If you ever wondered how the crude oil is sampled, you would be surprised by the “crudeness” of the act. Sample is typically taken from the tank by descending the probe into the liquid, pulling the plug at the desired level (depth) and then withdrawing it. Naturally for sampling to be valid oil needs to be homogenous and proper care needs to be taken during the process. This method of sampling however may yield rather poor results in subsequent tests for reid vapor pressure (RVP), which is a way to measure how quickly fuels evaporate. It is often used in determining petroleum product blends. The higher RVP, the more quickly it evaporates. The testing for RVP depends on scarce quantities of volatile components which may be easily lost in the sampling and subsequent testing.
Line samplers, which withdraw a continuous or intermittent sample (from a loading or discharge pipe) tend to bring better results, as far as RVP is concerned.
Another problem involving oil sampling relates to proper measurement of water content in oil. Water content is not evenly distributed through the pipe, can move in slugs and hence even a method of line sampling is not always accurate. The issue with water content measurement becomes less problematic in shore tanks, where it settles well and can be measured with a measuring tape (through the dipping). The problem reappears however if we receive the cargo. Then if we would like to measure a difference in a height of water content, in order to determine its amount in the newly received cargo, then it might be difficult considering the implied miniscule differences in a typically large storage tanks. Also considering the crude oil we need to watch out for its tendency to take time to settle and to distribute unevenly across the bottom of a storage facility.
Generally for loading and unloading of crude oil the water measurement is estimated to add an additional f 0.1 per cent error but in case of some crudes it might be considerably more.
Refineries in-house labs usually furnish a certificate of quality delivered with the products. It shows the results obtained on samples of the product certified against the full range of specification tests. Samples are also subsequently taken on shore and from ship tanks during loading and discharge. Then comparison of samples is made in order to confirm that a contamination is not occurring. It is a good practice to retaine these samples in case of any future dispute. It is almost always a case that a third party inspection company is hired to measure, sample and test the quality of a product at different stages of supply chain.
As far as crude oils are concerned, solely API gravity and water content are widely specified and tested. In the next post we will discuss which properties play a key role in which oil products.
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