Antiseismic response research of horizontal residual heat removal pump in different seismic spectrum input directions

3.1. Seismic excitation parameters and characteristics

Integrity and operability under seismic load according to the Règles de Conception et de Construction des Matériels Mècaniques des Ilots Nucléaires PWR or, in English, “Design and Construction Rules for the Mechanical Components of PWR Nuclear Islands” (RCC-M 2000) standard [16] are assessed by determining whether the deformation of the rotor surface is larger than the clearance between the rotor and the stator. The SSE seismic load is applied to the bottom surface of the base, which is attached to the ground. Safe Shutdown Earthquake is a jargon of American Nuclear Control Association, specifically referring to the earthquake with maximum ground shock which could still make sure the nuclear reactor safely shutdown bringing no danger to the public. It is the maximum possible earthquake confirmed by analyzing the geological conditions, seismic conditions and the material properties under the ground of the area where the nuclear power plant located. And in this case, the important structure, system and equipment for safety concern should also maintain the following functions: 1) the capability to maintain the integrity of the pressure boundary of the reactor coolant and 2) the capability to shut down the reactor and keep it in the state of safe shutdown. To estimate the maximum ground acceleration of safe shutdown earthquake (SSE) for nuclear power plant site is a most important work of the structural engineer, seismologist and geologist. The response spectrum method of modal decomposition is used to obtain the strength of the seismic response for each structural vibration mode; the intensity is regarded as the maximum projected value of the seismic effect on the vibration mode. Next, the value of the seismic force is obtained after combining these values of seismic response. Before using this method, the input direction of the seismic force should be assumed. The seismic effect varies with the spectrum input direction, because the seismic projection on the vibration mode changes with the directions of seismic input. In addition, the residual heat removal pump is placed on the floor of the reactor building and the spectra of three orthogonal accelerations are obtained after the SSE response spectrum is discretized. The maximum seismic response is computed based on the seismic input acceleration. The square root of the sum of the squares (SRSS) is used, which is based on probability, statistics of a random event, and complete independence between participating dates without a coupling relationship. The accelerations of the horizontal and vertical walls as presented in Fig. 2 were imposed on the integral structure. The acceleration spectrum is obtained from the response spectrum of the floor with the residual heat removal pump. Response spectrum analysis and time-history analysis both reflect the influence of seismic dynamic characteristics and structural dynamic characteristics on structural seismic response. But the calculation of the response spectrum analysis is with higher convenience and less calculation, which can calculate the maximum value of the structural seismic response efficiently. So this paper analyzed the seismic response of residual heat removal pump in nuclear power plant by response spectrum analysis method. The accelerations of the horizontal and vertical walls can be considered to be independent. The SRSS was used to analyze the maximum deformation in the section perpendicular to the shaft. As shown in Fig. 5(a), the clearance between the impeller and the diffuser should be guaranteed to ensure that there is no rub or disturbance between the rotor and the stator as required by RCC-M 2000.

Fig. 2a) Acceleration spectrum of horizontal floor, b) Acceleration spectrum of vertical floor

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The theory of seismic response spectrum takes account of the dynamic relations between the structural dynamic characteristics and the seismic dynamic characteristics, and remains the original static theoretical format. The maximum seismic base shear of the structure is as follows:

where the $\beta \left(T\right)$ is the ratio of the acceleration response spectrum $S_a\text{(}T\text{)}$ and the maximum acceleration of ground motion $a$, that is $\beta \left(T\right)=S_a\text{(}T\text{)/}a$, which means the magnification of acceleration of the structure.

3.2. Computational setup

The solid model of residual heat removal pump is created in Pro/Engineer and the CAD model of the complete structure is imported into ANSYS Workbench 14.5 preprocessing environment to create the finite element model required in the analyses.

Finite element model required in FE analysis is created by discretizing the geometric model shown in Fig. 1 into smaller and simpler elements. The discretization is performed in ANSYS environment. The three-dimensional solid model assembly of residual heat removal pump is transferred to ANSYS Workbench by direct interface. ANSYS Workbench automatically recognizes the contact conditions for corresponding contact surfaces. Completely bonded contact type is chosen as contact condition for contact surface.

Complete model consists of around 1891637 elements and 3127918 nodes. Fine mesh is applied to the implant models. The finite element model of the stator, rotor, and base and bearing box are shown in Fig. 3.

Fig. 3a) Finite element model of rotor, b) Finite element model of stator, c) Finite element model of base and bearing box

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A multiseismic spectrum, multiinput direction method is applied to analyze the dynamic characteristics and seismic response. The seismic excitation direction of the earthquake spectrum is shown in Fig. 4. The seismic spectra in the horizontal direction ($x$) and vertical direction ($y$) are input every 15 degrees and the total deformation response is obtained by the SRSS method. The vertical accelerations of seismic force have two directions, but only the downward vertical acceleration is computed for the consideration of conservation, because the self-weight is vertically propelled downward. The surface between the stationary component and the rotating component consists of the impeller surface, fitted with a wear ring, and the outlet surface of the impeller; therefore, the influence of their deformation is crucial to the use of the residual heat removal pump. The nuclear pump will leak if the deformation of the impeller surface fitted with a wear ring and the outlet surface of the impeller are larger than the nuclear RCC-M 2000 standard. Some monitoring points are set on the impeller surface fitted with a wear ring and the outlet surface of the impeller to study the deformation responses.

Fig. 4Seismic excitation direction of the earthquake spectrum

The essence of antiseismic analysis is to evaluate the operability of the pump structure by judging the existence of rubs or disturbances between the rotor and stator components. If there are rubs and disturbances, the pump structure will be damaged and at risk for accidents such as nuclear leakage. Therefore, the deformation of the rotor and stator components is calculated in ANSYS Workbench 14.5. The monitoring points on the impeller surface fitted with a wear ring, the pressure and suction sides of the blade, and the outlet surface of the impeller are shown in Fig. 5. Points 1-4 are distributed on the impeller surface fitted with a wear ring, points 5-12 are set every 45 degrees on the surfaces of the shroud and the hub, and points 13-16 are set on the pressure and suction sides of the blade.

Fig. 5a) Monitoring points on the surface corresponding with the wear ring, b) monitoring points on the outlet surface of the impeller, c) monitoring points on the pressure and suction sides of the impeller

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The deformation factor is defined as:

where $D_r$ is the deformation factor, $D$ is the deformation at each seismic input direction, and $\stackrel{-}{D}$ is the mean deformation based on all calculated deformation values.

4.1. Deformation of impeller surface fitted with wear ring

Fig. 6 shows the deformation response to different input directions under SSE seismic load. The response value varies in different directions, because the vibration frequency changes when the input direction changes. Although the vibration frequency is the same in different input directions, the deformation response is different because the pump structure is asymmetrical. As also shown in Fig. 6, the deformation of the impeller surface fitted with a wear ring in the section perpendicular to the shaft shows multiple extreme values. Furthermore, point 1 has the largest deformation, which is larger than that of point 2. This indicates that the deformation decreases along the direction of flow in different seismic spectrum input directions. In addition, the deformation varies greatly with the direction of the seismic spectrum input; therefore, the seismic response characteristic of the horizontal residual heat removal pump changes greatly for different directions of seismic spectrum input. Compared with previous work [13], which focuses on the characteristic of vertical pumps, and in which the seismic response characteristic displays a smooth curve, in this research, the model is horizontal and the seismic response characteristic displays some extreme deformation values. As a whole, this result indicates that the influence of the horizontal asymmetrical structure is different from that of the vertical asymmetrical structure.

Fig. 6Deformation of monitoring points on the impeller surface fitted with a wear ring

4.2. Deformation of impeller outlet surface

Fig. 7 shows the deformation of the impeller outlet surface. Points 5 and 9 are located on the outlet surface of the impeller on the shroud, while points 6 and 10 are located on the outlet surface of the impeller on the hub. As also shown in Fig. 7 the deformation of points 5 and 9 is better than that of points 6 and 10, which indicates that the deformation of the shroud is larger than that of the hub. The impeller edge connected with the shroud is larger than that of the impeller edge connected with the hub. The largest deformation occurs at an angle of approximately 135 degrees, which indicates that the antiseismic analysis should be conducted at this input angle to conservatively evaluate the antiseismic performance. Furthermore, the analysis of seismic effect on the integral pump structure in multiple input directions cannot be neglected when making an accurate prediction of the antiseismic performance of the pump, because different magnitudes of seismic effect can be found in multispectrum input directions. Thus, the antiseismic analysis procedure for a residual heat removal pump will not be accurate when using an FE tool if the multispectrum input directions are not taken into account.

Fig. 7Deformation of monitoring points on the impeller outlet surface

4.3. Deformation of pressure and suction sides of blade

Fig. 8 shows that the trends of the response curves of the blades to different seismic input directions are similar, but the deformation of the impeller edge connected with the shroud is larger than that connected with the hub. The deformation of the pressure and suction sides of the impeller is basically the same; however, the difference between the pressure and suction sides of the impeller is large, in the ranges of 45-60 degrees and 135-165 degrees, respectively. Furthermore, on the shroud, the deformation of the pressure side is larger than that of the suction side of the impeller; on the hub, the deformation of the suction side is larger than that of the pressure side of the impeller. Altogether, the results demonstrate that multiple extreme deformations of the impeller surface fitted with a wear ring, the pressure and suction sides of the blade, and the outlet surface of the impeller occur in the section perpendicular to the shaft. The largest deformation occurs at an angle of approximately 135 degrees for all components, which suggests that the antiseismic analysis should be conducted at this input angle to conservatively evaluate the antiseismic performance. In addition, the installation angle designed for frequent earthquakes should avoid 135 degrees to reduce the deformation effect caused by the seismic force. These results indicate that it is necessary to apply the response spectrum method in multiseismic spectrum input directions during the antiseismic analysis process to estimate the maximum seismic effect in residual heat removal pumps. Furthermore, this can provide a calculation method to determine the input direction with the maximum seismic effect to optimize the design and installation, to increase the antiseismic performance, and to increase the reliability of the pump.

Fig. 8Deformation of monitoring points on the pressure and suction sides of the blade

4.4. Maximum deformation of impeller outlet surface and impeller surface fitted with wear ring

Fig. 9 shows the largest deformation of the impeller surface fitted with the wear ring and the outlet surface of the impeller in different seismic input directions. As shown, the tendency of the response curves to the seismic force is the same. Furthermore, the largest deformation of the impeller surface fitted with the wear ring is 0.0088 mm, which is much smaller than the real clearance between the impeller and the wear ring. The largest deformation of the impeller outlet surface is 0.0079 mm, which is also much smaller than the real clearance between the impeller and the diffuser. Therefore, there is no rubbing or disturbance between the rotating component and the stationary component. These effects should be discerned when the pump structure is being designed and optimized. Compared with previous analytical methods for determining the seismic response in pumps, the result of clearance between the rotor and the stator can instantly and easily assess the integrity and operability of residual heat removal pumps under seismic load.

Fig. 9a) Maximum deformation of the impeller surface fitted with wear ring, b) maximum deformation of the impeller outlet surface

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5.1. Pump model

The residual heat removal pump is a single-stage, single-suction, horizontal-orientated low specific speed centrifugal pump. The design parameters of the pump are given in Table 2. The design specific speed of this pump is 104, calculated by Eq. (2):

5.2. Experimental protocol and experimental setup

Experimental system is composed of two parts, namely, the water circulation system and the data acquisition system. The water circulation system supplies the necessary environment for centrifugal pump operation, whereas the data acquisition system changes all kinds of physical quantities at different conditions to the corresponding electrical signals by using sensors. The final data are directly discernible after processing. The installation diagram of the test system is shown in Fig. 10. The pump, together with the motor, was fixed on a solid base. The inlet gate valve was kept open during the measurement and the outlet gate was used to regulate the flow. The experimental condition is shown in Table 3. The accuracy class could reach the first grade accuracy.

Fig. 10. Installation diagram of the test system