Cryogenic Liquid Flow
For the most part, civil and mechanical engineers deal with fluid flow that conforms to the rules professed by Bernoulli regarding steady flow of a continuous stream of fluid. The fluid normally is either all vapor or all liquid. An engineer with cryogenic experience evaluates fluid flow in a cryogenic piping system very differently.
Liquid fluids such as oil or water passing through a pipe or channel normally remain liquid throughout their entire journey. With normal liquids the pressure and temperature do not vary enough to change the fluid state. The cryogenic engineer, on the other hand, must constantly deal with a fluid stream that is both vapor and liquid with constantly changing proportions of each. They must deal with a fluid that is saturated and is therefore constantly on the verge of boiling. A slight change in pressure or heat influx will cause the liquid to boil until it reaches its new saturated condition within the slightly different environment. For example, liquid nitrogen stored within a bulk tank at 45 psig will boil until its saturated pressure reaches 45 psig. It then becomes stable within the tank. As it enters a piping system where frictional losses reduce the pressure (pressure drop) as it travels along, it becomes unstable and boils again.
A typical problem for a cryogenic engineer is the sizing of a piping system for a needed flow rate of a cryogen. The cryogenic engineer must take into account the two-phase condition of the fluid in the piping system because two-phase flow dramatically affects mass flow rate. Any point of flow restriction such as a fitting, orifice, elevation change, or rough bore pipe will cause pressure drop. Any heat influx into the cryogen through the pipe will also cause two-phase flow since the higher temperature cryogen will boil at a constant pressure condition. Two-phase flow is unavoidable in a cryogenic piping system. It can an only be minimized.
Two-phase fluid substantially reduces cryogenic system flow. Flow reduction can be estimated mathematically once total system pressure drop and heat leakage is known. A two-phase flow condition has a lower density than a pure liquid, resulting in a lower delivered mass flow rate.