Scope: This procedure provides guidelines to determine the volume of Tank provers
at base conditions of temperature and pressure by the waterdraw method.
TANK PROVER CALIBRATION BY WATERDRAW METHOD
Frequency As indicated in Procedure No. 049
Procedure Tank Prover Cleaning Procedure
If necessary, the tank prover should be cleaned from any oil, scale or hydrocarbon products.
1 Ensure the following items are available prior to commencement of the cleaning exercise:
a) A quantity of diesel, similar to the tank prover nominal volume, if the tank prover is contaminated with oil deposits.
b) Fresh, clean, water supply.
c) A de-greasing detergent and flexible cleaning brushes.
d) Facility to dispose of contaminated diesel and water.
2 Isolate and drain the process fluids from the tank prover.
3 If the tank prover is contaminated with oil deposits, fill the tank prover with diesel and allow soaking for twelve hours.
4 Drain the diesel from the tank prover and fill to approximately 20% total capacity, a solution of fresh clean water and degreasing detergent.
Add the correct quantity of de-greasing detergent, according to the detergent manufacturer’s instructions.
5 Using suitable flexible brushes clean the inside of the tank prover.
6 Flush with fresh clean water to remove traces of hydrocarbon and / or de-greasing detergent.
7 Drain the tank prover and confirm, by observation, that it is now clean.
Test Set-Up
The test set-up and piping installation for the calibration of the tank prover is as shown in Attachment I of this Procedure.
Procedure
Internally and externally inspect the tank prover for mechanical
damage. Dents in the neck of the tank prover, that may affect scale
readings, cannot be tolerated. Dents in the body of the tank prover
should be removed, if possible, before commencing the calibration. If
a dent is present during calibration it should be recorded on the
calibration report.
The following procedure describes the calibration, by Watedraw of a
tank prover using one Field Standard Test Measure. For example,
one 10-gallon test measure used to calibrate a 100-gallon tank
prover by 10 repeated pours. Saudi Aramco safety procedures and
work permit requirements must be observed at all times
1. The tank prover shall be internally cleaned and shall be
plumb and level in both X and Y-axis.
2. All devices and instruments that affect the internal
volume of the tank prover, such as spray lines, temperature
sensors and gauge glasses shall be in place.
3. Connect water supply, platform with handrails, hoses
and fittings etc. as shown in Attachment I of this Procedure.
4. The water quality must be checked. This is achieved by
filling the tank prover and test measure almost to the top of
their respective gauge glasses. If the observed water level
changes, due to bubbles being present, then the water cannot
be used and it will be necessary to drain all water from the
system and replace with fresh, clean, deaerated water.
5. The tank prover and test measure, including valves,
fittings and blinds that hold the test liquid, shall be checked for leaks. All fittings must be ‘bubble tight’. When full of water
confirm that the tank prover is still plumb and level in both X
and Y-axis.
6. Perform function test of the temperature instruments by
comparing temperature readings. Confirm agreement within
0.1ºF. Fill each thermowell to be used with a temperature
transmission liquid such as Glycol
7. Drain the water from the tank prover according to the following (typical) drain time which is shown on the previous
tank prover calibration certificate.
i) Five minutes from opening to closing the bottom drain valve.
ii) 30 seconds drip after cessation of main flow.
iii) Bottom neck level set to zero data after 8 minutes drain
down.
This is called a wetting run and establishes the ‘clingage’,
which must be present whenever the tank prover is used.
8. All tank prover drain, inlet or outlet valves must be
closed before the water pours from the test measure
commence. Tank prover valve arrangements may vary.
9. Fill the test measure and record the scale reading as
accurately as possible (usually 1 cubic inch if using a Field
Standard Test Measure), and the test measure temperature to
0.1 °F.
Note: If thermowells are not installed it may be
necessary to record the water temperature when
draining the test measure. In this case drain
immediately after recording the scale reading and
place the temperature device in the immediate
vicinity of the drain outlet.
10. Pour the water from the test measure into the tank prover allowing the drain time shown on the test measure
calibration certificate.
11. Repeat steps 9 & 10 until the water level can be
observed at the tank prover gauge glass. As previously stated
more than one test measure may be required. This depends
upon the volume of the tank prover, and the volumes of the
test measures available.
12. Record the water level reading on the scale of the tank
prover gauge glass as accurately as possible. This will
depend upon the scale units and graduation markings.
Measure and record the height of the water level (in mm)
above a fixed datum. (Example; the top face of the bottom
gauge glass support bracket).
13. Observe and record the tank prover temperature to the
0.1 °F. Depending upon the size and design of the tank prover,
there may be up to three thermowells installed to monitor
temperature. All temperatures should be recorded and an
average temperature calculated.
14. Drain the tank prover allowing the drain time shown on
the previous tank prover calibration certificate. The first fill
(run) of the tank prover is now complete.
15. Repeat steps 9 to 14 until three consecutive runs of the
tank prover have been completed and the comparison
between the total poured volumes and the indicated volumes
agree within 0.020%.
16. Calculate the total volumes of the three runs at base
conditions and the errors when compared to the scale
readings.
17. The final operation is to adjust and seal the gauge glass scale, using a unique seal and sealing wire, according to comparison between the average poured volume at base conditions and the average indicated volume.
18. Disconnect all the calibration equipment following confirmation that the calibration has been successful.
19. Drain and return the tank prover to service.
Tank Prover Neck Calibration Procedure
When a tank prover is fabricated it is necessary to perform a neck calibration to determine the neck value. This value is needed to produce a volumetric scale with accurate graduations. A neck recalibration will be necessary if the original scale is lost, there have been significant alterations to the neck of the tank prover or the validity of the scale requires checking.
A Field Standard Test Measure or Primary Measure is used to perform the neck calibration.
The volume of the test measure will depend upon the volume of the neck that can be observed in the sight glass. (Example: a two liters test measure can be used to calibrate a tank prover with a 10 liters neck volume).
1 Fill the tank prover with fresh, clean, deaerated water, up to the lowest visible level in the tank prover sight glass.
2 Record the height of the water above a fixed datum. (Example: 18 mm above the top edge of the bottom gauge glass support bracket).
3 Pour one known volume using the test measure.
Record the test measure temperature to 0.1F and
observe the drain time shown on the test measure
calibration certificate.
4 Record the new height of water, in the tank prover
neck, above the fixed datum.
5 Repeat steps 3 and 4 until the water level in the neck
of the tank prover is close to the top of the sight glass.
6 Complete the neck calibration certificate and
calculate the ‘neck value’ in SI liters per mm.
