1. SCOPE ……………………………………………………….
2. REFERENCES ……………………………………………………….
3. GENERAL ……………………………………………………….
3.1 Units of Measurement …………………………..
3.2 Equipment Performance …………………………..
3.3 Environmental Conditions …………………………..
3.4 Packaged Unit Concept …………………………..
3.5 Total Responsibility Approach …………………………..
3.6 Conflicts ……………………………………………………….
3.7 Buyer Exceptions …………………………..
3.8 Vendor Exceptions …………………………..
4. METER SELECTION …………………………..
5. METER STATION DESIGN …………………………..
5.1 General Design Requirements …………………………..
5.2 Orifice Meter Station Design …………………………..
5.3 Turbine Meter Station Design …………………………..
6. ACCESSORY EQUIPMENT…………………………..
6.1 General……………………………………………………….
7.
INSTALLATION ……………………………………………………….
7.1 General……………………………………………………….
8. MAINTENANCE AND TESTING …………………………..
8.1 General……………………………………………………….
9. DOCUMENTATION …………………………..
9.1 General……………………………………………………….
10. REVISION HISTORY …………………………..
1. Scope
This standard is one of the SES R05 series of Flow Measurement standards. It establishes the
minimum guidelines for the design, construction, and installation of custody transfer metering
stations in dry hydrocarbon gas service.
2. References
Reference is made in this standard to the following documents. The latest issues, amendments
and supplements to these documents shall apply unless otherwise indicated.
American Gas Association (AGA)
AGA Gas Measurement Committee Report No. 3, Orifice Metering of Natural Gas, (AGA-3)
National Association of Corrosion Engineers (NACE)
MR0175/ISO15156
Petroleum and Natural Gas Industries – Materials for Use in H2S
Containing Environment in Oil and Gas Production
3. General
3.1 Units of Measurement
3.1.1 SI Metric as set forth by the International Organization for Standardization will be used.
Piping will be ANSI Code with ANSI dimensions for pipe, valves, fittings, and flanges with
diameters in inches, but lengths in millimeters.
3.1.2 Engineering units for all calculations and reports for custody metering stations shall be as
follows:
Density kg/m3
Flow Rate Std m3/s(1)
Level Percent
Pressure kPa
Temperature deg C
Velocity m/s
Viscosity cP
Volume m3
(1) Standard Conditions 15 deg C and 101.325 kPa
3.2 Equipment Performance
3.2.1 Gas metering systems shall be made up of industrial-grade components with accuracy
ratings in a range of ±0.1percent to ±1percent of span, depending on the type of equipment
NOTE: Accuracy rating includes the combined effects of linearity, hysteresis, dead band and
repeatability errors.
3.2.2 Repeatability – The metering system shall be capable of reproducing its output within ±0.5
percent or less of meter reading under identical operating conditions in its normal operating
range.
3.3 Environmental Conditions
For details and descriptions refer to BEDD
3.4 Packaged Unit Concept
The packaged unit concept for custody transfer metering stations is preferred.
3.5 Total Responsibility Approach
Total responsibility for the whole metering system shall, where possible, be assigned to a single
vendor. This applies to all the major sections of the system such as the metering and automatic
sampling equipment, together with any control instrumentation.
3.6 Conflicts
Any conflicts between this standard, SES’s and industry standards, engineering drawings, and
contract documents shall be resolved at the discretion of the Buyer.
3.7 Buyer Exceptions
Buyer exceptions to this standard will be detailed on the purchase order.
3.8 Vendor Exceptions
Vendor exceptions to this standard to be included in the quote.
4. Meter Selection
Meter type shall be selected on the basis of accuracy, service conditions, rangeability and
other operational requirements, maintenance considerations, cost and flexibility for future
expansion.
First-consideration shall be given to selecting meters in accordance with the following meter
selection guide based on projected maximum/minimum uncorrected flow rates.
Meter Type Actual Flow Rates m3/s Remarks
(1) Turbine 0.008 to 0.4 Good rangeability (15-1 to 100-1 depending on pressure)
Orifice > 0.4 Wide rangeability through orifice plate changes or multiple DP measurements
NOTE: Venturi tubes may be considered for special situations. Venturi tubes are expensive
and inflexible.
(1) In-line types only. Insertion type turbine meters are not acceptable.
5. Meter Station Design
5.1 General Design Requirements
5.1.1 Standard (Base) conditions shall be designated as the following:
a. Base Pressure = 101.325 kPa
b. Base Temperature = 15 deg C
5.1.2 Stations shall be designed to operate as reliable, unattended gas flow measurement
systems for the custody transfer.
5.1.3 All gas shall be conditioned prior to measurement to the extent that there are no free liquids
or solid particles that will collect and change the operation of a meter or be large enough to
damage metering elements per manufacturers’ recommendations. Based on pipeline
history, scrubbers, filters and strainers shall be installed upstream of meters as required to
ensure the above.
5.1.4 Meter station piping arrangements shall be designed with a minimum number of bends and
fittings, commensurate with requirements. Upstream and downstream straight pipe lengths
shall be provided according to SABIC Standards.
5.1.5 All instrumentation equipment will be suitable for environmental conditions (See Paragraph
3.3). The primary elements shall be capable of being installed in hazardous classification
areas designated as Class I, Division 2, Groups C and D. Any secondary element such as a
flow computer shall be installed as close as possible to the primary element to minimize
errors.
5.1.6 Equipment arrangement shall ensure convenient access for operation, maintenance and
replacement.
5.1.7 When handling other than sweet gas, critical parts of turbine meters and orifice fittings must
be made of a corrosion resistant material such as Type 316 stainless steel or better,
consistent with design and NACE MR-01-75 / ISO 15156 requirements.
5.1.8 Meter stations shall be equipped with vent lines to facilitate a safe depressurization for
maintenance and inspections. Full opening drain valves shall be installed in the threaded
drain taps provided on the lower sides of orifice fittings.
5.1.9 When using a turbine meter as a primary flow device, the Vendor shall ensure pulse signal
integrity for the distance from the primary to the secondary flow element. This may warrant a
preamplifier to convert a low voltage sinusoidal signal to a square wave pulse form that can
increase the transmission distance of the primary device to the secondary flow computer.
5.2 Orifice Meter Station Design
5.2.1 Gas volumetric measurement accuracy shall range from ±0.5 percent to ±2.0 percent of
maximum meter capacity rating, depending on orifice meter rangeability (turn down ratio)
and gas relative density (specific gravity) fluctuation considerations. All measurement
components shall have a basic ±0.5 percent of span accuracy or better.
5.2.2 Standard electronically transmitted signals for static pressure, differential pressure and
flowing gas temperature shall be fed into dedicated gas flow computers for continuous real
time solving of the orifice meter flow formula and totalization of mass flow units.
5.2.3 The other required variable inputs to flow computers for mass flow computations (like
density) shall be made either by on-line instrument signals or by operator manual entries.
5.2.4 All design and flow calculation requirements in this section shall conform to the AGA-3.
5.2.5 For natural gas, flowing gas density calculations shall conform to AGA-3 including manual
for the determination of super compressibility factors for natural gas. For ethane, ethylene
and other materials, calculation procedures shall be approved by SABIC.
5.2.6 Design Parameters
a. Estimated accuracy (volume basis) ± 0.5 to ± 2 percent of range
b. Normal orifice plate beta ratio () limitation 0.20 min. to 0.60 max
c. Meter tube installation lengths Upstream and downstream for 0.75 β orifice plate usage
d. Differential pressure ranges 0 to 0.05 Bar through 0 to 0.5 Bar
e. Maximum Ratio Of Differential Pressure (hw) hw < 0.036 for bar
f. Flowing Pressure (Pf) Bar (abs)
g. Single tube turn-down ratio limit 3.5 to 1 w/single D/P
h. Compensation for pressure temperature, Continuous flow computer relative density (sp.gr.) and supercompressibility
i. Orifice fitting and meter tube 2 or 3 section meter tube with straightening vanes and 2 Chamber orifice fittings, or orifice Flanges
j. Temperature measurement RTD fully compensated system
k. Specific gravity) determination On line transducers
l. Flow data interpretation Flow computer is required
m. Pipe inside diameter (I.D.) Matched with orifice fitting bore, within tolerances
5.2.7 Orifice Plates
a. Orifice plates shall be of removable concentric disc type designed for use in orifice fittings
or flanges. They shall be constructed of Type 316 stainless steel and finished smooth
with surface roughness not to exceed 1.3 m (50 microinches). Weep holes shall not be
used.
b. Upstream edges of orifice plates shall be square and sharp so they will not show a beam
of light when checked with an orifice edge gauge or, alternatively, will not reflect a beam
of light when viewed without magnification.
c. Orifice edge thickness shall conform the dimensions published in AGA-3 Figure 1A, and
shall have the downstream edges beveled at a 45 degrees angle, as required. Orifice
plate thickness shall also be as recommended in AGA-3 Figure 1A.
d. Orifice plate flatness shall conform the requirements of AGA-3 Figure 1B and orifice
concentricity in the meter tube shall be maintained as set forth in paragraph 2.2.4 of AGA -3.
e. Orifice to pipe I. D. ratios d/D () shall normally be maintained within the 0.20 to 0.60
range for design purposes. It is preferable to have the () ratio between 0.4 and 0.6.
f. Orifice plates shall be sized to maintain a differential of at least three (3) square root chart
units (0-10 scale), or greater, for at least 95 percent of the time.
5.2.8 Orifice Fittings
a. For single tube installations, orifice fittings shall be the double chambered type in order to
allow for orifice plate inspections and changes while the meter tube is pressurized and
without interrupting the gas flow. Single chamber type may be used in multi-tube
installations.
b. Fittings shall be of the type that has the upstream end welded to the upstream section of
the tube and the downstream end flanged to the downstream section of the tube. The
internal diameter of the fitting shall be matched to that of the tube, within specified
tolerances. (See AGA-3 Figure 2 and paragraphs 23 and 24).
c. Flanges on orifice fittings shall be the male-female self aligning type or shall be doweled
with a minimum of three (3) alignment pins.
d. All double chambered fittings 12 inch and larger shall be installed in an upright position
for mechanical strength reasons. Orifice flange sets may be used on nitrogen and
hydrogen service.
5.2.9 Orifice Meter Tubes
a. Fabrication
Orifice meter tubes shall be purchased from qualified tube manufacturers. (Field
fabrication of meter tubes shall not be allowed). The meter run shall be constructed with
a pressure rating at least as high as the rating of the associated piping.
b. Pipe Selection
Upstream and downstream meter tube selections shall be constructed of seamless pipe
or cold drawn seamless tubing. (Seamed pipe may be used for 24 inch and 30 inch tubes
if the longitudinal welds are ground flush and smooth.) The tube shall be as nearly
cylindrical as commercially possible with out-of roundness deviations (difference between
the maximum and minimum I.D. Measurements) not to exceed 0.5 percent for upstream
sections and 1 percent for downstream, (See AGA-3 paragraph 2.3.4).
c. Pipe Wall Finish
The inside pipe walls shall be as smooth as commercially practicable, with wall
roughness not to exceed 7.5 m (300 microinches) AARH by visual comparison. Pipe
walls may be machined, ground, coated, honed and/or polished, but none is a
requirement unless the original finish does not meet the above test.
d. Connected Fittings
Flanges and orifice fittings welded to meter tubes shall be aligned and the inside
diameters shall match the internal bore of the tubes within specified tolerances. All
internal weld joints shall be ground to a smooth finish, flush with the internal diameter of
the pipe and free of sharp edges or abrupt changes in surface level or diameter. There
shall be no pipe connections within the upstream-downstream outboard meter tube ends
other than pressure taps, instrument taps or thermometer wells in accordance with
established standards. The meter tube shall give a smooth, uninterrupted flow channel
throughout its full length.
e. Length of Tubes
Meter tube lengths shall be determined on the basis of a 0.75 ratio to provide for
flexibility and possible future capacity expansion.
f. Spacer Rings
Upstream meter tube ends shall be flanged and equipped with a spacer ring so that the
tube sections can be easily removed for inspection and cleaning. When the meter run
has a flange type straightening vane, the spacer ring is to be installed at the
straightening vane flange and be wide enough (when removed) to allow for the
uncoupling of male-female flanges if the orifice fitting is so equipped. Spacer ring I.D.’s
shall be 3 mm (1/8 in) larger than the meter tube I.D. and the O.D.’s shall be 1.5 mm
(1/16 in) smaller than the inside bolt circle of the flange.
g. Gaskets
All gaskets shall be purchased or trimmed to have at least a 10 mm (3/8 in) larger inside
diameter (bore) than the meter tube for 12 inch tube size and above. For 10 inch and
below, gasket I.D.’s shall be at least 6 mm (1/4 in) larger. This is to prevent the gasket
from protruding into the tube interior through misalignment or compression.
h. Identification
The averages of the upstream and of the downstream I.D. micrometer readings shall be
stamped on a stainless steel data plate which will be permanently attached to the
upstream meter tube.
NOTE: The orifice fitting and meter tube shall be purchased as one unit whenever
possible to ensure correct alignment, smooth approach to the orifice, proper location and
finish of tap holes, together with conformance to published diameters and other
specifications. Only qualified and recognized manufacturers of such equipment shall be
considered as suppliers.
5.2.10 Straightening Vanes
a. Straightening vanes shall be inserted in the upstream section of the meter tube to
minimize flow pattern disturbances and shall be constructed and installed in accordance
with AGA -3. However, it is preferable to avoid the use of straightening vanes by
providing appropriate upstream and downstream straight pipe length.
b. Either flange type or line type straightening vanes are acceptable.
c. Straightening vanes and their flanges shall be constructed of carbon steel (stainless steel
is acceptable) in accordance with AGA- 3 and industry practice.
5.2.11 Instrument Connection Taps
a. Differential Pressure
Differential pressure taps shall be “Flange Taps”, having the center of the upstream tap
hole placed 25 mm (1 in.) from the upstream face of the orifice plate. The center of the
downstream tap hole shall be 25 mm (1 in.) from the downstream face of the orifice plate.
(See AGA-3 paragraph 2.8.1).
NOTE: Tap nipples shall not be seal welded in orifice fittings.
The tap holes shall be located at the 25 mm (1 in.) dimension within plus or minus (±) the
tolerances shown in AGA-3 Figure 5 Section 2. These tolerances are normally satisfied
by the pressure tap holes found in the orifice fittings described in AGA-3 paragraph 6.2.4.
b. Static Pressure
Static pressure measurements shall be taken from the downstream flange tap static
pressure line.
c. Temperature
Two 316 stainless steel thermowells shall be installed in the meter tube not less than five
(5) and not more than twenty (20) pipe diameters downstream from the orifice plate.
d. Sampler
One 1 inch NPT tap fitting for a sampler connection shall be installed 300 mm (12 in)
downstream of the second thermometer well or at some point on the header where flow
is always passing and a representative sample can be taken.
5.2.12 Secondary Instrumentation
a. General Requirements
(i) The secondary instrumentation system shall always provide for the gathering
of flow rate, pressure and temperature data, and as required, for density,
relative density (specific gravity) value. The system shall also provide for the
direct calculation of corrected flow volumes and total energy units through the
use of dedicated real time flow computers. Flow and/or pressure control
capability are also sometimes provided.
(ii) The instrument system must record or compensate for all variables significant
to the measurement accuracy level desired.
(iii) Measurement and control piping shall be separated, with one set of orifice
fitting taps dedicated for measurement and the other for control functions.
(iv) Any equipment that will be adversely affected by high temperatures and a
harsh environment shall be installed in a housing with controlled
temperatures (air conditioning) and filtered pressurization.
(v) Design of the housing should be such that accumulations of gas cannot
occur, with particular attention given to sour gas services.
(vi) As a minimum, transducers/transmitters and flow recorders in custody
measurement service shall be installed in well ventilated gauge houses or
otherwise be protected from the sun’s direct rays.
b. Differential Pressure Transducers/Transmitters
(i) Differential pressure transducers/transmitters and flow recorders shall be
fitted with 3-valve manifolds and mounted as close to the orifice fitting as
practical, preferably within 6 m (20 ft).
(ii) Piping (gauge
lines) between the orifice fitting taps and the
transducers/transmitters or recorders shall be ½ inch Type 316 stainless steel
tubing, installed with a slope of not less than 1:12 toward the orifice fitting.
Condensate chambers shall be installed at all gauge line low points if the
lines do not drain directly back into the orifice fitting.
(iii) Smart electronic type differential pressure transducers/transmitters shall be of
the 2-wire type, be fully field adjustable, have a 4 to 20 mA DC output and
possess an overall 4 to 20 mA DC accuracy capability of 0.2 percent of span
or better.
c. Static Pressure Transducers/Transmitters
(i) Static pressure transducers/transmitters shall be connected to the
downstream orifice fitting tap gauge line.
(ii) Smart electronic type pressure transducers/transmitters shall be of the 2-wire
type, be fully field adjustable, have a 4 to 20 mA DC output and possess an
overall accuracy capability of 0.25 percent of span or better.
d. Temperature Transducers/Transmitters and Recorders
Electronic type temperature transducers/transmitters shall have a linearized platinum
resistance temperature detector (RTD) sensing system, a 4 to 20mA DC output and an
accuracy capability of 0.2 percent of span or better. Normal span adjustment shall be 18
deg
C
to
66
deg
C
(0-150
deg
F).
Resistance
bulbs
shall
be
100
ohms
at
0deg
C,
3
wire,
DIN
43760.
Individual
3
point
calibration
shall
be
furnished.
e. Electronic Flow Computers
(i) Electronic flow computers or mainframe computer shall perform all
calculations necessary to convert the basic measured variables (differential
pressure, static pressure, temperature and relative density) and the manually
entered data (meter functions, meter connection data, and orifice plate data)
into gas custody transfer quality values of flow rate and accumulated
standard energy units (kJ).
(ii) The calculations shall be in accordance with the methods of AGA-3. For
natural gas mixtures, the super compressibility correction shall be made using
the Specific Gravity (Relative Density) Method, as stated in AGA-3, and the
supercompressibility equation for electronic computers from the AGA-3
Project NX-19.
(iii) Units may also possess the optional capabilities to receive and handle
variable input signals from densitometers, and to compute gas transfers using
these inputs. If required, the following data shall be displayed and furnished
to an interface terminal strip for retransmitting:
(iv) Instantaneous Flow Rate – Analog (Tons/Hr)
(v) Accumulated flow – ASCII/RS232* (compatible tons)
A code and a standard which allow different computers to communicate with
each other.
(vi) Gas flow in tons per hr shall be accumulated for billing period reports.
Calculations shall include all results required to support the reports. Overall
accuracy of calculations shall be better than 0.1percent of full scale.
5.3 Turbine Meter Station Design
5.3.1 General Requirements
5.3.2 Gas flow measurements shall be made with an accuracy of 0.5 percent of range or better.
Temperature, pressure and density corrections shall be made continuously with electronic
instruments.
5.3.3 Design Parameters
a. Estimated accuracy 0.5 percent of range or better
b. Minimum piping lengths 10 dia. upstream, 5 dia. downstream
c. Upstream straightening vanes
d. Upstream filter or strainer
e. Sun shade for transmitters
f. Turn down ratio 15-1 to 100-1 (Maximum flow to minimum) depending on meter type and
pressure
g. Compensation for pressure, continuous flow computers or temperature and density
recorders/computation
h. Temperature measurement RTD
i. Density determination
5.3.4 System Configuration
a. Two identical metering units shall be provided. The metering units shall be connected in
series. Each metering unit shall consist of upstream block valve, straightening valves,
turbine meter, pressure, temperature, density transducers/transmitter, downstream block
valve and unit by-pass valve.
b. It shall be possible to take any one unit out of service for repair and maintenance without
the need to shut down the system or effecting the operation of other metering unit.
Common upstream filters shall be provided for the two units.
c. One turbine meter shall serve as the custody transfer meter whereas, the other shall serve
as the check meter. The check meter shall be periodically removed for lab calibration.
d. The signals from the flow meter, pressure, temperature and density transmitter/transducer
shall be routed to the flow computer for mass flow calculations.
5.3.5 Electrical Supply Systems
Typical voltage supplies to be used are the following:
a. 480 V, 3-phase, 60 Hz power for motor operated valves, central hydraulic pump units,
motors, etc.
b. 120 V, single phase, 60 Hz power for some field and panel mounted instruments, lighting,
small motors and certain motorized valves, solenoids, etc.
c. 125 V, dc power for emergency lighting and equipment if required.
d. 24 V, dc power for digital flow computers and some field mounted instruments.
5.3.6 Panel-Mounted Instrumentation
a. Electrical supply for panel-mounted instrumentation (other than flow computers, if any)
shall be unstabilized 120 V, single phase, 60 Hz. This will apply also to any remote valve
indication system. In cases where a different voltage requirement is necessary (such as
for flow computers) integral or separate power packs, constant voltage transformers,
batteries, etc., shall be provided.
b. An uninterruptible power supply (UPS) shall be provided for flow computers to avoid
affecting the metering results during periods of power failure. Power supply back-up shall
be supplied from a transformer/rectifier/battery system.
c. A stabilized power supply shall be provided for special equipment where required.
6. Accessory Equipment
6.1 General
6.1.1 Gravitometers, calorimeters, densitometers, on-line chromatographs and moisture, H2S,
and sulfur analyzers shall:
a. Be of a type approved for the purpose by SABIC.
b. Be installed in accordance with recognized industry practice, manufacturer’s
recommendations and the standards of recognized authoritative bodies.
7. Installation
7.1 General
7.1.1 All fabrication and installation shall be done in accordance with SABIC specifications,
together with industry and government standards.
7.1.2 All control panels shall be provided with a sun shade to shield the components from direct
sun light.
8. Maintenance and Testing
8.1 General
8.1.1 Meter station design and construction shall facilitate ease of field inspection and
maintenance. See individual design sections and project specifications for special
requirements.
8.1.2 The calibration (establishing dimensional measurements) of the critical points of orifice
meter tubes, fittings and plates shall be done in accordance with AGA-3 requirements and
shall be witnessed by SABIC or its agent.
8.1.3 Piping and other component integrity testing shall be done in accordance with project
specification requirements. All the mechanical equipment shall be tested as a complete
system. All valves will be stroked full open and closed. All equipment on the metering skid
will be tested electrically with the skid pressurized and under a flowing condition.
8.1.4 All low points in the piping (the orifice fitting in particular) shall be drained after the
hydrostatic test and dry air/gas blown through the meter tubes and piping to dry up residual
water.
8.1.5 After factory hydrostatic tests, a coating of rust inhibiting oil shall be sprayed onto the inside
surfaces of the meter tubes and fittings following the above drying process to forestall
corrosion.
8.1.6 Factory Acceptance Test dates on metering systems shall be communicated to SABIC at
least two months in advance in order to coordinate their participation. The Factory
Acceptance Test shall individually check the functionality of all the flow computers and
associated peripheral equipment. Included in the testing will be a software checkout and a
complete test of any flow equations programmed into the flow computer.
9. Documentation
9.1 General
9.1.1 Orifice meter tube, fitting and plate calibrations and turbine meter flow calibrations shall be
documented on appropriate forms, which shall be sent to SABIC in advance of product
delivery for evaluation and filing.
9.1.2 Drawings, schematics, and product literature adequate to facilitate proper operation and
maintenance of the stations and components shall be provided in the numbers stipulated in
the project specifications.