Method and apparatus for pumping a cryogenic fluid from a...

Refrigeration – Storage of solidified or liquified gas – Liquified gas transferred as liquid

Reexamination Certificate

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C062S050700

Reexamination Certificate

active

06640556

ABSTRACT:

FIELD OF THE INVENTION
This invention relates in general to a method and apparatus for pumping a cryogenic fluid from a storage tank. The apparatus comprises a reciprocating pump and the method comprises controlling pump flow rate and vapor pressure within the storage tank by controlling the proportion off cryogenic liquid and vapor supplied to the pump during the induction stroke.
BACKGROUND OF THE INVENTION
Cryogenic fluids are defined as liquids that boil at temperatures of less than about 200° Kelvin at atmospheric pressure, such as hydrogen, helium, nitrogen, oxygen, natural gas or methane.
For containing cryogenic fluids, vacuum insulated storage tanks are known. For example, liquefied natural gas (LNG) at pressures of between about 15 and 200 psig (about 204 and 1580 kPa) can be stored at temperatures of between about 120° K and 158° K in vacuum insulated storage tanks.
A problem with known storage tanks is that heat leaks can cause vaporization of some of the cryogenic fluid within the storage tank and this reduces the time that cryogenic fluids can be held within such storage tanks. To prevent the vapor pressure from rising to undesirable pressures, cryogenic storage tanks are normally equipped with a pressure relief valve. When the vapor pressure rises to above the set point for the relief valve, the storage tank is vented. There is a need for a system that reduces the need for venting, since it may be undesirable to release some cryogenic fluids into the atmosphere and since venting is wasteful of cryogenic fluid.
Some cryogenic fluids such as hydrogen, natural gas, and methane are usable as fuels in internal combustion engines. In some engines, improved efficiency and emissions can be achieved if the fuel is injected directly into the cylinders under high pressure at the end of the compression stroke of the piston. The fuel pressure needed to inject fuel directly into the engine cylinder in this manner can be 3000 psig (about 23,700 kPa), or higher, depending upon the engine design. Accordingly, the cryogenic fuel cannot be delivered directly from a conventional storage tank and an apparatus is needed for delivering a cryogenic fluid to the engine at such high pressures. A pump is required to boost the pressure from storage pressure to injection pressure and to remove vapor from the storage tank to reduce the need for venting.
U.S. Pat. No. 5,411,374, and its two divisional patents, U.S. Pat. Nos. 5,477,690 and 5,551,488, disclose embodiments of a cryogenic fluid pump system and method of pumping cryogenic fluid. In one embodiment the disclosed double-acting piston pump may be employed as a mobile vehicle fuel pump. In this embodiment, the pump is employed to remove both cryogenic vapor and liquid from the tank in a manner whereby only liquid is removed when the pressure in the surge tank is low and vapor starts to be removed when pressure in the surge tank is sufficiently high for engine demand and the vapor pressure in the vehicle tank is higher than the set point. The cryogenic liquid and vapor are supplied from a storage tank through respective conduits communicating between the tank and the pump inlet. A liquid control valve is associated with the liquid supply conduit and a vapor control valve is associated with the vapor supply conduit. The liquid and vapor control valves are controlled in response to fuel demand and the vapor pressure measured within the cryogenic storage tank.
Co-owned U.S. Pat. No. 5,884,488, which is hereby incorporated by reference herein in its entirety, discloses a high-pressure fuel supply system for supplying cryogenic fluid from a storage tank to an engine. The '488 patent discloses, among other things, a multi-stage LNG pump that is capable of pumping liquid or a mixture of liquid and vapor. A metering valve is adjustable to control the amount of vapor drawn into the pump suction. In another embodiment, an orifice is provided in the vapor supply line for regulating the amount of vapor induced into the sump for the LNG pump. The technique disclosed herein permits increased holding times in the storage tank by providing a method and apparatus for removing vapor from the storage tank.
SUMMARY OF THE INVENTION
In the present method, cryogenic liquid and vapor is pumped from a storage tank with a reciprocating piston pump. The method comprises:
(a) In an induction stroke,
retracting a piston within the reciprocating pump and drawing cryogenic fluid from the storage tank into a piston chamber associated with the piston;
controlling flow rate through the pump by controlling the proportion of liquid and vapor supplied to the pump by supplying substantially only vapor during a selected portion of the induction stroke; and
(b) in a compression stroke, compressing and condensing any vapor and compressing any liquid within the piston chamber, and discharging compressed cryogenic fluid from the pump.
In a preferred method, flow rate through the pump is controlled to maintain pressure within a predetermined range at a point downstream from the pump. For example, the point downstream from the pump may be in an accumulator vessel, in a pipe, or in a manifold of a fuel system leading to an engine.
The method may further comprise monitoring vapor pressure within the storage tank and further controlling the proportion of vapor and liquid supplied to the pump to maintain vapor pressure within the storage tank below a predetermined value. For example, by changing pump speed, a constant flow rate may be maintained, while changing the proportion of liquid and vapor supplied to the pump. Similarly, when pressure downstream from said pump is within the desired predetermined range, the proportion of vapor supplied to the pump may be increased to reduce vapor pressure within the storage more quickly.
The proportion of liquid and vapor supplied to the pump during the induction stroke may be controlled by first supplying liquid until the piston reaches a position during the induction stroke that corresponds to a desired proportion of liquid and then supplying substantially only vapor to fill the piston chamber until the induction stroke is complete.
In a preferred embodiment, for each pump cycle, the minimum flow rate pumpable through the pump is determined by the minimum proportion of liquid that is needed during the compression stroke to allow condensation of the vapor within the piston chamber.
A liquefied gas occupies much less space than the same fluid in the gaseous state, so a storage space advantage may be realized by applications that use cryogenic systems to supply a gas. For high-pressure applications a cryogenic pump may be employed. After the liquefied gas is discharged from a cryogenic pump, the fluid may be directed to a heater for transforming it into a gas.
In one embodiment of the method, the desired proportion of liquid, measured by volume, is constant in each pump cycle. To achieve a constant proportion of liquid, vapor is supplied to the pump during a predetermined portion of the induction stroke. For example, liquid may be supplied to the pump initially from the beginning of the induction stroke and whenever the piston reaches a predetermined position, vapor is then supplied to the pump for the remainder of the induction stroke. The same result could be achieved by supplying substantially only vapor to the pump during any predetermined constant portion of the induction stroke, and substantially only liquid during the rest of the induction stroke.
When the cryogenic fluid is a combustible fuel, the present method may be employed to supply fuel to an engine.
In one embodiment, the supply of vapor to the piston chamber during the induction stroke is controlled by operating an automatically actuated valve associated with a vapor supply pipe that connects an ullage space of the tank with the pump. The method comprises opening the valve to supply substantially only vapor to the pump and closing the valve to supply substantially only liquid. The flow rate through the pump is controlled by controlling when the valve is opened with referenc

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