PROVING METERING SYSTEMS | PROCESS & CONTROL SYSTEM
10.1 General Meter Proving
The following are approved metering system proving methods:
1. In-line pipe proving
2. In-line master meter
3. Calibration using portable pipe prover/ small volume prover
4. Atmospheric portable Tank prover
The application of each method is to be conducted per SAES-Y-103. For certain
applications, the meter may be sent to a central proving facility for its calibration.
The flow range of the central facility must satisfy the flow range of the meter.
Calibration Frequency
All liquid meters that are in service shall be proved in accordance with the
proving schedule stated in Procedure No. 38 – Meter Proving Calibration Frequency.
Meters that are out of service or mothballed, shall be proved prior to return to
service. A meter not used for more than a year will be considered out of service
and should be proved before using it.
Criteria for Accepting Meter Factors
A meter factor shall also be established each time the meter is calibrated. The
criteria for acceptance are provided in Procedure No. 39 – Meter Factor
Acceptance Criteria.
Each meter shall have a separate meter factor. Average meter factors for multiple
meter installations shall not be used.
When it is necessary to complete a meter ticket and meter proving factors are not
available, a historical meter curve factor will be used on the ticket. A copy of all
supporting documents will be maintained for reference.
Meter Factor Application
The preferred method for applying a meter factor on continuously flowing streams,
(i.e., non-batching), is to apply a new meter factor to all volumes for the period
beginning at the time of completion of the meter proving. The new factor will be
applied until the next proving of the meter.
Meter factors may be applied to the entire batch interval or ticket period. A new
meter factor may be established any time during the batch interval or ticket period.
If multiple proofs are required, sub tickets shall be calculated for each batch interval
and the volumes summed for the custody transfer period.
Data to be recorded
The criteria for information required for reporting with the meter proving report
are provided in Procedure No. 39 – Meter Factor Acceptance Criteria.
Meter Curves
A calibration curve for each product to be measured shall be established for each
meter prior to placing the meter into service and after the meter is repaired. The
guidelines for number of calibration points, acceptability and break in period for
generating meter curves are specified in Procedure No. 40 – Meter Curves.
Statistical Quality Control
Meter factor control charts and/or records (API MPMS Chapter 13 Section 2
Methods of Evaluating Meter Proving Data) will be kept on all meters by the
operations personnel responsible for proving the meters so that they are
available to them.
The performance of the meter shall be monitored throughout its service in order
to detect any short or long-term change in its characteristics. This is normally
achieved by the use of a ‘Control Chart’, which is essentially a graph of the
turbine meter’s meter-factor history.
Requirements for performance monitoring of meter factors are specified in
Procedure No. 41 – Meter Performance Monitoring.
10.2 Meter Proving Methods
This section provides a general discussion to the use of pipe prover, small
volume prover, tank prover and master meter prover.
Pipe Prover
There are two basic types of conventional pipe provers:
Unidirectional pipe prover – the displacer travels in one direction only and a proving run consists of one trip of the ball through the calibrated section.
Bi-directional pipe prover – the displacer travels in both directions
alternately. A proving run consists of two consecutive trips through the
calibrated section in opposite directions called a round trip.
Conventional pipe provers (both bi-directional and unidirectional) are those that
have a volume between detectors that permits a minimum accumulation of
10,000 direct (unaltered) pulses from the meter. A pipe prover’s main advantage
over a tank prover is that its flow of liquid is not interrupted during proving. This
uninterrupted flow permits the meter to be proved under specific operating
conditions and at a uniform rate of flow without having to start and stop.
A pipe prover operation troubleshooting guide can be found in Procedure No. 42
– Operations Troubleshooting for the for Pipe Prover.
Small Volume Prover (SVP)
The SVP has a free flowing piston with two, fixed, shaft assemblies. One shaft
assembly incorporates the hydraulic and pneumatic systems required for piston
control and the other is the detector shaft with a ‘flag’ attached. The ‘flag’ on the
detector shaft passes through fixed optical sensors (switches) to detect the piston
position.
A small volume prover operation troubleshooting guide can be found in
Procedure No. 43 – Operations Troubleshooting for the Small Volume Prover.
Master Meter
Master meters shall conform to the requirements stated in Procedure No. 43 –
Requirements for Master Meter use.
When problems with proving are encountered, or where it can be shown that flow
rates on the installation concerned have declined to the extent that the existing
prover is over-sized, meter proving by use of a suitable master meter may be
considered.
Following provisions shall be considered when proving using a master meter:
The master meter shall be based on a different operating principle from the meters being proved.
The master meter shall be by-passed and isolated when not in use so that any long-term drift would be detectable when the ‘duty’ meter is compared with the ‘master’ meter.
The master meter shall be periodically removed and calibrated at a recognized facility.
The proposed master meter shall be appropriate for the nature of the fluids concerned.
A master meter installation shall include:
o Sufficient upstream filtering of the fluid to protect the master meter from damage
o Sufficient uninterrupted straight lengths of pipe upstream of the master meter to ensure unbiased flow at the meter
o Sufficient provisions for isolations valves to allow the master meter
removal for inspection and calibration without disturbing normal
flow.
o A dedicated master meter flow computer capable of determining
master meter flow, temperature, pressure, and density to a level of
accuracy equal to that of the flow computer used with the meter
being proved. Such a master meter flow computer shall ideally be
programmed to control proving sequences and to calculate meter
factors.
Per GI 405.001, the master meter shall normally be recalibrated at intervals not
exceeding 6 (six) months, or whenever its operation is thought to have
deteriorated. The meter shall be calibrated as a complete working unit –
combined spool and internals, along with any dedicated interface electronics as
required.
Master meters shall be calibrated in accordance with Procedure No. 46 –
Calibration of Master Meter Calibration by Master Tank Prover or Procedure No. 47 – Calibration of Master Meter Calibration by Master Displacement Prover.
The master meter curve shall be generated in accordance with Procedure No. 48 – Master Meter Curve.
The master meter shall be calibrated on the in-service fluid, where possible, across at least the range of flow rates commonly met in operation.
A spare calibrated master meter shall be held at the metering station, ready to be
placed in service during periods when the other master meter is being calibr ated
or inspected. Master meters not in service shall be placed in storage per
Procedure No. 49 – Master Meter Storage.
A master meter prover operation troubleshooting guide can be found in
Procedure No. – Prover Troubleshooting for Master Meter Provers.
Tank Prover
A tank prover is a volumetric vessel that has a reduced cross section or neck
located at both the top and bottom or, in some cases, at the top only. These
necks are equipped with gauge glasses and graduated scales. Tank provers may
be open to the atmosphere, or they may be closed pressurized vessels. Proving
by a tank prover employs the standing start-and-stop method (that is, the flow
through the meter must come to a complete stop at the beginning and end of
each proving run).
Tank provers shall conform to the requirements of API Manual of Petroleum
Measurement Standards, Chapter 4.4. Each tank prover shall have a volume
equal to twice the maximum volume that can pass through the meter in one
minute [maximum continuous flow rate (capacity) x 2 minutes]. Typical capacities
of tank provers required to meet this requirement for various sizes of positive
displacement meter are as follows:
Tank provers shall comply with the design requirements specified in 34-SAES-Y103.
A tank prover operation-troubleshooting
guide can be found in Procedure
No. 45 – Operations Troubleshooting for the Tank Prover.
10.3 Proving Pipeline and Marine Meter Systems
Pipeline and Marine metering systems shall be proved according to the table
below (SAES-Y-103 Section 7.2.1):
The procedures listed below provide instructions and guidelines based on the
API Manual of Petroleum Measurement Standards (MPMS), Chapter 4.8,
“Operation of Proving Systems” for meeting the above proving requirements:
Procedure No 51. – Proving by Pipe Prover
Procedure No. 52 – Proving by Small Volume Prover
Procedure No. 53 – Proving by Master Meter
Procedure No. 54 – Proving by Tank Prover
10.4 Proving Truck Loading and Unloading Metering Systems
Truck Loading and Unloading metering systems shall be proved according to the
table below (SAES-Y-103 Section 7.2.1):
X – Approved proving method.
WA – Proving method is acceptable only with the written approval from the Chairman, Custody
Measurement Standards Committee.
Notes:
(1) Small volume provers shall be limited to applications where the flow rate is stable and free from pulsation.
(2) Piston provers are limited to applications where the liquids are free of particulates.
(3) Small volume provers shall be used only if the positive displacement meters are equipped with direct drive
pulse transmitters.
10.5 Proving Air Fueling, Dispensing and Defueling Metering Systems
Air fueling, dispensing and defueling metering systems shall be proved according
to the table below (SAES-Y-103 Section 7.2.1):
X– Approved proving method.
WA – Proving method is acceptable only with the written approval from the Chairman, Custody
Measurement Standards Committee.
Notes:
(1) Small volume provers shall be limited to applications where the flow rate is stable and free from
pulsation.
(2) Piston provers are limited to applications where the liquids are free of particulates.
(3) Small volume provers shall be used only if the positive displacement meters are equipped with
direct drive pulse transmitters.
10.6 Proving of Coriolis Flow Meters
The methodologies used to prove a Coriolis meter are direct mass, inferred
mass, and volumetric. Calibration against a mass flow rate standard is preferred.
Where the Coriolis meter is calibrated against a volume flow rate standard, the
uncertainty in the density of the test fluid (at meter conditions) shall be
considered when interpreting the calibration results.
The proving conditions shall generally be as similar as practically possible to the
anticipated ‘in service’ conditions. This requirement does not extend to the
upstream pipe configuration, since Coriolis meters are relatively insensitive to
flow profile. The meter may be verified by periodic comparison with a prover or
recalibration at a recognized test facility.
Procedure No 55– Proving Coriolis Meter using Pipe Prover provides instructions
and guidelines based on the API Manual of Petroleum Measurement Standards
(MPMS), Chapter 4.8, “Operation of Proving Systems” for meeting the proving
requirements.
Following are provisions to be considered for each proving method:
Direct Mass Proving
In a direct mass proving, the mass of fluid in the prover (reference quantity) is
physically measured. The mass measured by the prover is then compared to the
mass measured by the meter to generate a meter factor.
The common methods used are:
a. Gravimetric: The reference quantity of fluid is weighed on a scale and
compared to a meter’s indication of mass. This method is not covered by
API MPMS standard.
b. Mass Master Meter: The reference quantity of fluid is obtained from a
mass master meter and compared to a meter’s indication of mass. This
method is not covered by API MPMS standard.
Inferred Mass Proving
In an inferred mass proving, the mass of the fluid in the prover (reference
quantity) is calculated rather than physically measured. The mass is calculated
by multiplying the volume and density of the reference fluid together.
The inferred mass is then compared to the meter’s indicated mass to generate a
meter factor. The accuracy of this method is equally dependent upon the
accuracy of both the volume and the density measurements.
The volumetric proving methods should be used to determine the reference volume in an inferred mass proving. The selection of a method to determine a reference density (density at the prover) is critical to a successful and accurate prove. These methods should be closely reviewed as to their accuracy and ability to measure the density under the conditions (pressure and temperature) present at the prover. If the density varies during a prove, it must be averaged for each prover run or pass (averaged between the prover switches). The sampling frequency and the density averaging method also influence the overall accuracy of this method.
For inferred mass proving, the preferred method for determining the fluid density
at the prover is to use an on-line density meter. The density meter must be
installed, operated, and calibrated per API MPMS Chapter 14.6. The resulting
output of this meter should be averaged during each prover run or pass.
Volumetric Proving
In a volumetric proving, the volume of fluid in the prover (reference quantity) is
determined by the methods listed is API MPMS Chapter 4.8, Appendix D. The
prover volume is then compared to the meter’s indicated volume to generate a
meter factor.
10.7 Proving of Ultrasonic Flow Meters
Ultrasonic meters are not authorized for custody/royalty liquid hydrocarbon
measurement. Ultrasonic meters shall under all circumstances be flow calibrated
at a recognized facility prior to their use for custody transfer. This applies equally
to ultrasonic master meters, where their use is approved.
The meter shall be calibrated over as much of the full anticipated flow range as
possible, with particular attention paid to the expected operating flow rate. The
meter shall normally be calibrated at six ‘nominal’ flow rates evenly-spaced within
the range, with interpolation of the calibration offset for flow rates not directly
covered. To maintain traceability, the calibration data and interpolation
calculations shall be stored within the flow computer rather than the meter
electronics.
The Ultrasonic flow meter shall be calibrated using a fluid that resembles, as
closely as possible, the in-service process fluid. Where this is not possible, There
may be SCOPE to use onshore terminal facilities as calibration sites. Metering
stations equipped with turbine meter and prover loop designed for ‘batch’ export
may be particularly suitable for this purpose.
In addition, the flow profile at the calibration shall be representative of that
predicted at the ‘in-service’ meter conditions. If this condition cannot be met then
the use of flow conditioners may be necessary.
Ultrasonic flow meter verification shall follow one of the three following methods:
o The use of a master meter
o Removal of the meter for calibration at a recognized test facility
10.8 Field Standard Calibrations
Per GI 405.001, Field test measures used to calibrate meter provers are
maintained and handled in such a manner so as to prevent damage and their
volumes are certified in accordance with API MPMS Chapter 4.7 by an approved
weights and measures agency at intervals not to exceed 5 years.
10.9 Prover Calibration
Provers shall be calibrated by the methods listed below:
The minimum frequency requirements for prover calibrations by prover type are
listed in Procedure No 56 – Prover Calibration Methods and Frequency.
Prover loops shall be calibrated at the manufacturer’s works by methods
described in SAES-Y-103 as part of their systems checks, and again after
installation on site. One copy of the calibration certificate for each of these and all
subsequent calibrations shall be sent to the Custody Measurement Unit.
Prover Calibration Preparation
Procedure No 57 – Prover Calibration Preparation, contains some important guidance to help ensure that the prover calibration proceeds as smoothly as possible.
Procedure No 58 – Pipe Prover Cleaning for Calibration provides guidance on
the requirements for cleaning a pipe prover prior to initiating calibration.
Inspection of all critical valves and instrumentation along with the sphere,
checking of sphere size, etc. shall take place prior to calibration. After calibration
the sphere detectors and all vents and drains shall be sealed. Prover spheres
shall be inspected and inflated in accordance with Procedure No. 59– Prover
Sphere Inspection.
Any maintenance work on the prover that could affect the swept volume (for
example, changes of sphere detectors and switches) shall not be undertaken
without prior notification to Custody Measurement Unit. Custody Transfer Unit
shall be given at least 30 days notice of all prover loop calibrations so that
arrangements for possible MINPET witnessing can be made.
Acceptance of Prover Calibration Results
For a prover base volume calibration to be acceptable, it shall be based on 3 (three) consecutive round trips where the range of volumes is within ±0.02% of the mean of these volumes.
Any shift of >0.025% shall be verified by a repeat calibration at a different flow
rate. The difference in flow rate shall be at least 25%.
When a volumetric (displacement or tank) prover is calibrated using any of the
approved methods, the new base volume from the previous calibration base
volume shall not exceed ±0.05%. If the new volume exceeds this limit, an
investigation shall be performed to determine the cause of this shift.
The new volume may be accepted if the prover is in good order and the check
calibration substantiates the change in volume. It is not uncommon for a
relatively new prover’s second volume to differ substantially from the initial
calibration. This may be due to excessive wear during the break-in period, or to a
poor initial calibration that was undetected due to lack of history.
Prover Recalibration
A prover shall also be recalibrated if:
1. Piping of calibrated section has been modified or worked on.
2. Flanges(s) of calibrated section have been disconnected.
3. Detector switches have been repaired or removed.
4. Evidence of prover volume change is seen
5. Design modifications affecting the prover volume are applied
6. The prover is relocated from its original site.
7. Calibration certificate expired in accordance with the frequency stated
in SACMM-DM-069 – Prover Calibration Methods and Frequency.
Prover Failure
In the event of the failure of any critical element of the prover, CMU shall be contacted so that an appropriate strategy for the reverification of the meters may be agreed.
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