Industry in South Africa is moving vast quantities of petroleum and chemical products for export, import, local trade, and storage. Typically, international movements are transported by sea in tankers carrying large quantities. Once a ship arrives berthing costs begin to apply requiring a speedy transfer of the product. Flowmeters are inadequate in size, and the alternative is custody transfer by tank gauge LTD. However, there are some complications.
Gone are the days when it was acceptable in South Africa to transfer the custody of any product by measure providing that both parties are satisfied with the measuring equipment used. It is now necessary for the apparatus to be in accordance with a South African national standard written to detail the equipment requirements, and the method of use for this equipment. However, there is a major difficulty with this in South Africa, and that is there is no standard that covers custody transfer by tank gauge LTD.
Why required?
An obvious question comes to mind. Why is it necessary, since there is a standard for flow metering, and the transfer can be suitably monitored using this tried and trusted method? The reply in part is: flowmeters and checking devices are not large enough for metering the discharge from ocean-going supertankers.
Points for consideration
It should be remembered that in tank gauge LTD, the level measuring instrument itself is not the difficult component in these applications. The following parameters also need consideration.
1. The tank itself:
a. The deformation of the sides due to product level, product temperature, and ambient temperature.
b. The deformation of the floor due to product level, product temperature, and ambient temperature.
c. The deformation of the roof due to tank movement and loading on the roof.
2. The product conditions:
a. The temperature.
b. The pressure
c. The transfer is normally specified in standard units rather than measured units. This requires a density correction to the measured volume to obtain standard volume. In South Africa correction is to 20°C, however, 15°C and 60°F are also used.
3. The gauge support structure.
4. The amount of water in the bottom of the tank.
5. The operator interface.
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