Patent Application
20110027071
An inducer comprising at least two sets of rotating and non-rotating helical inducer vanes. As the fluid enters the inducer, the fluid moves up through a first set of rotating vanes, and gains rotational momentum. The fluid then enters a second set of non-rotating vanes that use the rotational momentum of the fluid to progress the fluid forward while removing the rotation, which consequently decreases the net positive suction head required. The inducer is positioned at the inlet of a cryogenic centrifugal pump. Embodiments of the cryogenic centrifugal pump use a vertical rotational axis and include a thrust equalizing mechanism device to balance hydraulic thrust.
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A common problem with spiral inducers used within centrifugal pumps and similar devices is that the fluid in the tank in which the centrifugal pump is installed will begin to rotate in the same direction as, and along with, the inducer blades. When this occurs, the fluid does not move up through the inducer as efficiently. This phenomenon can also result in a change in pressure near the inlet of the inducer and increase the amount of net positive suction head (NPSH) required to make the pump continue to work efficiently or properly.
When the pressure of a liquid, such as a cryogenic fluid, falls below the vapor pressure, vapor bubbles will form in the fluid. As this liquid-vapor fluid combination is pumped through a machine, such as an inducer, impeller or pump, the fluid pressure increases. If the fluid pressure increases above the vapor pressure, the vapor bubbles in the fluid will collapse, which is called “cavitation.” It is desirable to prevent cavitation in devices because the collapsing bubbles can generate shock waves that are strong enough to damage moving parts around them. In addition, cavitation causes noise, vibration, and erosion of material from the device. Thus, the service life of a pump can be shortened due to cavitation.
However, it is desirable, when pumping cryogenic fluid from a tank to get the fluid pressure as close to the vapor pressure as possible, in order to pump more fluid from the tank. In other words, it is desirable for the net positive suction head available (NPSHA) in the tank to be greater than the net positive suction head required (NPSHR) of the pump. NPSHA is a function of the system in which the pump operates, such as the pressure of the fluid within a containment vessel, such as a tank, before it enters the inducer at the inlet of the pump, and the liquid depth of the vessel or tank housing the pump, among other factors.
Inducers are used in cryogenic systems, including storage tanks, rocket fuel pump feed systems, and other similar uses. Inducers are used in such systems to prevent the fluid being moved from cavitating in the impeller or pump, which can occur when there is not enough pressure to keep the liquid from vaporizing. Non-cavitating inducers are used to pressurize the flow of the fluid sufficient to enable the devices to which the inducer is attached to operate efficiently. An excellent discussion of the fluid dynamic properties of inducers is provided by B. Lakshminarayana, Fluid Dynamics of Inducers—A Review, Transactions of the ASME Journal of Fluids Engineering, December 1982, Vol. 104, Pages 411-427, which is incorporated herein by reference.
The techniques used to improve pump performance relative to the operation of inducers vary significantly. For example, Nguyen Duc et al., U.S. Pat. No. 6,220,816, issued Apr. 24, 2001, describes a device for transferring fluid between two different stages of a centrifugal pump through use of a stator assembly that slows down fluid leaving one impeller before entering a second impeller. A different technique is used in Morrison et al., U.S. Pat. No. 6,116,338, issued Sep. 12, 2000, which discloses a design for an inducer that is used to push highly viscous fluids into a centrifugal pump. In Morrison et al., an attempt is made to resolve the problem of fluids rotating with the inducer blades by creating a very tight clearance between the blades of the auger of the inducer and the inducer housing, and configuring the auger blades in such a way as to increase pressure as fluid moves through the device to the pump.
While grooves have been used in inducer designs in the past, they have not been used to help efficiently move the fluid through the inducer. For example, in Knopfel et al., U.S. Pat. No. 4,019,829, issued Apr. 26, 1977, an inducer is illustrated that has a circumferential groove around a hub at the front of the inducer. This design causes turbulence to develop within the grooves of the inducer hub rather than in the fluid outside of the grooves, thereby reducing the tendency of the fluid to pulsate and generate noise.
Grooves are also illustrated and described in Okamura et al., An Improvement of Performance-Curve Instability in a Mixed-Flow Pump by J-Grooves, Proceedings of 2001 ASME Fluids Engineering Division, Summer meeting (FEDSM '01), May 29-Jun. 1, 2001, New Orleans, La. In Okamura et al., a series of annular grooves are formed on the inner casing wall of a mixed-flow water pump to suppress inlet flow swirl and therefore passively control the stability performance of the pump. In particular, the J-grooves of Okamura et al. reduce the onset of back flow vortex cavitation and rotating cavitation that can be induced by the flow swirl at the inlet of the inducer.
Okamura et al. acknowledge, however, that increasing the specific speed of mixed-flow pumps has a tendency to make their performance curves unstable and to cause a big hump at low capacities, thus it is stated that it is doubtful that the illustrated technique would be effective for higher specific-speed (i.e., higher flow rate) pumps.
Contra-rotating blade rows on or around a horizontal shaft have been used for marine applications, specifically for propulsion of marine vessels. The goal in marine vessels is to improve aerodynamics and power generation. Most importantly, marine vessels generate and use high thrust forces in order to drive the marine vessels. Thus, maximizing thrust forces allows for faster and more powerful marine vessels.
An embodiment is directed to inducers, and more particularly to an inducer that incorporates sets of rotating helical inducer vanes and sets of non-rotating helical inducer vanes. A first set of rotating vanes move the fluid up along the vanes. The sets of helical vanes are set in alternating stages, with a rotating inducer vane stage followed by a non-rotating inducer vane stage, and so on. The number of stages used before the fluid leaves the inducer and enters the impeller, or some other structure, can be varied depending upon the fluid and the process conditions, such as the structure size, but should include at least two sets. Embodiments of the multi-state inducer can be positioned at the inlet of a cryogenic centrifugal pump. Alternative embodiments can be positioned at the inlet of a cryogenic centrifugal pump with a vertical rotational axis and a thrust equalizing mechanism device.
The fluid gains rotational momentum as a result of passing through the rotating vanes. Such rotational momentum can be detrimental to the net positive suction head (NPSH) if the fluid fails to actually move up through the inducer due to its rotation momentum. A set of non-rotating vanes is used to counter the rotational momentum gained by the fluid. The non-rotating vanes use the rotational momentum of the fluid to progress the fluid forward while removing the rotational momentum of the fluid, thereby increasing the NPSH. Embodiments of the present invention keep the NPSHR of the pump lower and provide a smooth and constant increase in fluid pressure, which makes the pump more efficient because it is capable of removing more fluid from the tank.
The substantially bell-shaped inlet 22 to the inducer 10 is raised off of the bottom surface of a tank or other structure (not shown) by the feet 24 so fluid (not shown) in the tank or structure can enter and be funneled toward the inducer 10 and be moved up into another device mounted above the inducer 10, such as an impeller.
The rotating blades 12 of
Alternative embodiments may have a different number of stages. For example, a first embodiment may consist of two stages: a rotating blade 12 stage near the inlet, and a non-rotating blade 14 stage on top of the rotating blade 12 stage, near the impeller or other structure. A second embodiment may consist of three stages: a rotating blade 12 stage near the inlet, a non-rotating blade 14 stage on top of the rotating blade 12 stage, and a second rotating blade 12 stage on top of the non-rotating blade 14 stage. Any other number of two or more rotating and non-rotating stages may also be used. Ideally the rotating and non-rotating stages alternate, enabling the non-rotating blade 14 stages to remove the rotational momentum of the fluid. However, as has been described above, a multi-stage inducer 10 may have either a rotating blade 12 stage or a non-rotating blade 14 stage as the last stage before the fluid leaves the inducer 10.
The width of the rotating blades 12 and the width of the non-rotating blades 14 can be different, with the difference depending upon the fluid or structure and the process conditions. For example, the first stage may consist of rotating blades 12 with a first width, followed by non-rotating blades 14 with a second width. The blade width of rotating blades 12 can also vary across stages. For example, if there are a total of four stages, consisting of two rotating blade 12 stages and two non-rotating blade 14 stages, then the first rotating blade 12 stage may have blades with a different width than the second rotating blade 12 stage. Similarly, the first non-rotating blade 14 stage may have blades with a different width than the second non-rotating blade 14 stage.
An alternative embodiment has a rotating blade 12 that has a different pitch from the pitch of the non-rotating blade 14. The blade pitch across rotating blade 12 stages can also be varied depending upon the fluid and the process conditions. For example, the blade pitch of a first rotating blade 12 stage can be different than blade pitch of a second rotating blade 12 stage. Similarly, the blade pitch across non-rotating blade 14 stages can be varied. Alternative embodiments may also design the rotating blades 12 differently than the non-rotating blades 14, such as using a different number of blades or having different blade lengths.
Accordingly, as noted above, the number of stages used can range from using at least two sets of rotating blade stages followed by non-rotating blade stages, to as many sets and stages as are necessary to produce an NPSHR of the pump that is less than the NPSHA of the tank or structure, which may vary based on the type of fluid being held by the tank, the liquid depth of the tank housing the pump, among other factors. In particular, the non-rotating blades 14 move fluid that is not being propagated up through the inducer 10 by the rotating blades 12 because the fluid is rotating with the blades 12. More efficiently moving the fluid up through the inducer increases the NPSH (head) so, for example, a pump attached to the inducer 10 can pump the fluid to a lower level within the tank or structure and thus increase the capability and efficiency of the pump. The lowest fluid level a tank or structure can be pumped to is related to the point at which NPSHA is equal to or greater than the NPSHR. However, when NPSHA and NPSHR are close to equal, it is likely that vapor bubbles will form, which can lead to cavitation as pressure is increased within the inducer. Stopping vapor bubbles from forming in the fluid, a focus of other inducers, is not a purpose of the combination of the rotating blades 12 and the non-rotating blades 14 described herein, since vapor bubbles can form in any tank when the level of the fluid is pumped to the point where there is not sufficient NPSHA. Rather, embodiments disclosed herein seek to lower the NPSHR of the pump and to increase the efficiency of the pump, or other structure, so that the fluid in the tank or structure can be pumped to a lower level. Embodiments also keep the NPSHR of the pump lower and provide a smooth and constant increase in fluid pressure, which prevents cavitation and makes the pump more efficient because it is capable of removing more fluid from the tank.
The stages of the alternating rotating blades 12 and non-rotating blades 14 can extend all of the way into the outlet 26 of the inducer 10.
Embodiments of at least two rotating blades 12 and at least two non-rotating blades 14 provide a lower suction head than is possible with a single set of alternating rotating blades 12 and non-rotating blades 14. However, using at least two sets of rotating blades 12 and non-rotating blades 14 increases the design complexity and the complexity of assembly. It also significantly increases the possibility for the pump to be damaged if any torque or other motion of the shaft of the pump causes a set of rotating blades to contact a set of non-rotation blades.
The pump 300 includes a motor 304 mounted on a motor shaft 306. The motor shaft 306 is supported by dry side ball bearings 308. The pump embodiment illustrated in
The pump shaft 314 transfers the rotational power to the inducer 302 and the impeller 320. The impeller 320 increases the pressure and flow of the fluid being pumped. After the fluid goes through the impeller 320, the fluid exits through the discharge flow path 322.
The magnetic coupling 312 consists of two matching rotating parts, one rotating part mounted on the motor shaft 306 and one rotating part mounted on the pump shaft 314 next to each other and separated by a non-rotating membrane mounted to the motor housing 310. In alternative embodiments, the non-rotating membrane can be mounted to the pump housing 315. The operation of a magnetic coupling is known in the art.
While the pump 300 is illustrated having a magnetic coupling 312, embodiments are not limited to pumps with a magnetic coupling 312. Other means for transferring the rotational energy from the motor shaft 306 to the pump shaft 314 are within the scope of embodiments. Similarly, embodiments are not limited to pumps with a motor shaft 306 and a pump shaft 314. Alternative embodiments can consist of a pump with a single shaft or with more than two shafts.
The pump 300 uses a Thrust Equalizing Mechanism (TEM) device 324 for balancing hydraulic thrust. The TEM device 324 ensures that the wet side ball bearings 316 are not subjected to axial loads within the normal operating range of the pump 300. The wet side ball bearings 316 are lubricated with the fluid being pumped. When using the fluid being pumped for lubrication, it is imperative that the axial thrust loads are balanced to prevent vaporization of the fluid in the bearings, thereby ensuring reliability. Axial force along the pump shaft is produced by unbalanced pressure, dead-weight and liquid directional change. Self adjustment by the TEM device 324 allows the wet side (product-lubricated) ball bearings 316 to operate at near-zero thrust load over the entire usable capacity range for expanding. This consequently increases the reliability of the bearings. The TEM device 324 also prevents damage to the alternating rotating blades 12 and non-rotating blades 14 due to unbalanced thrust loads. Unbalanced thrust loads can cause the rotating blades 12 to collide against the non-rotating blades 14, causing severe damage to the multi-stage inducer and the pump. Thus, the TEM device 324 increases the reliability of the various components of the pump, including the multi-state inducer, and reduces equipment maintenance requirements. Alternative embodiments of cryogenic pumps may not include the TEM device 324.
Embodiments of the multi-state inducer described herein improve on common centrifugal pumps and the use of contra-rotating blade rows in marine vessels in a number of ways. First, embodiments of the multi-stage inducer are directed to cryogenic applications, where the goal is to maintain fluid flow and prevent the cryogenic fluid being pumped from cavitating. Cavitation is prevented or reduced by having a low NPSHR. Reducing cavitation and lower NPSHR in a cryogenic centrifugal pump and maximizing thrust forces to drive a marine vessel are completely different hydraulic goals. In fact, embodiments of cryogenic centrifugal pumps that use the herein disclosed multi-stage inducer balance and counteract high thrust forces rather than maximizing them. Balancing thrust forces is important in embodiments because thrust forces can damage components of the pump and the vessel housing the pump. As discussed above, the TEM device balances the up-thrust generated by the pump impeller by counteracting the unbalanced pressure and resultant axial force across the impeller. Thus, rather than maximizing thrust loads as is typical of marine applications, embodiments of cryogenic pumps equipped with the TEM device balance thrust loads to prevent damage to the pump. Embodiments of cryogenic centrifugal pumps equipped with the multi-stage inducer also use a vertical rotational axis rather than the horizontal axis. It is more difficult to balance and manage thrust loads along a horizontal axis.
While a number of embodiments have been illustrated and described herein, along with several alternatives and combinations of various elements, for use in an inducer to a pump, impeller, or some other structure, it is to be understood that the embodiments described herein are not limited to inducers only used with pumps and impellers and can have a multitude of additional uses and applications. Accordingly, the embodiments should not be limited to just the particular descriptions, variations and drawing figures contained in this specification, which merely illustrate a preferred embodiment and several alternative embodiments.
This application claims priority from provisional patent application Ser. No. 61/273,377, filed Aug. 3, 2009, which is hereby incorporated herein by reference in its entirety to be considered part of this specification.
| Number | Date | Country | |
|---|---|---|---|
| 61273377 | Aug 2009 | US |
