Pipeline mixing (homogenisation)
Why do the standards demand that pipeline contents must be homogenous?
A sample is taken from a single point in the pipeline. Water and oil do not mix and therefore it is vital that the point of sampling is representative of a cross section of the pipeline. This can only be achieved by mixing. Natural mixing can be provided by valves, elbows and natural turbulence generated by the flow.
The standards dictate that the sample should be drawn from the central third of a pipeline in which the water concentration at the top of the pipeline (C1) should be 90% of the water concentration at the bottom (C2), i.e. the C1/C2 ratio must be at least 0.9. If this requirement is met then the location selected is deemed to have sufficient mixing for sampling. The standards define a set of calculations (ISO 3171 annexe A and API 8.2 annexe B) to assess the mixing at a potential sampling system location. Mixing and dispersion cannot be assessed by determining the Reynolds number alone.
The C1/C2 ratio must be above 0.9 for ALL flow rates and fluids that will be transferred through the pipeline. The calculation should be performed for the worst to best-case scenarios to assess the mixing requirements. The worse case scenario (i.e. where there is the least natural mixing) occurs at the lowest flow rate, density and viscosity. It is also extremely important that the lowest flow rate is not the lowest normal flow rate but the lowest actual flow rate as in many situations (such as marine unloading) the most water will be discharged at the beginning and end of a batch when the flow rate is very low. Which type of mixer is best?
The API, IP and ISO standards both include a table to use as an initial assessment of the suitability of a location. This table is calculated at a single density and viscosity and is therefore only valid as a guide. All the standards recommend that the full calculations be performed before selecting a suitable sampler location.