Dual-stage cryogenic pump

1. A pump, comprising:

a barrel;

a boost enclosure disposed around the barrel;

a boost plunger disposed inside the barrel and configured to discharge fluid into the boost enclosure; and

a main plunger disposed inside the barrel and configured to receive fluid from the boost enclosure and to increase a pressure of the fluid, wherein the main plunger is free floating and configured to be moved to a retracted position by a pressure of the fluid in the boost enclosure.

2. The pump of claim 1 , further including a tank, wherein:

the boost enclosure and the barrel are located inside the tank; and

the boost plunger is configured to draw fluid into the barrel from a location inside the tank and separate from the boost enclosure.

3. The pump of claim 2 , wherein leakage from the main plunger is directed to the boost plunger.

4. The pump of claim 3 , wherein leakage from the boost plunger is directed into the tank.

5. The pump of claim 2 , further including a plurality of inlet passages connecting the tank to the barrel at the boost plunger.

6. The pump of claim 5 , further including at least one check valve configured to selectively close the plurality of inlet passages.

7. The pump of claim 6 , wherein the at least one check valve has a ring-shaped body configured to simultaneously inhibit flow through the plurality of inlet passages.

8. The pump of claim 7 , further including at least one internal recess formed in the ring-shaped body and configured to engage a guide formed in the barrel.

9. The pump of claim 7 , wherein each of the plurality of inlet passages is circular.

10. The pump of claim 7 , wherein each of the plurality of inlet passages is arcuate.

11. The pump of claim 5 , wherein a combined cross-sectional area of the plurality of inlet passages is equal to 0.4-0.7 times an exposed cross-sectional area of the boost plunger.

12. The pump of claim 1 , further including a mechanical input device connected to the boost plunger.

13. The pump of claim 12 , wherein an extending movement of the boost plunger causes the main plunger to extend.

14. The pump of claim 12 , wherein the mechanical input device includes:

a rotatable load plate; and

a pushrod transmitting an undulating axial motion of the rotatable load plate to the boost plunger.

15. A pump, comprising:

a tank;

a barrel disposed inside the tank;

a boost enclosure disposed inside the tank;

a boost plunger disposed inside the barrel and configured to discharge fluid into the boost enclosure;

a mechanical input device connected to the boost plunger;

a plurality of inlet passages connecting a location inside the tank and separate from the boost enclosure to the barrel at the boost plunger; and

a main plunger disposed inside the barrel and configured to receive fluid from the boost enclosure and to increase a pressure of the fluid, the main plunger being free floating and configured to be moved to a retracted position by a pressure of the fluid in the boost enclosure and configured so that an extending movement of the boost plunger causes the main plunger to extend,

wherein a combined cross-sectional area of the plurality of inlet passages is equal to 0.4-0.7 times an exposed cross-sectional area of the boost plunger.

16. The pump of claim 15 , wherein:

leakage from the main plunger is directed to the boost plunger; and

leakage from the boost plunger is directed into the tank.

17. The pump of claim 15 , further including at least one check valve configured to selectively close the plurality of inlet passages, the at least one check valve having a ring-shaped body configured to simultaneously inhibit fluid flow through the plurality of inlet passages.

18. A pump, comprising:

a tank;

a barrel disposed inside the tank;

a boost enclosure disposed inside the tank and around the barrel;

a boost plunger disposed inside the barrel and configured to discharge fluid into the boost enclosure;

a plurality of inlet passages connecting a location inside the tank and separate from the boost enclosure to the barrel at the boost plunger;

at least one check valve configured to selectively close the plurality of inlet passages;

a free-floating main plunger disposed inside the barrel and configured to receive fluid from the boost enclosure and to increase a pressure of the fluid;

a rotatable load plate; and

a pushrod connected to the boost plunger and configured to transmit an undulating axial motion of the rotatable load plate to the boost plunger,

wherein:

a combined cross-sectional area of the plurality of inlet passages is equal to 0.4-0.7 times an exposed cross-sectional area of the boost plunger;

leakage from the free-floating main plunger is directed to the boost plunger; and

leakage from the boost plunger is directed into the tank.