Generally, the heavier material at the bottom of the tank is the least valuable, both in terms of cost and specification. Therefore, the entity receiving this portion will receive off specification product and will either reject the cargo or claim financial compensation. The balance of the tank’s contents will either stay in the tank or go to another receiver. In the latter situation, this entity will receive product that is of higher specification than that which was contracted for. While this entity will most likely not reject the cargo, it is unlikely that the shipper will have any recourse to charge more for the better product.
2.8.1 What is Stratification?
2.7.6 Summary
If an ATG is used for custody transfer purposes, it is necessary to verify and record that there is documentation to support: • A factory calibration showing an accuracy of ± 1 mm or ± 1/16 inch, if the unit was installed after June 2001, and for crude oil lease tanks a factory calibration showing an accuracy of ± 3 mm or ± 1/8 inch, if the unit was installed after 2018. • An initial verification at approximately upper, middle and lower levels showing an accuracy of ± 4 mm or ± 3/16 inch, except for crude oil lease tanks which require an accuracy of 6 mm or ± 1/4 inch. • Monthly periodic verifications at a single level within the normal operating range showing an accuracy of ± 4 mm or ± 3/16 inch, or 6 mm or ± 1/4 inch for crude oil lease tanks. 2.8 Stratification It is not unusual for the contents of a shore tank to be stratified. This is most common when dealing with heavy fuel oils; however, stratification can also occur in light products such as diesel, gasoline, or naphtha.
Stratification is the incomplete mixing of a tank’s contents, also referred to as non-homogenous.
This usually results in layers of material which have different compositions and specifications.
The layer with the highest density (lowest API gravity) will be at the bottom of the tank and the layer with the lowest density (highest API gravity) will be at the top of the tank.
2.8.2 Problems caused by Stratification?
There are several problems associated with stratification and incomplete mixing of tanks.
2.8.3 Sampling
Although sampling requirements are specified by the client(s); typically, an upper, middle, lower and/or running sample is taken from the custody transfer tanks. API MPMS Chapter 17.1, states, ‘When the material is known or suspected to be stratified, spot samples may be drawn and analyzed to determine the degree of stratification’. Some of our clients instruct us to take spot samples at different levels, however, some do not. When this is the case, we must advise the client that testing for stratification is recommended as good practice. An example of such an advice is:
• If less than the entire contents of the tank are transferred, the product delivered out of the tank will not have the same analytical specifications as those of the entire tank, which are determined from the representative tank sample. • Even if the entire contents of the shore tank are shipped onto a marine vessel, it is likely that product on the marine vessel will vary in specification from tank to tank. If this product is then discharged into more than one shore tank, the receiving shore tanks will have differing specifications. This situation becomes even more problematical when different receivers are involved.
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