14 3 2 e5 updated 2 fm Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids— Concentric, Square edged Orifice Meters Part 2 Specification and Installation Requirements AGA Report No 3[.]
General
This document establishes design and installation parameters for measurement of fluid flow using concentric, square- edged, flanged tapped orifice meters.
Construction and Installation Requirements
This document details the essential design parameters for metering facilities utilizing orifice meters, highlighting the mechanical tolerances applicable across a variety of orifice diameter ratios supported by experimental data.
The decision to upgrade existing installations to comply with the new standard is at the discretion of the involved parties It is important for these parties to recognize that failing to upgrade may result in measurement bias errors caused by insufficient flow conditioning and inadequate upstream straight pipe lengths.
It is advisable to utilize the calculation procedures and techniques outlined in API MPMS Ch 14.3.1/AGA Report No 3, Part 1 and API MPMS Ch 14.3.3/AGA Report No 3, Part 3, as they offer significant enhancements over earlier methods However, it is important to note that the uncertainty levels for flow measurement with existing equipment may differ from those specified in API MPMS Chapter 14.3.1/AGA Report No 3, Part 1.
To ensure accurate measurements, it is advisable to avoid using orifice meters at the extremes of their diameter ratio (β r ) ranges Adopting a conservative approach in metering design and practice, particularly by utilizing tighter tolerances within the mid-diameter ratio (β r ) ranges, significantly increases the likelihood of achieving optimal measurement accuracy This principle is supported by the uncertainty guidelines outlined in API MPMS Chapter 14.3.1/AGA Report No 3, Part 1.
The standard specifies that the β r value should be between 0.10 and 0.75, with optimal uncertainty for the orifice plate coefficient of discharge (C d) occurring at β r values between 0.2 and 0.6, and orifice bore diameters of at least 0.45 inches While diameter ratios and orifice bore diameters outside this range are permissible, users are advised to refer to the uncertainty section in API MPMS Chapter 14.3.1/AGA Report No 3, Part 1 for any limitations.
Achieving optimal measurement uncertainty starts with effective design, but it also requires careful consideration of the metering system's application and meter maintenance These critical aspects, which vary significantly based on flow rate, fluid type, and operational needs, are not directly addressed by a single standard Therefore, users must select the most suitable meter for their specific application and establish appropriate maintenance protocols for their measurement systems.
No other document is identified as indispensable or required for the application of this standard.
Definitions
The definitions are given to emphasize the particular meaning of the terms as used in this standard.
The calculated orifice plate bore diameter (d) divided by the calculated meter tube internal diameter (D).
The measured orifice plate bore diameter (d m ) divided by the measured meter tube internal diameter (D m ).
The reference orifice plate bore diameter (d r ) divided by the reference meter tube internal diameter (D r ).
The static pressure difference measured between the upstream and the downstream flange taps
A time mean of the static pressure difference measured between the upstream and downstream flange taps.
A single measurement of ΔP at any instance in time.
3.1.7 root mean square differential pressure (ΔP rms )
The r square root of the sum of squares of the difference between the instantaneous differential pressure (ΔP t) and time mean differential (ΔP avg).
The tap holes must be positioned with the upstream tap center 1 inch upstream from the nearest plate face and the downstream tap center 1 inch downstream from the nearest plate face, ensuring that both taps are aligned in the same radial position.
Flow conditioners can be classified into two categories: flow straighteners or flow conditioners.
Flow straighteners are essential devices designed to minimize or eliminate the swirl in a flowing stream However, their effectiveness in creating the specific flow conditions needed to accurately replicate the orifice plate coefficient of discharge database values may be limited For detailed installation requirements, refer to Table 8a and Table 8b.
Flow conditioners are devices that effectively eliminate swirl from a flowing stream, ensuring a pseudo fully developed flow profile By redistributing the flow, they accurately replicate the orifice plate coefficient of discharge database values, having successfully passed the performance test protocol outlined in Annex D.
The straight sections of pipe, including all segments that are integral to the orifice plate holder, upstream and downstream of the orifice plate, as specified in 5.1.
The internal diameter of the upstream section of the meter tube, determined at the flowing temperature (T f) as outlined in API MPMS Ch 14.3.1/AGA Report No 3, Part 1, is essential for calculating the diameter ratio and Reynolds number.
3.1.12 published meter tube internal diameter ( D i )
The inside diameter as published in standard handbooks for engineers This internal diameter is used for determining the required meter run length in Table 7, Table 8a, and Table 8b.
3.1.13 measured meter tube internal diameter ( D m )
The average inside diameter of the meter tube's upstream section was measured to be 1 inch, taken upstream of the orifice plate's adjacent face, and at the temperature of the meter tube (T m) during the internal diameter measurements, as outlined in section 5.1.2.
3.1.14 reference meter tube internal diameter ( D r )
The upstream section of the meter tube has an internal diameter calculated at the reference temperature (T r), as outlined in section 5.1.2 This certified internal diameter of the reference meter tube is crucial for accurate measurements.
A thin square-edged plate with a machined circular bore, concentric with the meter tube ID, when installed
The internal diameter of the orifice plate measuring aperture, calculated at the flowing temperature (T f) as outlined in API MPMS Ch 14.3.1/AGA Report No 3, Part 1, is essential for determining the flow rate This calculated orifice plate bore diameter (d) plays a crucial role in the flow equation.
3.1.17 measured orifice plate bore diameter ( d m )
The measured internal diameter of the orifice plate measuring aperture at the temperature of the orifice plate (T m ) at the time of bore diameter measurements, determined as specified in 4.4.
3.1.18 reference orifice plate bore diameter ( d r )
The internal diameter of the orifice plate measuring aperture at the reference temperature (T_r) is determined according to the specifications outlined in section 4.4 This diameter is recognized as the certified or stamped bore diameter of the orifice plate.
An orifice fitting, commonly referred to as an orifice flange union (OFU), is a crucial pressure-containing piping element designed to securely hold and position the orifice plate within a piping system.
The orifice plate, the orifice plate holder with its associated differential pressure sensing taps, the meter tube, and the flow conditioner, if used.
The roughness average (R a ) defined in this standard follows ANSI B46.1, representing the arithmetic average of the absolute values of the profile height deviations measured from the graphical centerline within the sampling length of the surface profile.
A radially drilled hole in the wall of the meter tube or orifice fitting should be perpendicular to the centerline of the meter tube or orifice plate holder, ensuring that the inside edge is flush and free of burrs.
The flowing fluid temperature measured at the designated location, as specified in 6.5.
In flow measurement, a temperature sensing device is placed in the flowing stream to measure the fluid's temperature Corrections for dynamic effects are necessary if the fluid velocity exceeds 25% of the sound speed, although this is a rare occurrence It is crucial to ensure that the temperature sensing elements are properly coupled to the flowing stream rather than the steel of the meter tube, a practice recommended for all orifice meter installations The measured temperature is considered the static temperature of the fluid.
The measured temperature of the orifice plate and/or the meter tube at the time of the diameter measurements, as specified in 4.4 and 5.1.2.
The reference temperature used to determine the reference orifice plate bore diameter (d r ) and/or the reference internal meter tube diameter (D r ), as specified in 4.4 and 5.1.2.
Symbols/Nomenclature
This standard reflects orifice meter application to fluid flow measurement with symbols in general technical use.
Symbol Represented Quantity c Speed of sound
C d Orifice plate coefficient of discharge
The coefficient of discharge for a flange tap orifice plate, denoted as ΔC_d (FT)/C_d, represents the percent difference between the baseline coefficient of discharge and the installation effect The orifice plate bore diameter is calculated at the flowing temperature, T_f, while the diameter measured at temperature, T_m, and the diameter calculated at the reference temperature, T_r, are also significant parameters in this context.
D Meter tube internal diameter calculated at flowing temperature, T f
D i Published meter tube internal pipe diameter
DL Meter tube length downstream of orifice plate in multiples of published internal pipe diameters (see
D m Meter tube internal diameter measured at T m
D r Meter tube internal diameter calculated at reference temperature, T r e Orifice plate bore thickness
E Orifice plate thickness f Frequency °F Temperature, in degrees Fahrenheit l Recommended lengths of gauge line
NPS Nominal Pipe Size ΔP Orifice plate differential pressure ΔP avg Average orifice plate differential pressure ΔP rms Root mean square of the fluctuating differential pressure ΔP t Instantaneous orifice plate differential pressure
P f Static pressure of the fluid at the pressure tap °R Temperature, in degrees Rankine
T f Temperature of fluid at flowing conditions
T m Temperature of the orifice plate and/or meter tube at time of diameter measurements
T r Reference temperature (68 °F) of orifice plate bore diameter and/or meter tube internal diameter
UL Meter tube length upstream of orifice plate in multiples of published internal pipe diameters (see
The UL2 meter tube length is measured from the flow conditioner exit to the orifice plate in multiples of the published internal pipe diameters The linear coefficients of thermal expansion for the orifice plate and meter tube materials are denoted as α₁ and α₂, respectively The beta ratio (β), which is the ratio of the orifice plate bore diameter to the meter tube internal diameter, is calculated at the flowing temperature (Tₓ) Additionally, the modified beta ratio (βₘ) is determined at temperature (Tₘ), while the reference beta ratio (βᵣ) is calculated at the reference temperature (Tᵣ) Other important parameters include the orifice plate bore eccentricity (ε) and the bevel angle of the orifice plate (θ).
General
The symbols for the orifice plate dimensions are shown in Figure 1.
Orifice Plate Faces
The orifice plate must have flat upstream and downstream faces, with allowable deviations from flatness not exceeding 1% of the dam height (0.010 in per in of dam height) under non-flowing conditions The dam height can be determined using the formula \((D_m - d_m)/2\) This flatness criterion applies to any two points on the orifice plate within the pipe's inside diameter Illustrations of the departure from flatness can be found in Figures 2a, 2b, and 2c.
The surface roughness of the upstream and downstream faces of the orifice plate shall have no abrasions or scratches visible to the naked eye that exceed 50 microinches R a
Figure 1—Symbols for Orifice Plate Dimensions
Mark outlet on orifice fitting plates
Mark inlet on paddle-type plates
Figure 2a—Orifice Plate Departure from Flatness (Measured at Edge of
Orifice Bore and Within Inside Pipe Diameter)
Figure 2b—Alternative Method for Determination of Orifice Plate Departure from Flatness (Departure from
Figure 2c—Maximum Orifice Plate Departure from Flatness
Orifice plate outside diameter Pipe inside diameter, D m
Orifice bore, d m Departure from flatness Maximum allowable departure from flatness = 0.005 (D m – d m)
Orifice plate outside diameter Pipe inside diameter, D m
To verify the surface roughness of an orifice plate, an electronic-averaging-type surface roughness instrument should be used, with a minimum cutoff value of 0.03 inch Alternatively, other devices, such as a visual comparator, may be acceptable for measuring surface roughness, provided they demonstrate equivalent repeatability and reproducibility to the electronic instrument.
To ensure optimal performance, it is essential to maintain the orifice plate in a clean condition, free from dirt, ice, grease, and other contaminants This can be achieved by implementing a regular inspection schedule tailored to service conditions, whether daily, weekly, monthly, or quarterly Neglecting to clean the plate can lead to increased uncertainty in the orifice plate coefficient of discharge, denoted as \$C_d (FT)\$ After each inspection, the plate must be thoroughly cleaned before being returned to service.
Orifice Plate Bore Edge
The upstream edge of the orifice plate bore must be square and sharp to ensure accurate flow measurement If the edge reflects light without magnification or shows a beam of light when inspected with an orifice edge gauge, it is deemed too dull for precise measurements.
To estimate the appropriate sharpness of an orifice plate, compare its bore edge with that of a properly sharpened reference orifice plate of the same nominal diameter The evaluated orifice plate's bore edge should visually and tactilely match the edge of the reference plate.
The orifice plate bore must have upstream and downstream edges that are free from any visible defects, including flat spots, feathered textures, roughness, burrs, bumps, nicks, and notches.
If there is any doubt about whether the edge has sufficient quality for accurate metering, the orifice plate should be replaced.
Orifice Plate Bore Diameter (d m , d r ) and Roundness
The orifice bore diameter (d m) is determined by calculating the mean of four or more evenly spaced diameter measurements taken at the inlet edge It is essential that none of these measurements deviate from the mean by more than the tolerances specified in Table 1 Additionally, the temperature of the orifice plate must be recorded at the time of these measurements, which should be conducted under thermally stable conditions, ensuring that the temperature remains constant within ± 1 °F (± 0.5 °C).
The orifice plate bore diameter (d r) is defined as the calculated reference diameter at reference temperature (T r) and can be determined using the following equation:
The linear coefficient of thermal expansion for the orifice plate material, denoted as \$\alpha_1\$, is referenced in Table 2 The orifice plate bore diameter at the reference temperature, \$d_r\$, is compared to the bore diameter measured at temperature \$T_m\$, denoted as \$d_m\$.
T m is the temperature of the orifice plate at time of diameter measurements;
T r is the reference temperature of the orifice plate bore diameter.
NOTE α 1 , T m , and T r have to be in consistent units For the purpose of this standard, T r is assumed to be 68 °F.
The reference diameter, denoted as \$d_r\$, is determined at temperature \$T_r\$ and is essential for calculating the bore diameter \$d\$ under flowing conditions, as outlined in API MPMS Ch.14.3.1 and AGA Report No 3, Part 1 The relationship is given by the formula \$d_r = d_m[1+\alpha_1(T_r–T_m)]\$.
Table 1—Roundness Tolerance for Orifice Plate Bore Diameter, d m
Diameters below 0.45 inches are permitted; however, their use may lead to uncertainties that exceed the limits outlined in API MPMS Chapter 14.3.1 and AGA Report No 3, Part 1.
Table 2—Linear Coefficient of Thermal Expansion
Linear Coefficient of Thermal Expansion ( α )
USC (in./in °F) Metric Units
NOTE For flowing temperature limits or other materials, refer to the American Society for Metals (ASM) Metals
The "Handbook of Engineering Properties of Steel" and the "Handbook of Stainless Steels" provide essential guidelines for various materials For stainless steels, optimal flowing conditions range from +32 °F to +212 °F, while Monel is effective between +68 °F and +212 °F Additionally, for flowing conditions between –7 °F and +154 °F, it is recommended to consult API MPMS Chapter 12.2.1 The linear coefficient of thermal expansion for Type 304/316 stainless steel is derived as the average of Type 304 and Type 316.
NOTE Over a temperature range from 32 °F to 130 °F the maximum difference in calculated flow between use of the 304/316 average coefficient and either the 304 or 316 coefficient is less than 0.005 % (50 ppm).
Orifice Plate Bore Thickness (e)
The orifice plate bore must feature a smooth, constant-diameter cylindrical interior, free from any visible defects like grooves, ridges, pits, or lumps The length of this cylinder corresponds to the thickness of the orifice plate bore (e).
The minimum allowable orifice plate bore thickness (e) is defined by e ≥ 0.01d r or e > 0.005 in., whichever is larger.
The maximum allowable value for the orifice plate bore thickness (e) is defined by e ≤ 0.02D r or e ≤ 0.125d r , whichever is smaller, but e shall not be greater than the maximum allowable orifice plate thickness (E).
When the thickness of the orifice plate (E) surpasses the thickness of the orifice bore (e), a bevel is necessary on the downstream side of the orifice bore An unbeveled orifice plate with a bore thickness (e) exceeding the limits outlined in Table 3 is not covered by this standard.
NOTE Existing orifice plates, whose edge thickness meets the value defined by e < 0.033D m , need not be rebeveled unless reconditioning is required for other reasons.
For ease in machining, the next smaller values of e, in multiples of 0.03125 ( 1 /32 in.), may be used.
Orifice plate bores that demonstrate any convergence from inlet to outlet are unacceptable.
Bi-directional flow measurement necessitates a uniquely designed meter tube and an unbeveled orifice plate, which means it is not included in the standard measurement protocols.
Orifice Plate Thickness (E)
The minimum, maximum, and recommended values of orifice plate thickness (E) for Types 304 and 316 stainless steel orifice plates are given in Table 3.
The maximum allowable differential pressures for the recommended orifice plate thicknesses in Table 3 are applicable for operating temperatures up to 150 °F For conditions not specified in Table 3, including orifice diameter ratios, meter tube sizes, and orifice plate thicknesses, refer to the tables in Annex E If your specific application is not addressed in Table 3 or Annex E, it is advisable to consult the orifice plate and/or holding device manufacturer for detailed information on deflection, as outlined in Annex F of the 2000 version of the AGA Engineering Technical Note on High Differential.
Pressure Across Orifice Fittings) for a given diameter ratio, temperature, orifice plate material, orifice plate holder, and differential pressure.
Using an orifice plate thickness that deviates from the recommended specifications is permissible for both new and existing orifice plate holding devices, provided the thickness falls within the maximum and minimum limits outlined in Table 3 Additionally, it is essential to ensure that the tolerances and limits for orifice plate eccentricity, bore thickness, differential pressure tap hole, and expansion-factor pressure ratio are met.
For incompressible fluids, the maximum differential pressure across the plate is constrained by the structural integrity of the fitting design, as detailed in Table 3 and Annex E If there is a need to exceed these specified limits, it is essential to consult the manufacturer for the allowable maximum pressure for the fitting design Furthermore, it is crucial that the flowing conditions downstream of the orifice plate remain above the local vapor pressure of the fluid.
Consulting orifice fitting manufacturers is essential to establish the maximum allowable differential pressure when changing orifice plates under flowing conditions High differential pressures can create significant forces that complicate the removal of the plate and may lead to potential damage to both the orifice plate and the fitting.
Table 3—Orifice Plate Thickness and Maximum Allowable Differential Pressure
Based on the Structural Limit
Published Inside Pipe Diameter Orifice Plate Thickness, E (inches) Maximum
Maximum Allowable Δ P (in.-H 2 0) (inches) (inches) Minimum Maximum Recommended Orifice Fitting Orifice Flanges
The use of high differential pressures (ΔP/P f > 0.7 in of water/psia, where the ΔP is in inches of water at 60 °F and
P f is in psia) will result in expansion factor uncertainties in excess of 0.1 % (see 12.4.2 of API MPMS Ch.14.3.1/AGA Report No 3, Part 1).
Operators must recognize that maintaining a constant orifice plate size can lead to significant measurement errors during low-flow periods, especially when there are drastic fluctuations between high and low flows It is generally advisable to operate within 10% to 90% of the calibrated differential span for optimal accuracy Additionally, modern digital (electronic) transmitters can enhance rangeability It is essential to assess the impact of transducer and transmitter accuracy for wide-ranging applications against the potential savings in installation costs.
For the full range of orifice plate thicknesses, the maximum allowable orifice plate differential pressure can be obtained from Annex E.
Higher differential pressures lead to increased gas velocities in meter runs and greater permanent pressure losses It is essential to assess gas velocities for each installation, considering factors like noise, erosion, and thermowell vibration Meter run velocity is influenced by various factors, and users may have unique practices and limits regarding velocity Notably, the maximum allowable differential pressures listed in Table 3 do not account for meter-run gas velocity.
The maximum allowable differential pressure is capped at 1000 inches of water column, which corresponds to the limit of the coefficient of discharge database For additional information regarding the maximum allowable differential pressure limit, please consult section 4.6.
NOTE 2 Maximum allowable differential pressure is calculated for worst-case diameter ratio (typically β = 0.55 – 0.65) Other diameter ratios may be able to go to higher differential pressures (see Annex E).
NOTE 3 The maximum differential pressure applies to stainless steel plates at a maximum temperature of 150 °F, and for the recommended plate thickness.
NOTE 4 Maximum allowable differential pressure for other plate thicknesses refer to Annex E.
For single- or dual-chamber fittings, it is assumed that the orifice plate seal ring deflects under axi-symmetric conditions without experiencing plastic deformation, and therefore, the impact on the seal ring was not examined.
When operating at very high differential pressures, it is crucial to account for thermodynamic effects, particularly the temperature changes caused by the Joule-Thompson effect as the stream flows through the orifice Additionally, attention must be paid to the limits on the pressure ratio (ΔP/Pf), especially at low pressures, as a sudden drop in pressure can lead to significant changes in temperature and density.
Table 3—Orifice Plate Thickness and Maximum Allowable Differential Pressure
Based on the Structural Limit (Continued)
Published Inside Pipe Diameter Orifice Plate Thickness, E (inches) Maximum
Maximum Allowable Δ P (in.-H 2 0)(inches) (inches) Minimum Maximum Recommended Orifice Fitting Orifice Flanges
The permanent pressure drop is crucial as it indicates the energy loss incurred during fluid transport through a pipeline Numerous technical references provide data on the relationship between permanent pressure loss and the β ratio for various types of orifice meters, including concentric, square-edged, and flange-tapped designs.
When selecting a β value of 0.30 with a ΔP of 400 inches of H2O, the resulting permanent pressure loss is roughly 91% of 400 inches, equating to about 364 inches of H2O or approximately 13 psi Conversely, opting for a β value of 0.50 at a ΔP of 100 inches of H2O leads to a permanent pressure loss of around 75% of 100 inches, which translates to about 75 inches of H2O, or roughly 3 psi.
Orifice Plate Bevel (θ)
The plate bevel angle (θ) refers to the angle formed between the bevel and the downstream face of the plate, with an allowable range of 45 degrees ± 15 degrees.
The surface of the plate bevel shall have no defects visible to the naked eye, such as grooves, ridges, pits, or lumps.
If a bevel is required, its minimum dimension, (E-e), measured along the axis of the bore shall not be less than 0.0625
Description
The meter tube comprises a straight upstream pipe of uniform diameter, with a length defined in installation Tables 7 and 8, along with a flow straightener or conditioner if applicable, an orifice plate holder, and a downstream pipe of specified length (DL) as indicated in Tables 7, 8a, and 8b The upstream section is characterized by the length of straight pipe that extends from the upstream face of the orifice plate to the nearest upstream alteration in cross-sectional area, excluding any flanged fittings permitted by the standard, or any change in the pipe's centerline axis.
Section 6.3.1 outlines the required lengths for the upstream and downstream pipe sections, while sections 5.1.1, 5.1.2, and 5.1.3 detail the tolerances for the diameter and the restrictions on the inside surface of the meter tube.
No pipe connections are allowed within the designated upstream and downstream meter tube sections, except for specified pressure taps, temperature probes, flow conditioner attachments, orifice plate holders, and necessary in-line meter tube flanges Any downstream flange connection or weld must be at least 2 inches away from the downstream face of the orifice plate Additionally, any downstream weld within 0.5D or 2 inches from the orifice plate must be ground or machined to comply with the required out-of-roundness and surface roughness standards Care should be taken to prevent gasket protrusion into the line for any downstream flange connection within this specified distance.
The permanent pressure loss ≅ ΔP(1 – β 2 ) β Losses as a % of Δ P
The 0.75 44 meter tube flange must be positioned at the orifice plate, with the designated flow conditioner located in the upstream meter tube section, or at a distance of 10D i for meter tubes lacking a flow conditioner (excluding permitted flanged fittings as per the standard) All flanges and attachments within the specified meter tube lengths must comply with the requirements outlined in sections 5.1.1, 5.1.2, 5.1.3, and 5.1.4.
The sections of the meter tube where the orifice plate holder is attached, as well as the adjacent pipe sections, must adhere to the specifications outlined in 5.1.1.1, 5.1.1.2, and 5.1.1.3 Due to the increased upstream meter tube length requirements specified in Table 7 and Table 8, the necessary upstream meter tube section must comply with these specifications and is limited to the lengths indicated in the tables or 17 internal pipe diameters, whichever is shorter Additionally, the piping roughness R a upstream of this length should not exceed 600 microinches (μin.).
The internal surface roughness of the meter tube must be measured at the same axial locations used for verifying the internal diameter The specified R a values are the arithmetic average roughness obtained with an electronic-averaging surface roughness instrument, which should have a cutoff value of at least 0.03 inch Alternative surface roughness devices may be used if they demonstrate equivalent repeatability and reproducibility A minimum of four roughness measurements is required for accurate assessment.
The mean (arithmetic average) of these four or more roughness measurements is defined as the meter tube internal surface roughness
For meter runs with nominal diameters of 12 inches or smaller, the maximum meter-tube roughness is limited to 300 microinches R_a for diameter ratios (β_r) less than 0.6, and to 250 microinches R_a for diameter ratios (β_r) of 0.6 or greater Additionally, the minimum roughness must not fall below 34 microinches for all diameter ratios.
For meter runs with nominal diameters exceeding 12 inches, the maximum meter-tube roughness is limited to 600 microinches R_a for diameter ratios (β_r) below 0.6, while it is capped at 500 microinches R_a for diameter ratios of 0.6 or higher Additionally, regardless of the diameter ratio, the minimum meter-tube roughness must be at least 34 microinches.
NOTE The use of lower diameter ratios (β r ) reduces the effect of pipe roughness on uncertainty.
Smooth commercial pipes that are meticulously chosen can be utilized, and to enhance the smoothness of the meter tube, the interior walls of the pipe may undergo machining, grinding, or coating to fulfill the necessary specifications.
Irregularities such as grooves, scoring, or ridges that exceed the tolerances specified in section 5.1.3 are not permitted, as they negatively impact the inside diameter While pits on the surface of the meter tube are undesirable, they are acceptable as long as their measurements remain within the surface roughness and diameter tolerance limits, and do not compromise the tube's pressure integrity Any irregularities that surpass these tolerances must be corrected.
To ensure accurate measurements, it is essential to maintain the interior of the meter tube clean and devoid of any dirt, ice, grit, grease, oil, free liquid, or other foreign materials Any damage or buildup of such extraneous substances can lead to increased uncertainty in the orifice plate coefficient of discharge, denoted as \$C_d(FT)\$.
The measured internal diameter of the meter tube (D m) shall be determined as specified in 5.1.2.1, 5.1.2.2, 5.1.2.3, 5.1.2.4, and 5.1.2.5.
A minimum of four equally spaced measurements of the internal diameter must be taken in a plane located 1 inch upstream from the orifice plate's upstream face The average of these measurements is referred to as the measured meter tube internal diameter (D m).
Individual check measurements of the internal diameter of the upstream section of the meter tube must be conducted at a minimum of two additional cross-sections, excluding the orifice plate gasket or sealing device diameter The specific locations for these measurements around the circumference and along the axis of the meter tube are not defined It is essential to perform these checks at points that will reveal the maximum and minimum dimensions of the internal diameter in the upstream section.
Individual check measurements must be conducted at least two pipe diameters away from the orifice plate or beyond the orifice plate holder weld or flange, depending on which distance is greater Additional measurements should be taken at specific points within the UL dimension.
Individual check measurements are utilized to ensure the consistency of the internal diameter in the upstream section of the meter tube, as outlined in section 5.1.3; however, these measurements do not contribute to the calculation of the average internal diameter of the meter tube.
Orifice Plate Holders
Orifice flanges, also known as Orifice Flange Unions (OFUs), serve as holders for orifice plates in orifice meter tube installations It is essential that these flanges are constructed and attached to the pipe in compliance with the mechanical specifications outlined in sections 5.1.1 and 5.1.4.
Table 5—Example Meter Tube Internal Diameter Roundness Tolerances—
All Upstream Meter Tube Individual Internal Diameter Measurements
Position Meter Tube Internal Diameter Measurements
Any distortion of the pipe resulting from welding the flange to the pipe shall be removed by machining or grinding to meet the limitations specified in 5.1.3.
Orifice fittings are widely utilized in the industry as a type of orifice plate holder, allowing for the reproduction of orifice coefficients as defined by API MPMS Ch.14.3.1/AGA Report No 3, Part 1, while maintaining the same uncertainty limits as traditional orifice plates held between flanges To ensure accuracy, these devices must be manufactured according to the specified tolerances in the standard It is important to recognize practical considerations and conduct critical inspections unique to orifice fittings The information provided is based on devices known at the time of the standard's development and may not encompass recent innovations, which can still comply with the standard if they meet all specified tolerances.
Orifice Fittings Considerations
When using an upstream flanged orifice fitting, the mean inside diameter of the connected meter tube must match the fitting's diameter within specified tolerances It is essential to connect the inlet side to the upstream section of the meter tube first and ensure it is properly centered, avoiding any sharp edges at the junction.
To ensure proper alignment at a flanged connection, it is recommended to ream two diametrically opposed bolt holes and install snug-fitting bolts, or alternatively, use dowel pins Various alignment methods can be employed as long as they achieve the same effective result.
When welding the upstream section of the meter tube to the orifice-fitting body, it is essential to eliminate any distortion caused by the welding process This can be achieved through machining or grinding to ensure compliance with the specified requirements.
Inspecting a weld neck orifice fitting can be more challenging than examining a conventional flanged orifice meter, especially when the fitting is already connected to the meter tube This difficulty is heightened if the meter tube is small, making it hard to take measurements near the orifice plate To facilitate easier inspections, it is advisable for the fitting to include at least one flanged side, ideally on the downstream side Users should consult the relevant pressure vessel and pipeline codes to ensure the design is suitable for their system Additionally, all mechanical tolerance measurements should be conducted after the fitting has undergone pressure testing at the maximum required test pressure.
In orifice fittings, fluid may potentially bypass the orifice plate After pressure-testing the meter run according to the relevant code, it is essential to conduct tests to verify that there is no communication or leakage in the differential pressure taps and that no fluid bypass occurs in the holding or sealing devices.
Pressure Taps
For meter tubes utilizing flange taps, the upstream pressure tap hole should be positioned 1 inch from the upstream face of the orifice plate, while the downstream pressure tap hole must be 1 inch from the downstream face Each tap hole's location should adhere to the specified 1-inch dimension within the tolerances illustrated in Figure 3 It is advisable to apply a maximum diameter ratio (β) of 0.75 for allowable variations in pressure tap hole locations during installation design Additional details can be found in Annex F.9 of the 2000 version of this document.
Orifice fittings may require different methods of confirming pressure tap hole location than orifice flanges.
Flange taps must only be used for measuring static and/or differential pressure, and any flow through or out of them for other purposes is strictly prohibited This includes any unintended flows caused by manufacturing defects that create communication between taps For any additional fluid needs for other instruments, it is essential to utilize taps located outside the specified dimensions of the meter tube.
The sharing of metering taps by multiple differential pressure devices may cause increased uncertainty and/or operational problems If possible, such a practice should be avoided
When using an orifice fitting, it is essential to take pressure tap hole measurements before the final fabrication of the meter tube, particularly when the fitting will be welded to one end of the piping These measurements can be accurately obtained using commercially available micrometers and gauges, and other valid techniques for verifying the pressure tap hole location are also acceptable.
In orifice fittings, the orifice plate is secured by a carrier mechanism that ensures accurate positioning relative to the pressure tap holes For pressure tap hole location testing, it is essential to use a plate/carrier combination that matches the design intended for practical application Additionally, if the internal mechanism of an orifice fitting is replaced, a thorough inspection must be conducted again.
Figure 3—Allowable Variations in Pressure Tap Hole Location
Flange Tap–Nominal 4-in and larger Flange Tap–Less than nominal 4-in.
For flange-tapped orifice fittings, it is crucial to maintain the correct location of the flange tap in relation to the orifice plate faces While using plates of varying thickness is permissible, they must fall within the specified maximum and minimum ranges outlined in Table 3, and adhere to the differential pressure tap hole tolerances indicated in Figure 3, or the fitting must be redrilled Additionally, seals and orifice holding devices should not interfere with the plate's positioning relative to the taps, and it is essential to verify that the seal/plate combinations comply with the tolerance requirements for flange tap locations.
When using orifice flanges (OFUs), the placement of the pressure tap hole is determined by measuring from the flange face to the center of the pressure tap hole It is essential to account for the thickness and compression of gaskets, o-rings, or other sealing mechanisms when the orifice plate is positioned between the two flanges.
Pressure tap holes must be drilled radially to the meter tube, ensuring that the centerline of the tap hole intersects and forms a right angle with the axis of the meter tube.
The pressure tap holes on the inner surface of the meter tube must have a diameter of 3/8 inches (0.375 in.) ± 1/64 inches (0.016 in.), allowing for a maximum diameter of 0.391 inches and a minimum of 0.359 inches for pipes with a nominal diameter of 2 inches or 3 inches For pipes with a nominal diameter of 4 inches or more, the diameter of the pressure tap holes shall be 1/2 inches (0.5 in.) ± 1/64 inches (0.016 in.), resulting in a maximum diameter of 0.516 inches and a minimum of 0.484 inches.
The orifice plate holder's pressure tap holes can be drilled and modified to accommodate the required size for pressure-sensing line connections, ensuring that the edges of the pressure tap holes are sharp and square.
The tap hole diameter must remain unchanged for a length of 2.5 times the tap hole diameter from the meter tube's inner surface Any reduction in the tap hole diameter during operation, caused by the accumulation of liquids or particulate contamination, is not permissible.
All pressure tap holes shall be round to a tolerance of ± 0.004 in throughout their length.
Similarly, the inside diameter of the gauge line should remain constant up to the differential pressure sensor and/or manifold.
To prevent resonance in the gauge line, it is essential to keep the gauge line length as short as possible or to specify lengths (l) based on the highest frequency (f) of concern, following the appropriate formulas.
To prevent resonance in the gauge line, it is essential to keep its length as short as possible If the gauge line exceeds the limits outlined in Equation 6, lengths should be determined based on the highest frequency of concern, as indicated in Equations 7 through 10 In cases where gauge line resonance is suspected or anticipated, additional testing may be necessary.
0 ≤ l 1 ≤ 0.25c / (2π f ) (6) l 2 = 2.5c / (2π f ) (7) l 3 = 5.5c / (2π f ) (8) l 4 = 8.5c / (2π f ) (9) l 5 = 11.5c / (2π f ) (10) where c is the speed of sound in the flowing fluid at operating conditions; f is the frequency of pulsation levels; π is the mathematical constant = 3.14159.
To prevent resonance and amplification of pressure pulsations in gauge lines, it is essential that both lines are of equal length and maintain a consistent internal diameter, particularly in low-pressure applications Additionally, using direct-mount manifolds can help mitigate the effects of pulsation.
See 6.4 for acceptable pulsation environment.
The edges of the pressure tap holes on the inner surface of the meter tube shall be free from burrs and may be slightly rounded.
Flow Conditioners
Flow conditioners can be classified into two categories: flow straighteners or flow conditioners.
Flow straighteners are essential devices designed to minimize or eliminate the swirl component in a flowing stream However, they may not fully correct a non-fully developed flow profile, which is crucial for accurately replicating the orifice plate coefficient of discharge database values For optimal performance, installation of flow straighteners should adhere to the guidelines provided in Table 8a or Table 8b.
Flow conditioners are devices that effectively eliminate swirl from flowing streams and redistribute the flow to create a pseudo fully developed flow profile They are designed to replicate the orifice plate coefficient of discharge database values, having successfully passed the performance test protocol outlined in Annex D.
This standard does not endorse any specific type of flow conditioner To minimize flow measurement bias in existing setups and enhance accuracy in new installations, it offers installation guidelines for the 19-tube uniform concentric tube bundle flow straighteners identified in the research on installation effects Due to significant variations in the coefficient of discharge caused by differences in straightening vane tube bundle construction, only those meeting specific criteria are deemed to produce "no additional uncertainty" when installed as recommended All other tube bundles are classified as "other" flow conditioners.
5.5.2 Description of the 1998 Uniform Concentric 19-Tube Bundle Flow Straightener
All tubes or thin wall pipes must maintain uniform smoothness, outer diameter, and wall thickness, arranged in a cylindrical pattern without any gaps between their outer walls To minimize swirl between the exterior tubes of the tube bundle flow straightener and the meter tube wall, the outer diameter of the tubes should be sized to ensure the tube bundle flow-straightener's outside diameter (OD) is a maximum of \(D_i\) and a minimum of \(0.95D_i\) The length of the vanes (LTB) should be calculated as follows: 3 × NPS for NPS of 2 inches, 2.5 × NPS for 2 < NPS ≤ 4 inches, and 2 × NPS for NPS greater than 4 inches.
5.5.3 Tubing of the 1998 Uniform Concentric 19-Tube Bundle Flow Straightener
The 1998 Uniform Concentric 19-Tube Bundle Straightener must have individual tube wall thicknesses not exceeding 2.5% of the inner diameter (D i) All tubes should be parallel and feature an internal chamfer at both ends, which must be at least 50% of the wall thickness and angled at 45 degrees Additionally, the tubes must be mounted axially with the pipe.
5.5.4 Fabrication of the 1998 Uniform Concentric 19-Tube Bundle Flow Straightener
The 1998 Uniform Concentric 19-Tube Bundle Straighteners must be robustly constructed, with individual tubes welded at the tangency points at both ends, ensuring that welds do not exceed 20 degrees around the tube circumference For tube bundles with a nominal pipe size (NPS) of 4 inches or less, the F areas may be filled with weld To aid installation, centering spacers can be added to the outside of the assembly for proper alignment within the meter tube Once inserted, the tube bundle must be securely fastened using a mounting flange or pinning arrangement to prevent vibration or displacement against the orifice plate, while ensuring that the fastening does not distort the symmetry of the tube bundle assembly within the meter tube.
Specifications for the description, installation, or uncertainty of other flow conditioners are not presented in this standard.
Flow straighteners that do not meet the specifications outlined in sections 5.5.2, 5.5.3, and 5.5.4 are classified as "Other Flow Conditioners," which means the installation requirements specified in Table 8a and Table 8b may not apply.
The selection of flow conditioners should rely on technical performance data from performance tests This standard establishes a consistent criterion for assessing the performance of installations and flow conditioners These tests are essential to verify the performance level achievable with an orifice meter installation that incorporates a flow conditioner For detailed information, refer to Annex C and Annex D The performance tests will validate the orifice meter diameter ratio (β), meter tube length, and the optimal location for the flow conditioner to ensure acceptable performance.
The performance criteria chosen, specifically \$\Delta C_d (FT)\$, are consistent with those utilized to assess the impact of installations in meter tubes, both without flow conditioners and with the 19-tube uniform cylindrical tube bundle flow straightener The deviation observed highlights the differences in performance under varying conditions.
Figure 4—1998 Uniform Concentric 19-Tube Bundle Flow Straightener
Flow straightener OD Centering spacer options
The performance of the flow conditioner should be assessed using the discharge coefficient values, denoted as [ΔC d (FT)], which are derived from reference values obtained through separate baseline calibrations with identical orifice plates.
Acceptable performance levels indicate that no further measurement uncertainty is required when the variation in ΔC d is 50% or less of the specified 2σ uncertainty in the Reader-Harris/Gallagher orifice equation at infinite Reynolds number, as outlined in API MPMS Ch.14.3.1 and AGA Report No 3, Part 1, Paragraph 12.4.
5.5.5.2 Required Elements of the Installation Performance Test
The installation performance tests are influenced by various flow conditions and disturbances Firstly, good flow conditions occur when a meter tube is installed in a piping configuration that maintains an ideal axial velocity profile, typically achieved with 75 or more diameters of straight pipe and minimal swirl (less than 2 degrees) Conversely, two close-coupled out-of-plane 90-degree elbows upstream of the meter tube can create significant swirl and alter the axial velocity profile, with swirl angles reaching up to ± 15 degrees Additionally, a 50% closed gate or ball valve positioned upstream can lead to a highly asymmetric axial velocity profile downstream Lastly, high swirl conditions, often found downstream of headers, can produce swirl angles of up to ± 30 degrees and may also result in an asymmetric axial velocity profile.
The detailed conditions of the performance test protocol can be found in Annex D.
General
The discharge coefficients of orifice plates [C d (FT)] outlined in this standard are derived from extensive experiments conducted in the United States and Europe These experiments ensured normal flow conditions by utilizing long straight lengths of meter tube both upstream and downstream of the orifice, or by employing flow conditioners upstream of the orifice meter (refer to API MPMS Ch.14.3.1/AGA Report No 3, Part 1, Paragraph 12.4.3) To achieve the specified uncertainty in the coefficient of discharge presented in API MPMS Ch.14.3.1/AGA Report No 3, Part 1, it is essential to replicate similar fluid dynamic conditions in practical applications.
Orifice Plate
The orifice plate bore must be concentric with the upstream and downstream inside bore of the orifice plate holder, with any eccentricity adhering to specified tolerances Specifically, the eccentricity parallel to the axis of the differential pressure taps (εx) in the x-y plane, as illustrated in Figure 5, should not exceed the tolerance defined by the relevant equation.
(11) where εx is the measurement (X – X')/2 as shown in Figure 5.
Table 6 presents the maximum allowable values for eccentricity, denoted as \$\epsilon_x\$ Additionally, the eccentricity perpendicular to the axis of the differential pressure taps, represented as \$\epsilon_y\$, can be significantly greater Specifically, for any eccentricity in the x-y plane illustrated in Figure 5, the orifice plate bore eccentricity component perpendicular to the differential pressure taps—measured as \$(Y - Y')/2\$ in Figure 5—may reach up to four times the value calculated using the relevant equation.
The maximum allowable orifice plate bore eccentricity calculated using Equation (11) can be doubled if flange taps
To achieve an average pressure, it is essential to connect two points that are 180 degrees apart using equal lengths of tubing with the same diameter, ensuring the nominal diameter is at least as large as the tap diameter Additionally, the connection to the differential pressure (ΔP) device should be positioned midway between the taps This method is not advisable in situations where pulsating or fluctuating flow may be an issue.
To ensure proper installation of an orifice plate in orifice flanges or OFUs when measuring eccentricity is not feasible, it is essential to use two accurately positioned alignment pins to support and center the plate during bolt tightening The eccentricity concerning the upstream side is deemed the most critical factor.
Figure 5—Eccentricity Measurements (Sample Method) ε x
Axis perpendicular to differential tap centerline
Axis parallel to differential tap centerline
Orifice borePlane of Taps
For optimal performance, alignment pins or positioning devices for the orifice plate should be installed to ensure the plate is centered with respect to the upstream section of the meter tube and the pressure tap.
Plate-centering techniques depend on design and are limited by the maximum allowable eccentricity In most orifice fittings, a carrier mechanism secures the orifice plate within the flow stream, ensuring consistent eccentricity It is essential to verify this consistency during multiple installations and removals of the plate The carrier used for testing should match the one employed in the field, and any replacement of internal mechanisms necessitates a repeat inspection For dual-chamber fittings ranging from 2 inches to 8 inches, it is advisable to conduct eccentricity tests during factory acceptance testing in both horizontal and vertical orientations.
The orifice plate holder should maintain the plane of the orifice plate at an angle of 90 degrees to the meter tube axis.
Meter Tube
To achieve accurate flow measurement, it is essential for the fluid to enter the orifice plate with a fully developed, swirl-free flow profile, which can be accomplished using flow conditioners and sufficient lengths of straight pipe both upstream and downstream of the orifice plate Distortions in the average flow profile or increased flow pulsation can lead to measurement errors, making it crucial to address pulsation at its source For more detailed information, refer to API MPMS Ch.14.3.1/AGA Report No 3, Part 1, Paragraph 12.4.3 regarding pulsation reduction measures and the operation of the orifice meter within specified flow pulsation limits.
Table 6—Maximum Tolerance of Orifice Plate Bore Eccentricity (ε x )
Dimensions in inches β m Meter Tube Inside Diameter
In various piping configurations, orifice meters often yield results that fall outside the uncertainty limits of established standards Research has been conducted on several common piping installations to assess their impact on metering accuracy The necessary lengths of meter tubes and the locations of the 1998 Uniform Concentric 19-Tube Bundle Flow Straighteners for the studied installations are detailed in Tables 7, 8a, and 8b.
For applications not specifically covered in Tables 7, 8a, and 8b, the required lengths and locations of the 1998 Uniform Concentric 19-Tube Bundle Straightener for the "any other configuration" classification should be adhered to This applies to multiple fittings upstream of the orifice plate, where the distance between fittings is 22D_i or less Most installations were tested to ensure fully developed flow at the inlet by utilizing a combination of flow conditioners and straight pipe Tests indicated that if the spacing between two fittings exceeds 22D_i, their interaction becomes negligible, resulting in a flow profile akin to fully developed flow Deviations from the specified inlet profile characteristics may render the recommended meter run lengths insufficient for optimal orifice-meter performance.
Meter run lengths for installations, whether they include flow straighteners or not, generally remain unaffected by variations in Reynolds numbers and roughness within standard limits However, exceptions arise in cases without flow conditioners, particularly when two 90-degree elbows in perpendicular planes are spaced 5D i or less apart, as well as in installations that create swirl, such as headers, eccentric expanders, and elbows combined with expanders These specific configurations are classified under "any other configuration" in Table 7.
Installation Table 7 provides the required minimum installation lengths for meter tubes without flow conditioners
For meter tubes without flow conditioners, the recommended upstream meter-tube length depends on the diameter ratio (\( \beta_r \)), with longer lengths needed for higher ratios When adjusting the orifice bore for varying flow conditions, the meter tube length should be based on the maximum diameter ratio (\( \beta_r \)) anticipated New installations should adhere to the lengths specified for a diameter ratio (\( \beta_r \)) of 0.75, and it is advisable to use upstream meter tubes that are longer than those indicated in Table 7.
Figure 6—Orifice Meter Tube Layout for Flanged or Welded Inlet
1998 Uniform Concentric19-Tube Bundle Flow Straightener
Installation Tables 8a and 8b provide the 1998 Uniform Concentric 19-Tube Bundle Flow Straightener allowable location range and the recommended location for two upstream meter-tube length categories, 17D i ≤ UL < 29D i , and
UL ≥29D i The standard does not address upstream meter-tube lengths of less than 17D i
Installations and flow conditioners not covered in Tables 7, 8a, and 8b can undergo flow testing either in situ or at a certified laboratory, adhering to the RG coefficient of discharge uncertainty outlined in Annex C This testing must utilize calibration devices and methods that comply with recognized national and international standards All instruments for monitoring flow parameters and calculating flow rates must be traceable to the relevant certifying organization for weights and measures The primary flow system may be either portable or permanently installed, and a calibrated master meter can be employed for flow testing, provided both the master meter and the proving system conform to established national standards.
For flow testing in a laboratory, it is essential that the installation includes the meter tube or meter station, along with a manifold and the correct upstream piping configuration This setup is crucial for accurately defining the flow signature, which encompasses the velocity profile and swirl entering the meter tube or station.
Flow testing should be conducted within the normal range of Reynolds numbers encountered during typical operations Acceptable performance levels, which indicate no need for additional measurement uncertainty, are defined as a variation in ΔC d of 50% or less of the stated 2σ uncertainty in the Reader-Harris/Gallagher orifice equation at infinite Reynolds number, as outlined in API MPMS Ch.14.3.1/AGA Report No 3, Part 1, Paragraph 12.4, and API MPMS Ch.14.3.2/AGA Report No 3, Part 2, Annex C.
Table 7—Orifice Meter Installation Requirements Without a Flow Conditioner D iam et er r at io β
The minimum straight unstructured meter tube length from the upstream side of the orifice plate should adhere to specific guidelines based on the number and arrangement of elbows For a single 90° elbow, the length must be in multiples of the published internal pipe diameter (D_i) When there are two 90° elbows in the same plane, the required length should exceed 30 times the internal diameter (S > 30 D_i) In cases where two 90° elbows are positioned in perpendicular planes, the minimum length should be greater than 15 times the internal diameter (S > 15 D_i).
Tw o 9 0° elbows i n the same plane “ S ” configurati o n sp ac er S ≤ 10 D i
Tw o 90° el bo ws in the s ame p lane, “ S ” con fig urati on 10 D i < S ≤ 30 D i
Tw o 90° el b o ws in perpendicul ar plane s, S < 5 D i *
Tw o 90° el bows i n perpen dicul ar pl ane s, 5 D i ≤ S ≤ 15 D i
Si ngl e 90 ° Te e used as an elbow but not as a header el ement a Si n g le 45 ° elbow b T w o 45 ° elbo w s in the same pla n e “ S ” confi guratio n S ≥ 22 D i
Gate va lve at least 50 % ope n Concentr ic re ducer
A n y other configurati o n (catch a ll ca te g o ry) *
D o wnstr eam mete r tube le ngt h UL UL UL UL UL UL UL UL UL UL DL ≤ 0.20 6 10 10 50 19 9 30 17 6 70 2.8 0 30 11 10 12 50 32 9 30 19 6 108 3.0 0 40 16 10 13 50 44 9 30 21 6 145 3.2 0 50 30 30 18 95 44 19 30 25 7 145 3.5 0 60 44 44 30 95 44 29 30 30 9 145 3.9 0 67 44 44 44 95 44 36 44 35 11 145 4.2 0 75 44 44 44 95 44 44 44 44 13 145 4.5 Recomm ende d len gth for ma xi mu m ran ge β ≥ 0 75
The tolerance for specified lengths for upstream (UL) and downstream (DL) meter tube lengths in internal pipe diameter (D_i) is ±0.25 UL refers to the minimum meter tube length upstream of the orifice plate, while DL indicates the minimum downstream meter tube length The straight length is measured from the downstream end of the nearest elbow or tee, or from the downstream end of the conical portion of a reducer or expander The separation distance (S) between piping elements is measured from the downstream end of the curved portion of the upstream elbow to the upstream end of the curved portion of the downstream elbow These installations are significantly influenced by Reynolds number and pipe roughness, with recommendations developed for high Reynolds numbers and smooth pipes to address the worst-case scenarios.
Table 8a—Orifice Meter Installation Requirements With 1998 Uniform Concentric 19-Tube Bundle Flow
Straightener for Meter Tube Upstream Length of 17 D i ≤ UL < 29 D i
Two 90° elbows out of plane
Single 90° tee used as an elbow but not as a header element
Partially closed valves (at least 50 % open)
High swirl combined with single 90° Tee
Any fitting (catch-all category)
UL 2 UL 2 UL 2 UL 2 UL 2 UL 2 DL
0.10 5 to 14.5 5 to 14.5 5 to 14.5 5 to 11 5 to 13 5 to 11.5 2.8
0.20 5 to 14.5 5 to 14.5 5 to 14.5 5 to 11 5 to 13 5 to 11.5 2.8
0.30 5 to 14.5 5 to 14.5 5 to 14.5 5 to 11 5 to 13 5 to 11.5 3.0
0.40 5 to 14.5 5 to 14.5 5 to 14.5 5 to 11 5 to 13 5 to 11.5 3.2
0.60 12 to 13 13.5 to 14.5 a Not allowed a Not allowed 3.9
0.67 13 13 to 14.5 Not allowed Not allowed Not allowed Not allowed 4.2 0.75 14 Not allowed Not allowed Not allowed Not allowed Not allowed 4.5
Recommended tube bundle location for maximum range of β
The lengths indicated in the UL2 column represent the dimensions illustrated in Figure 6, measured in terms of the number of published internal pipe diameters (D i) from the downstream end of the 1998 Uniform Concentric 19-Tube Bundle Flow.
Straightener and the upstream surface of the orifice plate.
NOTE 2 The tolerance on specified lengths for UL, UL2, and DL is ± 0.25D i
NOTE 3 Not allowed means that it is not possible to find an acceptable location for the 1998 Uniform Concentric 19-Tube
Bundle Flow Straightener downstream of the particular fitting for all values of UL.
S = Separation distance between elbows, measured as defined in Table 7.
UL1 = UL – UL 2 (see Figure 6). a 13D i allowed for up to β = 0.54. b 9.5D i allowed for up to β = 0.47. c 9.5D i allowed for up to β = 0.46.
Table 8b—Orifice Meter Installation Requirements With 1998 Uniform Concentric 19-Tube Bundle Flow
Straightener for Meter Tube Upstream Length of UL ≥ 29 D i
Two 90° elbows out of plane
Single 90° tee used as an elbow but not as a header element
Partially closed valves (at least 50 % open)
High swirl combined with single 90° Tee
Any fitting (catch-all category)
UL 2 UL 2 UL 2 UL 2 UL 2 UL 2 DL
0.10 5 to 25 5 to 25 5 to 25 5 to 13 5 to 23 5 to 13 2.8
0.20 5 to 25 5 to 25 5 to 25 5 to 13 5 to 23 5 to 13 2.8
0.30 5 to 25 5 to 25 5 to 25 5 to 13 5 to 23 5 to 13 3.0
0.40 5 to 25 5 to 25 5 to 25 5 to 13 5 to 23 5 to 13 3.2
0.50 11.5 to 25 9 to 25 9 to 23 7.5 to 15 9 to 19.5 11.5 to 14.5 3.5
0.60 12 to 25 9 to 25 11 to 16 10 to 17 11 to 16 12 to 16 3.9
0.67 13 to 16.5 10 to 16 11 to 13 10 to 13 11 to 13 13 4.2
0.75 14 to 16.5 12 to 12.5 12 to 14 11 to 12.5 14 Not allowed 4.5
Recommended tube bundle location for maximum range of β
NOTE 1 Lengths shown under the UL2 column are the dimensions shown in Figure 6 and as defined in Table 8a.
NOTE 2 The tolerance on specified lengths for UL, UL2, and DL is ± 0.25D i
NOTE 3 Not allowed means that it is not possible to find an acceptable location for the 1998 Uniform Concentric 19-Tube
Bundle Flow Straightener downstream of the particular fitting for all values of UL.
S = Separation distance between elbows, measured as defined in Table 7.
UL1 = UL – UL2 (see Figure 6).
This standard aims to offer comprehensive installation options without endorsing any specific flow conditioner, focusing instead on providing essential installation information to minimize systematic biases It specifies minimum required meter tube lengths for installations without flow conditioners and outlines location ranges for two meter tube length categories: 17D i ≤ UL < 29D i and UL ≥ 29D i, specifically for the 1998 Uniform Concentric 19-Tube Bundle Flow Straighteners Additionally, the standard includes a performance test to assess flow conditioners, other flow straighteners, and meter tube installations against the “no additional” uncertainty requirement For more details, please refer to Annex C and Annex D.
When assessing the necessity of flow conditioners, the key factor is not solely the nearest piping fitting at the meter tube's inlet The last fittings may not accurately reflect swirling flow or velocity profile asymmetry Each station design presents unique conditions, making it impractical to establish universal specifications The primary focus should be on minimizing flow disturbances at the orifice plate caused by upstream piping fittings.
Proper installation of flow conditioners can significantly minimize the straight pipe length needed before an orifice plate Their primary function is to mitigate the impact of velocity profile asymmetry and swirl caused by upstream pipe fittings and valves on flow measurement To ensure optimal performance, flow conditioners must be maintained clean and free from debris that may accumulate at the upstream end.
Proper installation and usage of flow conditioners are essential, as no device can completely eliminate profile effects To minimize flow disturbances and swirl in the metering system, especially upstream of the orifice, careful attention is required When correctly positioned in well-designed setups, a flow conditioner or the 1998 Uniform Concentric 19-Tube Bundle Flow Straightener can effectively eliminate bias in orifice measurements For detailed guidance on the proper installation of this flow straightener, consult Table 8a and Table 8b.