Method for identifying exhaust steam volume flow of low-pressure cylinder of steam turbine

Abstract

The invention relates to a turbine low-pressure cylinder exhaust steam volume flow identification method, which comprises the following steps of: calculating historical operation data of a unit by installing high-precision flow and pressure measuring points on site to obtain actual low-pressure cylinder exhaust steam mass flow and low-pressure cylinder efficiency under each working condition; the steam inlet pressure of the low-pressure cylinder can represent the steam exhaust mass flow of the low-pressure cylinder, the steam inlet pressure and the steam exhaust mass flow are in a typical linear direct proportion relation, and the influence of external factors on the steam exhaust mass flow of the low-pressure cylinder is overcome; the industrial problem that the precision of the sensor is insufficient under the small-volume flow working condition is solved, the characteristic that the low-pressure cylinder efficiency is small in the full-working-condition fluctuation range is verified through a large amount of data, the low-pressure cylinder efficiency is simplified into constant to participate in real-time calculation, the calculation burden of a complex efficiency model is avoided, and the real-time performance of an algorithm is remarkably improved; and meanwhile, a three-level safety interval of the per-unit value of the volume flow is established, fracture accidents are prevented, non-planned shutdown caused by blade damage is reduced, and the unit peak regulation income is greatly improved.

Description

Technical Field

[0001] The invention relates to a method for identifying the exhaust volume flow of a low-pressure cylinder of a steam turbine. Background Art

[0002] With the large-scale integration of renewable energy sources into the grid, coal-fired power is gradually shifting from a basic power source to a regulated power source. Deep peak shaving and frequent starts and stops have become the norm. At the same time, to meet the demand for flexibility and improve heating capacity, heating units have widely implemented "low-pressure cylinder zero (micro) output" modifications. Under deep peak shaving conditions, high-flow heating, and low-pressure cylinder switching, the low-pressure cylinder operates at a low volume flow rate. The steam flow field through the flow stage group, especially the last stage, is extremely uneven. Exhaust steam from the last blade experiences large-scale backflow along the blade height from the blade root, squeezing steam to the blade tip, creating a "blast" effect. Simultaneously, vortices form at the root of the last-stage stator blades and the tip of the rotor blades, creating a "vortex excitation" effect that significantly increases blade dynamic stress. The safety of the last blade of a steam turbine's low-pressure cylinder is directly related to the dynamic stress it withstands. Greater dynamic stress reduces the blade's safety margin and increases the risk of fracture damage. When the LP cylinder exhaust steam volumetric flow rate drops to 30% THA, the dynamic stress on the LP blade begins to increase significantly, reaching a peak in the 15%-20% THA range. When the volumetric flow rate further decreases to below 10% THA, the dynamic stress drops significantly due to the lower steam density, resulting in a "hump curve" relationship between the LP cylinder exhaust steam volumetric flow rate and the dynamic stress on the LP blade. Due to deep peak shaving and cylinder heating, incidents of LP blade breakage and cracking have occurred frequently in recent years.

[0003] From a design and manufacturing perspective, to adapt to the demands of the new situation, manufacturers generally adopt measures such as increasing root reaction, optimizing tie rod and shroud structure, and using shorter tail blades to further strengthen the strength of the low-pressure tail blades and better adapt to low-volume flow operating conditions. For example, from an operational control perspective, it is important to avoid operating the low-pressure cylinder tail blades in the dynamic stress "hump" and "blowing" zones. The low-pressure cylinder exhaust steam volumetric flow rate is the primary factor affecting the dynamic stress and "blowing" conditions of the tail blades. However, due to limited measurement capabilities, the low-pressure cylinder volumetric flow rate cannot be directly monitored. Furthermore, the low-pressure cylinder volumetric flow rate is affected by multiple boundary conditions, including unit load, low-pressure cylinder exhaust steam pressure, exhaust steam humidity, and low-pressure cylinder efficiency. Accurately calculating the volumetric flow rate is difficult, resulting in the current lack of accurate monitoring methods for the low-pressure exhaust steam volumetric flow rate in coal-fired power units. Summary of the Invention

[0004] In view of the above situation, in order to overcome the defects of the existing technology, the purpose of the present invention is to provide a method for identifying the exhaust volume flow of the low-pressure cylinder of a steam turbine, which can effectively solve the problem of real-time monitoring of the exhaust volume flow of the low-pressure cylinder according to the actual operating data of the unit.

[0005] The technical solution provided by the present invention is:

[0006] A method for identifying the exhaust volume flow rate of a low-pressure cylinder of a steam turbine is characterized by comprising the following steps:

[0007] Step 1: Collect historical operating data of the unit at 10-100% load for the past 180 days (see Appendix 1 for an example of the original operating data parameters). The data collection period is 1 minute, and data under the unit shutdown condition and the low-pressure cylinder extraction heater removal condition are excluded. Then, according to the steam turbine thermal performance calculation method provided in "GB / T 8117.1-2008 Steam Turbine Thermal Performance Acceptance Test Procedure Part 1: Method A Large Condensing Steam Turbine High Accuracy Test", calculate the steam turbine low-pressure cylinder exhaust mass flow rate and low-pressure cylinder efficiency under various operating conditions;

[0008] Appendix 1 Example of original parameter list of historical operation data:

[0009]

[0010]

[0011]

[0012]

[0013] Step 2: Based on the data calculated in step 1, with the low-pressure cylinder steam inlet pressure as the independent variable and the low-pressure cylinder exhaust steam mass flow rate as the dependent variable, a linear regression algorithm is used to perform linear fitting to obtain the relationship between the low-pressure cylinder steam inlet pressure and the low-pressure cylinder exhaust steam mass flow rate:

[0014] Q lpco =k1×p lpci +b1

[0015] Where: Q lpco is the exhaust steam mass flow rate of low pressure cylinder, t / h; p lpci Low-pressure cylinder inlet steam pressure, MPa; k1 is the slope obtained by fitting, b1 is the intercept obtained by fitting;

[0016] Step 3: Calculate the average value of the low-pressure cylinder efficiency based on the low-pressure cylinder efficiency under various working conditions calculated in step 1;

[0017] Step 4: Based on the low-pressure cylinder steam inlet pressure under the current actual working conditions, use the fitting formula obtained in step 2 to calculate the low-pressure cylinder exhaust steam mass flow rate under the current actual working conditions;

[0018] Step 5: Using the low-pressure cylinder steam inlet pressure and actual low-pressure cylinder steam inlet temperature under the current actual working conditions, and according to the characteristics of water vapor, calculate the low-pressure cylinder steam inlet enthalpy and low-pressure cylinder steam inlet entropy; then, using the low-pressure cylinder steam exhaust pressure and low-pressure cylinder steam inlet entropy, and according to the characteristics of water vapor, calculate the ideal enthalpy of the low-pressure cylinder exhaust steam, and then use the low-pressure cylinder efficiency to calculate the actual enthalpy of the low-pressure cylinder exhaust steam. The calculation formula is as follows:

[0019] h lpci =f(p lpci ,T lpci )

[0020] s lpci =f(p lpci ,T lpci )

[0021] h lpcos =f(p lpco ,s lpci )

[0022] h lpco =h lpci -(h lpci -h lpcos )×η lpco

[0023] Where: p lpci is the steam inlet pressure of the low-pressure cylinder, MPa; T lcpi is the steam inlet temperature of the low pressure cylinder, ℃; p lpco is the exhaust steam pressure of the low-pressure cylinder, MPa; s lpci is the steam entropy of the low-pressure cylinder, kJ / kg·K; h lpcos is the ideal enthalpy of low-pressure cylinder exhaust, kJ / kg; h lpco is the exhaust enthalpy of the low-pressure cylinder, kJ / kg; η lpco is the efficiency of the low-pressure cylinder, %;

[0024] Step 6: Using the low-pressure cylinder exhaust steam pressure and low-pressure cylinder exhaust steam enthalpy value, according to the characteristics of water vapor, calculate the low-pressure cylinder exhaust steam specific volume. The calculation formula is as follows:

[0025] v lpco =f(p lpco ,h lpco )

[0026] Where: v lpco is the exhaust volume of low-pressure cylinder, m 3 / kg; p lpco is the exhaust steam pressure of the low-pressure cylinder, MPa; h lpco is the exhaust enthalpy of the low-pressure cylinder, kJ / kg; Step 7: Calculate the exhaust volume flow rate of the low-pressure cylinder using the exhaust mass flow rate and the exhaust specific volume of the low-pressure cylinder. The calculation formula is as follows:

[0027]

[0028] Where: Gv lcpo is the exhaust volume flow of the low-pressure cylinder, m 3 / s;

[0029] Step 8: Based on the operating data of the THA condition, calculate the exhaust volume flow rate of the THA condition according to the above steps, and use the exhaust volume flow rate of the THA condition as the rated value to calculate the per unit value of the low-pressure cylinder exhaust volume flow rate under the actual working condition:

[0030]

[0031] Where: Gv lpco-s is the per unit value of the low-pressure cylinder exhaust volume flow rate under the current working condition, %; Gv lpco-1 is the actual value of the low-pressure cylinder exhaust volume flow rate under the current working condition, m 3 / s;Gv lpco-0 is the actual value of the exhaust volume flow of the low-pressure cylinder under THA working condition, m 3 / s;

[0032] Step 9: Set the per-unit alarm threshold of the low-pressure cylinder exhaust volume flow rate. When the per-unit value of the low-pressure cylinder exhaust volume flow rate is greater than 30%, it means that the last blade can operate safely for a long time. When the per-unit value of the low-pressure cylinder exhaust volume flow rate is in the range of 10%-30%, it means that the last blade is in the dynamic stress hump range, the stress is large, the blade operation safety margin is reduced, and there is a risk of fracture damage. This range cannot be operated for a long time, the continuous operation time shall not exceed 120 minutes, the cumulative operation time shall not exceed 800 minutes, and an alarm will be issued to the operating personnel. When the per-unit value of the low-pressure cylinder exhaust volume flow rate is lower than 10%, the last blade will experience blast overtemperature and cannot be operated for a long time. The continuous operation time shall not exceed 60 minutes, the cumulative operation time shall not exceed 800 minutes, and an alarm will be issued to the operating personnel.

[0033] The present invention installs high-precision flow and pressure measuring points on site, based on GB / T The general algorithm for thermal performance testing provided in the "8117.1-2008 Steam Turbine Thermal Performance Acceptance Test Procedure Part 1: Method A Large Condensing Steam Turbine High-Accuracy Test" calculates historical unit operating data to obtain the actual LP cylinder exhaust mass flow rate and LP cylinder efficiency under various operating conditions. Based on the Flügel formula, from 0-100% load, the LP cylinder inlet pressure can be used to represent the LP cylinder exhaust mass flow rate. The two show a typical linear positive proportional relationship, which can overcome the effects of external factors such as intermediate exhaust heating, heat reheating, LP cylinder cutting, boiler soot blowing, and auxiliary steam use on the LP cylinder exhaust mass flow rate. The LP cylinder flow efficiency varies slightly with load, within about 2%, while the exhaust specific volume is mainly affected by the exhaust pressure and is not sensitive to the exhaust enthalpy. For example, if the LP cylinder efficiency changes by 2%, the exhaust enthalpy changes by 10kJ / kg, and the exhaust specific volume changes by only 0.11m 3 / kg, the change in specific volume is only 0.4%. Compared with the existing technology, the present invention uses this linear relationship to replace direct flow measurement, solving the industry problem of insufficient sensor accuracy under small volume flow conditions. Through a large amount of data, the low-pressure cylinder efficiency has a small fluctuation range under all working conditions, which is simplified into a constant to participate in real-time calculation, avoiding the calculation burden of complex efficiency models and significantly improving the real-time performance of the algorithm. At the same time, a three-level safety interval of the per-unit volume flow value is established, namely 30%: safe operation zone; 10%-30%: hump risk zone (limited time 120min / accumulated 800min); <10%: blast overtemperature zone (limited time 60min / 800min cumulative); transform the theoretical "hump curve" into an operational operation control standard, filling an industry gap; integrate real-time operation data and historical fitting models to build a closed calculation loop, realize dynamic correction of exhaust steam specific volume, and ensure accuracy under all operating conditions; thereby improving the turbine safety risk prevention and control capabilities, accurately identifying the "hump area" of the last blade dynamic stress (10%-30% volume flow), preventing fracture accidents, reducing unplanned shutdowns due to blade damage, and greatly improving the unit's peak-shaving benefits. This is an innovation in the method of identifying the exhaust steam volume flow of the low-pressure cylinder of the turbine. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is the relationship curve between the dynamic stress of the typical low-pressure last blade and the volume flow rate. DETAILED DESCRIPTION

[0035] The specific implementation of the present invention will be further described in detail below with reference to the accompanying drawings and application examples.

[0036] Taking a 600MW unit as an example, we collected nearly 180 days of historical operating data and calculated the turbine thermal performance calculation method provided in "GB / T 8117.1-2008 Steam Turbine Thermal Performance Acceptance Test Procedure Part 1: Method A Large Condensing Steam Turbine High Accuracy Test" to obtain the turbine low-pressure cylinder exhaust mass flow rate and low-pressure cylinder efficiency.

[0037] Table 2 Original data, low-pressure cylinder exhaust mass flow rate and low-pressure cylinder efficiency value

[0038] Serial number load … Low pressure cylinder steam inlet pressure Low pressure cylinder exhaust steam mass flow Low pressure cylinder efficiency unit MW MPa t / h ％ 1 60 … 0.09 139.8359 86.32 2 100 … 0.16 206.9558 85.78 3 200 … 0.34 374.7554 86.42 4 300 … 0.52 542.555 86.04 5 400 … 0.7 710.3546 85.07 6 500 … 0.88 878.1542 85.72 7 600 … 1.06 1045.954 86.19 … … … … … … 15552000 300 … 0.51 540.423 86.30

[0039] (2) With the low-pressure cylinder steam inlet pressure as the independent variable and the low-pressure cylinder exhaust steam mass flow rate as the dependent variable, a linear regression algorithm is used for linear fitting to obtain the relationship between the low-pressure cylinder steam inlet pressure and the low-pressure cylinder exhaust steam mass flow rate.

[0040] Q lpco =932.22×p lpci +57.987

[0041] (3) According to the low-pressure cylinder efficiency under various working conditions calculated in step 1, the average low-pressure cylinder efficiency is calculated to be 85.37%.

[0042] (4) The low-pressure cylinder inlet steam pressure under the current actual 300MW operating condition is 0.513MPa. Using the fitting formula obtained in step 2, the low-pressure cylinder exhaust steam mass flow rate under the current actual operating condition is calculated to be 536.21t / h.

[0043] (5) Under the current actual 300MW working condition, the low-pressure cylinder inlet steam pressure is 0.513MP, the low-pressure cylinder inlet steam temperature is 375℃, and the low-pressure cylinder exhaust steam pressure is 6.7kPa. According to the characteristics of water vapor, the low-pressure cylinder inlet steam enthalpy is calculated to be 3219.82kJ / kg and the low-pressure cylinder inlet steam entropy is 7.70kJ / kg·K. Then, using the low-pressure cylinder exhaust steam pressure and the low-pressure cylinder inlet steam entropy, according to the characteristics of water vapor, the ideal enthalpy of the low-pressure cylinder exhaust steam is calculated to be 2387.91kJ / kg. The actual enthalpy of the low-pressure cylinder exhaust steam is calculated to be 2509.62kJ / kg using the following formula.

[0044] h lpco =h lpci -(h lpci -h lpcos )×η lpco =3219.82-(3219.82-2387.91)×85.37 / 100=2509.62

[0045] (6) Using the exhaust pressure and enthalpy of the low-pressure cylinder, and according to the characteristics of water vapor, the exhaust volume of the low-pressure cylinder is calculated to be 20.85m 3 / kg.

[0046] (7) Using the low-pressure cylinder exhaust steam mass flow rate and the low-pressure cylinder exhaust steam specific volume, the low-pressure cylinder exhaust steam volume flow rate is calculated using the following formula: 3105.76m 3 / s.

[0047]

[0048] (8) According to the actual operating data of THA working condition, according to the above steps, the exhaust steam volume flow rate of THA working condition is calculated to be 5515.28m 3 Taking the exhaust volume flow rate under THA conditions as the rated value, the per unit value of the low-pressure cylinder exhaust volume flow rate under actual conditions is calculated to be 56.31%.

[0049]

[0050] Where Gv lpco-s is the per unit value of the low-pressure cylinder exhaust volume flow rate under the current working condition, %; Gv lpco-1 is the actual value of the low-pressure cylinder exhaust volume flow rate under the current working condition, m 3 / s;Gv lpco-0 is the actual value of the exhaust volume flow of the low-pressure cylinder under THA working condition, m 3 / s.

[0051] (9) According to the calculation results, the current exhaust volume flow per unit value is 56.31%, which is greater than the 30% threshold. Under this operating condition, the blades can operate safely for a long time.

[0052] The results of other application examples are shown in the following table:

[0053]

[0054]

Claims

1. A method for identifying the exhaust volume flow rate of a low-pressure cylinder of a steam turbine, characterized in that: The following steps are involved: Step 1: Collect historical operating data of the unit at 10-100% load for the past 180 days (see Appendix 1 for an example of the original operating data parameters). The data collection period is 1 minute. Data under the unit shutdown condition and the low-pressure cylinder extraction heater removal condition are excluded. The steam turbine low-pressure cylinder exhaust steam mass flow rate and low-pressure cylinder efficiency under each condition are calculated. Step 2: Based on the data calculated in step 1, with the low-pressure cylinder steam inlet pressure as the independent variable and the low-pressure cylinder exhaust steam mass flow rate as the dependent variable, a linear regression algorithm is used to perform linear fitting to obtain the relationship between the low-pressure cylinder steam inlet pressure and the low-pressure cylinder exhaust steam mass flow rate: Q lpco =k1×p lpci +b1 Where: Q lpco is the exhaust steam mass flow rate of low pressure cylinder, t / h; p lpci Low-pressure cylinder inlet steam pressure, MPa; k1 is the slope obtained by fitting, b1 is the intercept obtained by fitting; Step 3: Calculate the average value of the low-pressure cylinder efficiency based on the low-pressure cylinder efficiency under various working conditions calculated in step 1; Step 4: Based on the low-pressure cylinder steam inlet pressure under the current actual working conditions, use the fitting formula obtained in step 2 to calculate the low-pressure cylinder exhaust steam mass flow rate under the current actual working conditions; Step 5: Using the low-pressure cylinder steam inlet pressure and actual low-pressure cylinder steam inlet temperature under the current actual working conditions, and according to the characteristics of water vapor, calculate the low-pressure cylinder steam inlet enthalpy and low-pressure cylinder steam inlet entropy; then, using the low-pressure cylinder steam exhaust pressure and low-pressure cylinder steam inlet entropy, and according to the characteristics of water vapor, calculate the ideal enthalpy of the low-pressure cylinder exhaust steam, and then use the low-pressure cylinder efficiency to calculate the actual enthalpy of the low-pressure cylinder exhaust steam. The calculation formula is as follows: h lpci =f(p lpci ,T lpci ) s lpci =f(p lpci ,T lpci ) h lpcos =f(p lpco ,s lpci ) h lpco =h lpci -(h lpci -h lpcos )×η lpco Where: p lpci is the steam inlet pressure of the low-pressure cylinder, MPa; T lcpi is the steam inlet temperature of the low pressure cylinder, ℃; p lpco is the exhaust steam pressure of the low-pressure cylinder, MPa; s lpci is the steam entropy of the low-pressure cylinder, kJ / kg·K; h lpcos is the ideal enthalpy of low-pressure cylinder exhaust, kJ / kg; h lpco is the exhaust enthalpy of the low-pressure cylinder, kJ / kg; η lpco is the efficiency of the low-pressure cylinder, %; Step 6: Using the low-pressure cylinder exhaust steam pressure and low-pressure cylinder exhaust steam enthalpy value, according to the characteristics of water vapor, calculate the low-pressure cylinder exhaust steam specific volume. The calculation formula is as follows: v lpco =f(p lpco ,h lpco ) Where: v lpco is the exhaust volume of low-pressure cylinder, m 3 / kg; p lpco is the exhaust steam pressure of the low-pressure cylinder, MPa; h lpco is the exhaust enthalpy of the low-pressure cylinder, kJ / kg; Step 7: Calculate the low-pressure cylinder exhaust steam volume flow rate using the low-pressure cylinder exhaust steam mass flow rate and the low-pressure cylinder exhaust steam specific volume. The calculation formula is as follows: Where: Gv lcpo is the exhaust volume flow of the low-pressure cylinder, m 3 / s; Step 8: Based on the operating data of the THA condition, calculate the exhaust volume flow rate of the THA condition according to the above steps, and use the exhaust volume flow rate of the THA condition as the rated value to calculate the per unit value of the low-pressure cylinder exhaust volume flow rate under the actual working condition: Where: Gv lpco-s is the per unit value of the low-pressure cylinder exhaust volume flow rate under the current working condition, %; Gv lpco-1 is the actual value of the low-pressure cylinder exhaust volume flow rate under the current working condition, m 3 / s;Gv lpco-0 is the actual value of the exhaust volume flow of the low-pressure cylinder under THA working condition, m 3 / s; Step 9: Set the per-unit alarm threshold for the low-pressure cylinder exhaust steam volume flow rate. When the per-unit alarm threshold is greater than 30%, the last blade can safely operate for a long time. When the per-unit alarm threshold is between 10% and 30%, the last blade is in the dynamic stress hump range, where the stress is high and the blade operation safety margin decreases, leading to a high risk of fracture damage. Long-term operation in this range is prohibited, with a continuous operation time of no more than 120 minutes and a cumulative operation time of no more than 800 minutes. An alarm will be issued to the operator. When the per unit value of the low-pressure cylinder exhaust volume flow rate is lower than 10%, the last blade will experience blast overtemperature and cannot operate for a long time. The continuous operation time must not exceed 60 minutes, and the cumulative operation time must not exceed 800 minutes. An alarm will be issued to the operating personnel.

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