NO20130059A1 - Durable pumps for abrasive materials - Google Patents

Abstract

Durable pumps are available for abrasive materials. In one embodiment, an example of a centrifugal pump or pump stage for operating below the surface has a pressure washer located within the perimeter of a tightly sealed clearance seal between a impeller shaft and the spreader. The displacement of the pressure disc allows the clearance seal to protect the pressure disc from abrasive materials, while the pressure disc supports the impeller against the reaction forces of axial fluid flow. In one embodiment, the radius or size of a pressure washer or other seal-like function is reduced to increase exposure of the lower impeller screen to pressurized fluid, thereby balancing pressure at the top and bottom of the impeller to reduce friction between the impeller and the impeller. Reducing the radius of the pressure washer also reduces the surface area of the base plate depending on friction and reduces the torque arm to a brake torque on the rotary impeller, thereby reducing power loss in the pump.

Description

DURABLE PUMPS FOR GRINDING MATERIALS

RELATED APPLICATIONS

[0001] This patent application claims the benefit of priority over US Provisional Patent Application No. 61/360,031, Docket No. 89,0608, filed Jun. 30, 2010, entitled “Device and Means to Reduce Downthrust in a Multistage Centrifugal Pump” and over US Provisional Patent Application No. 61/365,695, Docket No. 89.0627 filed Jul. 19, 2010 entitled “Centrifugal Pump with Increased Abrasion Resistance,” both of which are incorporated herein by reference in their entirety.

BACKGROUND

[0002] Oil fields sometimes use electric submersible pumps arranged in series to pump wellbore fluids. A series of centrifugal pump stages can be stacked together along the axial direction for coupled lift in subsurface environments. Such submersible multistage pumps are often used to transfer liquids consisting of liquid hydrocarbon mixtures which may have some mixed and suspended solids of soil. The liquid may also contain gaseous components and water. Particles and lumps of rock and sand are usually present to some extent. Such heterogeneous "liquid sandpaper" can result in cavitation and grinding problems for pumps, especially if the solids cause deposits to build up against surfaces in the pump, or if the liquid itself has a slurry-like consistency. The viscosity and other flow characteristics of a particular fluid mixture can result in high velocity flow of the abrasive fluid around certain pump parts. Impellers used in downhole centrifugal pumps are exposed to significant abrasive action from pressure washers (hereafter referred to as "pressure washers") when pump fluids contain abrasive substances. Therefore, pump designers try to minimize grinding effect and extend the life of the pump. The special composition and flow characteristics of the abrasive liquid to be pumped often allow special pumps to be specially designed and optimized for special types of unrefined liquids.

SUMMARY

[0003] Durable pumps for abrasives are provided. In one design, an example of a centrifugal pump has a thrust washer that sits inside the perimeter of a tight venting clearance seal between an impeller screen and the spreader. The displacement of the thrust washer allows the clearance seal to protect the thrust washer from abrasives while the thrust washer supports the impeller against the reaction forces of axial fluid flow. In one design, the radius or size of a thrust washer or other seal-like features is reduced to increase exposure of the lower impeller shield to pressurized fluid, thereby balancing pressure at the top and bottom of the impeller to reduce friction between the impeller and thrust washer. Reducing the radius of the thrust washer also reduces the surface area of the washer exposed to friction and reduces the torque arm of a braking torque on the rotating impeller, thereby reducing pressure loss in the pump.

[0004] This summary section is not intended to provide a complete description of wear-resistant pumps for abrasive materials. A detailed description with examples of designs follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Fig. 1 is a diagram of an example of a stage in a multi-stage subsurface pump for pumping liquids containing abrasive materials.

[0006] Fig. 2 is a diagram of an example of internal thrust washers for a subsurface abrasive pump.

[0007] Fig. 3 is a diagram of an example of reduction in diameter of an inner pressure disc.

[0008] Fig. 4 is a diagram of an example of reduction in diameter of a pressure washer to balance pressure areas to reduce friction.

[0009] Fig. 5 is a diagram of an example of a reduction in thrust disc diameter to reduce friction and reduce power loss.

[0010] Fig. 6 is a diagram of an example of reduction in pressure disc diameter to reduce torque arm of a braking torque to reduce power loss.

[0011] Fig. 7 is a flow diagram of an example of a method for making a subsurface abrasion resistant pump.

[0012] Fig. 8 is a flow diagram of an example of a method for increasing the durability and efficiency of an abrasion resistant pump.

DETAILED DESCRIPTION

Overview

[0013] This publication describes wear-resistant pumps for abrasive materials. The pumps described provide higher wear and longer life than conventional designs, especially when pumping subsurface fluids that contain solids that tend to be abrasive when pumped or when pumping slurries. Fig. 1 shows a cross-section of a centrifugal pump stage 100 of a multistage submersible pump stack 102. The multistage submersible pump stack 102 includes a series of centrifugal pump stages 100 stacked together along the axial direction for coupled lift to generate axial fluid flow 104 in a subsurface environment. Fig. 2 shows an example of internal pressure washers. Fig. 3-6 shows reduction of the diameter of an example of thrust washers and associated benefits. Fig. 7-8 shows examples of methods to increase the duration of pumps for abrasive liquids.

Example of svstem surroundings

[0014] Electric submersible pumps for abrasive liquids typically have at least one surface that is an impeller housing, or "screen," i.e., a fixed portion of the impeller assembly that extends radially outward from a more central hub to strengthen and secure the impeller blades on one side , and also serves to shield the impeller blades, at least partially, from the liquid on the other side of the screen, since the screen is fixed. The impeller blades are usually attached to the screen, and the screen is usually attached to a hub that receives the rotational drive of the pump, or, the screen is an extension of the hub itself. Such a screen may "underlie" the lower sides of the impeller blades, or two screens may enclose both the top and bottom sides of the impeller blades in a "closed impeller" or "enclosed" design where only the radial ends of the impeller blades are open, as opposed to to open-type impeller blades that are exposed to the fluid being pumped on all sides of the blades. Open-type impellers (without a screen) are more susceptible to abrasive wear than a shielded impeller, because high-velocity fluid on the impeller blades is very close to the liner walls ("spreaders" or housings), which create rotating eddies that accelerate wear when abrasive materials are present in the liquid.

[0015] Centrifugal pumps for transferring liquids can have abrasive features that usually include a simple screen that sits at the bottom of the impeller, or an enclosed design with both top and bottom screens. In abrasive environments, the screen(s) can also provide additional structural support and reinforcement to protect against blade collapse or deformation. Such enclosed or semi-open impeller designs are suitable for handling solids in use where the blades can face high impact loads from rock and solids. A semi-open impeller also has an ability to pass through solids in a similar way to an open type impeller. With a simple screen, the semi-open impeller is also relatively easy to fabricate.

[0016] High axial pressure is the primary disadvantage of semi-open and enclosed impeller designs: the rotating impeller creates a net fluid flow 104 along the axial direction, but also creates large reaction forces that push the shielded impeller back in the opposite direction of the axial fluid flow. On a semi-open impeller, the entire back surface of the screen is exposed to the full pressure given off by the impeller. The front of the screen is at suction pressure at the center of the impeller where the liquid enters, and increases along the impeller radius due to centrifugal action.

[0017] The differential between the pressure profiles along the two sides of the screen creates the imbalance in axial pressure, here referred to as down pressure. The downward pressure can be counteracted with a pressure disc that radially supports the back of the screen. There is also a loss of efficiency due to disc friction caused by the impeller rotating close to the stationary casing wall. The down pressure forces are resisted by thrust washers on each stage for float type pumps. Mixed flow type impellers usually have balances to help keep these forces within acceptable limits. However, radial flow impellers do not have such balances due to the need to minimize the axial length of the step. The height of wear rings, thrust washers or other balances in the axial direction is of primary importance since this height directly affects the overall height of each pump stage which is critical in many multistage pump designs. Radial impellers therefore tend to have high thrust loads which lead to high mechanical frictional force losses and a high wear rate for thrust washers.

[0018] In centrifugal pumps, a portion of the fluid exiting the rotating impeller usually leaks back to the pump suction by moving through the space between the impeller screen and the liner. A semi-open impeller usually has wear rings or a face seal to regulate this leakage. In some pump stage designs, the outer edges of the pressure washer can perform this leakage control role. Thereby, the thrust disc can also aim to provide a liquid seal.

[0019] The pressure washers regulate recirculation through flow restriction, and can also be used together with impeller balance holes to regulate axial pressure. However, the flow restriction created by the tight clearance between the rotating and stationary thrust disc surfaces causes a very high local fluid velocity and thus a high wear rate. Conventional thrust washers, since they are subjected to this high flow rate, have a short life in abrasive environments, even when hard materials and treated surfaces are used.

[0020] The flow restriction at the pressure disc also causes solids to dam up at this location. Conventionally, as shown at the top of fig. 2, an external thrust washer 202 is positioned radially outward from an impeller-to-spreader screen clearance seal 204 or other seal. Such a clearance seal 204 is typically a finely machined, tight-fitting, tight-fitting, metal-to-metal interface between the impeller shield 206 and the spreader (lining walls) 208 of the pump. The conventional wisdom of this arrangement is to support the impeller 210 against reactive forces from axial fluid flow 104, which supports the impeller 210 around a ring having a significant diameter below the impeller at a medial radius of the impeller shield 206. However, a weakness of the external thrust washer arrangement 202 is that abrasive particles carried by liquid leakage from the impeller tip tend to accumulate at deposit location 212 in fig. 2. This buildup of abrasive particles is due to the axial clearance of the thrust washer being greater than the radial clearance of the face seal action, and therefore the face seal acts as a particle dam. Accumulated abrasive particles quickly wear the outer thrust washer 202.

[0021] The pressure created by the impeller 210 on each stage of a submersible pump can be problematic in a variety of submersible pump types, including mixed flow stage pumps and radial flow stage pumps. In some float type designs, e.g. a significant part of the power loss in the pump due to that pressure friction occurs at an external pressure plate due to relatively high friction-induced torque in this radially outward position. If the outer pressure washer is removed for the float type stage, however, the lack of sealing function increases leakage losses.

Example of pump and impeller design

[0022] As shown in the lower part of fig. 2, in one embodiment of a pump stage 100, an example of a pump impeller 214 has an inner thrust washer (pad, ring) 216 that is displaced inwardly relative to a seal 204 that defines a boundary of a fluid chamber for the spreader (ie, the stationary housing around the impeller). The displacement of the inner thrust washer 216 "behind" the seal 204 protects the inner thrust washer 216 from abrasive fluids being pumped and thereby, from conventional abrasive action and wear. The term "internal," as used herein, means "radially inward, toward, or closer to the axial center of rotation of the pump," while "external" means "radially outward, away from, or further away from the axial center of rotation of the pump."

[0023] The previously mentioned seal 204 may be a wear ring, or may be a finely machined, tight-fitting interface between a rotating part of the impeller 214, usually an impeller screen 218, and the stationary spreader housing: Ie. an impeller shield-to-spreader clearance seal 204. With respect to abrasive fluid, since the protective seal 204 is upstream from the inner thrust washer 216 (with respect to fluid attempting to return from the impeller 214 to the pump inlet 220) the amount of abrasive particles that reach the protected internal thrust washer 216 greatly reduced or eliminated. In conventional designs, an external thrust washer 202 may be in direct contact or even fully submerged in the fluid being pumped. The thrust washer 216 thus offset and protected counteracts and supports against reactionary downforces generated by the pumping impeller while providing higher wear and longer life than conventional pumps used to pump abrasive fluids in multistage subsurface environments.

[0024] In the same or another design, as shown in fig. 3, the diameter (size or "ring size") of a seal or thrust washer at the bottom (ie, rear) of the impeller is strategically reduced to expose more surface area of the lower impeller shield to the pressurized fluid being pumped. In some designs, a seal, wear ring, or sealing interface between the movable impeller and the stationary spreader forms the extent of the fluid space below the impeller, while in other designs the thrust washer 202 itself performs this role. Thrust washer 202 may be used as an example in the description below, as it plays the additional role of a "wear ring" type seal in some pumps.

[0025] As shown in the upper part of fig. 4, a conventional pressure washer 202 defines the extent of a fluid chamber 402 on a lower impeller screen 206. The top of the impeller 210 has a fluid chamber 404 which exposes a greater portion of the surface area at the top of the impeller 210 to pressurized fluid and results in unbalanced pressure area 406 which presses the impeller 210 down into the thrust washer 202, where friction results in power loss. Downforces tend to be high because pressure acting on the surface of the lower impeller shield 206 is sealed at the outer diameter 408 of the thrust disc 202, while pressure acting on the surface 410 of the upper impeller shield is sealed at the spreader hub inside diameter 412.

[0026] In the lower part of fig. 4, reducing the diameter of the conventional thrust washer 202 to a smaller diameter thrust washer 216 in the design and manufacture of a pump increases the extent of the lower fluid chamber 414 and increases the amount of surface area of the lower impeller shield 416 that is exposed to the pressurized fluid below. With reference to fig. 4, reducing the diameter of pressure disc 216 effectively reduces the unbalanced pressure area 418, as given in equation (1): where dj is the conventional outside diameter of the unbalanced pressure area 406, and d2 is the outside diameter of that area 418 with reduced pressure imbalance. Reducing the area 418 of pressure imbalance in this way increases the pressure on the lower impeller screen 416 and thereby to some extent lift the impeller 420 off the pressure disc 216. The lifting is possibly not a physical movement of the impeller 420 off the pressure disc 216, but may be a reduction in the net downforce which acts on the impeller 420, or a reduction of the normal force Fn on the friction surface of the pressure disc 216, thereby saving the pressure disc 216. The friction on the surface of the pressure disc 216 can be approximated by the dry friction expressed in equation (2):

where F{ is the frictional force applied by each surface on the other, and is parallel to the surface in a direction opposite to the net applied force; u is the coefficient of friction, which is an empirical property of the materials used to make the thrust washer 216, and Fa is the normal force applied by each surface on the other, directed perpendicular (normal) to the surface.

[0027] The diameter of the thrust washer 216 (or other seal) can thereby be selectively reduced to strategically balance the exposed surface area and pressure at the bottom of the impeller 420 with the exposed surface area and pressure at the top of the impeller 420 to reduce friction and power loss. This balancing of pressure at the top and bottom of the impeller 420 through choice of seal or disc size also provides other benefits.

[0028] As shown in fig. 5, by reducing the diameter of the conventional thrust washer 202, the reduced diameter of the smaller thrust washer 216 also reduces the power loss due to smaller surface area where friction can occur on the smaller pressure disc 216. The reduction in surface area where friction can occur is given by equation (3) using the radii shown in fig. 5:

[0029] For a reduction in the outside diameter of a conventional thrust washer 202 where the new outside diameter of the smaller thrust washer 216 still remains larger than the original inside diameter of the conventional thrust washer 202, the reduction in surface area where friction can occur can be provided of equation (4):

where dj is the outside diameter of the conventional thrust washer 202 and d2 is the outside diameter of the new, smaller thrust washer 216.

[0030] Furthermore, as shown in fig. 6, since the radius of the circle or ring defined by the conventional pressure disc 202 is reduced to the radius of the new, smaller pressure disc 216, the torque arm 602 of the random braking force is reduced 604. The braking force is a high-friction induced torque acting between the rotating screen and the pressure disc 216 or between the stationary spreader and the pressure disc 216, depending on the setup, since the pressure disc 216 acts undesirably as elements of a disc brake. The friction torque is given by equation (5): where x is the friction braking torque, r is the moment arm 602 (or weight arm) and F is the friction force approximated by equation (2) above. Thereby, the reduction in the friction braking torque can be given by equation (6), by using the radii shown in fig. 6:

[0031] Displacing the seal or thrust washer can also increase pump efficiency and reduce wear by placing the thrust washer 216 or other seals where there is less shaking and turbulence in the abrasive fluid and/or where there is improved laminar flow away from closely interacting moving parts .

Examples of methods

[0032] Fig. 7 is an example of a method 700 for making an abrasion resistant pump for subsurface use. In the flowchart, the operations are given as summaries in individual blocks.

[0033] In block 702, a pump is created to transfer fluids containing abrasive materials at a subsurface location, including an impeller, a liner, and a thrust washer.

[0034] In block 704, a seal and a thrust washer are positioned relative to each other to resist a flow of the abrasive materials to the thrust washer.

[0035] Fig. 8 is an example of method 800 for increasing the durability and efficiency of an abrasion resistant pump. In the flowchart, the operations are given as summaries in individual blocks. 0036] In block 802, an impeller is made to pump liquid, including a thrust washer to support the impeller and to restrict a flow of the liquid.

[0037] In block 804, the diameter of the thrust washer is reduced to balance a first pressure at the bottom of the impeller with a second pressure at the top of the impeller, to reduce friction of the impeller on the thrust washer.

[0038] In block 806, the diameter of the thrust washer is reduced to reduce a surface exposed to friction and to reduce a torque arm of a braking torque on the impeller, to reduce power loss in the pump.

Conclusion

[0039] Although example systems and methods have been described in language specific to structural functions or techniques, the subject matter defined in the appended claims is not necessarily limited to the specific functions or actions described. Rather, the specific functions and actions are published as examples of forms of execution of the systems, methods and structures required.

Claims (20)

1. A pumping stage for a centrifugal pump for use in an underground hydrocarbon well comprising: a spreader; an impeller; a thrust washer and a clearance seal located upstream of the thrust washer. 2. The pump stage of claim 1, wherein the clearance seal is formed between the impeller and the spreader. 3. The pump stage in claim 1, in which the thrust washer sits radially inwards from the clearance seal with respect to a central axis of the pump stage. 4. The pump stage of claim 1, wherein the clearance seal resists a flow of liquid from the impeller back to a liquid inlet. The clearance seal sits between the liquid and the pressure disc. 5. The pump stage of claim 1, wherein the close-fitting metal-to-metal interface comprises a flat interface between the impeller and the spreader. 6. The pump stage in claim 5 in which the tight-fitting interface comprises a flat interface of two surfaces running parallel to an axial direction of the pump stage, and the flat interface is placed between a liquid and the pressure disc. 7. The pumping step of claim 1, wherein the fluid comprises an abrasive fluid and the clearance seal resists the flow of abrasive components in the abrasive fluid to the pressure plate. 8. The pump stage of claim 1, wherein a diameter of the thrust disc is reduced to locate the clearance seal upstream of the thrust disc, and wherein a reduced diameter of the thrust washer increases a fluid pressure on a lower side of the impeller to reduce friction of the impeller on the thrust washer. 9. A submersible pump stage for a subsurface hydrocarbon well comprising: an impeller; a spreader, and at least one thrust washer seated within at least one impeller-to-spreader clearance seal thereby reducing a down pressure load acting on the impeller. 10. The submersible pump stage of claim 9, wherein said placement of the thrust washer within the impeller-to-spreader clearance seal increases a first fluid pressure on a bottom of the impeller to reduce friction of the impeller on the thrust washer. 11. The submersible pump stage in claim 9, where a reduced surface area of the pressure disc reduces friction of the impeller on the pressure disc. 12. The submersible pump stage of claim 9, wherein a reduced diameter of the thrust washer reduces a moment arm of a braking torque acting between the impeller and the thrust washer, and reduces friction of the impeller on the thrust washer. 13. An impeller for a centrifugal pump for use in a subsurface hydrocarbon well, comprising: an impeller including impeller blades attached to a screen; a thrust washer that sits radially inward from an impeller-to-spreader clearance sealing function of the impeller. 14. The impeller of claim 13, wherein the centrifugal pump comprises a stage of a subsurface multi-stage pump for moving hydrocarbons containing abrasive solids. 15. The impeller of claim 13, wherein the impeller-to-spreader clearance seal forms a tight-fitting clearance seal with a stationary part of a spreader. 16. The impeller of claim 15, wherein said thrust washer seated radially inward from the impeller-to-spreader clearance seal causes a lower surface area of the screen to be exposed to an increased fluid pressure to reduce friction for the thrust washer. 17. The impeller of claim 13, wherein the thrust washer has a reduced diameter to locate the thrust washer inwardly from the impeller-to-spreader clearance seal function, and wherein the reduced diameter of the thrust washer reduces a surface area of the thrust washer to reduce friction on the thrust washer. 18. The impeller of claim 13, wherein the thrust washer has a reduced diameter to locate the thrust washer within the impeller-to-spreader clearance seal function, and wherein the reduced diameter of the thrust disc reduces a torque arm of a braking torque acting on the thrust disc. 19. A method comprising: making a pump stage for a multi-stage submersible pump for a subsurface hydrocarbon well, the pump stage including an impeller, a spreader and a thrust washer, and locating the thrust washer within an impeller-to-spreader clearance seal. 20. The method in claim 19 which further comprises selecting a diameter for the thrust disc, wherein the selected diameter reduces a surface area of the thrust disc to reduce friction on the thrust disc and reduce a torque arm of a braking torque acting on the thrust disc.