pour point is the lowest temperature at which a marine fuel can be handled
without excessive amounts of wax crystals forming so preventing flow. If the
temperature of a fuel is below the pour point, wax will begin to separate out
which will block the filters. The wax will also build up on tank bottoms and on
heating coils. However, when heat is adjusted properly it can be difficult to
get the wax to be dissolved because of its insulating nature. In extreme cases
hand cleaning of the tanks becomes necessary.
The actual pour point of a fuel depends on various factors, which include the source of crude oil and the refining processes used in manufacture. Although for the majority of residual grades in ISO 8217 (those with a viscosity higher than 15 cSt at 100oC - RM15 and above) the limit on pour point is 30oC, in practice the great majority have a pour point of less than 0oC.
If the pour point is known an informed operational decision can be taken as to a safe storage temperature of the fuel. A fuel is stored at a temperature at least 5oC to 7oC above the pour point in order to avoid the waxing problems described. Knowledge of the pour point is not usually available at the time of delivery, but it is one of the parameters routinely determined by fuel testing services.