Heat transfer analysis of pump sump for LNG.

Submerged pumps have been frequently used at LNG fuelling stations for vehicles. The pump including its motor is accommodated inside a cryogenic vacuum insulated vessel - the pump sump. The bottom of the pump sump is connected with the station LNG storage tank with a liquid inlet line, while the vapour return line is connected with the top of the storage tank. Circulation in this circuit enables filling of the pump sump with LNG before start of the pump. This ensures the needed net suction pressure head and enables easy start of the pump. However, during stand-by periods with closed liquid inlet, the LNG gets vaporized from the pump sump. For the new start, the pump sump has to be subcooled and filled again, before the pump is started. The filling time has to be sufficient for achieving the required liquid height but it has to be minimized for prevention of not needed prolongation of the vehicle delay at the station and consequential reduction of the fuelling capacity of the station. The pump sump filling time depends on the rest of liquid, if any, average temperature of the pump sump and the height of liquid in the station storage tank. Mathematical model was developed for identification of the filling time. Validation of the model was done on a stand with the real pump sump and connections, simulating the real circuit on the LNG station. Results of measurement, done for various liquid height and various initial thermal states of the pump sump, were aligned to calculated values with reasonable agreement.