AGA 3 PART 2 review

1. A Review of the Revisions to API 14.3 / AGA 3 – Part 2
Tom Cathey
Wedge Measurement & Control, Inc.
1415 Louisiana Suite 1500, Houston Texas 77056
Introduction
In April of 2000, revisions to the specification and
installation requirements for orifice meters was published
by the American Gas Association in the form of the AGA
Report No. 3 – Part 2, Fourth Edition. This purblication is
also correctly known as API 14.3, Part 2, ANSI/API 14.3,
Part 2-2000 and GPA 8185, Part 2. This report applies to
fluids that are considered to be clean, single-phase,
homogenous and Newtonian measured using concentric,
square edged, flange-tapped orifice meters.The
requirements for the construction of orifice meter tubes, as
defined in this publication, will be discussed in greater
detail throughout this paper.
Definition of Terms
Primary Element – Defined as the orifice plate, the orifice
plate holder with associated differential pressure sensing
taps, the meter tube, and flow conditioner if used.
Meter Tube – The straight sections of pipe, including all
segments integral to the orifice plate holder upstream and
downstream of the orifice plate.
Diameter Ratio (Beta Ratio) – For purposes of this
discussion, the definition is the measured orifice plate bore
diameter divided by the measured meter tube internal
diameter.
Orifice Plate – A thin square-edged plate with a machined
circular bore concentric with the meter tube I.D. when
installed.
Orifice Plate Holder – A pressure containing piping
element, such as a set of orifice flanges or an orifice fitting
used to contain and position the orifice plate in the piping
system.
Design
A beta ratio of.75 should be used as the design criteria for
new orifice meter installations. The 2000 publication
provides required minimum installation lengths UL and
DL for meter tubes with no flow conditioners in Table 2-7.
Minimum installation lengths for meter tubes with the
1998 Concentric 19-Tube Flow Straighteners are
provided in Tables 2-8a and 2-8b. You may be familiar
with the following drawing taken from Figure 2-6 of
the publication.
In many instances it may be difficult or impossible for
the engineer to control upstream piping configurations.
This is been a common problem for many companies
over the years and some have taken this in to
consideration when designing their measurement
packages. In all but two of the applications identified in
Table 2-7, a UL length of 44D at a beta ratio of .75
meets minimum requirements. The exceptions are as
follows:
o Two 90° elbows in perpendicular planes
where S<5Di
o Any other configuration (catch all
category)
For two 90° elbows in perpendicular planes, the
recommendation is 95 published inside diameters
beginning at beta ratios of .50 through .75.

2. For configurations not specifically addressed in Table 2-7,
145 published inside diameters are recommended for beta
ratios of .40 through .75. Some have incorporated a piping
segment upstream of the meter tube so that the “S”
dimension can be controlled.
For tubes with The Uniform Concentric 1998 19-Tube
Bundle, there are multiple configurations that are not
allowed at the higher beta ratios. This means that it is not
possible to find an acceptable location for the Concentric
19-Tube Bundle under these specific
conditions. A few examples follow:
• Single 90° tee used as an elbow but not as a
header element for beta ratios of .67 and higher.
• Partially closed valves (at least 50% open) for
beta ratios of .60 and higher.
Required minimum lengths for other types of flow
conditioners are not specified although flow testing in situ
and at flow-testing laboratories is addressed. Many
companies have now changed their engineering standards
to accommodate other types of one and two piece flow
conditioners such as those produced by Gallagher,
Canadian Pipeline Accessories and Daniel.
Flow Conditioners
Flow conditioners are divided into two categories:
flow straighteners and isolating flow conditioners. Flow
straighteners are defined as “Devices that remove or have
limited ability to accurately replicate the orifice plate
coefficient of discharge database values”. Isolating flow
conditioners are defined as “Devices that effectively
remove the swirl component from the flowing stream while
redistributing the stream to produce flow conditions that
accurately replicate the orifice plate coefficient of
discharge database values”.
The 1998 Uniform Concentric 19-Tube bundle is
considered a flow straightener and the physical
specifications are considerably different from the
requirements listed in the 1991 publication for
straightening vanes. The individual tubes must be of
uniform smoothness, outer diameter and wall thickness.
Commercially available seamless carbon steel tubing is
readily available and most commonly used. The individual
wall thickness of the tubes shall be less than or equal to
2.5% of the published internal diameter. For example:
The individual wall tube thickness for a 3” schedule 40
meter tube (3.068”) must be .025 X 3.068 or 0.0767” or
less. The individual tubes must be chamfered on both
ends not less than 50% of the wall thickness by 45
degrees. The tubes must be arranged in a cylindrical
pattern and the individual tube outer walls must come
in direct contact with each other. The outside diameter
of the tube bundle must be a minimum of 95% of the
published internal diameter of the meter tube and can
obviously be no greater than the published internal
diameter. In order to ensure that the bundle outer tube
walls come in direct contact with each other and
achieve an outside diameter greater than or equal to
95% of the published inside diameter of the meter tube,
the correct O.D. tubing must be used as illustrated in
the table below for schedule 40 piping.
Meter Vane Minimum Vane
Tube ID " Tube O.D. " Bundle O.D. "
3.068 19/32 2.9146
4.026 13/16 3.8247
6.065 1- 3/16 5.7618
7.981 1- 5/8 7.5820
10.020 2 9.5190
The required length of the tube bundles must be
as follows:
• 3 X NPS for 2”
• 2.5 X NPS for 3” & 4”
• 2 X NPS for 6” and above
Flow conditioners not meeting the requirements of the
1998 Uniform Concentric Tube Bundle are considered
as Other Flow Conditioners. While the 2000 publication
does not recommend any particular type of flow
conditioner, specific criterion for evaluation of
installation and/or flow conditioner testing is provided.
These tests define the meter tube lengths and flow
conditioner locations for acceptable performance.
Significant research and numerous flow studies have
been conducted since 1991 to test various flow
conditioner designs with repeatable and acceptable
results.

3. Meter Tube Surface Roughness
In the 2000 publication, sections 2.5.1.1 through 2.5.1.1.3
address the inside surface of meter tubes as follows:
For meter runs with nominal diameters of 12 inches or
smaller:
• 300 micro inches for diameter ratios equal to or
less than 0.6
• 250 micro inches for diameter ratios greater than
or equal to .60
• The minimum roughness shall not be less than 34
micro inches for all diameter ratios
• For meter runs with nominal diameters larger than
12 inches:
• 600 micro inches for diameter ratios equal to or
less than 0.6
• 500 micro inches for diameter ratios greater than
or equal to 0.6
• The minimum roughness shall no be less than 34
micro inches for all diameter ratios.
Readily available cold drawn seamless pipe used in the
construction of meter tubes generally falls within 75 to 150
microinches. When other types of commercial pipe are
used, honing to achieve the desired finish is required.
Digital profilometers are commonly used to accurately
determine this measurement
Meter Tube Internal Diameter and Roundness
Tolerance
This is specified in sections 2.5.1.2 through 2.5.1.4.5 and
includes general restrictions related to offsets, shoulders
and welding seams. Also included are recesses and
protrusions. Generally, single and dual chamber flangeneck
fittings are used in the United States in the construction of
orifice meter tubes.
There are exceptions such as weldnek fittings and
orifice flange unions but in custody transfer
applications, this is rare. When care is taken in the
selection of orifice fittings, pipe, and weld fittings and
with proper alignment, welding and grinding techniques
are applied, the requirements are not difficult to meet.
The equations used to determine roundness and
diameter tolerances are illustrated in the following:
Examples of the allowable tolerances for various
diameters are illustrated in the following table:
A =
(Any US diameter within one Dm )
- Dm ≤ 0.25%
Dm
B =
Maximum US Diameter – Minimum
US Diameter ≤ 0.50%
Dm
C = Any downstream diameter - Dm ≤ 0.50%
Dm
Note: Dm is equal to the average of the H,V,LV and RV
measurements taken 1" from the upstream face of the
orifice plate.
Nominal
Mean
ID Upstream Downstream
Pipe
Size ( Dm) A B C
2 1.939 0.005 0.010 0.010
2.067 0.005 0.010 0.010
3 2.900 0.007 0.015 0.015
3.068 0.008 0.015 0.015
4 3.826 0.010 0.019 0.019
4.026 0.010 0.020 0.020
6 5.761 0.014 0.029 0.029
6.065 0.015 0.030 0.030

4. Portrusions resulting from gaskets or sealing devices in to
the pipe bore are not permitted. Companion flanges are
used in conjunction with weldnek orifice fittings and the
correct gaskets to minimize the potential for this type of
problem. Utilization of incorrect piping schedule seal rings
can cause either a restriction or a recess in excess of 0.25
inches as specified in sections 2.5.1.4.2 and 2.5.1.4.3.
Pressure Taps
Meter tubes using flange taps shall have the the center of
the upstream tap hole placed one inch from the upstream
face of the orifice plate. The center of the downstream
pressure tap shall be one inch from the downstream face of
the orifice plate. There are allowable deviations specified
in Figure 2-3 of the April 2000 publication and her again
the recommendation is to use a Beta Ration of .75 to
determine the allowable tolerance. The information below
illustrates this:
Nominal
Pipe Figure 2-3 Minimum Maximum
Size Distance Distance
2" & 3" 1" +/- 0.015" 0.985" 1.015"
≥4" 1" +/- 0.035" 0.965" 1.035"
The tap holes shall be radially drilled radially to the meter
tube. The centerline of the tap hole shall intersect and form
a right angle with the axis of the meter tube. All pressure
tap holes must be round to to a tolerance of ± 0.004 inch
throughout their length. The diameter and diameter
tolerances are contained in the following table:
Nominal
Pipe Tap Hole Minimum Maximum
Size Diameter Diameter Diameter
2" & 3" 0.375" +/- 0.016" 0.359" 0.391"
4" or Larger 0.500" +/- 0.016" 0.484" 0.516"
Eccenricity
The orifice plate bore must be concentric with both the
upstream and the downstream inside bore of the orifice
plate holder. The orifice fitting manufacturers accomplish
this task for the smaller bore sizes by utilizing a metal to
metal centering technique. Of course the smaller the bore
size, the more difficult it is to meet the criteria set forth in
Section 2.6.
The eccentricity tolerances for various bore sizes are
illustrated in the following table:
X-X'/2 = Parallel to the Differential Pressure Taps
(Y-Y'/2)*4 Axis = Perpendicular to the Differential Pressure
Taps
Note: All Tolerances Listed are at 0.75 Beta Ratio
Value in Inches
Nominal Bore X Axis Y Axis
Pipe Size Tolerance Tolerance
2 1.939 0.006 0.024
2.067 0.006 0.024
3 2.900 0.009 0.036
3.068 0.009 0.036
4 3.826 0.012 0.048
4.026 0.012 0.048
6 5.761 0.017 0.068
6.065 0.018 0.072
8 7.625 0.023 0.092
7.981 0.024 0.096
10 9.750 0.029 0.116
10.020 0.03 0.120
12 11.374 0.034 0.136
11.626 0.035 0.140
11.938 0.036 0.144
16 14.312 0.043 0.172
14.688 0.044 0.176
15.000 0.045 0.180
Thermometer Well Location
Thermometer wells should be located to sense the
average temperature of the fluid at the orifice plate. The
wells may be placed on the downstream side of the
orifice plate not closer than DL or farther than 4DL as
illustrated in tables 2-7 and 2-7 of AGA 3, Report #3. If
a flow conditioner is used, the thermometer well may
be located no closer than 36 inches upstream of the
flow conditioner inlet.

5. Tom Cathey