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 Many
factors are involved in moving or trading bulk volumes of liquid
petroleum and other products. The value of the product is usually
determined by an independent inspection company who uses their
own equipment to calculate the volume, purity and other required
specifications. These inspection companies are responsible for
providing the most accurate information possible from circumstances
which are highly variable. Within the storage vessel, walls
are irregular, temperatures of the product are stratified, quality
of the product is stratified, and water may cover the bottom.
While gathering data on the stored product, caution must be
taken in this usually hazardous environment. All equipment used
in the hazardous location near the vessel must be intrinsically
safe†.
Petroleum products have a high rate of thermal expansion which
must be taken into account when several thousand barrels are
transferred or purchased. The temperature of a product can expand
or contract the total volume a significant amount and therefore
must be corrected to a standard volume at 60 degrees Fahrenheit.
To obtain the correct volume of product within a storage tank,
an average of temperatures taken at several levels must be obtained.
For most petroleum the instrument must have an accuracy of +
0.2 °F / 0.1 °C.
A “strapping table” of the vessel determines the volume
of the vessel at different levels. This table takes into account
any pipes, ladders, or objects which may be within the vessel.
The table also shows top and bottom reference points within
the vessel.
The level of the liquid within a vessel can be determined with
a steel engineering tape and bob, or other ullage measuring
devices. By measuring from a reference at the top of the vessel
to the surface of the product, the level and liquid volume can
be obtained with the aid of the strapping tables. After the
ullage is taken, then the water and sludge at the bottom must
be determined and subtracted. This can be done by coating the
gauging tape with a special paste that changes color when in
contact with water or by electronic means.
After the volume of the petroleum product within the vessel
is derived a temperature/volume correction must be made. Depending
on the classification of the petroleum being gauged, different
“API Tables” are used to calculate the volume correction
factor. After this the correct volume of the product is known.
Samples from within the vessel are sent back to the laboratory
for further information regarding the quality of the product.
The most commonly used instruments to determine the temperature
of petroleum inventories are the ThermoProbe digital thermometers.
The instruments are very accurate and tested to be intrinsically
safe under North American, European and Japanese standards.
All ThermoProbes are listed for Class I, Division 1, Groups
C&D Hazardous Locations or II B, while some models have
a listing for higher groups which allow safe use in the most
flammable gas mixtures. Depending on model and testing in progress,
these safety marks are displayed on ThermoProbe products:
ThermoProbe instruments are currently used by major international
inspection companies such as: Inspectorate, SGS Control Services,
ITS CalebBrett, Saybolt and DNV Petroleum Services.
The list of major petroleum producers which use ThermoProbes
includes, Pemex, BP, ChevronTexaco, ExxonMobil, Shell, Koch,
TotalFinaElf, Marathon, Ashland, BHP, Statoil, Conoco, Phillips,
Calte, and many others.
Many for-hire storage facilities and pipelines use the instruments
to monitor inventories and calibrate other equipment.
A relatively new application of ThermoProbe thermometers is
its use in calibrating fuel meters for propane, diesel and gasoline.
Regulatory and tax agencies in the states of California, Hawaii,
Tennessee, North Carolina and Florida use ThermoProbes to help
verify meter accuracy in order to protect the consumer and collect
fuel taxes.
ThermoProbe thermometers are marketed by our numerous representatives
located on six continents.
*
In this text the word Petroleum is used to describe any petroleum
product or chemical which is subject to volumetric expansion
due to temperature.
† Intrinsic Safety is a
protection concept employed in potentially explosive atmospheres.
‡ Unfilled space between
liquid level and top reference point.
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