Centrifugal pump with thrust balance holes in diffuser

A centrifugal pump has alternating impellers and diffusers. One or more vent holes extend through one or more vanes of one or more diffusers. In an upthrust condition, high pressure production fluid from an upper impeller is able to pass through the one or more vent holes, and thereby exert force on the preceding impeller. The force exerted on the preceding impeller offsets the upthrust force acting against the preceding impeller.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for manufacturing a centrifugal pump that mitigates the effects of upthrust. More specifically, the invention relates to a submersible centrifugal pump having one or more passages through one or more diffuser vanes to communicate pressure and thus reduce the effects of upthrust force.

2. Description of the Related Art

Centrifugal pumps can include a series of alternating impellers and diffusers. The impellers can rotate together by, for example, being connected to a common shaft. Fluid enters a base of each impeller and travels radially outward through a passage defined by vanes within the impeller. Centrifugal force, from the rotation of the impeller, accelerates the fluid through the impeller passages. The fluid exits the impeller and enters the diffuser.

Each diffuser is stationary relative to the adjacent impeller. The fluid moves through diffuser passages, which are passages defined by vanes within the diffuser. As the fluid moves through the diffuser, the fluid's velocity decreases as its pressure increases. The fluid exits the diffuser to enter into a subsequent impeller.

Upthrust and downthrust forces can act on each impeller during operation. Upthrust forces, being force acting in the direction of fluid flow, can occur from the pressure of the fluid below the impeller. Downthrust forces can occur from the head pressure of the fluid above the impeller. In some operating conditions, upthrust forces can exceed downthrust forces, thereby causing the impeller to move axially in a downstream direction. The operating conditions can be, for example, when little head pressure exists. Head pressure can be low when first starting the pump or at a maximum flow condition. It is desirable to reduce the upthrust forces during times when upthrust force exceeds downthrust force.

SUMMARY OF THE INVENTION

A centrifugal pump is used for pumping fluid. It can be used, for example, to pump fluid from a wellbore. In one embodiment, the pump includes a pump housing and a first and second diffuser located within the pump housing, each diffuser having a plurality of diffuser passages defined by a plurality of vanes. The pump can also include impellers located adjacent to or radially within each diffuser. An upper surface of a diffuser and a lower surface of an adjacent impeller can define an annular recess between the diffuser and impeller. Similarly, a void can be defined by a lower surface of a diffuser and an upper surface of an adjacent impeller. During operation, pressure within the annular recess may increase, contributing to an upthrust condition.

In one embodiment, a vent passage passes through the a diffuser to provide communication between the annular recess and the void. The vent passage can, for example, pass through a vane of the diffuser and, thus, not obstruct flow within the diffuser passage. In an upthrust condition, as pressure increases in the annular recess beneath the impeller, fluid can pass through the vent passage of the preceding diffuser into the void below the diffuser. The passage of fluid can reduce the pressure in the annular recess and, thus, reduce the pressure acting against the bottom side of the first impeller. The passage of fluid can also increase the pressure in the void and, thus, increase the force acting against the top of the preceding impeller. The upthrust force, thus, is reduced or offset for both impellers on either side of the diffuser having the vent passage. In some embodiments, a rotating seal can be located between the lower surface of the diffuser and the upper surface of the impeller to contain fluid within the void.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Referring toFIG. 1, an example embodiment of an electrical submersible pump (“ESP”)100is shown located in wellbore102. Casing may be used wellbore102. ESP100comprises pump assembly104, seal section106, and motor108. ESP100may be suspended from tubing110in wellbore102, wherein it is submerged in wellbore fluid. Wellbore fluid is drawn into pump inlet112on pump104and then pumped up to the surface through tubing110. The wellbore fluid can be any type of fluid including, for example, product fluid such as oil, natural water-drive fluid, or injected drive fluid.

Motor108may be any type of motor including, for example, an electric motor. Seal section106has a housing, a seal section shaft (not shown), and means for equalizing pressure (not shown) of the lubricant in motor108with the hydrostatic fluid in well102. Motor108has a shaft (not shown) that connects to seal section shaft. Seal section shaft passes through seal section106to the base of pump assembly104. Referring toFIG. 2, an exemplary embodiment of the pump assembly104ofFIG. 1is shown with an outer pump housing114, impellers116, and diffusers120located within pump housing114.

Referring toFIG. 2, pump housing114is a tubular member that forms an exterior of pump assembly104. Housing114may be made of metal, plastic, or any other suitably rigid material. Pump housing114can contain and protect components of pump assembly104.

For the sake of clarity in describing an embodiment of a centrifugal pump with upthrust balancing holes, some references refer to “upper” and “lower,” as though ESP100is in a substantially vertical position. These positional references are for description only, and should not be construed to limit the invention to an application wherein electrical submersible pump100is in a vertical orientation. Indeed, ESP100may be in a horizontal orientation or any other orientation.

Diffusers120can be stationarily located within pump housing114. One or more diffusers120may have a different design than one or more other diffusers120or they may have a substantially similar design. Diffusers120may be of volute, radial, mixed flow, or axial designs. Each diffuser120has a generally curved outer surface and an outer diameter sized to fit within the inner diameter of pump housing114. Diffuser120has central bore124defined by its inner diameter. The outer diameter of diffuser120is defined by diffuser sidewall126. Each diffuser120contains a plurality of passages128that extend through diffuser120. In one embodiment (not shown), wherein a pump housing114is not used, a stack of diffusers120can be connected by, for example, bolts to create a pump body. In this embodiment, diffuser sidewall126can be the exterior surface of pump assembly104.

Referring toFIG. 3, each passage128is defined by vanes130that extend helically outward. Diffuser120may be a radial flow type, as shown, with passages extending outward in a radial plane or a mixed flow type (not shown), with passages extending axially and radially. Passages128generally flow from an outer radial location132near the lower portion, or base, of diffuser120and then move inward, nearer the center of the diffuser120, as the passage128moves along the axial length of diffuser120. In some embodiments, the cross-sectional area of passages128also tends to increase as the passage128moves from the base of diffuser120toward the top of diffuser120. Thus fluid entering passage128near the periphery of diffuser120at high velocity is slowed to a lower velocity, but higher pressure, as the fluid moves radially, or both axially and radially, through passage128. Vanes130form the sidewalls of passages128.

Referring back toFIG. 2, upper shroud134defines the top of the passage128. Diffuser lower shroud136defines the bottom of the passage128. The bottom surface of diffuser lower shroud136may have annular groove138for engaging upthrust washer140, which could be, for example, a thrust bearing washer. Upper shroud134of diffuser may have eye washer142for engaging impeller116. The profile of upper shroud134may create an annular recess144relative to diffuser sidewall126and eye washer142. Annular recess144is a void space that may fill with fluid during operation.

Referring toFIG. 4, vent hole150is shown formed through diffuser lower shroud136. In some embodiments, vent hole150is a passage through diffuser vane130. In these embodiments, vent hole150has top opening152through upper shroud134and bottom opening154through diffuser lower shroud136, each connected by passage156. In some embodiments, vent hole150includes a tube158. In these embodiments, top opening152is in communication with tube158. Tube158passes through a portion of diffuser lower shroud136, or is in communication with passage160that passes through diffuser lower shroud136. Tube158may occupy a portion of passage128.

Each opening152,154may be round, square, elliptical, or any other shape. Furthermore, top opening152and bottom opening154need not have the same shape. The cross-section of passage156, tube158, and passage160may be round, elliptical, or any other shape. The overall dimensions of vent hole150may be any size. In embodiments wherein vent hole150is a passage through vane130, the size of vent hole150may be limited by the dimensions of vane130through which vent hole150passes. Embodiments using tube158are not so limited, as the outer diameter of tube158may be wider than the width of passage128.

Any number of vent holes150may be used. Some embodiments may have just one vent hole150through each diffuser120. Other embodiments may have multiple vent holes150equally spaced around diffuser120or unequally spaced around diffuser120. Indeed, some embodiments may have one or more vent holes150through each vane130. The vent holes150may be spaced at any distance between bore124and outer radial location132. In some embodiments, the radial location of vent hole150may be different than the radial location of an adjacent vent hole150. The number, size, and location of vents holes150may be calculated to allow a predetermined amount of fluid to pass through diffuser lower shroud136at a given pressure.

Referring back toFIG. 2, passages128may open to diffuser inlet162, located near diffuser lower shroud136, for receiving fluid from impeller116. In the example ofFIG. 2, diffuser passages128terminate at diffuser exit164. Diffuser exit164may be an annular groove defined by the upper, inner diameter portions of diffuser vanes130.

Diffuser lower shroud136of diffuser sidewall126may have downward facing lower interlocking member166, such as a shoulder or rabbet, for receiving a corresponding upper interlocking member168on the upper end of an adjacent diffuser120.

Upper sidewall170of diffuser120is a cylindrical member having an inner diameter greater than the largest outer diameter of impeller116. The inner diameter of upper sidewall170narrows at the impeller interface point176. The inner diameter of impeller interface point176is roughly the same as, or slightly larger than, the outer diameter of impeller116.

Referring still toFIG. 2, impeller116is a rotating pump member that uses centrifugal force to accelerate fluids. Impeller116has a central bore defined by the inner diameter of impeller hub178. Shaft180passes through central bore of impellers116. Impellers116may engage shaft180by any means including, for example, splines (not shown) or keyways (not shown) that cause impellers116to rotate with shaft. One end of shaft180may engage shaft (not shown) of seal section106(FIG. 1) or otherwise be coupled to shaft (not shown) of motor108. In some embodiments, two or more pump assemblies104may be used and thus shaft180may be coupled to a shaft (not shown) of an adjacent pump assembly (not shown).

Impeller vanes182may be attached to or integrally formed with impeller hub178. Vanes182may extend radially from impeller hub178and may be normal to shaft180, or may extend at an angle. In some embodiments, vanes182are curved as they extend from impeller hub178. Passages184are formed between surfaces of vanes182.

Lower shroud186forms an outer edge of impeller116and may be attached to or join an edge of vanes182. In some embodiments, lower shroud186is attached to impeller hub178, either directly or via vanes182. In some embodiments, impeller hub178, vanes182, and lower shroud186are all cast or manufactured as a single piece of material.

Impeller edge188is a surface on an outer diameter portion of impeller116. In an exemplary embodiment, outer edge188is the outermost portion of lower shroud186. Outer edge188need not be the outermost portion of impeller116. The diameter of edge188is slightly smaller than the inner diameter of impeller interface point176.

Lower shroud186may have lower lip190for engaging impeller eye washer142on diffuser120. Lower lip190may be formed on the bottom surface of lower shroud186. Lower shroud186defines impeller inlet192from below impeller116into the passages184formed between vanes182.

Impeller upper shroud194is located at the opposite end of vanes182from lower shroud186. Impeller upper shroud194may be attached to or join vanes182. Impeller upper shroud194generally defines an upper boundary of passages184between vanes182. Upper shroud194may have sealing surface196for sealing against upthrust washer140of diffuser120. Downthrust washer197may be located between a downward facing surface of impeller116and an upward facing surface of diffuser120.

Void198is a space bounded on the bottom by impeller upper shroud194and on the top by diffuser lower shroud136. Upthrust washer142or a portion of impeller hub178may form a boundary on one side of void198. Referring toFIG. 5, in some embodiments, rotating seal200may form a boundary on one side of void198. Rotating seal200is a seal for retaining fluid and pressure in void198. Shroud stationary seal lip202(FIG. 6) may be attached to or formed with the lower surface of diffuser lower shroud136. Similarly, impeller rotating seal lip204(FIG. 6) may be a seal formed with or attached to impeller upper shroud194. In some embodiments, rotating seal groove205is located on diffuser lower shroud136(as shown inFIG. 7) or on impeller upper shroud194(not shown). In these embodiments, rotating seal lip202or204fits into rotating seal groove205to retain pressure in void198. Other configurations of rotating seal200may be used. In some embodiments, rotating seal200is not used with void198.

Within a single pump housing, one or more of the plurality of impellers116may have a different design than one or more of the other impellers, such as, for example, impeller vanes having a different pitch.

A plurality of impellers116may be installed on shaft180. A plurality of diffusers120are installed, alternatingly, between impellers116. The assembly having shaft180, impellers116, and diffusers120is installed in pump housing114.

Referring toFIG. 8, two axial forces typically act on impeller116during operation—downthrust force206and upthrust force208. Downthrust force206is defined as a force on the impellers116acting against the direction of flow, thus urging impellers116in an upstream direction (away from discharge tubing110(FIG. 1)). Upthrust force208is defined as force acting on the impellers116in the same direction as the direction of flow, thus urging impellers in a downstream direction (towards discharge tubing110(FIG. 1)). Upthrust forces208occur, for example, when the discharge fluid from the first impeller116′ (FIG. 2) exerts force against the downstream impeller116. Low head pressure, such as during a high flow rate, may cause significant upthrust forces on impeller116.

Downthrust forces206occur, for example, when head pressure exerts force on impellers116, thus urging impellers116in a direction opposite the direction of flow (i.e., away from tubing110). Higher head pressure, such as in a no-flow condition, may exert the greatest amount of downthrust force206on impellers.

Thrust characteristics vary depending on stage design. In an exemplary embodiment, thrust characteristics acting on impeller116may vary from downthrust of approximately 40 pounds per stage when flow is zero to upthrust of approximately 15 pounds per stage when flow approaches approximately 1500 barrels per day. An example of thrust characteristics is shown inFIG. 9. Other impeller and pump designs may have different thrust and flow characteristics.

Under normal operating conditions, downthrust force206exceeds upthrust force208, thus urging impellers in an upstream direction (i.e. towards motor108), relative to flow (“downthrust condition”). In some circumstances, upthrust force208may exceed downthrust force206. This “upthrust condition” may occur during startup, before the pump develops head pressure, or during a maximum flow condition when there is little or no head pressure.

Referring back toFIG. 2, in operation, fluid enters impeller at impeller inlet192. Shaft180rotates, causing impellers116to rotate, while diffusers120remain stationary relative to pump housing114. Wellbore fluid entering pump inlet112(FIG. 1) is drawn through impeller inlet192and into passage184of impeller116. The rotation of impeller116accelerates fluid out of passage184into diffuser passage128. In diffuser passage128, the fluid velocity is decreased and pressure is increased. The fluid exits diffuser passage128, passing through the opening defined by lower shroud186as it enters the next impeller116. The wellbore fluid continues to pass through each subsequent diffuser120and impeller116until it reaches tubing110, wherein it is propelled up through tubing110.

Fluid may rotate in a plurality of locations within pump housing114. In recess144, for example, fluid may rotate below lower shroud186. The fluid, being located between rotating impeller116and stationary diffuser120, may rotate at approximately one half the rotational velocity of impeller116. Similarly, fluid in void198may rotate between impeller upper shroud194and diffuser lower shroud136. Like the fluid in annular recess144, fluid in void198may rotate at approximately one half the rotational velocity of impeller116.

In an exemplary multistage pump, each stage (impeller116and diffuser120) increases the pressure of the fluid as the fluid moves through the stage. By way of example, assume each stage increases the fluid pressure by 10 psi. If pressure at impeller inlet192′ is 50 psi, then pressure at the next impeller inlet192may be 60 psi. Fluid pressure at impeller exit210′ may be approximately 58 psi. Fluid pressure at annular recess144′ and void198′, being near to, and in communication with, impeller exit210′ may also be approximately 58 psi or slightly different than 58 psi.

In this example, pressure in the next state increases by approximately 10 psi, thus causing pressure at impeller exit210to be approximately 68 psi. Fluid in annular recess144and void198will also have a pressure of approximately 68 psi.

One or more vent holes152function to communicate pressure from annular recess144to void198′. The communication of fluid, and pressure, reduces the pressure in recess144. The reduction of pressure in recess144reduces the upthrust effect on impeller116. Furthermore, the increased of pressure in void198′ acts against impeller upper shroud194′ of impeller116′, thus increasing the downthrust force acting on impeller116′.

Due to the taper profile of diffuser sidewall126, wherein the inner diameter of diffuser sidewall126becomes smaller at impeller interface point176, downthrust conditions may decrease the clearance between edge188of impeller116and inner diameter of diffuser120. During upthrust, however, impeller116is urged up and away from diffuser116, thus causing a larger gap between impeller116and diffuser120at impeller interface point176. The gap may allow a greater portion of discharge from impeller116to pass into annular recess144between impeller116and the diffuser120below impeller116. The additional fluid in recess144may further contribute to the upthrust condition.

High pressure fluid in annular recess144may pass through vent hole150and exit below diffuser120. Pressure in annular recess144is generally higher than fluid pressure at the discharge of preceding impeller116′ because the fluid in annular recess144has been accelerated by impeller116. Thus fluid is able to pass from the area of higher pressure, within annular recess144, to the area of lower pressure (void198′), below the diffuser120. The movement of fluid results in less pressure acting against the under side of impeller116. Furthermore, as fluid passes through vent hole150into void198′, the higher pressure urges impeller116′ downward. A larger upthrust condition results in a greater amount of fluid passing into annular recess144, and thus a greater amount fluid and pressure are available to act against preceding impeller116′. Thus pressure and flow through vent hole150acts to offset upthrust forces acting on impeller116′.

As each impeller116is urged downward, the gap between the impeller116and the diffuser120is decreased, thereby reducing the flow, and pressure, from impeller116to annular recess144. Thus, when the upthrust condition ceases to exist, the flow and pressure through vent hole150is at a minimum and therefore the force acting on impeller116′, which is no longer necessary, is greatly reduced or eliminated.