Helium degassing/pressurization systems

Purge pump A with water and pump B with methanol. Dial-a-mix 20% B and equilibrate the column at 1.5mL/min (six column volumes or a stable baseline at 254 nm, about 10 min). (Lab note If the gradient system is a low-pressure mixing system, solvents must be degassed by purging or running under helium.)... 

Details of the operation of the vacuum system (Fig. 3) should be reviewed earefully in consultation with an instructor before beginning the experiment. In the following it is assumed that both the helium and nitrogen storage bulbs have already been filled to a pressure slightly over 1 atm, the diffusion pump is operating, and the sample has been degassed. 

The major operating problem of piston pumps is dissolved air and the formation of bubbles in the eluent. Bubbles in the pump heads cause pulsation of volume flow and pressure pulsation. Bubble formation and cavitation problems are promoted at the inlet check valve because the minimum pressure in the system is reached here. For this reason the eluent must be suitably degassed. This can be done online by a membrane degasser, by pearling helium offline through the eluent or by the use of an ultrasonic bath. 

From the degassing area discharge, each fuel stream flows to the suction of a variable speed centrifugal sump type pump. Each pump is designed to deliver about 9000 gpm at 20-ft head of pumped fluid. To obtain a reasonable pump speed, the net positive suction head requirement is 11.5 ft. A gas pressure (helium) is maintained o er the pump sump to prevent flooding of upper parts (motor windings, cooling system, etc.) of the pump. 

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