment closely resembles the “laboratory
” environment for proper performance of inferential flowmeters
like orifice
, turbine
, coriolis
, ultrasonic
etc.
in pipes can be classified as follows –
that carries a lot of liquids with it is known as wet gas
whereas natural gas that is produced without liquid is known dry gas
. Dry gas is also treated as to remove all liquids. The effect of flow conditioning for various popular meters which is used in gas measurement is explained below.
state referred to as reference conditions that are free of installation effects.
s, tees and reducer
s causes the flow conditions within the pipe to vary from the reference conditions. How these installation effects impact the meter is very important since devices which create upstream installation effects are common components of any standard metering design. Flow Conditioning refers to the process of artificially generating a reference, fully developed flow profile and is essential to enable accurate measurement while maintaining a cost-competitive meter standard design. 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.
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.
---- (1)
The value of n determines the shape of the velocity flow profile. The eq.(1) can be used to determine the flow profile's shape within the pipe by fitting a curve to experimentally measured velocity data. In 1993, the transverse velocities
were being measured within the high pressure natural gas environment using hot wire technology to accomplish the data fit. A fully developed flow profile was used as the reference state for meter calibration and determination of Coefficient of Discharge
(Cd). For Reynolds Number
to 
n is approximately 7.5; for Re of 
, n is approximately 10.0 where a fully developed profile in a smooth pipe was assumed. Since n is a function of Reynolds Number and friction factor, more accurate values of n can be estimated by using the eq.(2),
---- (2)
Where, f is friction factor. A good estimate of a fully developed velocity flow profile can be used for those without adequate equipment to actually measure the velocities within the pipe. The following straight pipe equivalent length in eq.(3) was utilized to ensure a fully developed flow profile exists.
---- (3)
In eq.(3) the pipe lengths required is significant, hence we need some devices that can able to condition the flow over a shorter pipe length allowing metering packages to be cost competitive and accurate. Here the velocity flow profile is generally three-dimensional. Normally the description requires no axial orientation indication if the profile is asymmetric and if it does exists, then axial orientation with respect to some suitable plane of reference is required. Asymmetry exists downstream of installation effects such as elbows or tees. Usually, the velocity flow profile is described on two planes 90° apart. Using the latest software technology a full pipe cross sectional description of the velocity profile is possible provided sufficient data points are given.
at times when the velocity profile was not fully developed. Hence this behavior was referred to the turbulence intensity of the gas flow that can cause metering bias error. This behavior accounts in part for the less than adequate performance of the conventional tube bundle.
----(4)
Where,
= Mass flow
= Coefficient of discharge
= Velocity of approach factor
Y = Expansion factor
d = orifice diameter
= density of the fluid
= differential pressure
Now to use the eq.(4), the flow field entering the orifice plate
must be free of swirl and exhibit a fully developed flow profile. API 14.3 (1990) and ISO standards
determined the Coefficient of Discharge by completing numerous calibration tests where the indicated mass flow was compared to the actual mass flow to determine coefficient of discharge. In all testing the common requirement was a fully developed flow profile entering the orifice plate. Accurate standard compliant meter designs must therefore ensure that a swirl free, fully developed flow profile is impinging on the orifice plate. There are numerous methods available to accomplish this. These methods are commonly known as “flow conditioning”.
The first installation option is to revert to no flow conditioning, but adequate pipe lengths must be provided by the eq.(2) mentioned above. This generally makes the manufacturing costs for a flow measurement facility unrealistic due to excessively long meter tubes; Imagine meter tubes 75 diameters long.
The second and most well known option is the 19-tube tube-bundle flow conditioner. The majority of flow installations in North America
contain the tube bundle. With the help of hot wire, pitot tube
and laser
-based computerized measurement systems which allow detailed measurement of velocity profile and turbulence intensity; we know that the tube bundle does not provide fully developed flow. Therefore, this device is causing biased orifice flow measurement. As a result of these recent findings, few tube bundles are specified for flow measurement and reduce the use of such device. Numerous references are available providing performance results indicating less than acceptable meter performance when using the conventional 19-tube test bundle. The individual results should be reviewed to ascertain details such as beta ratio, meter tube lengths, Re and test conditions.
The general indications are that the conventional tube bundle will cause the orifice installation to over register flow values up to 1.5% when the tube bundle is 1 pipe diameter to approximately 11 pipe diameters from the orifice plate. This is caused by a flat velocity profile that creates higher differential pressures than with a fully developed profile. There is a crossover region from approximately 10 to 15 pipe diameters where the error band is approximately zero. Then a slight under-registration of flows occurs for distances between approximately 15 to 25 pipe diameters. This is due to a peaked velocity profile that creates lower differential pressures than a fully developed profile. At distances greater than 25 pipe diameters the error asymptotes to zero. Fig.(3) showing the Conventional Tube Bundle Performance explaining typical characteristic behavior of the popular 19 tube, tube-bundle. An additional drawback of the conventional 19 tube, tube bundle is variation in sizing.
The conventional tube bundle provides errors very much dependent on installation details, that is, the elbows on and out of plane, tees, valves and distances from the last pipe installation to the conditioner and conditioner to the orifice plate. These errors have a great significance. Therefore the latest findings regarding conventional tube bundle performance should be reviewed prior to meter station design and installation.
The final installation option for orifice metering is perforated plate flow conditioners. There is a variety of perforated plates have entered the market. These devices generally are designed to rectify the drawbacks of the conventional tube bundle (accuracy
and repeatability
insufficiency). The reader is cautioned to review the performance of the chosen perforated plate carefully prior to installation. A flow conditioner performance test guideline should be utilized to determine performance. The key elements of a flow conditioner test are -
and rotor
configured devices. These devices are designed such that when a gas stream passes through them they will spin proportionally to the amount of gas passing over the blades in a repeatable fashion. Accuracy is then ensured by completion of a calibration, indicating the relationship between rotational speed
and volume
, at various Reynolds Numbers. The fundamental difference between the orifice meter and the turbine meter is the flow equation derivation. The orifice meter flow calculation is based on fluid flow fundamentals (a 1st Law of Thermodynamics
derivation utilizing the pipe diameter and vena contracta
diameters for the continuity equation
). Deviations from theoretical expectation can be assumed under the Coefficient of Discharge. Thus, one can manufacture an orifice meter of known uncertainty
with only the measurement standard in hand and access to a machine shop. The need for flow conditioning, and hence, a fully developed velocity flow profile is driven from the original determination of Cd which utilized fully developed or 'reference profiles' as explained above.
Conversely, the turbine meter operation is not rooted deeply in fundamentals of thermodynamics. This is not to say that the turbine meter is in any way an inferior device. There are sound engineering principles providing theoretical background. It is essentially an extremely repeatable device that is then assured accuracy via calibration. The calibration provides the accuracy. It is carried out in good flow conditions (flow conditions free of swirl and a uniform velocity flow profile) this is carried out for every meter manufactured. Deviations from the as-calibrated conditions would be considered installation effects, and the sensitivity
of the turbine meter to these installation effects is of interest. The need for flow conditioning is driven from the sensitivity of the meter to deviations from as calibrated conditions of swirl and velocity profile.
Generally, recent research indicates that turbine meters are sensitive to swirl but not to the shape of the velocity profile. A uniform velocity profile is recommended, but no strict requirements for fully developed flow profiles are indicated. Also, no significant errors are evident when installing single or dual rotor turbine meters downstream of two elbows out-of-plane without flow conditioning devices.
The resulting flow equation for the mean velocity experienced by the sound path is given by,
----(5)
The case of no flow gives the actual path of the sound when there is zero flow (by equating eq.(5) to zero). In case of theoretical flow profile, say a uniform velocity flow profile where the no-slip condition on the pipe walls is not applied, Fig.(6) shows Ultrasonic Meter sound path - uniform velocity profile which illustrates the resultant sound path.
A theoretical derivation of the Mean velocity equation for this sound path becomes much more complicated. In case of a perfect fully developed real velocity profile of Ultrasonic meter which is shown in Fig.(7) indicating a possible sound path as a result of an installation in a real flow.
Here a mathematical derivation for this Ultrasonic meter is also becomes very complicated. Developing a robust flow algorithm to calculate the mean flow velocity for the sound path can be quite complicated. Now add to this; sound path reflection from the pipe wall, multipaths to add degrees of freedom, swirl and departure from axisymmetric fully developed flow profile and the problem of integrating the actual velocity flow profile to yield volume flow rate can be an accomplishment. Hence the real performance of ultrasonic meters downstream of perturbations, and the need for calibrations is required.
The effectiveness of a flow conditioner can be indicated by the following two key measurements:
Laboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...
” environment for proper performance of inferential flowmeters
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
like orifice
Gas meter
A gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly...
, turbine
Gas meter
A gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly...
, coriolis
Gas meter
A gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly...
, ultrasonic
Gas meter
A gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly...
etc.
Types of Flow
Basically, FlowFluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
in pipes can be classified as follows –
- Fully developed flow (found in world class flow laboratories)
- Pseudo fully developed flow
- Non-swirling, non-symmetrical flow
- Moderate swirling, non-symmetrical flow
- High swirling, symmetrical flow
Types of Flow Conditioners
Flow conditioners shown in fig.(a) can be grouped into following three types –- Those that eliminate swirl only (tube bundles)
- Those that eliminate swirl and non-symmetry, but do not produce pseudo fully developed flow
- Those that eliminate swirl, non-symmetry and pseudo fully developed flow (high performance flow conditioners)
Natural gas measurement
Natural gasNatural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
that carries a lot of liquids with it is known as wet gas
Wet gas
A wet gas is any gas with a small amount of liquid present. The term "wet gas" has been used to describe a range of conditions ranging from a humid gas which is gas saturated with liquid vapour to a multiphase flow with a 90% volume of gas...
whereas natural gas that is produced without liquid is known dry gas
Dry gas
Dry gas is an alcohol-based additive used in automobiles to prevent any water in the fuel from freezing, or to restore combustive power to gasoline spoiled by water. The name Drygas is actually a brand name, owned by Cristy. It is a liquid that is added in to the fuel tank, that absorbs the water...
. Dry gas is also treated as to remove all liquids. The effect of flow conditioning for various popular meters which is used in gas measurement is explained below.
Pipe flow conditions
The most important as well as most difficult to measure aspects of flow measurement are flow conditions within a pipe upstream of a meter. Flow conditions mainly refer to the gas velocity profile, irregularities in the profile, varying turbulence levels within the velocity or turbulence intensity profile, swirl and any other fluid flow characteristics which will cause the meter to register flow different than that expected. It will change the value from the original calibrationCalibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
state referred to as reference conditions that are free of installation effects.
Installation effects
Installation effects such as insufficient straight pipe, exceptional pipe roughness or smoothness, elbows, valveValve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
s, tees and reducer
Reducer
A reducer is the component in a pipeline that reduces the pipe size from a larger to a smaller bore .The length of the reduction is usually equal to the average of the larger and smaller pipe diameters...
s causes the flow conditions within the pipe to vary from the reference conditions. How these installation effects impact the meter is very important since devices which create upstream installation effects are common components of any standard metering design. Flow Conditioning refers to the process of artificially generating a reference, fully developed flow profile and is essential to enable accurate measurement while maintaining a cost-competitive meter standard design. 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.
Law of similarity
The principle of Law of Similarity is used extensively for 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 transferCustody transfer
Custody Transfer in the oil and gas industry refers to the transactions involving transporting physical substance from one operator to another. This includes the transferring of raw and refined petroleum between tanks and tankers; tankers and ships and other transactions...
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.
Velocity flow profile
The most commonly used description of flow conditions within the pipe is the velocity flow profile. Fig.(1) shows the typical velocity flow profile for natural gas measurement. The shape of the velocity flow profile is given by the following equation, ---- (1) The value of n determines the shape of the velocity flow profile. The eq.(1) can be used to determine the flow profile's shape within the pipe by fitting a curve to experimentally measured velocity data. In 1993, the transverse velocities
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
were being measured within the high pressure natural gas environment using hot wire technology to accomplish the data fit. A fully developed flow profile was used as the reference state for meter calibration and determination of Coefficient of Discharge
Discharge coefficient
In a nozzle or other constriction, the discharge coefficient is the ratio of the mass flow rate at the discharge end of the nozzle to that of an ideal nozzle which expands an identical working fluid from the same initial conditions to the same exit pressures-References:, J. B. Calvert, 15 June 2003...
(Cd). For Reynolds Number
to n is approximately 7.5; for Re of , n is approximately 10.0 where a fully developed profile in a smooth pipe was assumed. Since n is a function of Reynolds Number and friction factor, more accurate values of n can be estimated by using the eq.(2), ---- (2) Where, f is friction factor. A good estimate of a fully developed velocity flow profile can be used for those without adequate equipment to actually measure the velocities within the pipe. The following straight pipe equivalent length in eq.(3) was utilized to ensure a fully developed flow profile exists.
---- (3) In eq.(3) the pipe lengths required is significant, hence we need some devices that can able to condition the flow over a shorter pipe length allowing metering packages to be cost competitive and accurate. Here the velocity flow profile is generally three-dimensional. Normally the description requires no axial orientation indication if the profile is asymmetric and if it does exists, then axial orientation with respect to some suitable plane of reference is required. Asymmetry exists downstream of installation effects such as elbows or tees. Usually, the velocity flow profile is described on two planes 90° apart. Using the latest software technology a full pipe cross sectional description of the velocity profile is possible provided sufficient data points are given.
Turbulence intensity
The second description of the flow field state within the pipe is the turbulence intensity. According to an experiment in 1994, the metering errors may exist even when the velocity flow profile is fully developed with perfect pipe flow conditions. Conversely, it was found zero metering errorError
The word error entails different meanings and usages relative to how it is conceptually applied. The concrete meaning of the Latin word "error" is "wandering" or "straying". Unlike an illusion, an error or a mistake can sometimes be dispelled through knowledge...
at times when the velocity profile was not fully developed. Hence this behavior was referred to the turbulence intensity of the gas flow that can cause metering bias error. This behavior accounts in part for the less than adequate performance of the conventional tube bundle.
Swirl
The third description of the flow field's state is swirl. Swirl is the tangential flow component of the velocity vector. The velocity profile should be referred to as the axial velocity profile. As the velocity vector can be resolved into three mutually orthogonal components, the velocity profile only represents the axial component of velocity. fig.(2) showing the Swirl Angle which explains the definition of flow swirl and swirl angle. Note that swirl is usually referenced to full body rotation (that which the full pipeline flow follows one axis of swirl). In real pipeline conditions, such as downstream of elbows two or more mechanisms of swirl may be present.Effects of flow conditioning on Orifice meter
The basic orifice mass flow equation provided by API 14.3 and ISO 5167 is given as, ----(4)Where,
= Mass flow = Coefficient of discharge = Velocity of approach factor Y = Expansion factor
d = orifice diameter
= density of the fluid = differential pressure Now to use the eq.(4), the flow field entering the orifice plate
Orifice plate
An orifice plate is a device used for measuring the volumetric flow rate. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which states that there is a relationship between the pressure of the fluid and the velocity of the fluid...
must be free of swirl and exhibit a fully developed flow profile. API 14.3 (1990) and ISO standards
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
determined the Coefficient of Discharge by completing numerous calibration tests where the indicated mass flow was compared to the actual mass flow to determine coefficient of discharge. In all testing the common requirement was a fully developed flow profile entering the orifice plate. Accurate standard compliant meter designs must therefore ensure that a swirl free, fully developed flow profile is impinging on the orifice plate. There are numerous methods available to accomplish this. These methods are commonly known as “flow conditioning”.
The first installation option is to revert to no flow conditioning, but adequate pipe lengths must be provided by the eq.(2) mentioned above. This generally makes the manufacturing costs for a flow measurement facility unrealistic due to excessively long meter tubes; Imagine meter tubes 75 diameters long.
The second and most well known option is the 19-tube tube-bundle flow conditioner. The majority of flow installations in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
contain the tube bundle. With the help of hot wire, pitot tube
Pitot tube
A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...
and laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
-based computerized measurement systems which allow detailed measurement of velocity profile and turbulence intensity; we know that the tube bundle does not provide fully developed flow. Therefore, this device is causing biased orifice flow measurement. As a result of these recent findings, few tube bundles are specified for flow measurement and reduce the use of such device. Numerous references are available providing performance results indicating less than acceptable meter performance when using the conventional 19-tube test bundle. The individual results should be reviewed to ascertain details such as beta ratio, meter tube lengths, Re and test conditions.
The general indications are that the conventional tube bundle will cause the orifice installation to over register flow values up to 1.5% when the tube bundle is 1 pipe diameter to approximately 11 pipe diameters from the orifice plate. This is caused by a flat velocity profile that creates higher differential pressures than with a fully developed profile. There is a crossover region from approximately 10 to 15 pipe diameters where the error band is approximately zero. Then a slight under-registration of flows occurs for distances between approximately 15 to 25 pipe diameters. This is due to a peaked velocity profile that creates lower differential pressures than a fully developed profile. At distances greater than 25 pipe diameters the error asymptotes to zero. Fig.(3) showing the Conventional Tube Bundle Performance explaining typical characteristic behavior of the popular 19 tube, tube-bundle. An additional drawback of the conventional 19 tube, tube bundle is variation in sizing.
The conventional tube bundle provides errors very much dependent on installation details, that is, the elbows on and out of plane, tees, valves and distances from the last pipe installation to the conditioner and conditioner to the orifice plate. These errors have a great significance. Therefore the latest findings regarding conventional tube bundle performance should be reviewed prior to meter station design and installation.
The final installation option for orifice metering is perforated plate flow conditioners. There is a variety of perforated plates have entered the market. These devices generally are designed to rectify the drawbacks of the conventional tube bundle (accuracy
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
and repeatability
Repeatability
Repeatability or test-retest reliability is the variation in measurements if they would have been taken by a single person or instrument on the same item and under the same conditions. A less-than-perfect test-retest reliability causes test-retest variability. Such variability can be caused by, for...
insufficiency). The reader is cautioned to review the performance of the chosen perforated plate carefully prior to installation. A flow conditioner performance test guideline should be utilized to determine performance. The key elements of a flow conditioner test are -
- Perform a baseline calibration test with an upstream length of 70 to 100 pipe diameters of straight meter tube. The baseline Coefficient of Discharge values should be within the 95% confidence interval for the RG orifice equation (i.e. the coefficient of discharge equation as provided by AGA-3).
- Select values of upstream meter tube length, and flow conditioner location, to be used for the performance evaluation. Install the flow conditioner at the desired location. First, perform a test for either the two 90° elbows out-of-plane installation, or the high swirl installation for
= 0.40 and for 
= 0.67. This test will show whether the flow conditioner removes swirl from the disturbed flow. If the 
is within the acceptable region for both values of 
i.e. 0.40 and 0.67, and if the Cd results vary as 
, then the conditioner is successful in removing swirl. The tests for the other three installations namely, good flow conditions, partly closed valve and highly disturbed flow) may be performed for 
= 0.67, and the results for other (i ratios predicted from the 
correlation. Otherwise, the tests should be performed for a range of p ratios between 0.20 and 0.75. - Perform test and determine the flow conditioner performance for the flow conditioner installed in good flow conditions, downstream of a half closed valve, and for either the double 90° elbow out-of-plane or the high swirl installation.
Effects of flow conditioning on turbine meter
The turbine meter is available in various manufacturer's configurations of a common theme; turbine bladesTurbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
and rotor
Rotor (electric)
The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...
configured devices. These devices are designed such that when a gas stream passes through them they will spin proportionally to the amount of gas passing over the blades in a repeatable fashion. Accuracy is then ensured by completion of a calibration, indicating the relationship between rotational speed
Rotational speed
Rotational speed tells how many complete rotations there are per time unit. It is therefore a cyclic frequency, measured in hertz in the SI System...
and volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
, at various Reynolds Numbers. The fundamental difference between the orifice meter and the turbine meter is the flow equation derivation. The orifice meter flow calculation is based on fluid flow fundamentals (a 1st Law of Thermodynamics
First law of thermodynamics
The first law of thermodynamics is an expression of the principle of conservation of work.The law states that energy can be transformed, i.e. changed from one form to another, but cannot be created nor destroyed...
derivation utilizing the pipe diameter and vena contracta
Vena contracta
Vena contracta is the point in a fluid stream where the diameter of the stream is the least, such as in the case of a stream issuing out of a nozzle,...
diameters for the continuity equation
Continuity equation
A continuity equation in physics is a differential equation that describes the transport of a conserved quantity. Since mass, energy, momentum, electric charge and other natural quantities are conserved under their respective appropriate conditions, a variety of physical phenomena may be described...
). Deviations from theoretical expectation can be assumed under the Coefficient of Discharge. Thus, one can manufacture an orifice meter of known uncertainty
Uncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...
with only the measurement standard in hand and access to a machine shop. The need for flow conditioning, and hence, a fully developed velocity flow profile is driven from the original determination of Cd which utilized fully developed or 'reference profiles' as explained above.
Conversely, the turbine meter operation is not rooted deeply in fundamentals of thermodynamics. This is not to say that the turbine meter is in any way an inferior device. There are sound engineering principles providing theoretical background. It is essentially an extremely repeatable device that is then assured accuracy via calibration. The calibration provides the accuracy. It is carried out in good flow conditions (flow conditions free of swirl and a uniform velocity flow profile) this is carried out for every meter manufactured. Deviations from the as-calibrated conditions would be considered installation effects, and the sensitivity
Sensitivity (control systems)
The controller parameters are typically matched to the process characteristics and since the process may change it is important that the controller parameters are chosen in such a way that the closed loop system is not sensitive to variations in process dynamics...
of the turbine meter to these installation effects is of interest. The need for flow conditioning is driven from the sensitivity of the meter to deviations from as calibrated conditions of swirl and velocity profile.
Generally, recent research indicates that turbine meters are sensitive to swirl but not to the shape of the velocity profile. A uniform velocity profile is recommended, but no strict requirements for fully developed flow profiles are indicated. Also, no significant errors are evident when installing single or dual rotor turbine meters downstream of two elbows out-of-plane without flow conditioning devices.
Effects of flow conditioning on ultrasonic meter
Due to the relative age of the technology, it may be beneficial to discuss the operation of the multipath ultrasonic meter to illustrate the effects of flow profile distortion and swirl. There are various types of flow measurements utilizing high frequency sound. The custody transfer measurement devices available today utilize the time of travel concept. The difference in time of flight with the flow is compared to the time of flight against the flow. This difference is used to infer average flow velocity on the sound path. Fig.(5) showing the Ultrasonic Meter sound path no flow which illustrates this concept.The resulting flow equation for the mean velocity experienced by the sound path is given by,
----(5)The case of no flow gives the actual path of the sound when there is zero flow (by equating eq.(5) to zero). In case of theoretical flow profile, say a uniform velocity flow profile where the no-slip condition on the pipe walls is not applied, Fig.(6) shows Ultrasonic Meter sound path - uniform velocity profile which illustrates the resultant sound path.
A theoretical derivation of the Mean velocity equation for this sound path becomes much more complicated. In case of a perfect fully developed real velocity profile of Ultrasonic meter which is shown in Fig.(7) indicating a possible sound path as a result of an installation in a real flow.
Here a mathematical derivation for this Ultrasonic meter is also becomes very complicated. Developing a robust flow algorithm to calculate the mean flow velocity for the sound path can be quite complicated. Now add to this; sound path reflection from the pipe wall, multipaths to add degrees of freedom, swirl and departure from axisymmetric fully developed flow profile and the problem of integrating the actual velocity flow profile to yield volume flow rate can be an accomplishment. Hence the real performance of ultrasonic meters downstream of perturbations, and the need for calibrations is required.
Effects of flow conditioning on Coriolis meter
Coriolis meter shown in fig.(8) is very accurate in single-phase conditions but inaccurate to measure two-phase flows. It poses a complex fluid structure interaction problem in case of two-phase operation. There is a scarcity of theoretical models available to predict the errors reported by Coriolis meter in aforementioned conditions. Flow conditioners make no effect on meter accuracy while using wet gas due to the annular flow regime, which is not highly affected by flow conditioners. In single-phase conditions, Coriolis meter gives accurate measurement even in presence of severe flow disturbances. There is no need for flow conditioning before the meter to obtain accurate readings from it, which would be the case in other metering technologies like orifice and turbine. On the other hand in two-phase flows, the meter consistently gives negative errors. The use of flow conditioners clearly affects the reading of the meter in aerated liquids. This phenomenon can be used to get fairly accurate estimate of flow rate in low gas volume fraction liquid flows.Fluid flow measurement
Flow conditioning makes a huge effect on the accuracy of liquid turbine meter which results into flow disturbances. These effects are mainly caused by debris on strainer screens, for various upstream piping geometries and different types of flow conditioners.The effectiveness of a flow conditioner can be indicated by the following two key measurements:
- Percentage variation of an average meter factor over the defined range of flow disturbances for a given flow rate and inlet piping geometry. The lesser the value of percentage variation of an average meter factor over the range of flow disturbances, the better will be the performance of flow conditioner.
- Percentage meter factor repeatability for each flow disturbance, at a given flow rate and inlet piping geometry. The lesser the value of percentage meter factor repeatability at a given set of installation/operating conditions, the better will be the performance of flow conditioner.
See also
- Flow measurementFlow measurementFlow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
- Orifice meterGas meterA gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly...
- Turbine meterGas meterA gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly...
- Ultrasonic flow meterUltrasonic flow meterAn ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in...
- Coriolis meter
- Fluid dynamicsFluid dynamicsIn physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
- Wet gasWet gasA wet gas is any gas with a small amount of liquid present. The term "wet gas" has been used to describe a range of conditions ranging from a humid gas which is gas saturated with liquid vapour to a multiphase flow with a 90% volume of gas...
- Dry gasDry gasDry gas is an alcohol-based additive used in automobiles to prevent any water in the fuel from freezing, or to restore combustive power to gasoline spoiled by water. The name Drygas is actually a brand name, owned by Cristy. It is a liquid that is added in to the fuel tank, that absorbs the water...
- Orifice plateOrifice plateAn orifice plate is a device used for measuring the volumetric flow rate. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which states that there is a relationship between the pressure of the fluid and the velocity of the fluid...
- Mass flow meterMass flow meterA mass flow meter, also known as an inertial flow meter is a device that measures mass flow rate of a fluid traveling through a tube. The mass flow rate is the mass of the fluid traveling past a fixed point per unit time....
- Mass flow rateMass flow rateMass flow rate is the mass of substance which passes through a given surface per unit time. Its unit is mass divided by time, so kilogram per second in SI units, and slug per second or pound per second in US customary units...
- Volumetric flow rateVolumetric flow rateThe volumetric flow rate in fluid dynamics and hydrometry, is the volume of fluid which passes through a given surface per unit time...