WO2015003650A1

WO2015003650A1 - Low pressure cryogenic pump assembly

Info

Publication number: WO2015003650A1
Application number: PCT/CN2014/082028
Authority: WO
Inventors: Ziyuan REN; Raymundo A. SAENZ
Original assignee: Westport Power Inc.
Priority date: 2013-07-11
Filing date: 2014-07-11
Publication date: 2015-01-15
Also published as: CN104279135B; CN104279135A

Abstract

A low pressure cryogenic pump assembly has components that define the dimensions of the pump's swept volume and dead volume which are made with manufacturing tolerances, that collectively, when compounded with each other, result in a dead volume of no more than 20% of the swept volume. The pump assembly comprises a pump piston that is reciprocable in a pumping chamber between a fully retracted position and a fully extended position to define a swept volume. The dead volume is defined by fluid volume remaining inside the pumping chamber when said pump piston is in its fully extended position.

Description

Low Pressure Cryogenic Pump Assembly

Technical Field

[0001 ] The present disclosure relates to a low pressure cryogenic pump assembly, more particularly, to a reciprocating piston pump for pumping a liquefied gas. Background of the Invention

[0002] Reciprocating piston pumpsare used for delivering pressurized fluids in a variety of applications. For example, reciprocating piston pumps are being applied to automotive applications to deliver fuel, such as liquefied natural gas (LNG), to an internal combustion engine. [0003] In known automotive gaseous fuel systems,reciprocating piston pumps have been developed by the applicant for engines that inject fuel directly into the combustion chamber late in the compression stroke or early in the power stroke. For this type of direct fuel injection, the fuel must be delivered at a high pressure to overcome the in-cylinder pressure, and to enable the required amount of fuel to be injected in a limited amount of time. Such high pressure systems deliver fuel to the engine at a pressure of at least around 3000 psi and sometimes up to 4000 or 5000 psi. Cryogenic pumps to raise fuel pressure to these levels require precision manufacturing and are typically expensive.

[0004] One characteristic of piston pumps is known as the dead volume, which is the volume that occupies the portion of the pumping chamber that is not swept by the pump piston. The fluid that remains in the dead volume is not discharged at the end of a compression stroke when the pump piston is fully extended. When the piston retracts to start the intake stroke, if the fluid is a compressible fluid, or one that is near its boiling point, the fluid can expand in volume and delay the entry of new fluid through the intake valve and into the pumping chamber. For cryogenic pumps the fluid is often at a temperature near its boiling point and a larger dead volume normally corresponds with a longer delay before new fluid is drawn into the pumping chamber and this results in a lower pumping efficiency. [0005] Accordingly, known cryogenic pumps use a variety of techniques to reduce the dead volume. To achieve a more precise construction with a reduced dead volume, many of the solutions for manufacturing a high pressure cryogenic pump employ components made to strict manufacturing tolerances, which adds to the cost of fabrication, and the assembly is designed to permit adjustments during manufacturing, which adds to the assembly costs. For example, threaded connections can be used to join pump pieces together so that fine adjustments can be made to dimensions between components by adjusting the threaded connection. For a high pressure pump, welded construction is avoided because this introduces more variability into the final geometry of the assembled pump because each piece that is welded together adds variability introduced by dimensional variations in the individual pieces, even when each piece is made within specified tolerances. Some amount of dead volume is unavoidable because even a high pressure pump assembly needs to have some allowances for variable effects such as thermal expansion and contraction. While it is not practical to achieve a zero dead volume, known high pressuepump designs teach techniques to reduce dead volume close to zero.

[0006] Natural gas is becoming more popular as a fuel for automotive applications, and now different types of engines are being designed to consume natural gas. When fuel is introduced into the engine's air intake system, or into the combustion chamber during the intake stroke or early in the compression stroke, the required fuel supply pressure is relatively low, compared to a direct fuel injection engine. For example a gaseous fuel pressure between around 100 psi and up to 300 psi is sufficient for most engines of this type.

[0007] There is a need for a low pressure cryogenic pump that satisfies the performance requirements for a low pressure fuel system and that is less expensive to manufacture, compared to a high pressure cryogenic pump that is designed for a high pressure fuel system.

Summary

[0008] A low pressure cryogenic pump assembly is disclosed which allows a larger dead volume at the end of a compression stroke. The low pumpassembly comprisesa pump piston that is reciprocable in a pumping chamber between a fully retracted position and a fully extended position to define a swept volume in the pumping chamber. The low pressure cryogenic pump assembly has a dead volume which is defined by fluid volume remaining inside the pump when said pump piston is in said fully extended position. The components of the low pressure cryogenic pump assembly that define the dimensions of the swept volume and the dead volume are made with manufacturing tolerances, that collectively, when compounded with each other, result in an assembled pumping chamber wherein the dead volume is no more than 20% of the swept volume. In preferred embodiments, the manufacturing tolerances of these components are specified to produce a dead volume between 5% and 10% of the swept volume. In other embodiments, the manufacturing tolerances are specified to produce a dead volume between 10% and 20% of the swept volume.

[0009] The low pressure cryogenic pump assembly can comprise an extension tube connecting a flange associated with a drive unit and a flange associated with a pump unit, wherein at least one end of said extension tube is welded to a respective one of these flanges.

[0010] In preferred embodiments, the described low pressure cryogenic pump assembly handles a liquefied gas, for example natural gas. The pump can deliver fuel such as liquefied gas to an engine.

[001 1 ] The low pressure cryogenic pump assembly can handle fluidswith a temperature that is less than minus 100 degrees Celsius.

Brief Description of the Drawings

[0012] The drawings illustrate specific preferred embodiments of the invention, but should not be considered as restricting the spirit or scope of the invention in any way.

[0013] Figure 1 shows a perspective view of a low pressure cryogenic pumpassembly of the present invention.

[0014] Figure 2 is a cross-section view of pump assembly illustrated in Figure 1.

[0015] Figure 3 is a schematic representation of the cross-sectional view of the pump assembly illustrated in Figure 2. [0016] Figure 4 illustrates a graph representing the correlation between the pump volumetric efficiency and the dead volume for different types of pumps.

Detailed Description of the Preferred Embodiments

[0017] Thelow pressure cryogenic pump illustrated in the preferred embodiment shown in Figures 1 to 3is a reciprocating piston pump assembly which handles fluids at low pressure, which is defined in this disclosure to mean pumps that can discharge fluid to a pressureof around 100 psi to 300 psi and can be designed to allow a maximum discharge pressure of up to around 600 psi. The example of a low pressure cryogenic pump that is described in this disclosure is made for delivering fuel such, as LNG, from a fuel tank to an internal combustion engine fuelled with natural gas, however, such a pump can be applied to other applications for low pressure pumps, and is particularly suited for cryogenic pumps that pump fluids to low pressures.

[0018] With reference now to Figures 1 and 2a subject low pressure cryogenic pump is shown in a perspective and across-section view, respectively. Pump assembly 100 comprises reciprocating pump 110 connected to drive unit 112 by extension tube 114. Because the fluid pumped by the pump is stored at cryogenic temperatures, reciprocating pump 110 is located at what is referred to herein as the cold end and drive unit 112, located at the opposite end, is referred herein as the warm end. Extension tube 114 reduces heat transfer between the warm end and the cold end by virtue of its length and structural design. Fluid enters reciprocating pump 110 through intake tube 118, and is discharged from reciprocating pump 110 through discharge tube 120. As shown in Figure 2, drive unit 112 can be a hydraulic drive that generates reciprocating motion by using hydraulic fluid pressure to cause drive piston 127 to move within the drive unit, for example, as described in the Applicant's co-owned United States Patent Application No. 7,739,941. Drive piston 127 is connected to pump piston 124 by shaft 122. Pump piston 124 is reciprocates within pump cylinder 126 and forms a dynamic fluid seal with the interior walls of pump cylinder 126, defining pumping chamber 128. Reciprocating pump 110 is preferably immersed in the fluid stored in a storage vessel or in a sump (not shown). During the intake stroke, when pump piston 124 moves in direction 130, fluid flows from the fluid supply through intake tube 118 in direction 123 and on through valve inlet port 119 into pumping chamber 128. During the compression stroke, when piston moves in direction 132, pressurized fluid exits pumping chamber 128 and is delivered through pump outlet 121 and further throughdischarge tube 120 to the user.

[0019] As noted with respect to high pressure pumps it is known that pumping efficiency drops significantly when the dead volume increases. This is illustrated in Figure 4 which shows the correlation between the dead volume, represented as a percentage of the swept volume, and the volumetric efficiency ("VE") of the pump. As shown by Figure 4, the smaller the dead volume, the higher the volumetric efficiency. While it is not practical to achieve a zero dead volume, as dead volume approaches 0%, volumetric efficiency approaches 100% meaning that there is no fluid in the pumping chamber at the beginning of the intake stroke, and all of the fluid that is drawn into the pumping chamber during the intake stroke is discharged at the end of the compression stroke.

[0020] The lines plotted in Figure 4 connect data points obtained using a model which computed the distance travelled by the pump piston to reach the discharge pressure and analyzed the volumetric efficiency obtained at the end of the following pump intake cycle using mathematical formulas and assumptions known in the industry. The model calculated the volumetric efficiency based on several parameters such as magnitude of the compression pressure and the normalized intake dimensions and took in consideration several assumptions, for example no piston seals leakage, no heat absorption or dissipation through the pump walls, an isenthalpic expansion of intake flow andan isentropic compression of the mass of fluid in the pumping chamber. The plotted lines show that there is a mostly linear relationship between dead volume and volumetric efficiency.

[0021 ] Line 410 illustrates the results of the model applied to a low pressure pump (500 psi), and lines 420, 430 and 440 correspond to a pump with a working pressure of 2,500 psi, and respectively 4,500 psi and 6,700 psi. The graphs shows that dead volume has less effect on the volumetric efficiency of a low pressure pump compared to a high pressure pump. That is, for higher pressure pumps the volumetric efficiency drops faster when the dead volume increases, compared to a pump operating with a relatively low working pressure (working pressures of 500 psi, for example). [0022] These results show that low pressure pumps have a higher tolerance for largerdead volumes, compared to high pressure pumps, and low pressure pumps can maintain an acceptable volumetric efficiency with a larger dead volume. For example with a dead volume of about 10% of the swept volume, a volumetric efficiency of around 90% can be achieved. Using the same model for estimating volumetric efficiency, line 410 can be extended to show that a volumetric efficiency of around 80% can be achieved by a pump with a dead volume that is about 20% of the swept volume, for a pump with a working pressure of about 500 psi. A volumetric efficiency of around 80% is an acceptable level of efficiency, but for a high pressure cryogenic pump, with a working pressure of 2500 psi, dead volume needs to be less than 4% of the swept volume to achieve 80% volumetric efficiency. The applicant's own high pressure cryogenic pump, which is designed with a working pressure about 2500 psi, and other known high pressure pumps are all designed with a dead volume that is much less than 4% of the swept volume.

[0023] Figure 3 is a schematic representation of the pump illustrated in Figures 1 and 2 showing the main components of a low pressure cryogenic pump assembly. The dimensions of each these components are referenced as Al to A7 and Bl to B3. Al is the length of compression cylinder 126, A2 is the dimension of flange 138 which connects compression cylinder 126 to extension tube 114, measured along the longitudinal axis of the pump, A3 is the length of extension tube 114, A4, A5 and A6 are the respective dimensions of the portions of flanges 140, 142 and 144 which are interposed between extension tube 114 and drive cylinder 116, measured along the longitudinal axis of the pump, A7 is the width the driving piston 127, Bl is the dimension of driving piston 127 measured along the longitudinal axis of the pump between the end of the driving piston 127 and the end of shaft 122 mounted in driving piston 127, B2 is the length of the shaft 122, and B3 is the dimension of pump piston 124 measured along the longitudinal axis of the pump between the end of pump piston 124 and the end of shaft 122 mounted in pump piston 124.

[0024] During the manufacturing process each component is made to a specification which stipulates the allowable manufacturing tolerances. Lower tolerates usually result in more precise dimensions but are also associated with higher fabrication costs. When the pump is assembled some of the dimensional variability between components can have an offsetting or compounding effect, unless the pump is designed with features that allow adjustments. As already mentioned, threaded connection are an example of connections that allow some adjustment. With the subject low pressure cryogenic pump, because a pump with a dead volume up to 20% of the swept volume can still achieve a volumetric efficiency of about 80%, this enables the specification of larger manufacturing tolerances and different manufacturing techniques, including welded construction, which would not be practical for a high pressure cryogenic pump when more precision is needed. For example, in a preferred embodiment, flanges 138 and 140 are welded to extension tube 114, the welding lines being illustrated in Figure 3 as numeral 150. In the subject low pressure cryogenic pump, the specified manufacturing tolerances for the dimensions for all of the pump components are calculated, so that collectively, their combined compounded effect results in a dead volume that is no more than about 20% of the swept volume to achieve a volumetric efficiency of about 80%. Furthermore, while a dead volume that is about 5% of the swept volume, or greater, would result in a poor volumetric efficiency for a high pressure pump, a dead volume that is between 5% and 10% of the swept volume yields a volumetric efficiency of between 90% and 95%. While manufacturing a low pressure pump with a higher volumetric efficiency requires lower manufacturer tolerances, a dead volume that is between 5% and 10% of the swept volume still allows some of the less expensive manufacturing methods taught by this disclosure, which would not be desirable in a high pressure cryogenic pump.

[0025] To better illustrate the claimed features of the pump, in the drawings, some of the details related to known elements that constitute said pump have been simplified. Actual working arrangements of the pump include more constructional details.

[0026] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.

Claims

We Claim:

A low pressurecryogenic pump assembly comprising a pump piston that is reciprocable in a pumping chamber between a fully retracted position and a fully extended position to define a swept volume in said pumping chamber, said low pressure cryogenic pump assembly further comprising a dead volume which is defined by fluid volume remaining inside said pump when said pump piston is in said fully extended position, and components of said low pressure cryogenic pump assembly that define the dimensions of said swept volume and said dead volume are made with manufacturing tolerances, that collectively, when compounded with each other, result in an assembled pumping chamber wherein said dead volume is no more than 20% of said swept volume.

The low pressure cryogenic pump assembly of claim 1 wherein said manufacturing tolerances are specified to produce a dead volume between 5% and 10%) of said swept volume.

The low pressure cryogenic pump assembly of claim 1 wherein said manufacturing tolerances are specified to produce a dead volume between 10%> and 20%) of said swept volume.

The low pressure cryogenic pump assembly of claim 1 further comprising an extension tube connecting a flange associated with a drive unit and a flange associated with a pump unit, wherein at least one end of said extension tube is welded to a respective one of said flanges.

The low pressure cryogenic pump assembly of claim 1 wherein said fluid is a liquefied gas.

The low pressure cryogenic pump assembly of claim 5 wherein said liquefied gas is natural gas.

The low pressure cryogenic pump assembly of claim 1 wherein said fluid has a temperature that is less than minus 100 degrees Celsius.

8. The low pressure cryogenic pump assembly of claim 1 wherein said low pressure cryogenic pump assembly is a fuel pump that delivers fuel to an engine.