The obvious: the boiling point is the temperature at which the fluid boils under atmospheric pressure! However, the pressure in a pipeline will change dramatically depending on where you are. Just behind a pump it will be higher than just before the next pump location due to pressure losses along the pipeline. Furthermore, the flow geometry plays an important role. The flow along a duct with constant cross-section is not very critical as long it is not exposed to heat sources. Much more critical are changes in cross-section as in the case of the deareator tank. This tank introduces large pressure losses into the system since the flow enters the tank in form of a jet which dissipates its kinetic energy almost completely. The average flow velocity in the tank is low, contrary to what would happen in a slow cross-section expansion (a diffusor) the pressure does not recover. The lossless bernoulli equation would say slow flow higher pressure. Therefore in the deareator the pressure is low and if it is working properly it has a free liquid surface. If the pressure in the gas phase is higher than the vapour pressure of the oil at the oil temperature it will not boil. A small amount of oil evaporates until an equilibrium is reached . The surface of the liquid is usually sufficient for this.

The idea behind a deareator tank is to give air bubbles time to rise in the oil . This can only happen if the flow velocity of the is much lower than the rising velocity of the bubbles.

The situation changes when the gas pressure is lower than the vapour pressure at the oil temperature. Real Systems are not clean and smooth. The probability that the oil can be superheated, i.e. that it has a temperature higher than that for boiling at the exsisting pressure is very small.

Summing up, the best location for an deareator tank is just behind the pump and shielded from any heat source, e.g. the sun.

For viscous fluid like oil, it is necessary to keep operating temperature above its pour point so to keep it flowing all time and avoid clogging. Below its boiling point, there is only liquid phase. At given pressure, there will be saturated liquid-vapor mixture above the saturated temperature and become super heated vapor at higher temperature than saturated temperature.

It will depend on the pressure profile. Some time if the pipe is so tricky and the pressure locally goes down could be a gas generation that could be reabsorbed when the pressure goes up, or in the deareation tank. So the answer is YEs, dependind in the pressure history inside the pipe.