What are the design basis and functionality of a pressure build regulator within an LH2 tank system?

The pressure regulator controls the liquid flow from the tank to the pressure build (PB) vaporizer. As the tank pressure falls, the pressure regulator opens. When the set pressure of the regulator is reached, then the regulator closes. The pressure build circuit depends largely on the required use pressure and the system house-line pressure drop, which is driven by peak flow rate, pressure, and temperature. The set point of this regulator is typically about ~10 to 20 psig or ~1 barg above the use pressure since the house-line regulator needs this pressure to supply hydrogen to the application and control accurately.  The PB design circuit requires knowledge about peak use flow from the tank, which in turn drives the size of the vaporizer heat transfer and the flow rate through the PB circuit. The flow through the PB circuit is based on the replacement rate of the withdrawn liquid with gas required to maintain constant tank pressure. As the liquid is removed from the bottom of the tank, the volume of the liquid removed must be replaced with the same volume of gas. Since density varies significantly with pressure (for both saturated liquid and gas), the flow through the pressure build unit is greatly affected by gas and liquid density, which is affected by the operating pressure/temperature of the tank.