when the pressure in tank 22 exceeds the pressure set point of regulator 38 vapor 36 may be withdrawn through the vapor line 40 and to the use device through the withdrawal line 28 this results in a lowering of the pressure in the tank 22 due to the horizontal nature of lng vehicle fuel tanks there is often sufficient hydrostatic pressure to cause liquid 26 to be withdrawn even when the regulator 38 is open therefore a biasing relief valve 42 is included in the liquid line 24 to cause the economizer circuit 34 to be the path of least resistance out of the tank 22 when the regulator 38 is open a small orifice 44 is located in parallel with relief valve 42 to allow back flow to the tank during transient periods of high to low use as shown in fig1 an active pressure building circuit 55 can be used to build head pressure in tank 22 the pressure building circuit 55 includes an inlet line 51 which branches off withdrawal line 28 downstream of the vaporizer 32 a flow inducer 52 causes vaporized gas to flow from the inlet line 51 to an outlet line 53 leading back to the tank 22 the outlet line 53 returns the gas to the vapor line 40 through a check valve 54 fig2 shows an example embodiment of a fuel delivery system with a reserve tank 62 and a main tank 22 the tank 22 and 62 may be constructed.