Patent Application
20130220429
The present invention relates to a system and method for delivering a pressurized gas from a cryogenic storage vessel. In particular, the disclosed system and method reduce thermal shock in the system by controlling a pump for cryogenic fluids so that the temperature of the gas does not drop below a predetermined temperature.
At cryogenic temperatures a gas can be stored in a storage vessel in liquefied form to achieve a higher storage density, compared to the same gas stored in the gaseous phase. For example, higher storage density is desirable when the gas is employed as a fuel for a vehicle because the space available to store fuel on board a vehicle is normally limited.
Another advantage of storing a gas in liquefied form is lower manufacturing and operating costs for the vessel. For example, storage vessels can be designed to store a liquefied gas at a cryogenic temperature at a saturation pressure less than 2 MPa (about 300 psig). Compressed gases are commonly stored at pressures above 20 MPa (about 3000 psig), but vessels that are rated for containing gases at such high pressures require a structural strength that can add weight and/or cost to the vessel. In addition, because of the lower storage density of gas stored in the gaseous phase, the size and/or number of vessels must be larger to hold the same molar quantity of gas and this adds to the weight, cost and space required to mount the storage vessels if the gas is stored in the gaseous phase. Extra weight also adds to operational costs if the vessel is used in a mobile application, since the extra weight adds to the load that is carried by the vehicle. For the same molar quantity of gas, the weight of the storage vessels for holding the gas at high pressure in the gaseous phase can be two to five times greater than the weight of the storage vessels for holding the same gas at lower pressure in liquefied form.
The desired temperature for storing a liquefied gas depends upon the particular gas. For example, at atmospheric pressure, natural gas can be stored in liquefied form at a temperature of minus 160 degrees Celsius, and a lighter gas such as hydrogen can be stored at atmospheric pressure in liquefied form at a temperature of minus 253 degrees Celsius. As with any liquid, the boiling temperature for the liquefied gas can be raised by holding the liquefied gas at a higher pressure. The term “cryogenic temperature” is used herein to describe temperatures less than minus 100 degrees Celsius, at which a given gas can be stored in liquefied form at pressures less than 2 MPa (about 300 psig). To hold a liquefied gas at cryogenic temperatures, the storage vessel defines a thermally insulated cryogen space. Storage vessels for holding liquefied gases are known and a number of methods and associated apparatuses have been developed for removing liquefied gas from such storage vessels.
When a gas is stored at cryogenic temperatures and the end user uses the gas in gaseous form at temperatures above zero degrees Celsius some of the challenges with such a system include supplying the gas without excessive thermal shock to components in the delivery system, reducing the temperature range for thermal cycling, and preventing freezing of the heat exchange fluid in the vaporizer. With regard to thermal cycling, the broader the temperature range, the more difficult it is for system components such as resilient seals that are exposed to such temperature cycling, and this can shorten the lifecycle of such components. In the example of a cryogenic fuel storage system for a vehicle engine that burns a gaseous fuel, the engine coolant can be used as the heat exchange fluid in a vaporizer to heat the fuel and regulate its temperature. However, vehicular fuel systems must be capable of performing under a range of operating conditions, and under some conditions, such as start-up when the engine is below normal operating temperature, or if there is a problem with the vaporizer that is used to vaporize the fuel, the engine coolant may not be able to provide enough thermal energy to keep the temperature of the delivered fuel above a desired temperature, resulting in a broader temperature range for thermal cycling, thermal shock to system components, and more difficult control of fuel combustion since there is more variability in fuel temperature and density. If measures are not taken to prevent the temperature of the delivered fuel from falling below threshold temperature levels, this can subject the system to further problems. For example, because of the cryogenic temperatures involved, moisture in the air can be frozen to cause ice build up on the fuel system components. In addition, if the heat exchange fluid is supplied to the vaporizer at a temperature that is lower than normal, because the cryogenic fluid can enter the vaporizer at temperatures at least as low as −160 degrees Celsius, there is also a danger of freezing the heat exchange fluid inside the vaporizer. If there is freezing up of the downstream components or freezing of the heat exchange fluid, it can take a long time for them to thaw if only the heat from the vaporizer is used to melt the ice build up or frozen heat exchange fluid, and this problem can be compounded by frozen heat exchange fluid restricting the flow of heat exchange fluid through the vaporizer. Thermal shock, thermal cycling, and freezing can each result in permanent damage to system components and/or degraded system performance.
Accordingly, to improve the operability, durability and lifecycle of systems that deliver a pressurized gas from a cryogenic storage vessel, there is a need to prevent thermal shock, freezing up of delivery system components, freezing of the heat exchange fluid in the vaporizer, and to reduce the temperature range for thermal cycling.
A method is provided of pumping a process fluid from a cryogenic storage vessel and delivering the process fluid to an end user in a gaseous phase. This method comprises:
The present technique further provides a method of pumping a process fluid from a cryogenic storage vessel and delivering the process fluid to an end user in a gaseous phase. The method comprises:
In this disclosure a distinction is made between a pump that has been “stopped” because process fluid pressure is at or above a predetermined high pressure threshold and a pump that is temporarily “suspended” from operation because process fluid temperature is less than a predetermined threshold temperature. When the pump is stopped, the method does not seek to restart the pump until process fluid pressure drops to the predetermined low pressure threshold. When that pump is temporarily “suspended” it can be restarted when at least one enabling condition is satisfied and the process fluid pressure is less than the predetermined high pressure threshold.
The method can comprise further conditions for temporarily suspending operation of the pump in addition to the enabling conditions for restarting the pump when it has been suspended from operation. For example, the method can comprise not suspending operation of the pump until the process fluid temperature is below the predetermined threshold temperature for a predetermined number of consecutive pump cycles. The number of consecutive pump cycles for this additional condition for temporarily suspending operation of the pump is a predetermined number and can be as low as two. Adding this condition can be advantageous for systems where the temperature sensor is susceptible to producing false temperature readings, which might otherwise result in unnecessarily suspending operation of the pump.
When operation of the pump is temporarily suspended, the method employs one or more predefined enabling conditions for determining when to re-start the pump. All of the disclosed predefined enabling conditions relate to strategies for preventing the temperature of the process fluid in the conduit from dropping below the predetermined temperature threshold. For example, whenever operation of the pump is temporarily suspended because the process fluid temperature is below the predetermined threshold temperature, one of the predefined enabling conditions can be satisfied when the pump has been suspended for a predetermined minimum length of time. This imposed delay provides a longer residency time for the process fluid that is in the vaporizer while the pump operation is suspended, helping to warm the process fluid to a temperature that is above the predetermined temperature threshold. After the predetermined minimum length of time has elapsed, if the process fluid pressure is still below the predetermined high pressure threshold, the pump can be restarted. Another enabling condition can relate directly to the temperature of the process fluid. For example, one of the predefined enabling conditions can be satisfied when process fluid temperature in the conduit downstream from the vaporizer is higher than the threshold temperature or if the process fluid temperature inside the vaporizer itself is higher than another predetermined temperature. Yet another enabling condition can be satisfied when the heat exchange fluid has a temperature measured downstream from the vaporizer that is above a predetermined temperature.
In a preferred method the process fluid is a fuel and the method further comprises delivering the fuel to a combustion chamber of an internal combustion engine. Because the pump in the disclosed system is capable of pressuring the gas to a high pressure, the method is particularly suited for systems in which at least some of the fuel is injected through a fuel injection valve directly into the combustion chamber. In the preferred method, when the process fluid is fuel for an engine, the heat exchange fluid can be engine coolant, wherein the method further comprises directing engine coolant from an engine cooling system to the vaporizer. In this embodiment, the method preferably comprises directing the engine coolant to the vaporizer from an outlet of a cooling jacket for the engine. Hotter heat exchange fluid temperatures improve the effectiveness of the vaporizer so it is preferable to direct the engine coolant to the vaporizer after it has been heated by flowing through the engine's cooling jacket.
The method can be applied to a system that has a plurality of storage vessels, each with a respective pump and vaporizer. For a system with two storage vessels, with the disclosed method the storage vessel is a first one of two storage vessels, the pump is a first one of two pumps, and the vaporizer is a first one of two vaporizers. With this system the method can further comprise:
With the system that has a plurality of storage vessels, each with a respective pump and vaporizer the method can further comprise:
In systems that comprise a plurality of pumps, one of the enabling conditions for restarting a pump that has been suspended from operation can be satisfied when another one of the plurality of pumps that are in the system performed the previous pump stroke. That is, when the pumps are reciprocating piston pumps that operate in parallel, the predefined enabling condition is satisfied when the suspended pump has been idle for at least the time it takes for another pump to complete an extension and retraction stroke. In some embodiments an additional predefined enabling condition for restarting a pump relates to directing a suspended pump to remain idle for a predetermined minimum length of time. Accordingly, in such embodiments of the method, even if a different pump performed the previous pump stroke, the controller is programmed to keep the suspended pump idle until this additional enabling condition is satisfied. That is, this additional enabling condition is satisfied when the suspended pump has been idle for a predetermined minimum length of time, and after the predetermined minimum length of time has elapsed the suspended pump can be restarted.
The method can comprise other predefined enabling conditions for restarting a suspended pump. For example, another predefined enabling condition for restarting a suspended pump can relate to process fluid temperature. This predefined enabling condition can be satisfied when process fluid temperature measured downstream from the suspended pump is greater than the predetermined temperature threshold. Another predefined enabling condition for restarting a pump, also relating to process fluid temperature, can be satisfied when process fluid temperature measured inside the vaporizer that is associated with the suspended pump is above a predetermined temperature. This predetermined temperature is preferably higher than the predetermined threshold temperature, so that restarting the suspended pump introduces warmer process fluid into the conduit downstream from vaporizer. This embodiment of the method requires a temperature sensor associated with each vaporizer to measure process fluid temperature inside the respective vaporizer and to send signals representative of the temperature to the controller for processing.
Yet another predefined enabling condition for restarting a pump that has been suspended can relate to the temperature of the heat exchange fluid. This predefined enabling condition can be satisfied when heat exchange fluid temperature measured at the outlet of the vaporizer that is associated with the suspended pump is above a predetermined temperature. The temperature of the heat exchange fluid can be an indirect indication of the process fluid temperature inside the vaporizer, and like in the embodiment that measures process fluid temperature inside the vaporizer directly, an enabling condition for restarting a suspended pump can be that process fluid temperature inside the vaporizer is greater than the predetermined threshold temperature.
A fluid delivery system is provided that comprises components that cooperate with one another to store a liquefied process fluid and deliver the process fluid in a gaseous phase to an end user. In a preferred embodiment, the fluid delivery system comprises:
The controller can be programmed such that one of the predefined enabling conditions dictates that a temporarily suspended pump be idle for at least a predetermined minimum length of time, hi another embodiment, the controller can be programmed to suspend operation of the pump until the process fluid temperature in the conduit is above the predetermined threshold temperature. In another embodiment the system can further comprise a temperature sensor disposed in a process fluid passage inside the vaporizer, from which electronic signals representative of the process fluid temperature can be sent to the controller. In this embodiment, one of the predefined enabling conditions that is programmed into the controller is satisfied when process fluid temperature inside the vaporizer is above a predetermined temperature. In yet another embodiment, the system can further comprise a temperature sensor disposed in or near an outlet conduit for heat exchange fluid exiting the vaporizer. This temperature sensor measures the temperature of the heat exchange fluid and emits electronic signals representative of the measured temperature. In this embodiment, the controller is programmable to keep the pump idle until the heat exchange fluid has a temperature that is above a predetermined temperature. The controller can be programmed to use one or a combination of the described approaches for determining when to restart a pump that has been temporarily suspended from operating.
The disclosed fluid delivery system preferably further comprises an accumulator vessel for holding pressurized gas downstream from the vaporizer and upstream from the end user. An accumulator vessel helps to ensure a sufficient supply of pressurized gas especially when the rate at which gas is consumable by the end user is variable, and when the availability of the pump to be operated is dependent upon factors such as process fluid temperature downstream from the vaporizer, process fluid flow rate, and heat exchange fluid temperature.
The fluid delivery system preferably further comprises a pressure regulator associated with the conduit for regulating gas pressure before it is delivered to the end user. For some systems a pressure regulator is not needed because the delivery pressure during system operation is not important. For example, a system that is used to fill pressure vessels with high pressure gas does not need a regulator, since the system is operated until the pressure vessel is filled; pressure increases as the pressure vessel is filled, and the system is stopped when the pressure in the pressure vessel reaches the desired pressure. However, in other systems, such as a fuel delivery system for an internal combustion engine, a pressure regulator is needed because the pressure of the gas that is delivered to the end user is important for controlling the amount of fuel that is delivered to the engine.
In a preferred embodiment of the fluid delivery system the end user is an internal combustion engine, and the process fluid is a combustible fuel, with the conduit delivering the fuel to a fuel injection valve. In a preferred embodiment the fuel injection valve has a nozzle disposed in a combustion chamber of the engine whereby the fuel is introducible directly into a combustion chamber of the engine. In this preferred embodiment, the engine can be the primer mover for a vehicle. The heat exchange fluid can be engine coolant and the system can further comprise piping connecting a cooling jacket of the engine to a heat exchange fluid inlet of the vaporizer.
In a preferred embodiment of the fluid delivery system the pump is disposed within the cryogen space of the storage vessel. This helps to keep the pump chamber at cryogenic temperatures so that there is no need to cool down the pump when starting up the system.
The storage vessel, the pump, and the vaporizer can each be one of a plurality of like components arranged in parallel, with each one of the vaporizers comprising an outlet in communication with the conduit for delivering process fluid to the end user. In this embodiment, the controller can be programmed to start one of the plurality of pumps that is idle when operation of another one of the pumps is temporarily suspended if at least one predefined enabling condition is satisfied and process fluid pressure is less than the predetermined high pressure threshold. Each one of the vaporizer outlets can be associated with a respective temperature sensor for measuring process fluid temperature in-between each one of the vaporizer outlets and respective one-way valves upstream from the conduit.
As disclosed in describing the method, and as with a single pump system, a multi-pump fluid delivery system can further comprise additional temperature sensors associated with each of the vaporizers to assist with determining when to restart a pump that has been suspended. For example, the system can further comprise a temperature sensor for each vaporizer that measures process fluid temperature inside the vaporizers, and the controller can be programmed to enable operation of a pump that has been suspended if process fluid temperature inside a respective vaporizer is above a predetermined value. In another embodiment, the system can further comprise a temperature sensor for each vaporizer that measures heat exchange fluid temperature near a heat exchange fluid outlet, and the controller can be programmed to enable operation of a pump that has been suspended if heat exchange fluid temperature for a respective vaporizer is above a predetermined value.
Cryogenic storage vessel 110 comprises a double-walled vacuum insulated cryogen space 112, pump 114, which is shown disposed within cryogen space 112, drive unit 116, and level sensor 118. In other embodiments, pump 114 can be disposed outside cryogen space 112 and connected thereto by an insulated suction pipe. Pump 114 can be designed to supply gaseous fuel to the engine at high pressures (above 14 MPa) and at temperatures above zero degrees Celsius. Accordingly, because the disclosed system is capable of supplying a gas at such high pressures, the illustrated liquefied gas supply system 100 is particularly suitable for supplying gaseous fuel to a direct injection engine, in which the gaseous fuel is injected directly into the combustion chamber, since the gaseous fuel pressure in such systems must be higher than the in-cylinder pressure, and fuel temperature must not be so low as to undesirably cool the combustion chamber.
In the illustrated embodiment, drive unit 116 is hydraulically driven. Hydraulic pump 120 supplies high pressure hydraulic fluid to flow switching device 122 through pressure line 124, and hydraulic fluid is returned to a hydraulic fluid reservoir or directly back to the hydraulic circuit through return line 126. Flow switching device 122 comprises valves for switching fluid connections to opposite ends of the hydraulic cylinder between pressure line 124 and return line 126 to cause reciprocating movement of a hydraulic piston disposed within the hydraulic cylinder. Other types of variable speed drive units can be employed. For example, instead of a hydraulic drive unit, the drive unit could be pneumatic, electric, electromagnetic, or another type of linear motor, or a rotary drive unit with a transmission device, such as crank and rod arrangement, for converting rotary motion into linear motion.
Cryogenic fluid pumped from storage vessel 110 is discharged through conduit 130 and flows into vaporizer 132. Vaporizer 132 is operable to raise the temperature of the fluid and shift it into the gaseous phase, so that a high pressure gas exits vaporizer 132 and flows to fuel conditioning module 140 through conduit 135. Vaporizer 132 is typically a heat exchanger designed to vaporize the cryogenic fluid by transferring heat energy to the cryogenic fluid from a warmer heat exchange fluid that is supplied through conduit 133. In the described example of the fuel delivery system for an engine, the warmer heat exchange fluid can be the engine coolant that is directed to conduit 133 from the engine's cooling jacket. In a typical engine the coolant exits the engine's cooling jacket with a temperature of between 80 and 95 degrees Celsius when the engine is operating under normal conditions. The engine coolant exits vaporizer 132 through conduit 134 and can be returned to a reservoir from which it can be recirculated through the engine's cooling system. Engine coolant temperature can vary depending upon many factors such as ambient air temperature, vehicle speed, and how long the engine has been running. If all other variables remain constant, cooler engine coolant temperatures result in a cooler fuel stream exiting vaporizer 132. An objective of the present invention is to prevent the temperature of the process fluid from dropping below a predetermined value.
The disclosed apparatus comprises temperature sensor 136 that measures the temperature of the gas that exits from vaporizer 132 in conduit 135. The instrumentation can optionally also include temperature sensor 132A that measures the temperature of the process fluid inside and near the outlet of vaporizer 132 and temperature sensor 139 that measures the temperature of the heat exchange fluid that exits vaporizer 132. The temperatures measured by sensor 136 and/or sensor 132A and/or sensor 139 can be relayed to controller 150, which processes that information as described below when the method is discussed.
Fuel conditioning module 140 can perform a number of functions. As discussed in the previous paragraph, one of the main functions of fuel conditioning module 140 can be to control the pressure of the fuel in conduit 142, which supplies fuel gas to fuel injection valve 144. Fuel conditioning module 140 can comprise pressure sensors for measuring the gas pressure in conduit 135 and/or conduit 142, a filter for separating solid contaminants, and/or safety devices such as a pressure relief valve for preventing over-pressurization of fuel conduit 142 and/or to reduce the fuel pressure in fuel conduit 142 when the engine is shut down. The components of fuel conditioning module 140 are preferably integrated to reduce the number of connections where leaks can develop, to reduce the size, and to reduce the labor needed to assemble this module.
Even with integration of the individual components that make up fuel conditioning module 140, there are a number of seals and moving parts in fuel conditioning module 140 that can be permanently damaged or otherwise suffer from a reduction in their lifecycle if exposed to temperatures below their prescribed operating range. Further damage or temporary inoperability can result if components downstream from vaporizer 132 are allowed to freeze up. For example if the temperature of the fuel flowing from vaporizer 132 is below zero degrees Celsius, moisture in the air can freeze on the components downstream from vaporizer 132 resulting in a build up of ice that can inhibit the operation of the fuel delivery system.
Controller 150 can be part of the engine controller or a separate controller that works in cooperation with the engine controller. In a preferred embodiment, controller 150 is an electronic control module that receives input signals representative of operational parameters, processes such input signals, and emits control signals to control the operation of the fuel delivery system. Responsive to the processed input signals, controller 150 is programmed to send predetermined control signals to hydraulic pump 120, flow switching device 122, and fuel conditioning module 140. When controller 150 is integrated with the engine controller it also sends control signals to fuel injection valve 144. In
In the embodiment of
The heat exchange fluid flows through inner channel 306 and outer channel 309 in the same general direction as the pressurized fluid flowing through inner tubular coil 308 and then outer tubular coil 310. Depending on the operating conditions for the particular application for which the apparatus is employed, and, in particular, the temperature of the pressurized fluid and the temperature of the heat exchange fluid, the length of the pressurized fluid coil within the heat bath is determined so that the pressurized fluid exits vaporizer 300 as a gas that has been heated to a temperature within a pre-determined temperature range.
As already described above in discussing the application of the disclosed system to deliver fuel to an engine, when the system is employed for this application, the engine coolant is an example of a suitable and convenient heat exchange fluid that can be delivered to the vaporizer. In such an embodiment, engine coolant that has been heated after passing through the cooling jacket of the engine can be delivered to the heat bath in vaporizer 300 where it is cooled prior to being returned to the engine cooling system. In the described system, the quantity of engine coolant that is diverted to the vaporizer can be only a relatively small portion of the total engine coolant flow, such that there is not a significant change to the overall heat balance within the engine cooling system compared to a conventional engine cooling system that does not divert any engine coolant to a vaporizer.
After controller 150 determines that n is greater than N, controller 150 can impose a predetermined wait time before resetting n to zero and then commanding the pump to stroke, or as shown in
Like the methods of
When process fluid pressure P is less than PL and Tf1 is less than TL, then controller 250 leaves pump 214A idle and commands pump 214B to stroke. The process for operating pump 214B is the same as the process for operating pump 214A except that after stroking pump 214B and controller 250 checks whether process fluid pressure P is less than PH, controller 250 checks process fluid temperature Tf2 (not Tf1) before determining which pump to stroke, where process fluid temperature T is measured by temperature sensor 236B downstream from pump 214B. That is, if P is less than PH, pump 214B is commanded to take another stroke if Tf2 is not less than TL. If Tf2 is less than TL, then controller 250 commands pump 214A to stroke. If, after stroking pump 214B process fluid pressure P is not less than PH, then controller 250 waits until P is less than PL before again considering whether to command another stoke of pump 214B or to shift to pump 214A if Tf2 is less than TL. With this embodiment the minimum time that each of the pumps is idle is the time that it takes for the other pump to complete an extension and retraction stroke. The idle time for each pump can be longer than this minimum time and typically is longer depending upon a number of system characteristics such as the flow capacity of the pumps relative to the normal consumption rates by the end user, the size of the accumulator volume, and the efficiency of the vaporizer. Longer idle times for one pump can be achieved, for example, if the other pump is stroked for a plurality of consecutive strokes, or if the other pump raises process fluid pressure P to PH and there is no need to stroke either pump until P is less than PL.
In the methods just described with reference to
At time zero, the temperature downstream from both pumps is about minus 5 degrees Celsius. The process fluid temperature at the respective outlets of the vaporizers associated with pumps 214A and 214B are represented by lines 810 and 820 respectively. Since this temperature is initially much higher than threshold temperature TL for both pumps, and since at start up, process fluid pressure P is typically less than PL, first pump 214A is commanded to start, as indicated at the ten second mark by line 812. The peaks of lines 812 represent when the pump piston is fully extended and the baseline indicates when the piston is fully retracted. Line 812 shows that first pump 214A is operated for six consecutive pump strokes until, as indicated by line 810, the temperature downstream from pump 214A drops to below threshold temperature TL. Then controller 250 commands pump 214A to temporarily suspend operation, thereby increasing residency time in the associated vaporizer, which results in an increase in the process fluid temperature. In this example, process fluid pressure is still below the desired system pressure, and since process fluid temperature downstream from second pump 214B, as indicated by line 820, is higher than threshold temperature TL, controller 250 commands second pump 214B to stroke, as indicated by line 822. Like line 812, peaks in line 822 correspond to when the pump piston is fully extended and the baseline corresponds to when the pump piston is fully retracted. Initially, the temperature downstream from second pump 214B is at about minus 5 degrees Celsius, but after four piston strokes, as shown by line 820, process fluid temperature downstream from second pump 214B drops below threshold temperature TL, and controller 250 commands second pump 214B to temporarily suspend operation. After second pump 214B is suspended, process fluid temperature downstream from second pump 214B begins to rise. Meanwhile, in the time that first pump 214A has been suspended, line 810 shows that process fluid temperature downstream from first pump 214A has risen above TL, enabling first pump 214A to be ready to be restarted when needed. As shown in this example, when pump 214B is suspended, at about the 35 second mark, controller 250 commands first pump 214A to restart and stroke again. After the second stroke it is commanded to suspend operation because process fluid temperature downstream from first pump 214A is again below the threshold temperature TL. However, by this time the system pressure has exceeded high pressure set point PH and another piston stroke is not commanded until around the 70 second mark when process fluid pressure drops to the predetermined low pressure threshold PL. Because first pump 214A was last suspended because process fluid temperature downstream from it was below TL, when system pressure drops below the predetermined low pressure threshold, controller 250 commands second pump 214B to operate at around the 77 second mark. At this point, system pressure is within the desired operating range and less frequent pump strokes are required to maintain system pressure, allowing more residency time for the process fluid in the vaporizers. As well, after the engine has reached its normal operating temperature, the engine coolant is warmer, and that also helps to keep process fluid temperature above threshold temperature TL.
A large accumulator volume can reduce the frequency of operating the pump, allowing more residency time of the process fluid in the vaporizer. However, if the accumulator volume is excessively large, it can be difficult at start up to pressurize the system. Under normal operating conditions, the pump is stroked when system pressure drops to low pressure threshold PL and as long as process fluid temperature remains above threshold temperature TL the pump can be commanded to stroke until system pressure reaches a predetermined high pressure set point, thereby maintaining system pressure between a predetermined high pressure set point and a predetermined low pressure threshold. However, at times such as start up, if process fluid temperature drops below threshold temperature TL and the pump can be temporarily suspended before system pressure reaches the high pressure set point, system pressure can fluctuate between the predetermined low pressure threshold and an intermediate system pressure.
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.
This application is a continuation of U.S. patent application Ser. No. 12/117,930, having a filing date of May 9, 2008, entitled “System And Method For Delivering A Pressurized Gas From A Cryogenic Storage Vessel”. The '930 application is, in turn, a continuation of International Application No. PCT/CA2006/001838, having an international filing date of Nov. 8, 2006, also entitled “System And Method For Delivering A Pressurized Gas From A Cryogenic Storage Vessel”. The '838 international application claimed priority benefits, in turn, from Canadian Patent Application No. 2,523,732 filed on Nov. 10, 2005. Each of the '930 application and the '838 international application is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12117930 | May 2008 | US |
| Child | 13856350 | US | |
| Parent | PCT/CA2006/001838 | Nov 2006 | US |
| Child | 12117930 | US |
