Pumps – Processes
Reexamination Certificate
2001-11-26
2003-12-16
Freay, Charles G. (Department: 3746)
Pumps
Processes
C417S213000, C417S286000, C417S297500, C417S298000, C417S521000, C062S050600
Reexamination Certificate
active
06663350
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not Applicable
SEQUENCE LISTING
Not Applicable
BACKGROUND
Field of the Invention
This invention relates to the apparatus and methods suitable for very low Net Positive Suction Head (NPSH) cryogenic (and other low temperature boiling liquids) pump systems, either mobile or stationary, able to operate under a wide variety of liquid supply conditions. These conditions include where the pump is above, at or below the source of liquid; whether pumping to low or medium or high pressures; start-stop against discharge pressure; and from zero to low to high NPSH at the pump's intake. Thus it can operate under conditions including where NPSH varies during use from none, to little, to much while pumping. One example of such zero NPSH difficult pumping applications is where the pump is located above a saturated or near saturated cryogenic liquid source carried in a small tank located on a vehicle (the vibration of the vehicle/motor tending to destroy any liquid stratification or pressure building result), thus providing near zero Net Positive Suction Head (NPSH) or less at the intake of the pump's inlet conduit, a condition under which most known cryogenic pumps cannot reliably operate, especially as the tank becomes nearly empty. Furthermore, for many reasons, it may not be desirable to vent to the atmosphere vapor from the cryogenic or liquefied gas storage system; accordingly many traditional methods/techniques utilized in the cryogenic pump industry incorporating pressure building or similar techniques to provide prime or NPSH to a pump are not appropriate. A still further problem is that many such cryogenic systems are self-refrigerating, depending upon the repetitive delivery of cold liquid cryogen to provide the system's refrigeration needs, thus such pressure building methods are undesirable, as they add heat to the system.
One specific application where all these pumping abilities would be desirable is when using LNG (liquefied natural gas) as an on-board fuel for large trucks or buses (or other large mobile powered units), using a LNG fueled engine. Typically the LNG must be delivered by a LNG tank truck or rail car from the producing point to a bulk dispensing station having a large LNG storage vessel; from which the LNG is transferred into each truck's on-board fuel tank. Low pressure fuel storage tanks are desired so as to minimize their weight and costs. Then, as the truck's engine requires fuel, the LNG is vaporized and supplied to the engine at a pre-determined pressure, with the desired pressure being a function of the engine's specific design. Some engines are designed to operate at pressures below about 200 psig while others above about 2,000 psig, and still others at an intermediate pressure. One special difficulty presented at a LNG bulk station is that it is frequently desired for safety reasons to locate the LNG storage vessel underground and thus it is very inconvenient to locate a transfer pump underneath it, as is normal practice with many cryogens when stored in aboveground vessels. Depending upon a number of individual operational factors (and vessel design), the LNG in the underground vessel can be sub-cooled and/or pressurized and the vessel nearly full, thus offering substantial NPSH to an above-ground pump (once primed); or the opposite—at equilibrium conditions and an almost empty vessel, thus offering no NPSH to an above-ground pump; and accordingly the pump is subject to a variety of constantly changing, but normal, operating conditions.
Almost similar type difficulties and conditions are presented on the LNG fueled truck itself in providing NG to the engine. The most favored location on the truck or bus for the on-board fuel (LNG) tank(s) is low, and it would be inconvenient and unsafe to position a pump below the tank in an attempt to provide NPSH to the pump. In addition, the almost constant movement of a truck or bus (and of consequence the LNG fuel tank) causes the LNG throughout the tank to be at near equilibrium conditions, again making the provision of NPSH difficult.
Furthermore, it is desirable to be able to utilize as fuel nearly all the LNG in the tank, thus the ability to pump from a near empty fuel tank is desirable. A special difficulty of cryogenic and liquefied gas systems is that it is desirable to conserve the refrigeration potential of the stored liquid to the greatest extent possible, so that no venting of the cryogen or liquefied gas occurs, either when fuel is being used or when the truck is at rest; accordingly any heat conductive connections to the pump should be such that the heat leak caused by the pump is minimized. A still further difficulty is the wide range of fuel (LNG) supply rates required by trucks or buses and thus pumping capabilities required as the vehicle's engine goes from no use to idle to mid speed and to high speed in highly variable sequences on an as-needed basis. Different engines have different desired supply/injection pressures, but one current desire is to favor injection at higher pressures because of increased efficiency and reduced pollutants in the engine's exhaust gas. While it is theoretically possible to inject the LNG into the engine while in the liquid state (as with diesel fuel), the problems of variable volumetric efficiency associated with cryogenic pumps and the variation in LNG's density associated with its saturation pressure have made this unfeasible. Accordingly, designers have favored vaporizing the LNG after pumping it to the desired pressure and then supplying/injecting the natural gas (NG) to the engine as compressed natural gas (CNG). This typically requires a vaporizer (using the atmosphere and/or waste engine heat or other heat source) for warming the now pressurized LNG thus forming CNG, which is then stored in a small pressure vessel maintained between two pressures, the lower pressure of which is the minimum supply/injection pressure and the upper pressure of which is determined by system capabilities or other factors and delivered through a pressure regulator at the desired pressure; all controlled by a device to monitor the pressures and cause the pump to operate.
The U.S. Dept. of Energy (DOE) in a Small Business Innovation Research Program Solicitation No. DOE/ER0686 identified “Liquid Natural Gas Storage for Heavy Vehicles” as a technical topic in which DOE has a R & D mission. In this Solicitation, on-board medium pressure (about 500 psig) and high pressure (about 3,000 psig) cryogenic pumps for LNG fueled vehicles were identified as specific areas where innovation was specifically desired. A related pump use is where it is desired to also be able to provide CNG or LNG at the bulk dispensing station for charging the truck's small pressure vessel or similar uses, thus a high pressure transfer pump is needed capable of pumping from a LNG source lower than itself (underground).
While LNG in mobile applications is used as an example herein, almost every cryogenic liquid being pumped from storage under conditions wherein a reduction in pressure below the liquid's equilibrium pressure or where the incursion of heat into the liquid, causes part of the intake liquid to vaporize would present similar difficulties. This includes cryogens which vaporize easily from heat incursion, and also liquefied gases, which while less sensitive to heat incursion, vaporize readily from a reduction in pressure.
These problems have generally been addressed by pumps characterized by the term “low NPSH” pumps. Included in previous low NPSH designs are U.S. Pat. No. 3,011,450 issued Dec. 5, 1961; U.S. Pat. No. 3,023,710 issued Mar. 6, 1962; U.S. Pat. No. 3,263,622 issued Aug. 2, 1966; U.S. Pat. No. 3,277,797 issued Oct. 11, 1966; and U.S. Pat. No. 6,006,525 issued Dec. 28, 1999—all to the present inventor. Also U.S. Pat. No. 5,188,519 issued Feb. 23, 1993 to I. S. Spulgis. In particular, these patents illustrate a type of reciprocating
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