Cryogenic Liquid Flow

A Mechanical or Civil Engineer with no experience in cryogenic fluids typically views fluid flow very differently than an engineer with cryogenic experience. This paper is written to reveal how additional considerations are necessary to evaluate fluid flow in a cryogenic piping system.

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.

Liquid fluids such as oil or water passing through a pipe or channel normally remain liquid throughout their entire journey since environmental changes to 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. Most frequently the cryogenic engineer 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 its new 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. It then enters a piping system where frictional losses reduce it pressure (pressure drop) as it travels along. It again becomes unstable and boils.

A typical problem encountered by the cryogenic engineer is sizing of a piping system for a given flow rate of a given cryogen. The cryogenic engineer must take into account the two-phase condition of the fluid in his 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.

For a given delivery pressure, the mass flow rate is substantially reduced due to the two-phase flow.¹ It is possible to mathematically estimate the flow reduction by calculation when total system pressure drop and heat leak is known. In a two-phase flow condition the mixture of vapor and liquid, having a lower density than that of pure liquid, results in a reduced mass flow rate.

There are three kinds of two-phase flow. One is a rather homogeneous mixture of vapor bubbles and liquid. The second is “slug” flow consisting of alternating sections of pure vapor and pure liquid. The third is annular flow where the liquid flows along an annular region adjacent the pipe wall and the vapor, moving at much higher velocity, moves through the center of the pipe. From a practical point of view, condition one can exist without dramatic reduction in mass flow. Condition two results not only in a major mass flow rate reduction; it also can damage piping and equipment due to dynamic loading and vibration. Condition three results in very low mass flow rate. This is the result of the liquid clinging to the pipe walls while the vapor rapidly moves through the center of the channel. This condition renders the piping system virtually ineffective.

Technifab’s experience has been that any cryogenic piping system with a pressured drop greater than 10 psig 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.