8 EQUIPMENT CALIBRATION
All newly installed and following major repair flow meters and auxiliary
equipments (temperature transmitters, pressure transmitters, densitometer, flow
computers, totalizers and meter calibration/proving devices shall be calibrated or
verified in service for their accuracy according to the procedures set forth in this
manual and API Manual of Petroleum Measurement Standards. Acceptable
methods of meter calibration and proving and calibration of meter accessories
are provided in Section 2 of this Manual.
Saudi Aramco custody and royalty liquid metering measurement systems shall
be operated within ±0.25% system error. All newly installed and following major
repair flow meters and auxiliary equipment shall be calibrated or verified in
service for their accuracy according to the procedures set forth in this manual
and API Manual of Petroleum Measurement Standards.
8.1 CALIBRATION FREQUENCY AND TOLERANCES
Instruments used in the calibration of measurement systems shall be certified or
verified against a certified standard at the time of purchase, and recertified
periodically as established in Procedure No. 4 – Equipment Calibration
Frequency.
All master measurement standards used for calibration or verification of custody
transfer equipment shall be traceable to National Institute of Standards and
Technology and shall be no more than three levels.
8.2 EQUIPMENT
8.2.1 Temperature Measurement
All temperature measuring equipment used for custody transfer
measurement shall be verified against a National Institute of Standards
and Technology standard.
Electronic temperature loops
The electronic temperature loops used for custody transfer shall be
calibrated quarterly using a National Institute of Standards and
Technology certified reference standards within the tolerance established
in this manual.
In addition, electronic temperature loops used for custody transfer shall
also be monitored once during each proving using a glass or electronic
thermometer installed in-line on the meter and prover piping. If the
electronic temperature loop differs by more than 0.2ºF from the monitoring
device, the electronic temperature loop shall be verified using the National
Institute of Standards and Technology certified reference thermometer
and recalibrated as necessary.
Resistive Temperature Devices (RTDs)
Electronic resistive temperature devices (RTD) used to establish on-line
temperatures for custody transfer shall have their resistance certified to a
National Institute of Standards and Technology standard by the
manufacturer at a minimum of three points. A calculated resistance vs.
temperature table shall be supplied. This table shall be used for calibrating
the electronic temperature loops. The resistance of the certified RTD shall
be checked using a certified glass thermometer or a precision resistive
thermometer bridge any time the temperature loop is calibrated. The
difference between the measured resistance at test temperature and the
certified resistance shall be recorded and used to adjust the calculated
resistance vs. temperature table for the correct calibration value. The RTD
accuracy shall be maintained within the tolerance in the stated procedures
found in Procedure No. 9 – RTD Temperature Input Calibration.
Electronic Temperature Probes
Electronic temperature probes (hand held) inserted in-line for monitoring
on-line temperature measurement shall be verified quarterly or any time
there is a difference of more than 0.2ºF indicated on the on-line
temperature measurement, whichever is less, against a National Institute
of Standards and Technology reference standard at three points in the
normal operating temperature range.
Handheld electronic temperature probes shall be calibrated in accordance
with the stated procedures in Procedure No. 10 – Portable Electronic
Thermometer Calibration.
Reference Glass Thermometers
Certified reference glass thermometers used for monitoring on-line
temperature loops must be certified at three points (32ºF, 100ºF, and
150ºF) against a National Institute of Standards and Technology certified
reference thermometer when purchased and verified quarterly.
Thermometers whose mercury column is split shall not be used. If the
thermometer reading varies more than 0.1ºF from the standard, the
thermometer shall be replaced.
Certified reference glass thermometers shall be calibrated and verified in
accordance with the stated procedures in Procedure No. 11 – Calibration
and Verification of Reference Glass Thermometers.
8.2.2 Pressure Measurement
The pressure of the hydrocarbon liquids in the meter run shall be taken
either from the pressure transmitter installed on the upstream header of
the metering skid or from the transmitter installed on the individual meter
run for computation. If an outlet header pressure transmitter is installed, it
should be used for checking purposes only.
Pressure Instruments
Pressure instruments shall be checked quarterly with a handheld precision
calibrator or dead weight. Pressure instruments shall be calibrated in
accordance with the guidelines stated in Procedure No. 12 – Pressure
Indicator Calibration.
Pressure Transmitters
Pressure transmitters shall be calibrated in accordance with the guidelines
stated in Procedure No. 13 –Pressure Transmitter Calibration.
8.2.3 Density Measurement
The primary method for determining relative density (Specific Gravity)
shall conform to API MPMS Chapter 9, Density Determination. The
hydrometer shall be checked before initial use by the Saudi Aramco
Laboratory or as specified in the sales agreements.
Density Determination using Manual Samples
Manual density determination shall be performed according to the
guidelines stated in Procedure No. 14 – Density by Laboratory Hydrometer
or Procedure No. 15 – Density by Laboratory Thermohydrometer.
Automatic Determination of Specific Gravity
On-line densitometers installed in the fast loop of each custody transfer
metering system shall comply with API Manual of Petroleum
Measurements Standards, Chapter 14.6, “Continuous Density
Measurement.”
The flow-through pycnometer is the most accurate device for calibrating a
density meter under flowing conditions. Discrete digital density meters
which require a grab sample can also be used as density proving devices.
Densitometers shall be verified by Digital Density Meters which have been
verified using Procedure No. 16 – Digital Density Meter Verification or
calibrated according to Procedure No. 17 – Density Meter Proving by
Pycnometer.
The on-line densitometer shall be verified to ensure accuracy within ±0.20
API or ±0.0014 specific gravity using a manual spot sample as outlined in
Procedure No. 19 – Liquid Densitometer Verification Test.
Liquid densitometer calculations shall be checked according the
Procedure No. 20 – Liquid Densitometers Standard/Meter Computation
Test.
Liquid densitometers shall be removed from service according to
Procedure No. 21 – Liquid Densitometer Changeout.
Densitometers operated in fast loops shall have the fast loop flow rates tested to ensure the flowing velocity of the loop is representative of the velocity expected in the main pipeline per Procedure No. 22 – Fast Loop Flow Rate Check.