The Impact and analysis of nuclear power plant’s steam extraction and heating modification on the turbine

A certain nuclear power unit uses the circulating water heated by the steam extraction from the high-pressure cylinder exhaust pipe of the steam turbine through modification, and the pressurized heating circulating water is supplied to the outside of the plant. As it is the first large-scale model of steam extraction heating in domestic nuclear power, the heating will have a certain impact on the steam turbine. Therefore, it is necessary to analyze the impact of large-scale steam extraction on the steam turbine and propose corresponding countermeasures to provide relevant references for subsequent nuclear power extraction and heating modification.


Introduction to the turbine system
The nuclear power turbine consists of one high-pressure cylinder and three low-pressure cylinders, with two high-pressure main steam regulating valves arranged on both sides of the high-pressure cylinder.The two main steam pipes from the nuclear island are divided into four main steam pipes after passing through the main steam header, which are respectively connected to the inlet of the four main steam valves (MSV).The steam enters the main control valve (GV) through the main steam valve, and the main steam coming out of the GV enters the high-pressure cylinder from the upper and lower halves of the middle of the high-pressure cylinder.The high-pressure cylinder is arranged in a symmetrical double-split configuration, and the steam powered by the high-pressure cylinder is discharged through three exhaust ports at the regulating end and three exhaust ports at the electrical end.It enters the lower three steam inlets of the steam water separation reheater (MSR) shell arranged on both sides of the turbine through six steam pipes.After passing through the separator in the MSR, approximately 97.8% of the moisture is separated.The separated steam is heated upwards through the first and second-stage reheaters of the MSR to become dry superheated steam.The reheated steam enters the low-pressure cylinder through the low-pressure inlet pipe via the reheat steam valve (RSV) and reheat control valve (ICV).
The structural diagram of the steam turbine is shown in Figure 1.

Introduction to the heating system (VQS) modification
The heating system uses extraction steam (cold section) from the exhaust pipe of the high-pressure cylinder of the steam turbine to heat the circulating water of the heating network, and the pressurized and heated circulating water of the heating network is supplied to the secondary heating station outside the plant [2].The heating system includes a steam side system for releasing heat and a circulating waterside system for absorbing heat, specifically including a heating steam system for the heating network, a drainage system for the heating network heater, and a circulating water system for the heating network [3]. Figure 2 shows the heating process diagram.

Heating steam system for heating network
The function of the heating steam system of the heating network is to send the heating steam extracted from the turbine to the heating network heater for heating the circulating water of the heating network during the heating period.The heating steam system mainly consists of steam water heat exchange heaters, pipelines, valves, and instruments.Heating steam is led out from the exhaust pipe of the high-pressure cylinder of the steam turbine and supplied to four parallel rows of heating network heaters [4].Two half-capacity pipelines for heating and steam extraction are equipped with electric isolation valves and manual isolation valves near the steam extraction port of the turbine, for isolating heating steam during nonheating seasons; Two heating and extraction half-capacity pipelines are also equipped with one pneumatic extraction check valve each.Two half-capacity heating and steam extraction pipelines are divided into two 1/4 branch pipes, and a set of extraction hydraulic control butterfly valves, electric control butterfly valves, and electric shut-off butterfly valves are installed on each of the four branch pipes to prevent water from entering the turbine.They can also isolate the heating and steam extraction during nonheating seasons or when the heater needs to be isolated, and are used for regulating the amount of heating steam.

Drainage system of heating network heater
The function of the drainage system of the heating network heater is to recover the condensed water released in the heating network heater into the secondary circuit.The drainage system of the heating network heater ensures that the amount of heating steam supplied by the unit is consistent with the amount of drainage from the heating network heater to the regeneration system, thereby maintaining the balance of the working fluid [5].

Heating network circulating water system
The function of the heating network circulating water system is to pressurize the circulating water of the heating network through the heating network circulating water pump, send it to the heating network heater for heating, and then supply it to the secondary heat exchange station.The heating network circulating water system mainly includes heating network circulating water filters, heating network circulating water pumps, heating network circulating water pipelines, valves, instruments, etc.

Impact and analysis of the steam extraction heating modification on low-pressure cylinder
Because the heated steam mainly comes from the exhaust of the high-pressure cylinder, the heating modification has little impact on the high-pressure cylinder.This article focuses on analyzing the impact of the extraction heating modification on the low-pressure cylinder of the steam turbine and corresponding countermeasures.

Analysis of steam turbulence at the inlet of the low-pressure cylinder
Under heating conditions, the ICV changes from full opening of pure condensation to partial opening.When the ICV is at the partial opening, steam will experience turbulence.The excitation force is used as a characteristic parameter for steam turbulence analysis to compare and analyze the steam turbulence at the inlet of the low-pressure cylinder before and after heating.The random vibration excitation force caused by steam turbulence is directly proportional to the dynamic pressure.By calculating and comparing the average dynamic pressure of each heating condition, it is determined that the operating condition with 100% thermal power of the reactor and a heating steam flow rate of 1, 500 t/h is the maximum excitation force during heating, that is, the maximum steam turbulence operating condition, as shown in Table 1.We use CFD software to simulate the excitation force of the hot section pipeline between the outlet of the steam water separation heater and the inlet of the low-pressure cylinder (Figure 3) and compare the excitation force of the pipeline after the ICV valve heating modification, as shown in Figure 4 and Figure 5.    From Figure 5, after the heating modification, due to the partial opening of the ICV, the throttling of the ICV valve will cause steam turbulence.At the first elbow (Elbow 1 and Elbow 2 in Figure 4) immediately after the ICV valve, the fluid excitation force after heating is much greater than before, mainly because the elbow is close to the ICV valve and is greatly affected by the flow turbulence caused by the throttling of the ICV valve.Along the steam pipeline behind the ICV valve, when steam enters the pipe section (Pipe 1) where the elbow converges, the steam turbulence caused by ICV throttling is alleviated.At the inlet of the low-pressure cylinder (Pipe 3) and the inlet of the low-pressure cylinder (Inlet 1), the fluid excitation force is the same before and after the heating modification, and there is no turbulence effect caused by partial opening throttling of the ICV.Therefore, after the heating modification, although the ICV is in a partially opened position, the turbulence caused by it is reduced.It will not affect the inlet of the low-pressure cylinder, nor will it affect the flow and bearings of the low-pressure cylinder.In addition, there is a circumferential imbalance at the inlet of the low-pressure cylinder under the original pure condensation condition, which also exists under the heating condition.However, no significant effect of ICV valve throttling was observed.The nondimensional dynamic pressure at the inlet of the low-pressure cylinder for simulation analysis is detailed in Figures 6 and 7.

Analysis of differential expansion and bearing vibration effects
3.2.1 Analysis of the impact of differential expansion.The analysis of differential expansion was based on pure condensation conditions, compared with the operating conditions of 100% reactor thermal power and heating steam flow rate of 1, 500 t/h, and estimated based on the measured differential expansion of the unit.In the heating condition, the theoretical calculation of the expansion difference at the regulating end of the steam turbine deviates downwards compared to the pure condensing condition, with a maximum deviation of about 0.5 mm.The theoretically calculated differential expansion curve is shown in Figure 8 (dashed line).Figure 8 shows the differential expansion curve of the turbine control end.In the heating condition, the theoretically calculated expansion difference of the steam turbine's electrical end is also offset downwards compared to the pure condensing condition, with a maximum offset of about 1 mm.The expansion curve of theoretical analysis is shown in Figure 9 (dashed line).
Based on the measured expansion difference curve of the unit, an estimate was made for the expansion difference between the regulating end and the electrical end of the steam turbine.The estimated results are shown in Figure 10    Based on the theoretical calculations and actual measurements, it can be inferred that: (1) Theoretical analysis shows that the differential expansion curve under heating conditions deviates downwards compared to pure condensation conditions.The maximum deviation value of the adjustment end is 0.5 mm, and the maximum deviation value of the electrical end is 1 mm, both of which are far away from the expansion differential alarm value.
(2) The expansion difference at the adjustment end is about 6.2 mm in the pure condensation condition and about 5.7 mm in the heating condition.It is offset downwards by 0.5 mm from the pure condensation condition and away from the expansion/contraction expansion difference alarm value (12.5 mm/-4.5 mm).The differential expansion of the electric end under pure condensation conditions is about 17.2 mm.Under heating conditions, the differential expansion of the electric end is about 16.2 mm, with a downward deviation of about 1 mm from pure condensation, which is far from the alarm value of expansion/contraction differential expansion (21.5 mm/-1 mm).
(3) We estimate the difference in expansion between the heating condition adjustment end and the electrical end and keep away from the alarm value for expansion/contraction expansion difference.In summary, the differential expansion curve under heating conditions is offset downwards compared to pure condensation conditions, with a maximum offset value of 1 mm.The theoretical and measured estimated values of the expansion difference between the regulating end and the electrical end are far from the expansion/contraction alarm value, and there is no problem with the expansion difference under steady-state heating conditions.

Analysis of the impact of axial vibration
The analysis condition of bearing vibration is 100% reactor thermal power with a heating steam flow rate of 1, 500 t/h and a rotor speed of 1, 500 rpm.
(1) Lateral vibration evaluation  The rotor imbalance caused by thermal bending remains unchanged. The dynamic stiffness remains unchanged after heating input. There is no change in the bearing characteristics after heating input. The temperature of the bearing pads affected by the alignment does not change after heating input.
 The axial force is generated by the expansion difference after the heating input. There is no problem with the clearance, and there is no change in the radial clearance.
(2) Torsional vibration assessment  There is no change in external forces caused by electrical interference after heating input. After the heating is put into operation, the steady-state shaft torque decreases due to the decrease in load on the low-pressure cylinder.
 There is no change in the natural frequency of torsional vibration of the blade shaft system after heating input.

Analysis of the impact on the last stage blades of the low-pressure cylinder
Before the heating project is put into operation, it is not possible to qualitatively analyze the water erosion status of the last stage moving blades of the low-pressure cylinder through actual operation and cylinder opening inspection [1].Only by calculating the water erosion index of the blades through empirical formulas and comparing it with the benchmark water erosion index, can the water erosion status of the blades be determined.The formula for calculating the blade water erosion index based on their experience is: where H B is the blade hardness, W w is the relative velocity of hydrophobic impact on moving blades in wet steam, C s is the absolute velocity of wet steam, G is the mass flow rate of steam, M is the steam humidity, and β W is the relative inlet angle of the last stage moving blade for drainage [6].The relative relationship between the velocities in the formula is shown in Figure 12, where U is the blade speed, C w is the absolute velocity of hydrophobic motion in wet steam, and W S is the relative velocity of steam [7].The water erosion index calculated according to the formula for water erosion index in the previous text under various working conditions is shown in Table 2.
Table 2 According to the formula, the increase in blade water erosion index under heating conditions is mainly caused by the decrease in hydrophobic relative inlet angle β w and the increase in relative velocity W w of impact on the final stage moving blades.As can be seen from the velocity triangle of the drainage in steam in Figure 13, during the heating operation, the steam flow rate passing through the last stage of moving blades decreases.Therefore, the absolute velocity C w of the impact of the drainage on the moving blades in wet steam decreases, while the turbine speed U remains unchanged.Therefore, the relative velocity W w of the impact of the drainage on the moving blades correspondingly increases, and the relative inlet angle of the drainage increases [8].The corresponding decrease in w leads to an increase in the water erosion index.The decrease in β w means that there is more hydrophobic impact on the last stage moving blade area of the same area size, resulting in more severe water erosion [9].

Conclusion
(1) Although the ICV is partially open after the heating transformation, the turbulence it causes will not affect the inlet of the low-pressure cylinder.This will not affect the flow and bearings of the low-pressure cylinder.
(2) There is no problem with the expansion difference under steady-state heating conditions.
(3) When the power of the unit is the same, water erosion intensifies with the increase of heating extraction volume.
(4) Compared to the operating conditions without heating at rated load, heating is different.There is a risk of intensified water erosion on the last stage moving blades of the low-pressure cylinder under operating conditions.Therefore, during the first major overhaul period after the heating operation, it is necessary to conduct major inspections and necessary maintenance on the last stage blades of the low-pressure cylinder.It is recommended to conduct relevant inspections on the last stage blades during the first major overhaul after the heating operation.The inspection locations are as follows: Table 3. Inspection parts of last stage blades of low-pressure cylinder Table 3.

No. Checkpoint Inspection & maintenance guideline 1 Blade tip section
Blade replacement is recommended when the erosion of the material on the tip side above the satellite has progressed and exceeded 6.4 mm past the trailing edge of the satellite piece.

Stellite portion
Blade replacement is recommended when the erosion has progressed to 3.2 mm or less from the trailing edge of the Stellite piece. 3

Stellite lower edge
Blade replacement is recommended when the basic material at the lower edge of the Stellite alloy is eroded to 3.2 mm or less from the tail edge of the alloy because of water erosion.

Erosion depth
Blade replacement is recommended when the erosion depth of the blade base material has exceeded 1/2 of the blade thickness.

Figure 5 .
Figure 5. Schematic diagram of fluid force RMS coefficient for 100% reactor, 1, 500 t/h thermal power, and pure condensing rated conditions.

Figure 6 .
Figure 6.Non-dimensional dynamic pressure diagram in the inlet of the low-pressure cylinder without VQS.

Figure 7 .
Figure 7. Non-dimensional dynamic pressure diagram in the inlet of the low-pressure cylinder at maximum VQS flow rate.

Figure 8 .
Figure 8.The differential expansion curve of the turbine control end.

Figure 9 .
Figure 9. Differential expansion curve of steam turbine electric end.
Figure8shows the differential expansion curve of the turbine control end.In the heating condition, the theoretically calculated expansion difference of the steam turbine's electrical end is also offset downwards compared to the pure condensing condition, with a maximum offset of about 1 mm.The expansion curve of theoretical analysis is shown in Figure9(dashed line).Based on the measured expansion difference curve of the unit, an estimate was made for the expansion difference between the regulating end and the electrical end of the steam turbine.The estimated results are shown in Figure10and Figure11(dashed lines), respectively.

Figure 10 .
Figure 10.Prediction of differential expansion at the adjusting end of the steam turbine.

Figure 11 .
Figure 11.Prediction of differential expansion at the electric end of a steam turbine.

Figure 12 .
Figure 12.Final stage moving blade steam and drainage velocity triangle.

Table 1 .
Average dynamic pressure under different heating steam flow rates.NSSS thermal power (%)VQS extraction (t/h)

.
Calculation results of the water erosion index for the last stage of moving blades of the low-pressure cylinder.