Liquid Transfer Techniques Avoiding 2-Phase Flow

6 Liquid Transfer Techniques Avoiding 2-Phase Flow 

Starting with a stored liquid at pressure Pq and saturation temperature To there are a number of mefliods for creating the necessary positive pressure differential and associated liquid subcooling for transfer as follows  

This is because the transfer introduces motion into the whole of the liquid contents and causes some mixing of the warm upper stratified layer into the colder lower layers. The surface temperature falls, there is some condensation of vapour and the vapour pressure also falls. At the same time, expansion of vapour into the volume previously occupied by flie exiting liquid also contributes to the fall in pressure. 

self-heating is adequate for transferring relatively small quantities of liquid from a large efficiently insulated tank, say 5-10 % in a period of 24 h. 

In all cases, the liquid transfer will stop, or perhaps not even start, if the liquid state crosses the saturation curve and creating 2-phase flow at any point in the transfer process. 

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If the mass flow rate in Eq. (7.99) is not attained, a condition known as zero delivery results. In order to avert zero delivery, several techniques have been employed. Typically, vents are installed at intermediate points along the transfer line to release some of the trapped vapor, thereby increasing the mass flow rate into the line. Of course, a disadvantage of this method is that some of the cooling capacity is lost in the vented gas. Another technique that has been exploited is to install gas-liquid separators along the transfer line and allow most of the vapor formed in the line to escape continuously at these sites during transfer. Though two-phase flow cannot be avoided, this second technique will reduce the time for liquid delivery at the exit of the transfer line. 

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