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.
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.
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.
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
Design o 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.
Two 90° elbows in perpendicular planes where S<5Di 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.
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 Tube ID "
Vane Tube O.D. "
Minimum Vane 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.
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: •
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:
300 micro inches for diameter ratios equal to or less than 0.6
(Any US diameter within one Dm ) - Dm
A=
•
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
Dm Maximum US Diameter – Minimum US Diameter
B=
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.
≤ 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.
Examples of the allowable tolerances for various diameters are illustrated in the following table:
Nominal Pipe Size 2
Meter Tube Internal Diameter and Roundness Tolerance
≤ 0.25%
3
4
6
Mean ID
Upstream
( D m)
A
B
C
1.939
0.005
0.010
0.010
2.067
0.005
0.010
0.010
2.900
0.007
0.015
0.015
3.068
0.008
0.015
0.015
3.826
0.010
0.019
0.019
4.026
0.010
0.020
0.020
5.761
0.014
0.029
0.029
6.065
0.015
0.030
0.030
Downstream
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.
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: Figure 2-3
Minimum Distance
Maximum 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
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 Nominal Pipe Size
Pressure Taps
Nominal Pipe Size
The eccentricity tolerances for various bore sizes are illustrated in the following table:
0.500" +/- 0.016"
0.484"
0.516"
2
3
4
6
8
10
12
16
Value in Inches Bore X Axis Tolerance
Y Axis Tolerance
1.939
0.006
0.024
2.067
0.006
0.024
2.900
0.009
0.036
3.068
0.009
0.036
3.826
0.012
0.048
4.026
0.012
0.048
5.761
0.017
0.068
6.065
0.018
0.072
7.625
0.023
0.092
7.981
0.024
0.096
9.750
0.029
0.116
10.020
0.03
0.120
11.374
0.034
0.136
11.626
0.035
0.140
11.938
0.036
0.144
14.312
0.043
0.172
14.688
0.044
0.176
15.000
0.045
0.180
Eccenricity Thermometer Well Location 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.
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.
Tom Cathey
