A three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device

Abstract

The present invention specifically relates to a three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device, including a No. 1 throttle, a No. 2 throttle, a No. 3 throttle and a No. 4 throttle, wherein the No. 1 throttle and the No. 3 throttle are respectively connected to the secondary mechanical seal chamber, the No. 2 throttle and the No. 4 throttle are respectively connected to the tertiary mechanical seal chamber, and the No. 1 throttle, the No. 2 throttle, the No. 3 throttle and the No. 4 throttle distribute the pressure of the primary mechanical seal chamber, the secondary mechanical seal chamber and the tertiary mechanical seal chamber to 4:4:2. The three-stage fluid dynamic pressure nuclear main pump mechanical seal pressure dividing device of the present invention reduces the pressure borne by the third-stage seal by adjusting the pressure distribution ratio of the three-stage fluid dynamic pressure mechanical seal, so as to reduce the low-pressure leakage flow rate and alleviate the problem of high low-pressure leakage flow.

Description

Technical Field

[0001] The invention relates to the technical field of mechanical seals, and in particular to a three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device. Background Art

[0002] The nuclear power plant reactor main coolant pump (hereinafter referred to as the nuclear main pump) provides driving pressure for the nuclear power plant reactor main coolant, sends the heat generated by the nuclear power plant reactor to the evaporator, and finally sends it back to the nuclear power plant reactor for circulation by the nuclear main pump. It is one of the key equipment in the nuclear power plant.

[0003] The three-stage fluid dynamic pressure mechanical seal is the most important component of the nuclear main pump structure, which is directly related to whether the nuclear main pump can operate safely and reliably in the nuclear power plant. If the seal leaks, it will directly lead to reactor shutdown, causing huge economic losses to the nuclear power plant, and in serious cases, it will also cause environmental radiation pollution.

[0004] See also Figure 1 The structural features of the three-stage fluid dynamic pressure mechanical seal are non-contact, balanced, static, multi-spring, multiple large corrugations on the end face of the dynamic ring, and additional support rings on the inner diameter of the static ring. The first two stages of the three-stage fluid dynamic pressure mechanical seal are assembled together, and the third stage is a separate assembly. The interface size of each stage with the pump body increases step by step, and the interface size with the pump shaft decreases step by step, which makes installation more convenient and quick.

[0005] The three-stage structure of the dynamic pressure shaft seal is similar and is installed in series on the shaft. As shown in the figure, the first sleeve 44, the second sleeve 50 and the third sleeve 55 of each stage are directly sleeved on the shaft 28, and each sleeve is transmitted through the fork lug and the groove. Taking the first stage as an example, the dynamic ring seat 248.8 is sleeved on the sleeve and is positioned by the pin 248.14. The sealing dynamic ring is assembled as a whole by the inner sleeve ring 248.9, the graphite ring 248.13 and the dynamic ring outer sleeve 248.6. The sealing dynamic ring is clamped on the dynamic ring seat 248.8 by the dynamic ring clamping ring 248.7. The static ring is assembled by the static ring seat 248.1, the carbide ring 248.12 and the static ring outer sleeve 248.5, and the static ring clamping ring 248.2 presses it on the static ring seat 248.1 to form a static ring assembly. The static ring assembly is sleeved on the static ring guide ring 43. The stationary ring guide ring 43 is pressed and fixed to the sealing chamber to keep it stationary.

[0006] During operation, the dynamic ring assembly rotates relative to the static ring assembly driven by the shaft and the sleeve. Since the static ring end face is provided with a fluid dynamic pressure groove, an opening force is formed to disengage the dynamic and static ring end faces.

[0007] The primary circuit system pressure of a nuclear power plant is 15.4MPa. Under normal operating conditions of the nuclear main pump, the system pressure is reduced to atmospheric pressure through a three-stage fluid dynamic pressure mechanical seal. The three-stage fluid dynamic pressure mechanical seal has three identical sealing pairs to share the sealing pressure. Each stage can withstand the full pressure. The three-stage sealing pressure distribution is 3:3:3, that is, each stage withstands about 5-5.2MPa pressure, and the leakage rate of each stage is about 5-15L / h.

[0008] After the high-pressure shaft seal injection water with a flow rate of 1920L / h and a pressure of 15.4MPa provided by the centrifugal charging pump of the nuclear power plant RCV system enters the shaft seal system, since the pressure of the shaft seal injection water is higher than the pressure of the primary circuit system, the injection water is divided into three flows. One flow is 1115L / h to the water-lubricated radial bearing and enters the primary circuit system to prevent the reactor coolant from entering the mechanical seal cavity. The other two flows enter the secondary mechanical seal and the tertiary mechanical seal at a flow rate of 400L / h, and their pressures are 10.4MPa and 5.4MPa respectively.

[0009] This directional flow is completed by four throttles 1, 2, 3, and 4. When the pressure of the injected water drops to 10.3MPa after throttle 1, the pressure difference between the front and rear ends of the first-stage shaft seal is 5.1MPa. At this time, the leakage of the first-stage shaft seal surface is 5-15L / h, which is mainly due to the lubrication and heat dissipation of the seal surface. Before this 5-15L / h leakage flow flows to the second-stage shaft seal, the pressure has dropped to 10.3MPa. In the same principle, under the joint action of the 2nd and 3rd stage seals and throttles, the pressure of the 15.4MPa high-pressure seal injection water is also reduced to 0.2MPa~0.3MPa, and is injected into the nuclear power plant RCV system respectively, and the 5-15L / h leakage flow leaking from the third-stage shaft seal seal surface flows into the low-pressure leakage system.

[0010] Due to design reasons, the fluid dynamic pressure type nuclear main pump mechanical seal has experienced low-pressure leakage and high flow events in nearly 20 nuclear main pump mechanical seals in just 5 years since its commissioning, causing the nuclear power plant to shut down for maintenance or overhaul and main line delays, seriously affecting the nuclear safety and economic benefits of the nuclear power plant. In order to ensure the safe and stable operation of the main pump, solving this problem has become a very urgent task. Summary of the invention

[0011] Based on this, it is necessary to provide a fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device to address the problem of high low-pressure leakage flow of the existing fluid dynamic pressure type nuclear main pump mechanical seal. The pressure dividing device adjusts the pressure distribution ratio of the three-stage fluid dynamic pressure mechanical seal to reduce the pressure on the third-stage seal, thereby reducing the low-pressure leakage flow rate and alleviating the problem of high low-pressure leakage flow.

[0012] In order to achieve the above object, the present invention provides the following technical solutions:

[0013] A three-stage fluid dynamic pressure core main pump mechanical seal pressure dividing device comprises a No. 1 throttle, a No. 2 throttle, a No. 3 throttle and a No. 4 throttle, wherein the No. 1 throttle and the No. 3 throttle are respectively connected to the secondary mechanical seal chamber, and the No. 2 throttle and the No. 4 throttle are respectively connected to the tertiary mechanical seal chamber, and the characteristic is that the No. 1 throttle, the No. 2 throttle, the No. 3 throttle and the No. 4 throttle distribute the pressure of the first-stage mechanical seal chamber, the second-stage mechanical seal chamber and the third-stage mechanical seal chamber into 4:4:2.

[0014] Furthermore, the throttle includes a throttle joint and a throttle body, the throttle joint is welded at one end of the throttle body, the throttle is inserted into the injection hole of the mechanical sealing chamber, and then the throttle joint groove is positioned and clamped by a pin to achieve the connection between the throttle and the mechanical sealing chamber.

[0015] Furthermore, the throttle body is prepared by the following method: a sufficiently long pipeline is wound several times on a special tool to obtain the throttle body.

[0016] Furthermore, the pressure distribution of the first-level mechanical seal chamber, the second-level mechanical seal chamber and the third-level mechanical seal chamber is achieved by adjusting the sizes of the No. 1 throttle, the No. 2 throttle, the No. 3 throttle and the No. 4 throttle. The change to the mechanical seal of the nuclear main pump is small, but it can significantly reduce the low-pressure leakage flow rate and achieve better benefits.

[0017] Furthermore, the pressure of the three-stage mechanical sealing chamber is 3-3.1 MPa.

[0018] Furthermore, the sizes of the throttle No. 1, throttle No. 2, throttle No. 3 and throttle No. 4 are obtained through the following steps: 1. According to the pressure distribution of the first-level mechanical seal chamber, the second-level mechanical seal chamber and the third-level mechanical seal chamber being 4:4:2, and the pressure difference being 6.16MPa, 6.16MPa and 3.08MPa, the calculated sizes of the throttle No. 1, throttle No. 2, throttle No. 3 and throttle No. 4 are obtained through the pipe pressure difference model; 2. According to the requirement of 800L / h output flow rate of high-pressure shaft seal injection water, the throttle length is truncated from long to short on the basis of the calculated size, and a special tool is used for pressure calibration until the pressure gauge measurement value meets the pressure distribution of the first-level mechanical seal chamber, the second-level mechanical seal chamber and the third-level mechanical seal chamber being 4:4:2, and the pressure difference being 6.16MPa, 6.16MPa and 3.08MPa.

[0019] Furthermore, the length of the No. 1 throttle is 2.04 m, the length of the No. 2 throttle is 4.1 m, the length of the No. 3 throttle is 2.96 m, and the length of the No. 4 throttle is 0.92 m.

[0020] Furthermore, the throttle No. 1, throttle No. 2, throttle No. 3 and throttle No. 4 are all made of 304 material.

[0021] The preparation method of the above-mentioned three-stage fluid dynamic pressure core main pump mechanical seal pressure dividing device comprises the following steps:

[0022] 1. Wrap a sufficiently long pipe around the special tooling several times and weld a throttle joint at one end.

[0023] 2. According to the pipe network pressure difference model, calculate the length of each throttle when the pressure distribution of the first-level mechanical seal chamber, the second-level mechanical seal chamber and the third-level mechanical seal chamber is 4:4:2 and the pressure difference is 6.16MPa, 6.16MPa and 3.08MPa.

[0024] 3. According to the flow rate requirement of 800L / h, cut the throttle length from long to short based on the calculation results, and use special tooling to calibrate the pressure until the pressure gauge measurement value meets the pressure difference described in the previous section.

[0025] 4. Install the manufactured No. 1 throttle, No. 2 throttle, No. 3 throttle and No. 4 throttle into the original mechanical seal;

[0026] 5. Complete the pressure test of the third-level container mechanical seal.

[0027] Beneficial technical effects of the present invention:

[0028] The three-stage fluid dynamic pressure nuclear main pump mechanical seal pressure dividing device of the present invention is applied to the three-stage fluid dynamic pressure nuclear main pump mechanical seal, which can solve the problem of high low-pressure leakage flow of the three-stage fluid dynamic pressure nuclear main pump mechanical seal. The double units can reduce the discharge of about 300 tons of radioactive waste water each year, and have good social and economic benefits; in addition, because the pressure borne by the third-stage seal is reduced from 5MPa to 3MPa, the load is reduced, and the end surface pressure reduction is reduced by 40%, which is conducive to improving the reliability and service life of the third-stage seal, and is also of great significance to ensuring the safety of nuclear power plants, and has strong practicality and promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic diagram of the mechanical seal structure of the existing nuclear power plant fluid dynamic pressure type nuclear main pump;

[0030] Figure 2 This is a schematic diagram of the pressure distribution of the three-stage mechanical seal chamber;

[0031] Figure 3 It is the schematic diagram of throttle pressure division;

[0032] Figure 4 Calculate the model for the throttle network;

[0033] Figure 5Depressurization model for existing throttle pipe network;

[0034] Figure 6 The throttle pipe network pressure reduction model of the present invention;

[0035] Figure 7 It is a schematic diagram of the structure of the No. 1 throttle tube of the present invention;

[0036] Figure 8 It is a schematic diagram of the structure of the No. 2 throttle tube of the present invention;

[0037] Fig. 9 It is a schematic diagram of the structure of the No. 3 throttle tube of the present invention;

[0038] Fig.10 It is a schematic diagram of the structure of the No. 4 throttle tube of the present invention.

[0039] In the figure, 248.1, stationary ring seat; 248.2, stationary ring clamping ring; 248.3, stationary ring pin; 248.4, stationary ring spring; 248.5, stationary ring outer ring; 248.6, movable ring outer ring; 248.7, movable ring clamping ring; 248.8, movable ring seat; 248.9, inner ring; 248.10, first O-ring; 248.11, second O-ring; 248.12, carbide ring; 248.13, graphite ring; 248.14, movable ring pin; 248.15, movable ring pin; 248.16, support ring; 248.17, third O-ring; 248.18, dynamic ring spring; 28, shaft; 43, stationary ring guide ring; 44, first shaft sleeve; 50, second shaft sleeve; 55, third shaft sleeve; 57, stationary ring guide sleeve; 1, high-pressure shaft seal injection water; 2, lower guide bearing; 3, No. 1 throttle; 4, No. 2 throttle; 5, No. 3 throttle; 6, No. 4 throttle; 7, high-pressure leakage system; 8, low-pressure leakage system; 9, first-stage mechanical seal chamber; 10, second-stage mechanical seal chamber; 11, third-stage mechanical seal chamber; 12, throttle body; 13, throttle joint. DETAILED DESCRIPTION

[0040] In the description of the present invention, it is necessary to understand that the orientations or positional relationships indicated by terms such as “left end”, “right end”, “above”, “below”, “outside”, “inside”, etc. are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0041] In the prior art, in the three-stage fluid dynamic pressure type nuclear main pump mechanical seal, the No. 1 throttle 3, the No. 2 throttle 4, the No. 3 throttle 5 and the No. 4 throttle 6 distribute the pressure of the first-stage mechanical seal chamber 9, the second-stage mechanical seal chamber 10 and the third-stage mechanical seal chamber 11 to 3:3:3.

[0042] In the initial stage, each stage of the mechanical seal cavity is designed to withstand the full system pressure of 15.4MPa. However, when the nuclear main pump is operating normally, each stage of the mechanical seal cavity can only withstand 1 / 3 of the full system pressure, about 5-5.2MPa.

[0043] See also Figure 2-3 The centrifugal charging pump of the RCV system of the nuclear power plant provides high-pressure shaft seal injection water 1 with a flow rate of 1920L / h and a pressure of 15.5MPa.

[0044] After the high-pressure shaft seal injection water 1 with a flow rate of 1920L / h and a pressure of 15.5MPa enters the shaft seal system from the injection port, the high-pressure shaft seal injection water 1 with a flow rate of 1115L / h and a pressure of 15.4MPa flows downward to the lower guide bearing 2; after the high-pressure shaft seal injection water 1 with a flow rate of 400L / h and a pressure of 15.4MPa passes through the No. 1 throttle 3, the pressure drops to 10.3MPa, and then enters the secondary mechanical seal chamber 10; the high-pressure shaft seal injection water 1 with a flow rate of 400L / h and a pressure of 15.4MPa passes through the No. 2 throttle 4, the pressure drops to 5.3MPa, and then enters the tertiary mechanical seal chamber 11; the No. 3 throttle 5 is additionally connected from the secondary mechanical seal chamber 10, the pressure drops to 0.2MPa, and then enters the high-pressure leakage system 7; the No. 4 throttle 6 is additionally connected from the tertiary mechanical seal chamber 11, the pressure drops to 0.2MPa, and then enters the high-pressure leakage system 7; thereby completing the arrangement of the throttle network pipe.

[0045] The controllable leakage of each stage of mechanical seal chamber is 5-15L / h. The leakage flow from the sealing surface of the first-stage mechanical seal chamber 9 flows into the second-stage mechanical seal chamber 10, the leakage flow from the sealing surface of the second-stage mechanical seal chamber 10 flows into the third-stage mechanical seal chamber 11, and the leakage flow from the sealing surface of the third-stage mechanical seal chamber 11 flows into the low-pressure leakage system 8.

[0046] The aperture of the throttle is fixed, the inner surface roughness is certain, and the radius of gyration is the same. The inlet and outlet pressures are fixed. Therefore, the length of the throttle can be determined based on the pressure difference before and after the throttle.

[0047] Specific parameters of the No. 1 throttle 3, the No. 2 throttle 4, the No. 3 throttle 5 and the No. 4 throttle 6 are shown in Table 1.

[0048] Table 1 Performance parameters of throttle

[0049]

[0050] Since the commissioning of the existing three-stage fluid dynamic pressure type nuclear main pump mechanical seal, in just 5 years, there have been nearly 20 nuclear main pump mechanical seal low-pressure leakage events (the leakage from the three-stage mechanical seal is as high as 50L / h flowing into the low-pressure leakage system 8), causing the nuclear power plant to shut down for maintenance or overhaul main line delays, seriously affecting the nuclear safety and economic benefits of the nuclear power plant. In order to ensure the safe and stable operation of the main pump, solving this problem has become a very urgent task.

[0051] The present invention adjusts the pressure distribution of the first-level mechanical seal chamber 9, the second-level mechanical seal chamber 10 and the third-level mechanical seal chamber 11 to reduce the low-pressure leakage flow rate and alleviate the problem of high low-pressure leakage flow.

[0052] In order to achieve the above object, the present invention provides the following technical solutions:

[0053] A three-stage fluid dynamic pressure nuclear main pump mechanical seal pressure dividing device comprises a No. 1 throttle 3, a No. 2 throttle 4, a No. 3 throttle 5 and a No. 4 throttle 6, wherein the No. 1 throttle 3 and the No. 3 throttle 5 are respectively connected to the secondary mechanical seal chamber 10, and the No. 2 throttle 4 and the No. 4 throttle 6 are respectively connected to the tertiary mechanical seal chamber 11, and is characterized in that the No. 1 throttle 3, the No. 2 throttle 4, the No. 3 throttle 5 and the No. 4 throttle 6 distribute the pressure of the primary mechanical seal chamber 9, the secondary mechanical seal chamber 10 and the tertiary mechanical seal chamber 11 into 4:4:2.

[0054] Furthermore, the throttle includes a throttle joint 13 and a throttle body 12, the throttle joint 13 is welded to one end of the throttle body 12, the throttle is inserted into the injection hole of the mechanical sealing chamber, and then the throttle joint 13 is positioned and clamped in the groove by a pin to achieve the connection between the throttle and the mechanical sealing chamber.

[0055] Furthermore, the throttle body 12 is prepared by the following method: a sufficiently long pipeline is wound several times on a special tool to obtain the throttle body 12 .

[0056] Furthermore, the pressure distribution of the first-level mechanical seal chamber 9, the second-level mechanical seal chamber 10 and the third-level mechanical seal chamber 11 is achieved by adjusting the sizes of the No. 1 throttle 3, the No. 2 throttle 4, the No. 3 throttle 5 and the No. 4 throttle 6. The change to the mechanical seal of the nuclear main pump is small, but it can significantly reduce the low-pressure leakage flow rate and achieve better benefits.

[0057] Furthermore, the pressure of the three-stage mechanical sealing chamber is 3-3.1 MPa.

[0058] Furthermore, the sizes of the No. 1 throttle 3, No. 2 throttle 4, No. 3 throttle 5 and No. 4 throttle 6 are obtained through the following steps: 1. According to the pressure distribution of the first-level mechanical seal chamber 9, the second-level mechanical seal chamber 10 and the third-level mechanical seal chamber 11 being 4:4:2, and the pressure difference being 6.16MPa, 6.16MPa and 3.08MPa, the calculated sizes of the No. 1 throttle 3, No. 2 throttle 4, No. 3 throttle 5 and No. 4 throttle 6 are obtained through the pipe pressure difference model; 2. According to the requirement of 800L / h output flow rate of high-pressure shaft seal injection water 1, the throttle length is truncated from long to short on the basis of the calculated size, and a special tool is used for pressure calibration until the pressure gauge measurement value meets the pressure distribution of the first-level mechanical seal chamber 9, the second-level mechanical seal chamber 10 and the third-level mechanical seal chamber 11 being 4:4:2, and the pressure difference being 6.16MPa, 6.16MPa and 3.08MPa.

[0059] Further, the length of the first throttle 3 is 2.04m, the length of the second throttle 4 is 4.1m, the length of the third throttle 5 is 2.96m, and the length of the fourth throttle 6 is 0.92m. See Table 2 for the performance parameters of the throttle of the present invention.

[0060] Table 2 Performance parameters of the throttle of the present invention

[0061]

[0062] Furthermore, the No. 1 throttle 3, the No. 2 throttle 4, the No. 3 throttle 5 and the No. 4 throttle 6 are all made of 304 material.

[0063] A method for preparing a three-stage fluid dynamic pressure core main pump mechanical seal pressure dividing device comprises the following steps:

[0064] 1. Wrap a sufficiently long pipe around the special tool several times and weld a throttle joint 13 on one end.

[0065] 2. According to the pipe network pressure difference model, calculate the length of each throttle when the pressure distribution of the first-level mechanical seal chamber 9, the second-level mechanical seal chamber 10 and the third-level mechanical seal chamber 11 is 4:4:2 and the pressure difference is 6.16MPa, 6.16MPa and 3.08MPa.

[0066] 3. According to the flow rate requirement of 800L / h, cut the throttle length from long to short based on the calculation results, and use special tooling to calibrate the pressure until the pressure gauge measurement value meets the pressure difference described in the previous section.

[0067] 4. Install the prepared No. 1 throttle 3, No. 2 throttle 4, No. 3 throttle 5 and No. 4 throttle 6 into the original mechanical seal;

[0068] 5. Complete the pressure test of the third-level container mechanical seal.

[0069] The existing No. 1 throttle 3, No. 2 throttle 4, No. 3 throttle 5 and No. 4 throttle 6, as well as the No. 1 throttle 3, No. 2 throttle, No. 3 throttle 5 and No. 4 throttle 6 of the present invention are installed into the original mechanical seal to complete the three-stage container mechanical seal pressure test. The test results are shown in Table 3.

[0070] Table 3 Low pressure leakage flow table

[0071] Original design After Improvement Low pressure leakage flow 13.6L / h 7.9L / h Third stage sealing pressure 5MPa 3MPa

[0072] From Table 3, it can be seen that compared with the prior art, the three-stage fluid dynamic pressure nuclear main pump mechanical seal partial pressure of the present invention reduces the low-pressure leakage flow of each nuclear main pump by 5.7l / h, which is about 42%. It is roughly estimated that the discharge of two units can be reduced by about 300 tons of radioactive wastewater each year, which has good social and economic benefits. In addition, since the pressure of the third-stage seal is reduced from 5MPa to 3MPa, the load is reduced, and the end surface specific pressure is reduced by 40%, which is conducive to improving the reliability and service life of the third-stage seal.

[0073] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device, comprising a No. 1 throttle (3), a No. 2 throttle (4), a No. 3 throttle (5) and a No. 4 throttle (6), wherein the No. 1 throttle (3) and the No. 3 throttle (5) are respectively connected to the secondary mechanical seal chamber (10), and the No. 2 throttle (4) and the No. 4 throttle (6) are respectively connected to the tertiary mechanical seal chamber (11), characterized in that: The first throttle (3), the second throttle (4), the third throttle (5) and the fourth throttle (6) distribute the pressure of the first-stage mechanical seal chamber (9), the second-stage mechanical seal chamber (10) and the third-stage mechanical seal chamber (11) to 4:4:

2.

2. The three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device according to claim 1 is characterized in that: The pressure distribution of the first-stage mechanical seal chamber (9), the second-stage mechanical seal chamber (10) and the third-stage mechanical seal chamber (11) is achieved by adjusting the sizes of the first throttle (3), the second throttle (4), the third throttle (5) and the fourth throttle (6).

3. The three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device according to claim 2 is characterized in that: The sizes of the No. 1 throttle (3), the No. 2 throttle (4), the No. 3 throttle (5) and the No. 4 throttle (6) are obtained by the following steps: (1) According to the pressure distribution of the first-stage mechanical seal chamber (9), the second-stage mechanical seal chamber (10) and the third-stage mechanical seal chamber (11) being 4:4:2 and the pressure difference being 6.16 MPa, 6.16 MPa and 3.08 MPa, the calculated dimensions of the No. 1 throttle (3), the No. 2 throttle (4), the No. 3 throttle (5) and the No. 4 throttle (6) are obtained through the pipe pressure difference model; (2) According to the requirement of 800L / h output flow rate of high-pressure shaft seal injection water (1), the throttle length is cut from long to short based on the calculated size, and pressure calibration is performed using a special tool until the pressure gauge measurement value satisfies the pressure distribution of the first-stage mechanical seal chamber (9), the second-stage mechanical seal chamber (10) and the third-stage mechanical seal chamber (11) of 4:4:2, and the pressure difference is 6.16MPa, 6.16MPa and 3.08MPa.

4. The three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device according to claim 3 is characterized in that: The length of the No. 1 throttle (3) is 2.04 m, the length of the No. 2 throttle (4) is 4.1 m, the length of the No. 3 throttle (5) is 2.96 m, and the length of the No. 4 throttle (6) is 0.92 m.

5. The three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device according to claim 1 is characterized in that: The throttle comprises a throttle joint (13) and a throttle body (12); the throttle joint (13) is welded to one end of the throttle body (12); the throttle is inserted into the injection hole of the mechanical sealing chamber, and then the throttle joint (13) is positioned and clamped in the groove by a pin to achieve connection between the throttle and the mechanical sealing chamber.

6. The three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device according to claim 5 is characterized in that: The throttle body (12) is prepared by the following method: a sufficiently long pipeline is wound several times on a special tool to obtain the throttle body (12).

7. The three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device according to claim 1 is characterized in that: The pressure of the third-level mechanical sealing chamber is 3-3.1 MPa.

8. The three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device according to claim 1 is characterized in that: The first throttle (3), the second throttle (4), the third throttle (5) and the fourth throttle (6) are all made of 304 material.

9. The method for preparing the three-stage fluid dynamic pressure type nuclear main pump mechanical seal pressure dividing device according to any one of claims 1 to 8, characterized in that: The steps include: (1) Wrap a sufficiently long pipe around a special tool several times and weld a throttle joint (13) at one end; (2) According to the pipe network pressure difference model, the pressure distribution of the first-stage mechanical seal chamber (9), the second-stage mechanical seal chamber (10) and the third-stage mechanical seal chamber (11) is calculated to be 4:4:2, and the length of each throttle when the pressure difference is 6.16 MPa, 6.16 MPa and 3.08 MPa; (3) According to the requirement of flow rate of 800L / h, the length of the throttle is cut from long to short based on the calculation results, and pressure calibration is carried out using a special tool until the pressure gauge measurement value meets the pressure difference described in the previous section; (4) Installing the prepared No. 1 throttle (3), No. 2 throttle (4), No. 3 throttle (5) and No. 4 throttle (6) into the original mechanical seal; (5) Complete the pressure test of the third-level container mechanical seal.

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