3. If the entire tank contents will not be used, but only a portion of it, the samples taken which will be subsequently analysed, should only be taken from the levels which will be transferred out of the tank. 4. Individual ship tank samples must be drawn on every job. If we only draw ship composite samples, we will be unable to localize a problem after the event. If a one-quart (which is 0.946 litres) tank composite sample is to be made from individual quart upper, middle and lower samples, one third of a quart should be taken from each level sample. The balance should be retained as individual samples should they need testing (individually) in the event of a dispute. 5. If a client asks us to determine the density of a tank, something that is almost universal whatever our nomination for trade work and a prerequisite to even quantity jobs, we MUST determine if that tank is homogenous or not. 6. The standards are clear. If upper, middle, and lower samples indicate that a tank is not homogeneous then additional samples are to be drawn until an adequate tank profile is obtained and reported. This is not optional; it is a 100% binding obligation under the method and standards. If we agree under a contract or nomination to establish a tank density, then this is the way we must proceed.
‘ We wish to draw to your attention that the material we are inspecting on your behalf could be susceptible to stratification. For this reason, and as recommended in API MPMS Chapter 17.1, we strongly recommend that spot samples be taken, at a minimum of upper, middle, and lower, and individually tested for evidence of stratification ’. NOTE: homogeneity is only defined for petroleum products in one place, and then only for one product, that is aviation fuel. For all other types of products, it is a rather unclear concept. The common assumption made is that homogeneity is established by reference to a narrow band of density or API between samples taken at various points (often upper, middle, and lower) or between tanks (e.g. on a ship); please be careful. Homogeneity with respect to density is no guarantee of homogeneity with respect to another analyte. For example, fuel oil density, viscosity, metals content and water within a large shore tank; the profiles for each property may be very different in each case, and not linked. Do not assume that because a tank is homogenous with respect to density it is also homogenous with respect to all other analytes.
capable of retaining water and sediments in suspension such as crude oils and heavy fuel oils, please bear in mind that any kind of petroleum product can stratify, resulting commonly from differences in cargo temperature and density. If we assume tanks to be homogeneous when they are not, then we will receive claims and complaints on a regular basis.
To manage this issue, it is our responsibility to:
a) Take adequate samples to establish if the contents of any tank are homogeneous or not - it is not the client’s obligation to tell us to do this. b) Retain sufficient sample to demonstrate after the fact if any given tank was homogeneous or not.
c) Communicate and document clearly if any cargo/tank is found to be non-homogeneous.
Please be mindful that:
1. Running or all levels samples WILL NOT highlight if a tank is homogeneous or not. Upper, middle, and lower spot samples or even more extensive layer / zone sampling can. 2. If we fail to take and to retain upper, middle, and lower spot samples or similar individual level samples we cannot demonstrate either before or after the fact that a tank was homogeneous or not.
Whilst stratification is most prominent when blending products or when working with materials
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