ISO 6578:2017 pdf free
ISO 6578:2017 pdf free.Refrigerated hydrocarbon liquids一Static measurement 一Calculation procedure
Although the principles of calculating the quantity of a static refrigerated hydrocarbon liquid are basically similar to those for petroleum liquids at ambient temperatures, there are differences caused by the low temperature and the physical properties of refrigerated hydrocarbons. These include the following.
a) The liquid product is at or near a temperature at which bubbles of vapour are first formed within the liquid (bubble point). In a tank containing refrigerated liquid, there will always be a small inward flow of heat through the insulation, which will cause a continuous vaporization of the product. The vapour will contain a higher concentration of more volatile constituents than the liquid. To avoid over-pressure, this vapour is vented from the tank and can be compressed, cooled and re-liquefied for re-introduction into the tank.
b) When a liquid product is transferred from one tank to another, additional heat inflow will occur in the pipeline and also from work done by the pump, causing additional evaporation in the receiving tank.
c) For custody transfers from a supply to a receiving tank, it is normal practice to provide a vapour return line linking the tanks to avoid displacement of vapour to the atmosphere. Build-up of pressure in the interlinked system is avoided by reliquefaction.
d) After a partial filling, stratification into different temperature and density layers may occur in the liquid contents of a tank. Therefore, a number of temperature measuring points and a special sampling system may be necessary. If the filling operation is such as to ensure mixing, these needs may be reduced.
e) There is considerable evidence that large temperature gradients exist in the vapour space of any tank containing a refrigerated hydrocarbon liquid. These gradients may not be linear. Suitable compensation (physical or by calculation) should be made if the reading of the level-measuring device is affected by differential contraction of the level-sensor suspension.ISO 6578 pdf download.
