Cryogenic Liquid Flow
For the most part, civil and mechanical engineers deal with the fluid flow that conforms to the rules professed by Bernoulli regarding the steady flow of a continuous stream of fluid. The fluid is typically 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 usually remain liquid throughout their journey. The pressure and temperature do not vary enough with normal liquids to change the fluid state. On the other hand, the cryogenic engineer 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 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 a slightly different environment. For example, liquid nitrogen stored within a bulk tank at 45 psig will boil until its saturation 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 consider the two-phase condition of the fluid in the piping system because two-phase flow dramatically affects the mass flow rate. Any point of flow restriction, such as a fitting, orifice, elevation change, or rough bore pipe, will cause a pressure drop. Any heat influx into the cryogen through the pipe will also cause a two-phase flow since the higher-temperature cryogen will boil at a constant pressure. Two-phase flow is unavoidable in a cryogenic piping system. It can only be minimized.
Two-phase fluid substantially reduces cryogenic system flow. Flow reduction can be estimated mathematically once the total system pressure drop and heat leakage are known. A two-phase flow condition has a lower density than a pure liquid, resulting in a lower delivered mass flow rate.
There are three kinds of two-phase flow:
- A relatively homogeneous mixture of vapor bubbles and liquid. This condition can exist without a dramatic reduction in mass flow, although some equipment may not respond favorably to a vapor/liquid mixture.
- “Slug” flow, consisting of alternating sections of pure vapor and pure liquid. This condition results in a significant mass flow rate reduction and can damage piping and equipment due to dynamic loading and vibration.
- Annular flow is where the liquid moves along an annular region adjacent to the pipe wall, and the vapor flows through the center of the pipe. The vapor moves at a much higher velocity through the center of the pipe, while the liquid moves at a slower velocity along the walls. This condition results in a very low mass flow rate, rendering a piping system virtually ineffective.
Two-Phase Cryogenic Fluid Flow Through Pipe
Technifab’s experience has been that any cryogenic piping system with a pressure drop more significant than 10 psig (.7 bar) from the bulk tank to the end use point will not achieve flow rates predicted by standard hydraulic flow rate calculations. These conditions require a more thorough analysis of the two-phase flow and must account for the pressure drop and heat leak to prevent surprises in reduced flow when the piping system becomes operational.
Several ways are used to reduce two-phase flow. They fall into two major categories, improving the system performance and venting or separating the vapor from the liquid.