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Liquid cargo should only be
trim and/or list corrected if the liquid is in contact with all
bulkheads. When the liquid is not in contact with all
bulkheads, a wedge correction should be applied. In all
circumstances, the cargo documents should include the vessel's list and
trim. The nature of the material in the tank should be
described in detail, and the conditions of measurement and other
pertinent information should be noted. For calculation of
small quantities, refer to API M-PMS Chapter 17.4.
Note: Wedge, trim, and list
corrections do not normally apply to sediment and sludge but may apply
to solidified (non-liquid) cargo. In addition, when the wedge
formula or wedge tables are used, extreme care must be exercised to
ensure that wedge does exist, that the measured material is not just a
puddle under the gauge hatch, and that the formula used is applicable
to the actual shape of the tank (that is, it accounts for the curve of
the bilge). Measures to be taken in such a case should
include-but are not necessarily limited to-taking ROB measurements at
more than one point in the tank. This would verify the
existence of a wedge and the extent of cargo solidification.
Open
Temperature Determination The temperature of the cargo
being measured is one of the most important elements needed to
accurately determine its volume. This section fully describes
the equipment and procedures that should be used to manually obtain the
cargo's temperature.
Open
Temperature Measurement Equipment All temperature
equipment must be safe for use with the material whose temperature is
to be obtained. The preferred method of obtaining
temperatures of the liquid in a vessel's tanks is to use a portable
electronic thermometer (PET). Alternately, a mercury-in-glass
thermometer with etched glass face may be used.
Thermometers used for custody
transfer should be properly calibrated and their accuracy verifiable
and traceable to a NlST standard thermometer and meet the requirements
specified in API MPMS Chapter 7.
Thermometers
Thermometers are precision
instruments and should be handled with care. For a technical
description of the specifications of each type, refer to API MPMS
Chapter 7.
Field
Verification of Temperature Equipment
All thermometers used for
custody transfer measurements should be verified for accuracy before
initial use, and at least once a year thereafter. In
addition, before each use or once per day (whichever is less frequent)
the thermometer should be spot-checked. For full details of
thermometer verification, please refer to API MPMS Chapter 7.1
Mercury-in-Glass
Thermometers
Glass
stem thermometers should be verified for accuracy before initial use
and at least once a year thereafter. In addition before each use or
once a day (whichever is less frequent) the thermometer should be field
checked by visually checking the glass capillary for breakage and
separation of the mercury column. Glass stem thermometers
with
abnormally worn etched faces or broken mercury column should not be
used. If the column is rejoined, it may be used provided that
it
successfully passes a bench inspection. For additional
technical
details see API MPMS Chapter 7.1.
Portable
Electronic Thermometers (PETs)
Before
initial use, and at least once a year thereafter, all electronic
thermometers shall be re-standardized in a laboratory or other
qualified facility. For full details see API MPMS Chapter
7.3. In addition before each use, or once a day (whichever is
less frequent), PETs should be spot-checked by comparing the ambient
reading against an ASTM glass stem thermometer in liquid. If
the
readings differ by more than 1°F or 0.5°C, the PET should be
re-standardized before it is used for custody transfer. For
details on verification of the PET see API MPMS Chapter 7.3.
Open
Temperature Measurement Procedures
Manual temperature measurement
is the determination of the temperature of a liquid in a vessel's tank,
using the appropriate devices. The primary considerations of accurately
determining temperature are
-
the size and location of cargo
tanks,
-
whether or not heat has been
applied to the cargo,
-
the atmospheric and seawater
temperatures, and
-
the degree of temperature
stratification within the cargo. Temperatures should be taken and
should be clearly designated as degrees Fahrenheit or Celsius, as
appropriate.
Temperatures should be
determined at the same time gauging is performed.
Temperatures should be taken in all tanks, and upper, middle, and lower
temperatures should be taken in each tank whenever the liquid level is
greater than 10 feet (3 metres). For vessel tanks with less
than 5000 barrels (795 cubic metres), a single temperature measurement
at the middle of the liquid will suffice (see Table 4). The
total vessel volume should be corrected to the standard temperature on
a tank-by-tank basis, using the average temperature determined for each
tank. By agreement of all parties involved, more or less than
three temperatures may be taken to calculate an average tank
temperature.
It
may be noted that when temperature differentials greater than 5°F (3°C)
are found, additional temperatures should be taken. The
number of
additional temperatures will vary with the temperature
differential. However, they must always be equally spaced and
averaged accordingly.
The immersion time required for
the thermometer reading to reach equilibrium will vary depending on the
type of liquid and equipment. For more specific guidelines on
immersion times, see Tables 5a and 5b and refer to API MPMS Chapters
7.1 and 7.3.
Portable
Electronic Thermometers (PETs)
In addition to the steps
described earlier, the following procedure is recommended for measuring
temperatures with a portable electronic thermometer (PET):
-
Attach an electrical ground
between the thermometer and the tank before the hatch is opened. Check
the ground to ensure that it is securely attached to the thermometer.
-
Set the temperature range
selector as appropriate.
-
Lower the sensing probe to the
predetermined level.
-
Raise and lower the probe 1
foot (0.3 metre) above and below the predetermined level to allow rapid
stabilization.
-
After stabilization, read and
record individual temperatures to the nearest 0.1°F or 0.°C,
-
Determine the average tank
temperature to a tenth of a degree.
-
Round off and report the
average tank temperature in accordance with the most recent edition of
API MPMS round off and report average tank temperature to 1°F or
0.5°C [round 0.5°F up]. Temperatures may be reported in units
less than whole degrees by mutual agreement.
If
the probe is allowed to remain stationary, contact with a convection
current of colder oil will cause low readings. With a moving probe,
however, the thermometer may be considered stabilized if the readout
varies by no more than 0.2°F (0.1°C) for 30 seconds.
Mercury Thermometers In
addition to the steps described earlier, the following procedure is
recommended for measuring temperatures with a mercury thermometer.
-
Lower the thermometer assembly
through the gauge hatch to the required level.
-
Repeatedly raise and lower the
thermometer 1 foot (0.3 meter) above and below the required level so
that the equilibrium temperature will be reached more rapidly.
-
Withdraw the thermometer after
the required immersion time.
-
Round off and report the
average tank temperature in accordance with the most recent edition of
API MPMS Chapter 7.1 (round off and report average tank temperature to
1°F or 0.5°C [round 0.5°F up]). Temperatures may be reported
in units less than whole degrees by mutual agreement.
-
Report the temperature to the
nearest 1°F or 0.5°C.
-
Repeat items a through e for
every tank to be 'temperatured'
Sections of text taken from API - MPMS Chapter 17 Marine Measurement
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