LNG delivery system with saturated fuel reserve

1. A cryogenic fluid delivery system comprising:

a main tank system, the main tank system including a main tank adapted to contain a first supply of cryogenic liquid, the main tank including a head space adapted to contain a vapor, the main tank system further including a main liquid withdrawal line and a main economizer circuit in communication with the main liquid withdrawal line and the head space of the main tank, the main economizer circuit including a main regulator configured to open to direct the vapor from the head space of the main tank to the main liquid withdrawal line when a pressure in the main tank rises above a predetermined main tank maximum pressure;

a reserve tank system, the reserve tank system including a reserve tank adapted to contain a second supply of cryogenic liquid, the reserve tank including a head space adapted to contain a vapor, the reserve tank system further comprising a reserve liquid withdrawal line and a reserve economizer circuit in communication with the reserve liquid withdrawal line and the head space of the reserve tank, the reserve economizer circuit including a reserve regulator configured to open to direct the vapor from the head space of the reserve tank to the reserve liquid withdrawal line when a pressure in the reserve tank rises above a predetermined reserve tank maximum pressure, wherein the predetermined reserve tank maximum pressure is less than the main tank maximum pressure;

a pressure building circuit adapted to deliver vapor to the head space of the main tank to build pressure in the main tank;

a fuel delivery line;

at least one fuel selection valve in communication with the fuel delivery line, wherein the main tank is in fluid communication with a use device through the main liquid withdrawal line and the fuel delivery line and the reserve tank is not in fluid communication with the use device through the fuel delivery line when the at least one fuel selection valve is in a first state and the reserve tank is in fluid communication with the use device through the main liquid withdrawal line and the fuel delivery line and the main tank is not in fluid communication with the use device when the at least one fuel selection valve is in a second state;

a reserve fill line in fluid communication with the reserve tank and the main liquid withdrawal line of the main tank; wherein said reserve fill line, said reserve liquid withdrawal line, and said fuel delivery line are configured to deliver a liquid portion of the first supply of the cryogenic liquid from the main tank to the reserve tank via the reserve fill line as the cryogenic liquid flows from the reserve tank to the use device via the reserve liquid withdrawal line and the fuel delivery line when the at least one fuel selection valve is in the second state.

2. The system as in claim 1 ,

further comprising:

a main vaporizer positioned within the main liquid withdrawal line; and

wherein the pressure building circuit comprises a flow inducer and a gas return pathway in fluid communication with an outlet of the main vaporizer and the head space of the main tank, wherein the flow inducer causes gas to flow from the main vaporizer to the head space of the main tank.

3. The system as in claim 1 , further comprising a controller, a main tank pressure sensor, a reserve tank pressure sensor, a main tank liquid level sensor and a reserve tank liquid level sensor configured to sense fill levels and pressure levels of each of the main tank and the reserve tank and automatically switch the at least one fuel selection valve between the first state and the second state based on the sensed fill levels and pressure levels.

4. The system as in claim 3 , wherein the controller is configured to automatically switch the at least one fuel selection valve from the second state to the first state when the main tank reaches a predetermined pressure as determined by the main tank pressure sensor and to automatically switch the at least one fuel selection valve from the first state to the second state when a liquid level in the main tank drops to a predetermined main tank liquid level as determined by the main tank liquid level sensor.

5. The system as in claim 1 , wherein at least one of the main tank and the reserve tank is formed or an outer tank and an inner tank inside the outer tank with a vacuum space between the outer tank an inner tank.

6. The system as in claim 1 , wherein a main biasing relief valve is included in the main liquid withdrawal line to cause the main economizer circuit to be a path of least resistance out of the main tank when the main regulator is open.

7. The system as in claim 6 , wherein a main small orifice is located in parallel with the main biasing relief valve to allow back flow to the main tank during transient periods of high to low use.

8. The system as in claim 1 wherein a reserve biasing relief valve is included in the reserve liquid withdrawal line to cause the reserve economizer circuit to be a path of least resistance out of the reserve tank when the reserve regulator is open.

9. The system as in claim 8 , wherein a reserve small orifice is located in parallel with the reserve biasing relief valve in the reserve liquid withdrawal line to allow back flow to the reserve tank during transient periods of high to low use.

10. The system of claim 3 wherein the controller is configured to automatically switch the at least one fuel selection valve from the second state to the first state when the main tank reaches a predetermined pressure as determined by the main tank pressure sensor and a liquid level within the reserve tank drops to a predetermined reserve tank liquid level as determined by the reserve tank liquid level sensor.

11. The system of claim 2 wherein the reserve fill line is in liquid communication with the main liquid withdrawal line between the main tank and an inlet of the main vaporizer.

12. The system of claim 1 further comprising a check valve positioned within the reserve fill line, said check valve configured to prevent flow from the reserve tank towards the main.