Design method of high-lift and high-speed two-stage canned motor pump capable of balancing axial forces

Abstract

The present invention discloses a design method for a high-lift, high-speed, two-stage shielded pump capable of balancing axial forces. By differentiating and refining the front and rear cover plates of the centrifugal impeller, the force difference between the front and rear cover plates of the two-stage impeller is used to balance the axial force directed toward the inlet of the first-stage impeller due to the high pressure at the inlet of the secondary impeller of the two-stage shielded pump. The steps are as follows: determining the basic performance parameters of the pump to be designed; determining the basic structure of the pump; allocating the outlet head of the two-stage impeller; determining the impeller parameters; respectively calculating the axial force generated by the asymmetry of the front and rear cover plates of the two-stage impeller, the fluid dynamic reaction force on the impeller, and the axial force generated by the pressure difference between the inlets of the two-stage impeller; calculating the axial resultant force. If it is less than the maximum pressure that the thrust bearing assembly can withstand, the design is completed. Otherwise, the outlet head of the two-stage impeller is redistributed and the impeller geometric parameters are adjusted until the requirements are met. This method does not add additional axial force balancing measures. The centrifugal pump designed by this method is highly efficient and reliable in operation.

Description

Technical Field

[0001] The invention belongs to the field of shielded pump design, and in particular relates to a design method of a high-lift and high-speed two-stage shielded pump capable of balancing axial forces. Background Art

[0002] Low-specific-speed centrifugal pumps have broad application prospects due to the demands of aerospace and underwater vehicle attitude adjustment. Traditional methods for axial force balancing generally include opening balancing holes and adding back vanes, but these methods often result in significant energy losses. For example, opening balancing holes increases pump leakage, while adding back vanes significantly increases pump shaft torque.

[0003] Currently, domestic researchers have proposed many new axial force balancing methods or devices. For example, patent application number 202010794591.X, "Multi-stage Shielded Pump with Balanced Disc Structure," utilizes a multi-stage balanced disc structure to effectively balance axial forces, offering strong adjustability and reliable operation. However, due to the uniform orientation of the impellers, the axial forces are superimposed, increasing the axial forces that the balancing discs must balance. Patent application number 202011347412.4, "An Axial Force Balancing Structure for an Internal Circulation Centrifugal Pump with Multiple Rear Port Rings," proposes an axial force balancing structure for an internal circulation centrifugal pump with multiple rear port rings, effectively reducing leakage and minimizing the impact of the balancing holes on the impeller inlet flow pattern. However, low-specific speed centrifugal pumps have low flow rates and high head, and single-stage designs often require large impeller diameters. Therefore, due to manufacturing challenges, semi-open impellers are often designed. The lack of a front cover in semi-open impellers presents a series of challenges with axial force balancing, and their efficiency is very low, making them generally unsuitable. In order to increase the specific speed of the impeller, low specific speed centrifugal pumps are usually designed in a two-stage form. However, since the fluid at the secondary impeller inlet of the two-stage structure has been pressurized by the first stage, an axial force pointing to the first-stage impeller inlet will exist in the rotor shaft system, which brings challenges to the axial force balance of high-head and high-speed two-stage canned motor pumps.

[0004] In summary, the current design methods have the problem of difficult to balance axial force or excessive sacrifice of efficiency. How to solve the axial force problem of high-lift and high-speed shielded pumps from the source is an issue that needs to be solved urgently. Summary of the Invention

[0005] The purpose of the present invention is to solve the technical problem of excessive axial force in existing low-specific-speed centrifugal pumps, and to provide a design method for a high-lift, high-speed, two-stage canned motor pump that can balance the axial force. The canned motor pump designed by this method has a good balancing effect, has no effect on the hydraulic performance and cavitation performance of the pump, and is safe and reliable in operation.

[0006] The object of the present invention is achieved by following steps:

[0007] A design method for a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces. The method utilizes the difference in axial forces generated by the asymmetry of the front and rear covers of the centrifugal impeller to balance the axial forces directed toward the first-stage impeller inlet due to the high pressure at the secondary impeller inlet of the two-stage canned motor pump. The method comprises the following steps:

[0008] Step 1: Determine the basic performance parameters of the pump to be designed, including the pump's design flow rate Q des , design head H and rated speed n;

[0009] Step 2: Determine the basic structure of the two-stage canned motor pump. The impellers are two-stage and arranged back to back. The impeller near the pump inlet is the first-stage impeller, and the impeller near the pump outlet is the secondary impeller. The outlet of the first-stage volute is connected to the inlet of the secondary impeller.

[0010] Step 3: Allocate two-stage head, where the first-stage impeller outlet head H1 and the secondary impeller outlet head H2 satisfy H1<H2, and H1=(0.2~0.6)H, H2=(0.35~0.85)H;

[0011] Step 4: Determine the impeller parameters and optimize the impeller blade shape through finite element simulation;

[0012] Step 5: Calculate the axial forces F1 and F2 generated by the asymmetric cover plates of the first-stage impeller and the secondary impeller respectively; calculate the dynamic reaction forces T1 and T2 of the fluid on the first-stage impeller and the secondary impeller respectively; calculate the axial force ΔF generated by the pressure difference between the secondary impeller and the first-stage impeller;

[0013] Step 6: When the pump to be designed is a horizontal pump, calculate the axial force acting on the shaft system according to the following formula:

[0014] F 合 =F2-F1-T2+T1-ΔF

[0015] When the pump to be designed is a vertical pump, the axial resultant force on the rotor shaft system is calculated according to the following formula:

[0016] F 合 =F2-F1-T2+T1-ΔF±G

[0017] Where G is the rotor's own weight. If the first-stage impeller is at the bottom, it takes a negative sign; if the first-stage impeller is at the top, it takes a positive sign.

[0018] If the axial force is less than the axial force borne by the thrust bearing assembly, the design meets the requirements; if not, return to step 3 and adjust the lift or impeller geometric parameters of the first-stage impeller until the requirements are met.

[0019] Furthermore, in order to fully reflect the throttling effect of the mouth ring, the single-side gap between the front and rear mouth rings of the two-stage impeller is not greater than 0.2mm, and the mouth ring length is not less than 15% of the inner diameter of the mouth ring.

[0020] Furthermore, in order to ensure that the pressure difference between the canned motor rotors is small, the canned motor main shaft gap should be no less than 0.8 mm.

[0021] Furthermore, in step 4, the impeller parameters include the first-stage impeller outer diameter 2×R o1 , the inner diameter of the first stage impeller ring is 2×R m1 , the first stage impeller inlet diameter 2×R j1 , the first stage impeller hub diameter is 2×R h1 , secondary impeller outer diameter 2×R o2 , inner diameter of secondary impeller ring 2×R m2 , secondary impeller inlet diameter 2×R j2 , secondary impeller hub diameter 2×R h2 ; and R h1 =R h2 .

[0022] Furthermore, in step 5, the magnitude of F1 is the force F acting on the rear cover of the first-stage impeller. 1后 Subtract the force F on the front cover 1前 , the direction of F1 is toward the inlet of the first-stage impeller; the magnitude of F2 is the force F acting on the rear cover of the secondary impeller 2后 Subtract the force F on the front cover 2前 , the direction of F2 is toward the secondary impeller inlet, and the calculation formula is as follows:

[0023]

[0024] Where g is the local acceleration of gravity, ρ is the density of the pumped liquid; H p1 is the outlet head of the first-stage impeller, H p2 is the potential head at the secondary impeller outlet, and its calculation formula is as follows:

[0025]

[0026] Furthermore, the directions of the dynamic reaction forces T1 and T2 of the fluid acting on the first-stage impeller and the secondary impeller are away from the corresponding impeller inlet, and are calculated as follows:

[0027] T1=ρQ t1 V s1

[0028] Among them, V s1 is the flow velocity at the first-stage impeller inlet, calculated by the following formula

[0029]

[0030] Q t1 is the design flow of the first-stage impeller, A j1 -A h1 is the area of ​​the first-stage impeller inlet;

[0031] T2=ρQ t2 V s2

[0032] Among them, V s2 is the flow velocity at the secondary impeller inlet, calculated by the following formula

[0033]

[0034] Q t2 is the design flow of the secondary impeller, A j2 -A h2 is the area of ​​the secondary impeller inlet.

[0035] Furthermore, the design flow rate Q of the first-stage impeller t1 and the design flow rate Q of the secondary impeller t2 Calculated by the following formula:

[0036] Q t1 =Q t2 =Q des

[0037] Furthermore, in step 5, the axial force ΔF generated by the pressure difference between the secondary impeller and the primary impeller inlet is mainly considered to be the difference in axial force acting on the secondary impeller and the primary impeller mounting nuts. The primary impeller mounting nuts and the secondary impeller mounting nuts are designed to be consistent. ΔF is calculated by the following formula:

[0038] ΔF=F 次级螺母 -F 首级螺母 =ξ×H1×ρ×g×A 叶轮螺母

[0039] Among them, F 首级螺母 Indicates the axial force of the pumped fluid acting on the first-stage impeller mounting nut, F 次级螺母 A represents the axial force of the pumped fluid acting on the secondary impeller mounting nut. 叶轮螺母 It represents the projected area of ​​the first-stage or secondary impeller mounting nut in the plane perpendicular to the axial direction. Taking into account the losses in the first-stage volute and the intermediate flow channel, ξ is taken as 0.8. The direction of ΔF points to the first-stage impeller inlet.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The design method of this invention utilizes the pressure differential between the front and rear covers of the centrifugal pump impeller to cleverly balance the axial force directed toward the primary impeller inlet, generated by the high pressure at the secondary impeller inlet of a two-stage canned motor pump. This design eliminates the need for traditional methods such as adding back blades or creating balancing holes to balance axial forces, resulting in high efficiency and reliable operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 A basic flow chart of a design method for a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces provided by an embodiment of the present invention.

[0043] Figure 2 This is the centrifugal pump rotor structure in an embodiment of the present invention, 1-first-stage impeller, 2-first-stage thrust bearing, 3-shielded motor main shaft, 4-secondary thrust bearing, 5-secondary impeller.

[0044] Figure 3 Schematic diagram of the axial force generated by the impeller due to the asymmetry of the cover plate in an embodiment of the present invention.

[0045] Figure 4 Schematic diagram of the pressure difference generated at both ends of the shaft system in an embodiment of the present invention.

[0046] Figure 5 A centrifugal pump is designed for a method according to an embodiment of the present invention.

[0047] Figure 6 This is a comparison chart of the wear of the thrust bearing assembly of a centrifugal pump designed using the method of the embodiment of the present invention and not using the design method of the embodiment of the present invention; wherein, Figure 6 (1) is the first-stage bearing seat designed by the method of this embodiment after the centrifugal pump has been running for 30 hours, (2) is the secondary bearing seat designed by the method of this embodiment after the centrifugal pump has been running for 30 hours, (3) is the first-stage bearing seat designed by the method of this embodiment after the centrifugal pump has been running for 30 hours, and (4) is the secondary bearing seat designed by the method of this embodiment after the centrifugal pump has been running for 30 hours. DETAILED DESCRIPTION

[0048] The present invention will be described in further detail below with reference to the accompanying drawings and examples. It should be noted that the following examples are intended to facilitate understanding of the present invention and do not have any limiting effect on the present invention.

[0049] like Figure 1 As shown, the design method of a high-lift and high-speed two-stage canned motor pump capable of balancing axial forces according to an embodiment of the present invention includes the following steps:

[0050] Step 1: Clarify the basic parameters of the centrifugal pump, including the pump's design flow rate Q des , design head H and rated speed n.

[0051] The design flow rate of the two-stage canned motor pump designed in this embodiment is Q des =9.1m 3 / h, the rated speed is n=22000r / min, and the design head is H=610m.

[0052] Step 2: Determine the basic structure of a two-stage canned motor pump. The impellers are arranged back-to-back in two stages. The impeller near the inlet is the primary impeller, and the impeller near the outlet is the secondary impeller. The outlet of the primary volute is connected to the inlet of the secondary impeller. Specifically, to ensure adequate flow control, the clearance between the front and rear rings of the two impeller stages should be no greater than 0.2mm, and the ring length should be no less than 15% of the ring inner diameter. Furthermore, to minimize the pressure differential between the canned motor rotors, the canned motor shaft clearance should be no less than 0.8mm.

[0053] Figure 2 This is a schematic diagram of the centrifugal pump rotor structure of this embodiment. The primary impeller 1 and the secondary impeller 5 are located at both ends of the shielded motor main shaft 3. The two-stage impellers are arranged back to back. The primary thrust bearing 2 and the secondary thrust bearing 4 are installed in the middle of the shielded motor main shaft 3. Figure 5 As shown in the figure, the outlet of the first-stage volute is connected to the inlet of the secondary impeller. The single-side clearance of the front and rear rings of the two-stage impeller is 0.15mm, the ring length is 6.5mm, and the shielding clearance of the motor main shaft is 1mm.

[0054] Step 3: Allocate the two-stage head, where the first-stage impeller outlet head H1 and the secondary impeller outlet head H2 satisfy H1 < H2, and H1 = (0.2-0.6) H, H2 = (0.35-0.85) H. In this embodiment, the first-stage impeller outlet head H1 = 300 m, and the secondary head is recorded as H2 = 440 m.

[0055] Step 4: Determine the impeller parameters and optimize the impeller blade shape through finite element simulation. The impeller parameters include the first-stage impeller outer diameter 2×R o1 , the inner diameter of the first stage impeller ring is 2×R m1 , the first stage impeller inlet diameter 2×R j1 , the first stage impeller hub diameter is 2×R h1 , secondary impeller outer diameter 2×R o2 , inner diameter of secondary impeller ring 2×R m2 , secondary impeller inlet diameter 2×R j2 , secondary impeller hub diameter 2×R h2 ; and R h1 =R h2 .

[0056] like Figure 3 As shown, in this embodiment, the outer diameter of the first-stage impeller is 2×R o1 =0.07m, inner diameter of mouth ring 2×Rm1 =0.0352m, inlet diameter 2×R j1 =0.03m, hub diameter 2×R h1 =0.018m; outer diameter of secondary impeller 2×R o2 =0.072m, inner diameter of mouth ring 2×R m2 =0.0352m, inlet diameter 2×R j2 =0.03m, hub diameter 2×R h2 =0.018m. After the basic size is determined, the blade shape is optimized according to the flow field through computational fluid dynamics (CFD) simulation.

[0057] Step 5: Calculate the axial forces F1 and F2 generated by the asymmetric cover plates of the first-stage impeller and the secondary impeller respectively; calculate the dynamic reaction forces T1 and T2 of the fluid on the first-stage impeller and the secondary impeller respectively; calculate the axial force ΔF generated by the pressure difference between the secondary impeller and the first-stage impeller.

[0058] In this embodiment, Figure 3 As shown, calculate the axial force generated by the impeller due to the asymmetry of the cover:

[0059] The axial force F1 generated by the asymmetric cover of the first-stage impeller is:

[0060]

[0061] The direction is toward the first-stage impeller inlet.

[0062] The axial force F2 generated by the asymmetric cover of the secondary impeller is:

[0063]

[0064] The direction is toward the secondary impeller inlet.

[0065] Calculate the fluid dynamic reaction force on the impeller:

[0066] First stage impeller inlet flow rate

[0067] Dynamic reaction force T1 = ρQ t1 V s1 =998.2×9.1÷3600×5.58=14.1N

[0068] Secondary impeller inlet flow rate

[0069] Dynamic reaction force T2 = ρQ t2 V s2 =998.2×9.1÷3600×5.58=14.1N;

[0070] Direction away from the corresponding impeller inlet.

[0071] The axial force ΔF generated by the pressure difference between the secondary impeller and the first-stage impeller is mainly considered to be the difference in axial force acting on the secondary impeller and the first-stage impeller mounting nuts. The first-stage impeller mounting nuts and the secondary impeller mounting nuts are designed to be consistent, such as Figure 4 ΔF is calculated using the following formula:

[0072] ΔF=F 次级螺母 -F 首级螺母 =ξ×H1×ρ×g×A 叶轮螺母

[0073] =0.8×300×998.2×9.81×π×0.009 2 =598.04N

[0074] The direction points to the first-stage impeller inlet.

[0075] Step 6: When the pump to be designed is a horizontal pump, calculate the axial resultant force on the rotor shaft system according to the following formula:

[0076] F 合 =F2-F1-T2+T1-ΔF

[0077] When the pump to be designed is a vertical pump, the axial resultant force on the rotor shaft system is calculated according to the following formula:

[0078] F 合 =F2-F1-T2+T1-ΔF±G

[0079] Where G is the rotor's own weight. If the first-stage impeller is at the bottom, it takes a negative sign; if the first-stage impeller is at the top, it takes a positive sign.

[0080] If the axial force is less than the axial force borne by the thrust bearing assembly, the design meets the requirements; if not, return to step 3 and adjust the lift or impeller geometric parameters of the first-stage impeller until the requirements are met.

[0081] In this embodiment, Figure 5 As shown in the figure, the designed two-stage canned motor pump is a horizontal pump. Therefore, the axial force acting on the rotor shaft system is calculated as follows:

[0082] F 合 =F2-F1-T2+T1-ΔF

[0083] =1601.13-1143.38-14.1+14.1-598.04

[0084] =-140.29N

[0085] The maximum thrust that the thrust bearing assembly selected in this embodiment can withstand is 500N, and the axial force is less than the axial force that the thrust bearing assembly can withstand, so the design meets the requirements.

[0086] Figure 6 The figure compares the wear of the thrust bearing assembly of a centrifugal pump designed using the method of an embodiment of the present invention and not using the design method of this embodiment, wherein (1) and (2) are the wear conditions of the thrust bearing assembly of the centrifugal pump designed in the specific implementation mode of the present invention after running for 30 hours, and (3) and (4) are the wear conditions of the thrust bearing assembly without using the design method of the present invention. It can be seen from the figure that the first-stage bearing seat is severely worn when not using this method.

[0087] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A design method for a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces, characterized in that: This method uses the difference in axial force generated by the asymmetry of the front and rear covers of the centrifugal impeller to balance the axial force generated by the high pressure at the secondary impeller inlet of the two-stage canned motor pump and directed toward the first-stage impeller inlet. It includes the following steps: Step 1: Determine the basic performance parameters of the two-stage canned motor pump to be designed, including the design flow rate of the pump , Design lift and rated speed ; Step 2: Determine the basic structure of the two-stage canned motor pump. The impellers are two-stage and arranged back to back. The impeller near the pump inlet is the first-stage impeller, and the impeller near the pump outlet is the secondary impeller. The outlet of the first-stage volute is connected to the inlet of the secondary impeller. Step 3: Allocate two-stage head, where the first-stage impeller outlet head and secondary impeller outlet head satisfy ,and , ; Step 4: Determine the impeller parameters and optimize the impeller blade shape through finite element simulation; Step 5: Calculate the axial forces generated by the asymmetric cover plates on the first and secondary impellers respectively and ; Calculate the dynamic reaction force of the fluid on the first-stage impeller and the secondary impeller respectively and ; Calculate the axial force due to the pressure difference between the secondary and primary impeller inlets ; Step 6: When the pump to be designed is a horizontal pump, calculate the axial resultant force on the rotor shaft system according to the following formula: ； When the pump to be designed is a vertical pump, the axial resultant force on the rotor shaft system is calculated according to the following formula: ； in, is the rotor's own weight. If the first-stage impeller is at the bottom, the sign is negative; if the first-stage impeller is at the top, the sign is positive. If the axial force is less than the axial force borne by the thrust bearing assembly, the design meets the requirements; if not, return to step three and adjust the two-stage head distribution and impeller geometric parameters until the requirements are met.

2. The design method of a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces according to claim 1 is characterized in that: In order to fully reflect the throttling effect of the mouth ring, the single-sided gap between the front and rear mouth rings of the two-stage impeller shall not be greater than 0.2mm, and the mouth ring length shall not be less than 15% of the inner diameter of the mouth ring.

3. The design method of a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces according to claim 1 is characterized in that: In order to ensure that the pressure difference between the shielded motor rotors is small, the shielded motor main shaft gap is not less than 0.8mm.

4. The design method of a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces according to claim 1 is characterized in that: In step 4, the impeller parameters include the outer diameter of the first stage impeller , inner diameter of first stage impeller ring , first stage impeller inlet diameter , first stage impeller hub diameter , secondary impeller outer diameter , inner diameter of secondary impeller ring , secondary impeller inlet diameter , secondary impeller hub diameter ;and .

5. The design method of a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces according to claim 4 is characterized in that: In the step five, The magnitude is the force on the first stage impeller rear cover Subtract the force on the front cover , The direction is toward the first-stage impeller inlet; The magnitude is the force acting on the secondary impeller rear cover Subtract the force on the front cover , The direction is toward the secondary impeller inlet, and the calculation formula is as follows: ； ； in, is the local gravitational acceleration, is the density of the pumped liquid; is the outlet head of the first-stage impeller, is the potential head at the secondary impeller outlet, and its calculation formula is as follows: ； 。 6. The design method of a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces according to claim 1 is characterized in that: The first-stage impeller and the secondary impeller are subjected to the dynamic reaction force of the fluid and The direction is away from the corresponding impeller inlet and is calculated as follows: ； in, is the density of the pumped liquid; is the flow velocity at the first-stage impeller inlet, calculated by the following formula ； is the design flow of the first-stage impeller, is the area of ​​the first-stage impeller inlet; ； in, is the flow velocity at the secondary impeller inlet, calculated by the following formula ； is the design flow rate of the secondary impeller, is the area of ​​the secondary impeller inlet.

7. The design method of a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces according to claim 6, characterized in that: Design flow of first-stage impeller and the design flow of the secondary impeller Calculated by the following formula: = = 。 8. The design method of a high-lift, high-speed, two-stage canned motor pump capable of balancing axial forces according to claim 1 is characterized in that: In step 5, the axial force generated by the pressure difference between the secondary impeller and the primary impeller is calculated. The main consideration is the difference in axial force acting on the secondary impeller and the primary impeller mounting nuts. The primary impeller mounting nuts and the secondary impeller mounting nuts have the same structural design. Calculated by the following formula: ； in, Indicates the axial force of the pumped fluid acting on the first-stage impeller mounting nut. It represents the axial force of the pumped fluid acting on the secondary impeller mounting nut. It indicates the projected area of ​​the first or secondary impeller mounting nut in the plane perpendicular to the axial direction; taking into account the loss of the first volute and the intermediate flow channel, Pick ; The direction points to the first-stage impeller inlet.

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