PROVING CORIOLIS METERS USING PIPE PROVER

Scope
This procedure defines the process required to properly perform the meter
proving of mass “Coriolis” meters using a pipe prover.

Procedure

GENERAL CONSIDERATIONS

Field (in-situ) meter proving provides a means of establishing the meter
factor for the Coriolis meter under actual operating conditions. There are
various methods of applying the meter factor to indicate the actual quantity
measured through the meter. The adjustment from indicated to actual
quantity can be made by varying the meter factor or K-factor.

These factors can reside in either the Coriolis transmitter or accessory
equipment or be applied manually. The preferred method is to apply a
meter factor in the accessory equipment because of its audit trail
capability. It is important that the method selected be used consistently.

Note: A Coriolis meter is calibrated by the manufacturer to determine one
or more calibration factors that are entered into the Coriolis transmitter.
These factors, although adjustable, should remain unchanged.

Any factor changed that can affect the quantities measured by the meter
must be retained in the audit trail. In applications where the flow rate varies
during normal operation, it may be desirable to determine meter factors
over a range of flow rates. The various meter factors can then be used to
linearize the output from the Coriolis meter at varying flow rates. If the
meter is used to measure bidirectional flow, a meter factor should be
developed for each direction.

In addition to the initial proving of a Coriolis meter when installed in the
field, periodic provings are necessary to confirm or reestablish the
performance accuracy of the Coriolis meter. Meter provings should be
performed if any of the following events occur:

a. Anytime the meter is rezeroed.
b. When the flow sensor installation or mounting conditions are
modified.
c. When the Coriolis meter density measurement is calibrated, if the
Coriolis meter is configured to indicate volume.

d. When the meter assembly is repaired.
e. When any of the assembly components have been replaced.
f. If a change in the fluid temperature, pressure, or density occurs
beyond user-defined limits as determined from field experience.
g. When a flow rate change occurs that will cause a shift in the meter
factor in The meter factor shift due to flow rate shall be determined
from field proving experience.
a. At the request of parties involved in custody transfer.
b. On a schedule based on throughput, elapsed time, or contract.
c. Anytime the accuracy of a meter is in question.
d. When a change in the direction of flow through the meter occurs, if a
meter factor has not been determined for the new direction.

PROVING CONSIDERATIONS

Proving conditions should be as close to the actual metering conditions as
practical. Occasionally there may be exceptions to this requirement;
however, the essential purpose of proving is to confirm the meter assembly
performance at normal operating conditions.

The conditions under which a meter is proven are:
a. Stable product composition.
b. Stable product temperature and pressure.
c. Stable flow rate.
d. System valves and seals have been checked to ensure there is no
leakage.
e. Trial runs have been conducted to evacuate any air/gas from the
system.

Requirements for stability of temperature, pressure, and product composition
will vary, depending on the proving method being employed and the
properties of the fluid being measured.

If the Coriolis meter is configured to indicate mass and is being proven
against a gravimetric tank prover, then the stability of the fluid properties is
less critical because there is no need for a density determination.

If the Coriolis meter is configured to indicate mass and is being proven
against a volumetric standard (volumetric tank prover, conventional pipe
prover, small volume prover, or volumetric master meter), it is essential

that the density remain stable. Stabilizing the density minimizes variations
in density between the prover, meter and the density determination used in
the calculation. Since the measured flowing density will be used to convert
the prover volume to a mass or the Coriolis meter mass to a volume, any
difference in the density and the true flowing density during the proving will
result in errors in the calculations. This in turn will result in an error in the
meter factor. Therefore, to minimize errors, it is extremely important that
the density remains stable during the proving.

As an alternative, the proving system may incorporate an online
densitometer, calibrated at regular intervals. This density reference is
particularly useful in eliminating errors, if the density varies during a proving.

The need for stable fluid density also applies to a Coriolis meter configured to
indicate volume being proven against a gravimetric tank prover. If there are
density variations during the proving, it is likely that additional proving runs
will be required to obtain an acceptable meter factor. The data should be
reviewed for outliers.

Any outlying points should be scrutinized to determine if they were caused by
density variations during the proving. These points may not be valid and
may result in an incorrect meter factor if used in the average. Good proving
practices and good judgment will be required when trying to compare mass
measurements versus volume measurements.

To determine fluid properties:

a. Pressure and temperature measurement devices
should be installed as close to the prover and/or
flow sensor as practical, so the measured
temperature and pressure is representative of the
fluid temperature and pressure in the prover and/or
flow sensor.
b. If the density is calculated based on temperature and pressure,
additional pressure and temperature measurements upstream and
downstream of the Coriolis meter flow sensor may be required to
determine the average density in the flow sensor.
c. Density measurement devices, if used, should be installed as close to
the prover and/or flow sensor as practical, so that the measured
density is representative of the fluid density in the prover and/or flow
sensor.

Meter Proving Data

Density

When discussing density measurement it is important to distinguish between
base density and flowing density, and when each is applied.

Base density, rb, is the density of the fluid at the base conditions of
temperature and pressure. The base density is needed to determine the
required correction factors for temperature and pressure, when both the
prover and the Coriolis meter are configured to indicate volume.

Flowing density, rf, is the density of the fluid at actual flowing temperature
and pressure.

Accurate determination of the flowing density, rf, is critical to successfully
prove a Coriolis meter whenever the proving device and the Coriolis meter
do not measure in the same flow units: one measure mass, the other
measures volume.

Variations in density and errors in the determination of density are the
greatest source of error when performing volumetric versus mass proving.
Proving a Coriolis meter on a mass-to-mass or a volume-to-volume basis
will reduce the uncertainty caused by errors in density determination.

Determination of a Coriolis meter density factor is not needed if the Coriolis
meter is configured to measure volume and is being proved against a
volumetric prover. For this case, the Coriolis meter factor will include the
combined errors for the mass flow measurement and the density
measurement.

The purpose of determining a density factor would be to identify what
portion of the meter factor can be attributed to each component: the mass
flow and the density measurement.

Even in this case, care must be taken to ensure that the density used for
calculating the correction factors (CTL, CPL) is accurate. In addition, in
some cases an inaccurate density may result in worsened linearity of the
output volume flow rate signal.

When required, means to determine density of the flowing fluid during
proving shall be incorporated in the metering system or in the prover. If the
density measurement is being used to convert a volume to a mass or a
mass to a volume then the accuracy of the meter factor determination will
be a reflection of the accuracy and precision of the density measurement.

The following methods are available to determine the fluid
density:
a. Hydrometer—Refer to API MPMS Chapters 8 and 9.
b. On-line density from either a Coriolis meter or a
separate density meter, which has been verified against
an accepted density reference. Refer to API MPMS
Chapter 14.6.
c. Sample run on a laboratory density meter. Sampling
practices should be performed in accordance with API
MPMS Chapter 8.
d. Sample, composition analysis, and calculated density.
This is limited to light or pure hydrocarbons whose
composition and physical properties are well known.

e. Pycnometer—Use of the pycnometer should follow API
MPMS Chapter 14.6. The use of a pycnometer may not
be practical for every liquid hydrocarbon application.
f. Equation of state if fluid composition is consistent.

Temperature and Pressure

The temperature and pressure measurements should be precise enough
to allow accurate determination of the applicable correction factors for both
the prover and the fluid. The requirements for temperature and pressure
measurement precision will vary depending on which correction factors are
being applied in the determination of the meter factor. For determination of
corrections for the thermal expansion of the liquid CTLp or CTLm, the
required temperature measurement precision will be determined based on
the thermal expansion properties of the liquid.

For determination of corrections for the pressure effect of the liquid CPLp
or CPLm, the required pressure measurement precision will be determined
based on the compressibility of the liquid. Experience with the specific

liquid will be necessary to establish requirements for temperature and
pressure measurement precision. Refer to API MPMS Chapter 7 for
information on temperature determination.

Number of Runs for a Proving

The required number of test runs for each proving varies depending on:
a. Type of proving method being employed.
b. Coriolis meter type and size.
b. Operating flow rate and quantity of fluid accumulated
during each proving run.

Experience with the meter/proving system will ultimately establish the
number of runs required. Typical examples of the number of runs
performed for each proving method are given below:

PROVING CORIOLIS METERS USING PIPE PROVER

* run defined as round trip for bidirectional prover

Refer to API MPMS Chapters 12.2.3 and 13.1 for more details regarding
the number of runs required to achieve the same uncertainty as five runs
at 0.05% repeatability.

The number of runs required to achieve the desired tolerance for meter
factor uncertainty should be defined and agreed to by all contractual
parties. Once established, the same procedure should be followed
consistently in order to better track the performance of the meter. These
requirements should not differ from other custody transfer meters for
similar applications.

Repeatability

The repeatability is used as an indication of whether the proving results are
valid. There are two general methods of calculating the repeatability: one
associated with the Average Data Method and the other associated with
the Average Meter Factor Method as described in API MPMS Chapter
12.2.3.

The Average Meter Factor Method is recommended for determining
repeatability because it reduces the influence of changing fluid density
and/or prover volume from the repeatability calculations. There can still be
other sources of non-repeatability. If a density measurement device is
used or density is determined from tables or equations, the repeatability
will reflect the repeatability of the density determination along with the
repeatability of the Coriolis meter.

Reproducibility

Reproducibility is defined as the ability of a meter and prover system to
reproduce results over a long period of time in service where the range of
variation of pressure, temperature, flow rate, and physical properties of the
metered fluid is negligibly small. The expected reproducibility is generally
determined from experience with each individual proving system.

A change in the meter factor greater than the user defined limits should be
considered suspect and every effort should be made to ensure that the
Coriolis meter and proving system are functioning properly. Statistical
control charting of meter factors will be valuable in analyzing the
reproducibility of Coriolis meters and determining the required frequency of
proving.

If the Coriolis sensor, transmitter, or Manufacturer Calibration Factors have
changed since the last prove, especially if the Calibration Factors are not
consistent with those provided by the manufacturer for the sensor in use, a
large unexpected variation in meter factor may occur. In this case, a
careful review of the sensor serial number, the Calibration Factors supplied
by the manufacturer for that sensor, and the Calibration Factors actually
contained in the Coriolis transmitter should be performed.