Sampling is done due to the fact that you cannot use the entire tank material to determine if it passes all required specifications, as well as to assess conformation with specification or other quality requirements.
Quality specifications are produced to ensure that, as far as possible, the product purchased is fit for its intended purpose. If a sample does not meet specifications or quality standards then solutions must be found to ensure that it passes all required prerequisites. Contamination is often the biggest and most costly problem faced by a tank farm and can only be determined by sample analysis.
How Well Does the Sample Represent the Tank’s Contents?
All samples taken from a homogeneous bulk of liquid should have the same quality. Non-homogeneous liquids are difficult to sample correctly, and sample representivity is far from a ‘given’. These liquids include crude oil and fuel oil, two of the most widely-traded petroleum commodities.
It is important to ensure that the sample is as representative as possible, however, it is important to understand that sampling requires product knowledge, skill, care and the right equipment.
Shore Tank Sampling
Upper, Middle and Lower samples should be taken and these samples should be checked for density (at least) to ascertain if the tank contents are homogeneous (well mixed).
If it is homogeneous then a composite can be prepared for further testing. If non-homogeneous, then it is necessary to take further samples (i.e. samples at metre intervals).
A sample is taken from a tank by lowering a stoppered bottle or flask to a designated point in the tank, removing the stopper with a sharp jerk, and allowing the bottle or flask to fill completely before withdrawing it.
Bottom samples are often the most problematic as any foreign material tends to settle at the bottom of the tank. Many depots fail to take “dead bottom samples”. This is the reason most auditors carry bottom samplers with them for audits.
Sampling is a critical aspect of tank management. While many may not realised it this is one of the most vital steps to ensuring a smooth and accurate sampling process.
Proper labelling, sealing, documentation, retention and preservation is required post sampling. Poor quality management can lead to big, unnecessary costs.