CHAPTER FOURTEEN
Orifice Flowmeter Design
T o accurately measure merchantable natural gas downstream of on-
shore gas plants, the connecting party may elect to use orifice,
multipath ultrasonic (MUSM), turbine, or rotary displacement flow-
meters. To accurately measure merchantable natural gas upsweam of
onshore gasplants, the connecting party is limited to using orifice flow-
meters due to the presence of excessive pipeline rouge, condensate,
and retrograde condensate.
14.1 General
T h e measurement system should be designed, operated, and main-
tained to provide measurements within +0.50% or *1.00% overall
uncertainty.
T h e concentric, square-edged, flange-tapped orifice flowmeter
should be installed in accordance with API MPMS Chapter 14.3
(A.G.A. Report No. 3 ) , latest edition as a minimum.
The orifice flowmeter should comprise a dual-chamber orifice
fitting equipped differential pressure (dP) sensing flange taps.
The orifice flowmeter should pass the fitting integrity (dP tap
communication) test specified in API MPMS Chapter 14.3 (A.G.A.
353
354 Natural Gas Measurement Handbook
Report No. 3), latest edition as a minimum. The seal ring should pass
the integrity (leak) test specified in API MPMS Chapter 14.3 ( A G A
Report No. 3), latest edition as a minimum. The orifice seal ring and
plate should pass the eccentricity test specified in API MPMS Chap-
ter 14.3 (A.G.A. Report No. 3), latest edition as a minimum.
The orifice meter should conform to the dimensional toler-
ances for the orifice fitting and meter tube specified in API MPMS
Chapter 14.3 (A.G.A. Report No. 3), latest edition as a minimum.
If the connecting party wishes to employ a second set of dP
instrumentation, then the orifice fitting should be equipped with two
sets of dP sensing ports located on the same side of the orifice fitting.
Where the delivery is required to be continuous and it is im-
practical to shut down in the event of a flowmeter failure, spare
flowmeter assembly(s), complete with associated accessories and
symmetrical with the initial flowmeter assembly, should be installed.
Where parallel flowmetering is likely, special consideration
must be given to balanced flow and pressure control. For parallel
flowmetering installations, the design must be symmetrical with
and of a similar size to the other flowmeter assemblies. Automated
valve sequencing should be installed to provide for accurate meas-
urement: turndown and flowmeter failure. For parallel orifice
flowmeters, flow control valves should be installed in a station con-
trol valve manifold to allow for control valve maintenance. Alterna-
tively, individual flowmeter control valves may be installed inside
the DB&B valves to allow for control valve maintenance.
Bidirectional flow is not permitted for orifice flowmeters. If
bidirectional flow is necessary, then orifice flowmeter assemblies
should be dedicated for a specific flow direction.
All LACT/ACT units should shut down any flowmeter assem-
bly that exhibits failure in a flowmeter or critical secondary or terti-
ary device (dP, $-,Tf, and flow computer).
14.2 Velocity and Piping Insulation
The maximum flowmeter assembly velocity (P&) should not exceed
50 fps under normal conditions due to accelerated erosion rates and
audible noise generation. If normal operation is below 7 fps, then
Orifice Flowmeter Design 355
insulation of the flowmeter assembly may be required to ensure
thermal homogeneity along the radial and axial coordinates of the
flowmeter assembly. Insulation of the flowmeter assembly may be
required due to climatic variations and flowing gas conditions.
14.3 Strainers
For orifice flowmeters, strainers may be required to ensure the
mechanical condition of the flowmeter. The manufacturer should
be consulted for sizing requirements of the strainer.
14.4 Flowmeter Assembly
The flowmeter assembly (Figure 14-1) consists of the upstream
meter run with a HPFC, the flowmeter body, and the downstream
meter run with appropriate sensing taps (dP, 9,
Tf) and sampling
connections.
T h e physical location of the high-performance flow condi-
tioner and the unobstructed upstream and downstream piping
should be determined in accordance with API MPMS Chapter 14,
Part 3 (A.G.A. Report No. 3) testing procedures as a minimum.
A conservative design establishes the required orifice flowmeter
assembly based on the following criteria:
A 0.75 p installation is required.
The orifice flowmeter should be sized for a p ratio from 0.20
to 0.65 for normal operations.
Larger p ratios are allowed with the written approval of the
affected parties at the time of their application.
The orifice flowmeter should employ a dual-chamber fitting
for ease of plate inspection and changeout.
At least 17 nominal pipe diameters of unobstructed straight
pipe are needed between the orifice plate (upstream meter run)
and the inlet of the flowmeter assembly.
The upstream unobstructed straight pipe should contain a
HPFC located seven nominal pipe diameters upstream of the
orifice plate.
Figure 14-1 Orifice flowmeter assembly: (1) dual-chamber orifice fitting, "flangenek"; (2) isolating flow conditioner, consisting
of an antiswirl device, a setting chamber, and a profile device; (3) spacer plate with paddle handle, 0.500 in. thick; (4) large
female-faced weld neck to match raised face on orifice fitting; (5) flange, raised-face weld neck (RFWN); (6) thread-0-let, 1.000
in.. welded vertical on top centerline of tube (venting/filling connection); (7) stud bolts for RFWN flanges; (8) stud bolts for
orifice fitting; (9) raised face, spiral wound metallic gasket; (10) nonasbestos ring gasket; (11) API 5L line pipe in conformance
with API MPMS Chapter 14.3 (A.G.A. Report No. 3), latest edition; (12) thread-0-let, 0.75 in.,welded vertical on top centerline
of tube (for thermowell).
OrificeFlowmeter Design 357
The high-performance flow conditioner should consist of an
antiswirl device, a settling chamber, and a profile device.
At least six nominal pipe diameters of unobstructed straight
pipe are needed between the orifice plate outlet and the first
thermowell (downstream meter run).
Isolation valves should be provided at the dP sensing taps.
The sharing of dP sensing taps or gauge lines (dP) is not
acceptable.
The maximum differential pressure (dP) across any orifice
plate shall not exceed 250 in. of H,O at 60°F due to seal ring
limitations.
The minimum differential pressure (dP) across any orifice
plate must not be below 20 in. of H,O at 60"E
The static pressure (9)sensor should be connected to the
upstream dP sensing line.
(7
A minimum of two thermowells and master) should be
installed approximately six nominal pipe diameters down-
stream of the orifice plate on each flowmeter assembly in
accordance with MI MPMS Chapter 14, Part 3 (A.G.A.
Report No. 3), latest edition. If additional thermowells are
needed, a maximum of four may be installed.
Sampling probes should be located downstream of any ther-
mowells (primary flowmeter assembly).
A valved 1-in. tap should be provided on both ends of the
flowmeter assembly to allow atmospheric venting of the flowmeter
assembly.
For RF flange installations, a spacer plate should be located at
the inlet of the flowmeter assembly for ease of replacement and
maintenance. Alternatively, an inspection tee design may be in-
stalled to allow for ease of replacement and maintenance.
For RTJ flange installations, an inspection tee design is re-
quired to allow for ease of replacement and maintenance.
The flowmeter assembly should be designed for ease of visual
or optical internal inspection of the meter body and adjacent pipe
358 Natural Gas Measurement Handbook
spool surfaces (foreign matter, rouge, condensate, and so forth).
Flanges or end closures should be installed at the ends of each flow-
meter assembly.
14.5 Flowmeter, Mechanical
The flowmeter’s average internal diameter is calculated in strict
accordance with API MPMS Chapter 14.3 (A.G.A. Report No. 3),
latest edition. The roughness of the flowmeter assembly should not
exceed 2 50 microinches at any point. The mechanical specifications
for proper application of the artifact compliance method requires
documentation of strict conformance to the multitude of specifica-
tions and tolerances contained in API MPMS Chapter 14.3 (A.G.A.
Report No. 3), latest edition.
The mechanical measurements and tests (tap communication,
seal ring leakage, eccentricity) should be performed using devices of
sufficient precision and traceable to National Institute of Standards
and Technology. The dimensional measurements and calculations
should be documented on a certificate, along with the name of the
manufacturer, flowmeter model, flowmeter serial number, flowme-
ter body temperature at the time the dimensional measurements
were performed, date, name of individual making the measurements,
and name of inspectors or witnesses present.
14.6 Piping Spools, Mechanical
The flowmeter assembly(s) should comply with the numerous spec-
ifications and tolerances contained in API MPMS Chapter 14.3
(A.G.A. Report No. 3), latest edition.
14.7 Secondary and Tertiary Devices
The facility should be equipped with microprocessor-based flow com-
puters that accept the following inputs: orifice’s differential pressure
(dP), flowing temperature (Tf), and flowing pressure (9) to generate
the mass flow (qJ and standard volumemc flow (qyd computations.
Orifice Flowmeter Design 359
Secondary Devices: dP, P, and 7;
Smart transmitters (static pressure, differential pressure, and static
temperature) should be utilized to ensure accuracy as follows:
A smart differential pressure transmitter (dP) should be
installed across the flange taps for each orifice meter. If addi-
tional transmitters are needed for rangeability, a maximum of
three differential pressure transmitters may be stacked.
A smart static pressure transmitter (5)should be installed on
the upstream flange tap for each orifice meter. A common
static pressure measuring point for all meter tubes is not
acceptable.
A smart temperature transmitter (T)should be installed on
each downstream meter tube. If additional transmitters or
probes are needed, a maximum of two probes may be installed.
A common temperature measuring point for all meter tubes is
not acceptable.
Secondary Devices: Sampling Probes
Gas sampling probes should be installed as follows:
A properly sized sample probe should be installed for the con-
tinuous sampler in accordance with API MPMS Chapter 14,
Section 1, preferably in the primary flowmeter assembly.
A properly sized sample probe should be installed for the man-
ual spot sampler in accordance with API MPMS Chapter 14,
Section 1, preferably in the primary flowmeter assembly.
A properly sized sample probe should be installed for the gas
chromatograph and online moisture monitor in accordance
with API MPMS Chapter 14, Section 1, preferably in the pri-
mary flowmeter assembly (if applicable).
If liquid (pipeline condensate, free water, oil, methanol, glycol)
is anticipated or experienced, then installation of a static mixer
upstream of the sample probes should be required to ensure
accurate mixing at the sample probe(s) location(s).
360 Natural Gas Measurement Handbook
For this application, the sample probes should be installed in
the common pipe entering or exiting the parallel flowmeter
arrangement.
If two or more gas streams are combined upstream of the
metering facility, then a static mixer should be installed to
ensure thermal and compositional homogeneity through the
measurement facility.
Tertiary Devices: Flow Computers
The flow computers should be programmed with the latest edition
of A.G.A. Report No. 8 for the computation of natural gas density
in the gaseous and dense phase region. The flow computers should
comply with API MPMS Chapter 2 1.1, “Electronic Gas Measure-
ment,” latest edition as a minimum.
