Method and apparatus for extending the duration of operation of a

Abstract

The duration of a pumping operation from a fixed supply of cryogenic liquid is extended by minimizing the amount of cryogenic liquid which is recirculated between a primary cryogenic pump and a storage tank for containing the cryogenic liquid. The primary cryogenic pump is characterized by a minimum necessary net positive suction head at its intake. The net positive suction head of the cryogenic liquid is directly measured at the inlet of the primary cryogenic pump by means of a differential pressure gauge. The total pressure on the cryogenic liquid at the inlet of the primary cryogenic pump is then maintained by means of a cryogenic booster pump to always exceed the measured vapor pressure at the inlet by an amount just equal to the minimum net positive suction head required by the primary cryogenic pump. Therefore, the minimum amount of pressure necessary to allow the primary cryogenic pump to operate is supplied to its inlet and thereby the amount of cryogenic liquid which is circulated from the storage tank to the primary cryogenic pump and then recirculated to the storage tank by means of a return line is minimized. Thus, the amount of energy absorbed by the recirculated cryogenic liquid is similarly minimized and the duration of the pumping operation is extended.