General Remarks on Subcooled Liquids and 2-Phase Flow

2 General Remarks on Subcooled Liquids and 2-Phase Flow 

We are all used to handling and transferring water, whether we are using a hosepipe to wash flie car, or filling a kettle, or turning taps to have a bath or shower. In all cases, whether the water comes from the cold tap at ambient temperature or from the hot tap at, say, 60 °C, the water is subcooled way below its normal boiling point 

LPG and Other Cryogenic Liquid Mixtures, SpringerBriefs in Energy, DOI 10.1007/978-3-319-20696-7 8 

Transferring LNG and LPG is straightforward as long as the liquids are above saturation pressure at the transfer device (e.g. a pump right through to the end of any transfer process). However when 2-phase flow occurs, the problem is a major one and the transfer may slow down, or stop altogether. 

When any liquid evaporates, whether cryogenic or otherwise, a very large volume of vapour is generated at the same pressure. In the case of liquid methane or LNG, the volume of vapour at ambient temperature and pressure is some 600 times the volume of the liquid before it evaporates. This means that, say, if only 1 % of the mass of the liquid in a transfer line evaporates, the volume occupied by the vapour is six times the total volume of unevaporated liquid. Although the vapour mass is 99 times less than the liquid mass, the vapour volume occupies six sevenths of the volume of the line, and the mass flow almost stops. 

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