CN101126118A - Prestressing method for fluid ends of pumps - Google Patents

Abstract

The present invention provides a multi-step prestressing method for preconditioning the fluid end of a multi-cylinder reciprocating pump having a central cylinder and at least two side cylinders, the method comprising prestressing the central cylinder treatment; and prestressing the at least two side cylinders, wherein the prestressing of the center cylinder and the prestressing of the at least two side cylinders are performed independently. By adopting the invention, the service life of the fluid end of the multi-cylinder reciprocating pump can be prolonged.

Description

Prestressing method for fluid ends of pumps

technical field

In general, the present invention relates to an autofrettage process for mechanically preconditioning the fluid ends of multi-cylinder reciprocating pumps in order to introduce residual compressive stresses within the fluid ends of the cylinders.

Background technique

Hydraulic fracturing of downhole formations is an urgent task to induce oil flow. Typically, this is done at higher pressures downhole by vacuum pumping oil to fracture the adjacent downhole soil and rock. Oil can then flow downhole through these faults to significantly increase well productivity. Reciprocating pumps, especially triplex pumps, are generally used to pump high-pressure fracturing fluids downhole. However, repeated exposure of the fluid end of the pump to high pressures will render the fluid end cylinder susceptible to fatigue damage. Accordingly, it is required to enhance the fatigue resistance of the fluid-end cylinders of multi-cylinder reciprocating pumps.

Contents of the invention

Prestressing methods can be used to create compressive residual stresses in the inner wall of the fluid end of a multi-block reciprocating pump so that the fluid end experiences minimal tensile stress during the pumping cycle. In the prestressing method, the cylinder cavity of the fluid end is exposed to high hydrostatic pressure, which causes the inner region of the fluid end to yield plastically, while the deformation of the outer region is elastic. When the pressure is removed, the outer region of the fluid end elastically returns, while the plastically deformed inner region is now under compressive stress. This compressive stress increases the fatigue resistance of the fluid end.

The purpose of the present invention is to provide a prestress treatment method that can improve the service life of the fluid end of the multi-cylinder reciprocating pump. In one embodiment, the present invention includes a multi-step prestressing method for preconditioning the fluid end of a multi-cylinder reciprocating pump having a central cylinder and at least two side cylinders, wherein the method includes subjecting the central cylinder to prestressing; and prestressing at least two of the side cylinders. In this method, the prestressing of the central cylinder and the prestressing of at least two side cylinders are carried out independently.

Description of drawings

These and other features and advantages of the present invention may be better understood by referring to the following detailed description in conjunction with the accompanying drawings. In the attached picture:

Fig. 1 is the perspective view of the multi-cylinder reciprocating pump used in the prestressing treatment method of the present invention;

Figure 2 is a cross-sectional view of one of the fluid end cylinders of the multi-cylinder reciprocating pump shown in Figure 1;

Fig. 3 is the schematic diagram of an embodiment of the prestress treatment method of the present invention;

Fig. 4 is the schematic diagram of another multi-cylinder reciprocating pump used in the prestressing treatment method of the present invention;

Fig. 5 is the schematic diagram of another embodiment of the prestress treatment method of the present invention;

Fig. 6 is a schematic diagram of another embodiment of the prestressing treatment method of the present invention.

Detailed ways

As mentioned above, in oil and gas wells, multi-cylinder reciprocating pumps are often used to pump bottomhole high-pressure fracturing fluids to increase well productivity. One embodiment of such a pump 10 is shown in FIG. 1 . In the illustrated embodiment, the pump 10 is a triplex pump having three cylinders 12A-12C, each cylinder having a corresponding piston 14A-14C arranged to move relative thereto. For the purposes of this document, the central cylinder of the three cylinders will be referred to as the central cylinder 12B, while the remaining two cylinders will be referred to as the side cylinders 12A, 12C. However, as discussed further below, the pump 10 may be a pump having any suitable number of cylinders, such as a five cylinder pump (pentaplex pump) or a seven cylinder pump (septax pump).

As will be described further below, in the embodiment shown in this figure, the pump 10 includes two parts, a power end 16 and a fluid end 18 . The power end 16 includes a crankshaft 20 powered by a motor assembly (not shown) to drive the pump's pistons 14A-14C; the fluid end 18 includes cylinders 12A-12C in which the pistons 14A-14C reciprocate to inhale low pressure fluid and discharge high pressure fluid.

For simplicity, FIG. 2 only shows a cross-section of one cylinder 12 of the fluid end of the reciprocating pump. However, the illustrated cylinder 12 is representative of any of those cylinders in a multi-cylinder reciprocating pump such as a triplex, quintuplex, or sevenx pump, among other suitable pumps. Accordingly, any discussion below regarding fluid end cylinder 12 applies equally to all three cylinders 12A-12C of triplex pump 10 in FIG. Any discussion regarding piston 14 applies equally to all three pistons 14A-14C of triplex pump 10 shown in FIG. 1 , or to any piston in five-cylinder and seven-cylinder pumps.

As shown in FIG. 1, and as discussed below, the illustrated triplex pump 10 each fluid end cylinder 12A-12C includes a piston 14A-14C arranged to move relative to the cylinder. Typically, each piston 14A-14C has a diameter of about 4.5 inches to about 6.5 inches and each piston 14 can generate pressures up to about 12,000 psi (12 kpsi) when used for reservoir fracturing .

As shown in FIG. 2 , each cylinder 12 includes a fluid chamber 22 . Each piston 14 is slidably mounted in its corresponding cylinder 12 and reciprocates in the fluid chamber 22 . The effect of the reciprocating movement of the piston 14 is to change the volume of fluid in the fluid chamber 22 . Cylinder 12 also includes check valves, such as suction valve 24 and discharge valve 26, which control the flow of fluid into and out of fluid chamber 22 as piston 14 reciprocates.

As mentioned above, the piston 14 is reciprocated by an electric motor driving the rotation of the crankshaft 20 . The actions of the suction valve 24 and the discharge valve 26 are controlled by fluid and spring force. For example, the suction valve 24 is biased towards the suction valve seat 28 , ie towards the closed position, by means of a spring 30 positioned between the suction valve 24 and a spring stop 32 . Similarly, the discharge valve 26 is biased toward the discharge valve seat 34 , ie toward the closed position, by means of a discharge valve spring 36 positioned between the discharge valve 26 and a spring stop 38 .

As the piston 14 moves outward (toward the left in FIG. 2 ) through a packing bore 40 , a pressure drop is created in the fluid chamber 22 . This pressure drop causes the suction valve 24 to move to the open position against the bias of the spring 30 and allows fluid to flow through the suction tube 25 through the suction valve 24 into the fluid chamber 22 . This phase of the movement of the piston 14 may be referred to as the "suction stroke".

When the piston 14 moves in the opposite direction (to the right in FIG. 2 ) through the sealing hole 40 , the spring 30 closes the suction valve 24 and the pressure in the fluid chamber 22 increases. The increase in pressure causes discharge valve 26 to open and forces fluid from fluid chamber 22 outward through discharge valve 26 and out discharge tube 35 . Discharge valve 26 remains open while piston 14 continues to pressurize fluid in fluid chamber 22 (typically about 2 kpsi to about 12 kpsi). This high pressure phase of movement of the piston 14 and in which fluid is expelled through the discharge valve 26 is known as the "discharge stroke".

Setting the pumping frequency at 2 Hz (ie, two pressure cycles per second), the fluid end 18 is subjected to a very high number of stress cycles over a relatively short operating life. These stress cycles will result in fatigue failure of the fluid end 18 . Fatigue consists of a damage process in which small cracks initiate on the free surface of a component under cyclic stress. The crack grows at a rate determined by the cyclic stress and material properties until the crack is large enough to demonstrate damage to the part. Since fatigue cracks usually initiate at the surface, a strategy to counteract this failure mechanism is to prestress the surface in compression.

This can be accomplished by a prestressing process that involves applying residual compressive stress to the fluid end 18 at its internal free surface (i.e., the surface exposed to the fracturing fluid within the fluid end cylinder 12). Perform mechanical pretreatment. During the prestressing process, the fluid end cylinder 12 is under high hydrostatic pressure. The pressure during the prestressing process causes plastic yielding of the inner region of the fluid end cylinder 12 wall. Because the stress level decays along the wall thickness, the deformation of the outer region of the wall remains elastic. When the hydrostatic pressure is removed, the outer regions of the walls tend to return to their original structural state.

However, the plastic deformation of the inner region of the same wall limits this deformation. As a result, the inside region of the wall of the fluid end cylinder 12 continues to carry residual compressive stress. This compressive stress increases the fatigue resistance of the fluid end. The effect of the prestressing treatment method depends on the range of residual stresses on the inner wall and their magnitudes.

The prestressing method consists of a single hydrostatic step acting on each cylinder of a multicylinder pump, that is, in the case of a triplex pump, all three cylinders are deformed simultaneously. The pressure depends on the size of the pump, for example in a multi-block reciprocating pump with a 5.5 inch piston diameter a prestress pressure of about 55 kpsi may be used.

However, computer modeling showed that this single-step prestressing approach was not optimal, producing small residual compressive stresses in the central cylinder at the fluid end. The reason for this is that, since the deformation of the central cylinder is constrained by the joint deformation of the side cylinders of the multi-cylinder pump, a small plastic strain and subsequently a small residual compressive stress is generated in the central cylinder during the prestressing treatment. As a result, tensile stresses within the central cylinder may be higher, resulting in a shorter operating life for the fluid end 18 .

In one embodiment, the above-described method of prestressing the fluid end 18 of the multi-cylinder pump 10 includes a two-step process, in which one step is performed on the central cylinder 12B separately from the remaining cylinders 12A, 12C. prestressing, and in a further step, either the remaining cylinders 12A, 12C or all cylinders 12A-12C are prestressed simultaneously. Computer modeling shows that this two-step approach improves the residual stress distribution of the fluid end 18 , resulting in increased fluid end 18 useful life.

FIG. 3 shows a schematic diagram for preconditioning the fluid end 18 of a multicylinder reciprocating pump 10 having at least three cylinders (cylinders 12A-12C in the case of the triplex pump 10 shown in FIG. 1 ). Multi-step prestressing method 300 . The method shown in FIG. 3 in combination with the pump 10 shown in FIG. 1 will now be described. In one embodiment, the prestressing method 300 includes a first step 310 comprising prestressing the central cylinder 12B separately from the remaining cylinders, in this case the side cylinders 12A, 12C. Step 310 includes applying hydrostatic pressure only on the center cylinder 12B and then releasing the static pressure. In one embodiment, the hydrostatic pressure may be in the range of about 55 kpsi to about 65 kpsi.

The second step 320 consists in simultaneously prestressing the remaining cylinders, in this case simultaneously prestressing the side cylinders 12A, 12C, but not prestressing the central cylinder 12B. This step 320 involves applying hydrostatic pressure only to the side cylinders 12A, 12C and then releasing the hydrostatic pressure. In one embodiment, the hydrostatic pressure may be in the range of about 55 kpsi to about 65 kpsi.

In one embodiment, the order of the above steps, that is, steps 310 and 320 can be reversed, that is, step 320 of prestressing the side cylinders 12A, 12C can be implemented first; secondly, the central cylinder 12B can be prestressed Step 310 of the process. In either sequence of steps, the prestressing pressure for the center cylinder 12B may be higher than the prestressing pressure for the opposite cylinders 12A, 12C. While exemplary prestressing pressures have been given above, other suitable pressures may be used, even if these pressures are outside the above ranges. In one embodiment, the optimal prestressing pressure can be determined from a suitable computer model, which takes into account the mechanical properties of the fluid end material, the pressure of the prestressing method, the pressure applied by the prestressing pressure on the fluid end Area, one of the other factors.

By correspondingly increasing the number of prestressing steps, the multi-step prestressing method can be applied to triplex pumps or pumps with more than three cylinders. For example, FIG. 4 schematically shows a fluid end 418 of a five cylinder pump having five cylinders 412A-412E. The multi-step prestressing method 500 shown in FIG. 5 illustrates one embodiment of the prestressing steps for such a pump.

As shown, in one embodiment, the first step 510 includes prestressing the center cylinder 412C separately from the remaining cylinders. In this case, the remaining cylinders include a first set of side cylinders 412B, 412D immediately adjacent to the central cylinder 412C and a second set of side cylinders 412A, 412E one cylinder away from the central cylinder 412C. Step 510 includes applying hydrostatic pressure only to the center cylinder 412C and then withdrawing the hydrostatic pressure. In one embodiment, the hydrostatic pressure may be in the range of about 55 kpsi to about 65 kpsi.

The second step 520 includes simultaneously prestressing the first set of side cylinders 412B, 412D without prestressing the center cylinder 412C and the second set of side cylinders 412A, 412E. This step 520 involves simultaneously applying and then withdrawing hydrostatic pressure to only the first set of side cylinders 412B, 412D. In one embodiment, the hydrostatic pressure may be in the range of about 55 kpsi to about 65 kpsi.

The third step 530 includes simultaneously prestressing the second set of side cylinders 412A, 412E without prestressing the center cylinder 412C and the first set of side cylinders 412B, 412D. This step 530 includes simultaneously applying and then withdrawing hydrostatic pressure to the second set of side cylinders 412A, 412E. In one embodiment, the hydrostatic pressure may be in the range of about 55 kpsi to about 65 kpsi.

For each group of side cylinders newly added from the central cylinder 412C, it can be realized by adding a prestressing treatment step. In one embodiment, the steps 510, 520 and 530 described above may be reversed and/or performed in any order. While exemplary prestressing pressures have been given above, other suitable pressures may be used, even if these pressures are outside the above ranges. As noted above, in one embodiment, an optimal prestressing pressure can be determined from a suitable computer model.

FIG. 6 illustrates a multi-step prestressing method 600 for preconditioning the fluid end 18 of a multi-cylinder reciprocating pump having at least three fluid end cylinders. As shown, in one embodiment, the first step 610 includes simultaneously prestressing all cylinders at the fluid end (e.g., all cylinders 12A-12C of the triplex pump shown in FIG. all cylinders 412A-412E of the five-cylinder pump shown). This step 610 includes applying hydrostatic pressure to all cylinders simultaneously and then withdrawing the hydrostatic pressure. In one embodiment, the hydrostatic pressure may be in the range of about 55 kpsi to about 65 kpsi.

The second step 620 includes prestressing only the center cylinder (eg, center cylinder 12B of a three-cylinder pump shown in FIG. 1 , or center cylinder 412C of a five-cylinder pump shown in FIG. 4 ). This step 620 includes applying hydrostatic pressure only to the center cylinder and then removing the hydrostatic pressure. In one embodiment, the hydrostatic pressure may be in the range of about 55 kpsi to about 65 kpsi. While exemplary prestressing pressures have been given above, other suitable pressures may be used, even if these pressures fall outside the above ranges. As noted above, in one embodiment, an optimal prestressing pressure can be determined from a suitable computer model.

Each of the multi-step prestressing methods 300, 500, and 600 described above results in improved distribution of residual stress in the preconditioned pump compared to a single-step process, as well as improved residual stress distribution under compressive residual stress conditions. The area inside the central cylinder is larger. This minimizes the tensile stress experienced by the fluid end during pumping and extends the operating life of the fluid end. It should be noted that while the above discussion has primarily focused on the use of multi-step prestressing methods for preconditioning multi-cylinder pumps in reservoir fracturing applications, such preconditioning pumps could also be used in any other suitable occasions. For example, exemplary applications in the oil well industry include coil tubing applications, cement applications, one of other suitable applications.

The above description is made in connection with the preferred embodiments of the present invention. Those skilled in the art will appreciate that modifications and changes can be made to the described structures and methods of operation without departing from the spirit and scope of the present invention. Therefore, the above description should not be regarded as applicable only to the certain structures illustrated and illustrated in the accompanying drawings, but should be understood as consistent with the appended claims that most fully and clearly describe the scope of protection. in support of these claims.

Claims (28)

1. one kind is used for the triplex pump fluid end that comprises a center cylinder body and two side cylinder bodies is carried out the rapid autofrettage method of pretreated multistep, and wherein, this method comprises:

Described center cylinder body is carried out autofrettage; And

Described two side cylinder bodies are carried out autofrettage, wherein said the center cylinder body is carried out autofrettage and described two side cylinder bodies are carried out autofrettage is independent the execution.

2. the method for claim 1, wherein describedly two side cylinder bodies are carried out autofrettage comprise simultaneously these two side cylinder bodies are carried out autofrettage.

3. the method for claim 1, wherein, the autofrettage pressure that described center cylinder body is carried out putting on during the autofrettage this center cylinder body is greater than the autofrettage pressure that described two side cylinder bodies is carried out put on during the autofrettage these two side cylinder bodies.

4. the method for claim 1, wherein before described step of two side cylinder bodies being carried out autofrettage, carry out described step of the center cylinder body being carried out autofrettage.

5. the method for claim 1, wherein before described step of the center cylinder body being carried out autofrettage, carry out the described step that two side cylinder bodies are carried out autofrettage.

6. one kind is used for the triplex pump fluid end that comprises a center cylinder body and two side cylinder bodies is carried out the rapid treatment process of pretreated multistep, and wherein, this method comprises:

Described center cylinder body is applied hydrostatic pressure to produce compressive residual stress therein;

Discharge the hydrostatic pressure on the cylinder body of described center;

Described two side cylinder bodies are applied hydrostatic pressure to produce compressive residual stress therein; And

Discharge the hydrostatic pressure on described two side cylinder bodies, wherein said the center cylinder body is applied hydrostatic pressure and described two side cylinder bodies are applied hydrostatic pressure is independent the execution.

7. method as claimed in claim 6 wherein, describedly applies hydrostatic pressure to two side cylinder bodies and comprises simultaneously these two side cylinder bodies are applied hydrostatic pressure.

8. method as claimed in claim 6 wherein, is applied to hydrostatic pressure on the cylinder body of described center greater than the hydrostatic pressure that is applied in described two side cylinder bodies on any.

9. method as claimed in claim 6 wherein, carried out described the center cylinder body is applied step with the release fluids static pressure before the described step that two side cylinder bodies is applied with the release fluids static pressure.

10. method as claimed in claim 6 wherein, carried out described two side cylinder bodies are applied step with the release fluids static pressure before the described step that the center cylinder body is applied with the release fluids static pressure.

11. one kind is used for the multi-cylinder body reciprocation pump that comprises a center cylinder body and at least two group side cylinder bodies is carried out the rapid autofrettage method of pretreated multistep, wherein, this method comprises:

Described center cylinder body is carried out autofrettage;

Carry out autofrettage to first group in the described at least two group side cylinder bodies; And

Carry out autofrettage to second group in the described at least two group side cylinder bodies, wherein said the center cylinder body is carried out autofrettage and described to carry out autofrettage to first and second groups at least two group side cylinder bodies be independent the execution.

12. method as claimed in claim 11, wherein, first group of second side cylinder body that comprises the first side cylinder body of first side setting that is close to described center cylinder body and be close to second side setting of described center cylinder body in the described at least two group side cylinder bodies.

13. method as claimed in claim 12, wherein, second group of the 4th side cylinder body that comprises the 3rd side cylinder body of the side setting that is close to the described first side cylinder body and be close to the side setting of the described second side cylinder body in the described at least two group side cylinder bodies.

14. method as claimed in claim 13, wherein, simultaneously the described first and second side cylinder bodies are carried out autofrettage, wherein simultaneously the described third and fourth side cylinder body is carried out autofrettage, wherein said the first and second side cylinder bodies are carried out autofrettage and described the third and fourth side cylinder body is carried out autofrettage is independent the execution.

15. method as claimed in claim 11, wherein, the autofrettage pressure that described center cylinder body is carried out putting on during the autofrettage this center cylinder body is greater than the autofrettage pressure that described first and second groups of side cylinder bodies is carried out put on during the autofrettage these first and second groups of side cylinder bodies.

16. method as claimed in claim 11 wherein, was carried out described step of the center cylinder body being carried out autofrettage before described step of first and second groups of side cylinder bodies being carried out autofrettage.

17. method as claimed in claim 11, wherein, described multi-cylinder body reciprocation pump is five cylinder pumps.

18. method as claimed in claim 11, wherein, described multi-cylinder body reciprocation pump is seven cylinder pumps.

19. one kind is used for the multi-cylinder body reciprocation pump fluid end that comprises at least three fluid end cylinder bodies is carried out the rapid autofrettage method of pretreated multistep, this method comprises:

Simultaneously all described at least three fluid end cylinder bodies are carried out autofrettage; And

Center cylinder body in described at least three fluid end cylinder bodies is carried out autofrettage, wherein said all at least three fluid end cylinder bodies are carried out autofrettage and described the center cylinder body is carried out autofrettage is independent the execution.

20. method as claimed in claim 19, wherein, the autofrettage pressure that described center cylinder body is carried out putting on during the autofrettage this center cylinder body is greater than the autofrettage pressure that all described at least three fluid end cylinder bodies is carried out put on during the autofrettage these all at least three fluid end cylinder bodies.

21. method as claimed in claim 19, wherein, described multi-cylinder body reciprocation pump is a triplex pump, so that described at least three fluid end cylinder bodies comprise three cylinder bodies.

22. method as claimed in claim 19, wherein, described multi-cylinder body reciprocation pump is five cylinder pumps, so that described at least three fluid end cylinder bodies comprise five cylinder bodies.

23. method as claimed in claim 19, wherein, described multi-cylinder body reciprocation pump is seven cylinder pumps, so that described at least three fluid end cylinder bodies comprise seven cylinder bodies.

24. one kind is used for the multi-cylinder body reciprocation pump fluid end that comprises at least three fluid end cylinder bodies is carried out the rapid autofrettage method of pretreated multistep, this method comprises:

Simultaneously all described at least three fluid end cylinder bodies are applied the first fluid static pressure to produce compressive residual stress thereon;

Discharge the hydrostatic pressure on all described at least three fluid end cylinder bodies;

Center cylinder body in all described at least three fluid end cylinder bodies is applied second hydrostatic pressure to produce compressive residual stress thereon; And

Discharge the hydrostatic pressure on the cylinder body of described center, wherein said step and the described step that applies second hydrostatic pressure that applies the first fluid static pressure is independent the execution.

25. method as claimed in claim 24, wherein, described second hydrostatic pressure is greater than described first fluid static pressure.

26. method as claimed in claim 24, wherein, described multi-cylinder body reciprocation pump is a triplex pump, so that described at least three fluid end cylinder bodies comprise three cylinder bodies.

27. method as claimed in claim 24, wherein, described multi-cylinder body reciprocation pump is five cylinder pumps, so that described at least three fluid end cylinder bodies comprise five cylinder bodies.

28. method as claimed in claim 24, wherein, described multi-cylinder body reciprocation pump is seven cylinder pumps, so that described at least three fluid end cylinder bodies comprise seven cylinder bodies.

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