Reprint - Analysis and solution measures for unplanned shutdown caused by maloperation

Source: Thermal Power Thermal Automation

Abstract:In view of the unplanned shutdown event caused by the failure of on-site thermal control equipment, the cause is analyzed as the short circuit of water accumulation of the protection switch with low anti-combustion pressure, which triggers the action shutdown of the steam turbine emergency blocking protection system (ETS), and proposes the rational use of intelligent integrated actuators to strengthen the physical protection of on-site measurement points, and points out that the reliability of field equipment has an important impact on thermal control protection.

Keywords: thermal power plants, thermal control equipment, unplanned shutdowns, reliability

With the improvement of the control level of steam turbine generator sets, the impact of thermal control equipment on the safety of the unit is also increasing, and in recent years, the unplanned shutdown of the unit caused by thermal control (hereinafter referred to as non-shutdown) events has been on the rise. Among the 13 non-stop events that occurred in a thermal power plant in the first quarter of 2017, 7 were caused by thermal control, accounting for 53.84%, so increasing the reliability of thermal control equipment and reducing protection malfunction have become the top priority of thermal control management at this stage.

The causes of thermal control protection malfunction are various, among them, the field measuring element and the actuator involved in the protection are distributed in every corner of the production site, the installation environment is complex, and it is easy to be affected by the surrounding working conditions, and the probability of malfunction is high, therefore, it is of great significance to strengthen the reliability of field equipment to reduce the probability of malfunction of thermal control protection. This paper analyzes a non-stop event caused by a thermal control equipment failure in a thermal power plant, points out the importance of strengthening the reliability of field equipment to prevent the malfunction of thermal control protection, and discusses the methods of strengthening the protection of field equipment. ［1］

1 Events

The No. 4 C260/N350-16.7/537/537 steam turbine of a thermal power plant is a subcritical, one-time intermediate reheat, two-cylinder, two-row steam and condensing steam turbine.

Before 17:07:00 on October 20, 2019, Unit 4 maintained stable operation of 202MW load, and at 17:07:51, the electric door opening feedback of the condensate outlet of Unit 5 low-pressure heater (hereinafter referred to as low-plus) of Unit 4 suddenly disappeared, and the interlocking closed 5-stage exhaust to low-plus exhaust backstop door and 5-stage extraction to low-plus extraction electric door;17 At 08:01, the opening feedback of the electric door with low water addition at No. 5 was restored; At 17:24:51, the feedback from the electric door closing at No. 5 low water outlet was issued, and the switch feedback continued until 17:34:51, and the switch feedback of the electric door with low water injection at No. 5 disappeared at the same time; at 17:35:32, the condensate flow rate dropped from 734t/h to 30t/ h, the pressure of the condensate tube suddenly rises from 1.5MPa to 2.95MPa, and at 17:45:20, the turbine critical cut-off protection system (ETS) trips, and the anti-fuel oil pressure is low. 17:45:22, Boiler Total Fuel Trip (MFT), tripping for the steam turbine.

After being processed by the operation and maintenance personnel, at 19:57:00, Unit 4 was connected to the grid.

2 Cause analysis

Check that the status of the operator station of the No. 5 low condensate outlet door is gray (gray means that the switch feedback has disappeared), and the 380V power supply air circuit breaker of the No. 5 low condensate outlet door is found in the cabinet of the steam turbine electric actuator (MCC) of the steam turbine workshop, and the state of the No. 5 low condensate outlet door is green after power transmission (green represents the state of the valve is fully closed), and the valve is in the full closed position on the spot, and it can be determined that the condensate interruption is caused by the false closure of the No. 5 low condensate outlet door. Afterwards, I communicated with the manufacturer's personnel and judged that the valve was accidentally closed due to the failure of the control board. After the No. 5 low water outlet electric door was closed by mistake, the condensate was interrupted, and the pressure of the condensate jellyfish tube rose to 2.95MPa, and the increase of system pressure led to the cracking of the flange of the refined yin bed resin trap located at zero meters of the steam engine, and a large amount of condensate was sprayed directly to the anti-fuel tank located around the yang bed, which caused the low anti-combustion pressure protection switch to accumulate water and short circuit, causing the ETS action to stop.

Improper protection of the anti-fuel pressure low protection switch was an important cause of this incident. The anti-fuel system is provided by the steam turbine manufacturer, and the anti-fuel pressure low switch for thermal control protection is also installed on the nearest anti-fuel tank according to the manufacturer's requirements. The four pressure switches do not have a completely independent sampling device, but are installed on the oil tank through the valve group, and the electrical connection plug of the pressure switch is upward, although there is a sheath pipe protection, but with the lack of multiple overhaul and disassembly and maintenance technology, the protection level of the plug has not reached the design requirements. When a large amount of condensate is sprayed for nearly 10 minutes, the switch is short-circuited and causes the ETS protection to malfunction and shut down.

3 Solution

Actuators and pressure switches are the most common thermal control equipment, they are installed in every corner of the site, the environment is complex and changeable, and the thermal control equipment mostly uses electronic components as the control core, and has strict requirements for temperature, humidity, vibration and other indicators, and the reliability of thermal control equipment in the existing environment should be comprehensively considered from the following aspects [2].

a. Rational use of intelligent integrated actuators. At present, intelligent integration is the mainstream of electric actuators, but there are many problems in the process of use, and the main reason is the poor adaptability of the control circuit board to the on-site environment. The price of imported actuators (such as ROTORK, ABB, Auma and other brands) is quite high, and they are generally used in important adjusting valves, and the cost of large-scale use of shut-off valves is too high; Vibration and other interference to electronic components, with the increase of use time, the failure rate will gradually increase, although there are split intelligent actuators can improve the temperature adaptability to the use environment, but in general, the life of electronic components limits the service life of intelligent actuators. The first phase of the plant has been used for more than 15 years with the valve electric device with the operating cabinet, such as the DWZ type produced by a factory in Yangzhou, the failure rate and maintenance amount is only one-tenth of the second phase of intelligent integration, and after the failure, the team can repair it in time, which is conducive to the recovery of the equipment as soon as possible. In some harsh environments and low requirements for valve switching speed, the reliability and practicability of this valve electric device are far greater than that of the intelligent integrated actuator.

b. Strengthen the physical protection of on-site measurement points, especially the switching measurement points involving protection. From the strict requirements of the maintenance process to ensure that the protection index is qualified; from the sampling method, to ensure that the independent three take two channels, and the measurement points are scattered and installed in different positions to reduce the influence of environmental factors; through the eye-catching warning signs to distinguish the protection of the measurement points, reduce the possibility of personnel misoperation; the linear variable differential transformer (LVDT) of large and small steam turbines are regularly checked and fastened; in winter, electric heating is added to the sampling, Increase insulation and regular inspection to prevent the sampling pipe from freezing and blocking due to cold temperature, regular oil discharge and drainage of the compressed air system, and all terminal boxes and junction boxes of the coal conveying system are tightly wrapped to prevent water from washing and polluting the equipment. Strengthen inspection, so that the equipment person in charge can grasp the latest status of each on-site equipment as soon as possible, and deal with problems in time.

c. The equipment provided by the manufacturer should be transformed as soon as possible, such as anti-fuel, top shaft oil, hydraulic oil station of the induced draft fan, etc. Most of the equipment from the manufacturer is mainly used to realize the maintenance function, and the materials and reliability of the control equipment are worrying. In particular, the anti-fuel system, top shaft oil system, etc., should be reasonably modified as soon as possible together with the maintenance profession.

d. The supporting thermal control equipment after the technical transformation of the machine maintenance should be protected, especially the new equipment after the transformation of low nitrogen, denitrification and ultra-low emission in recent years. The reliability of the actuator, measuring instrument and other ancillary equipment is very important, once the failure occurs, it may cause the environmental protection index to exceed the standard, the consequences are very serious, so we must grasp the health of these equipment, find and deal with equipment failures in time, and avoid environmental protection incidents.

e. Rational use of low-cost bid-winning procurement of materials, so as to make the best use of materials. Receive the heat tracing, high-pressure needle valve, pressure gauge, etc. that are lower than the average price of the industry, and reasonably apply them to the site, such as using the batch of heat tracing in the occasion of short laying length, using the needle valve in the place where the working pressure is low, and installing the pressure gauge in the peripheral sewage system, ash removal system and other places that do not need continuous output, and grasp the equipment status by strengthening inspection and regular testing, so as to ensure that the equipment abnormality can be found and dealt with in time, and the impact of equipment failure on system safety can be reduced.

4 Conclusion

In the current form of production and operation, how to improve the reliability of equipment is not simply a technical problem, but a comprehensive management problem. With the rise of coal prices, the business situation of thermal power enterprises is becoming increasingly severe, and how to improve the reliability of equipment as much as possible and ensure the operation safety of the thermal system under the condition of tight maintenance costs is worthy of serious discussion. Only by conscientiously summarizing the work, carefully maintaining the equipment, carefully reviewing the relevant protection logic, and working in strict accordance with the relevant standards and procedures, will the probability of thermal protection misoperation or rejection accidents be greatly reduced, and the safe and stable operation of the unit will be ensured.

Bibliography:

[1] Sun Changsheng.Fault analysis and reliability control of power plant thermal control system[M].Beijing:China Electric Power Press,2016.

[2] Si Ruicai, Zheng Yuan, Wang Changli, et al.Analysis of main protection of circulating fluidized bed boiler for 350MW supercritical unit[J].Power Generation Equipment,2016,30(4):274-277.