Gel point (petroleum)
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Gel point is the temperature at which diesel or biodiesel fuel freezes solid and can no longer flow by gravity or be pumped through fuel lines. This phenomenon happens when a fuel reaches a low enough temperature whereby enough wax crystals have formed to prevent any movement in the oil. For #2 diesel this is usually around 17.5 °F (−8.1 °C).
For the fuel to become pumpable again, it needs to be brought above the gel point temperature to the Ungel point, which is typically near its pour point. However, most of the waxes will still remain in solid form and the fuel has to be warmed up further until its Remix temperature in order to completely remelt and redissolve the waxes.
Anti-gel additives are therefore commonly added to diesel or biodiesels where cold temperature is expected. They act to reduce the formation of wax crystals in the fuel, thereby lowering the pour point and the gel point of the fuel. Anti-gel additives may not necessarily affect the cloud point.