【Haichuan Safety and Environment Cup】--4 insights on the boiler MFT accident caused by tripping during the switching of the feed water pump 0009+, + Guan Gongyu
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This post was last edited by zhaolijun on 2015-8-8 at 14:26. Insights from an MFT accident in the boiler caused by a trip during the switching of the feed water pump. I. Accident sequence: At 13:00 on a certain day, the turbine was operating at a load of 90 MW. Pump A is in standby mode while pump B is in operation; the feedwater flow rate is 301 t/h, and the outlet pressure is 11.3 MPa. Conduct a regular switching test for the feed water pump. Contact the 0 m inspection team to check the feed water pump system: At 13:21, the turbine operator in the control room started Pump A. Check the operation status after startup from 0 m. Increase the speed once everything is normal. At a speed of 1,000 r/min, the current, outlet pressure, and speed were checked, and all parameters showed normal values. Open the electric valve at the outlet of Pump A for Feed Water in Unit 2, and leave it to the boiler operator to continue increasing the speed. At L3:24, the speed of feedwater pump A was 3,485 r/min, with an outlet pressure of 11.2 MPa and a flow rate of 223 t/h; whereas the speed of feedwater pump B was 3,490 r/min, with an outlet pressure of 11.2 MPa as well. Flow rate: 222 t/h. The boiler feedwater flow rate is normal. At 13:25, contact the turbine operator to first close the recirculation valve of pump A. The recirculation valve of the B feedwater pump was opened again, and the boiler side used the scoop tube of the A feedwater pump to adjust the drum water level. During this adjustment process, the speed of pump A dropped to 3,600 r/min, the outlet pressure was 10 MPa, and the flow rate was 120 t/h. The recirculation valve of the A feedwater pump was closed, triggering a protection mechanism that caused the A feedwater pump to trip. At the same time, the check valve at the outlet of pump A for feedwater became stuck, causing feedwater to flow back. At 13:26, the flow rate of pump B increased to 420 t/h, while the flow rate to the boiler dropped rapidly, falling to 130 t/h at 13:29. At 13:31, the drum water level dropped to -295 mm, triggering the low water level protection mechanism; the boiler’s MFF system was activated to extinguish the fire. II. Detailed causes of the accident and lessons learned 1. Improper operation by the operators: When switching between the operating pump and the standby pump, it is essential to proceed slowly and carefully, ensuring that the pressure in the main pipeline does not fluctuate as a result of such operations. When the boiler crew used the scoop tube of feed pump A to adjust the drum water level, improper adjustment caused the protection system to activate, resulting in the tripping of feed pump A. Based on the description of the accident, I believe that when the boiler operator was adjusting pump A, it was likely due to an excessive amount of water supplied to the steam drum; as a result, the speed of pump A was reduced in order to control the flow rate. During this adjustment, the flow rate of pump A dropped to the level at which the recirculation valve opens. At that moment, the recirculation valve of the A feed pump was closed, which triggered the protective mechanism and caused the pump to trip. Since it is about shutting down pump B and starting pump A, when the water supply volume is high, any adjustment should be made by reducing the speed of pump B, rather than pump A. 2. The check valve of Pump A got stuck, causing water to flow back, and the operators failed to make the correct assessment. When Pump A tripped, the check valve at the outlet of Pump A’s feed water pump got stuck, causing feed water to flow in reverse. The operators failed to make the correct assessment; instead of closing the outlet valve of Pump A, they increased the flow rate of Pump B’s feed water pump, which resulted in a large amount of water flowing back into Pump A. As a result, the flow rate to the boiler dropped rapidly, triggering the low water level protection system, and the boiler’s MFF system was activated to extinguish the fire. If the operator remains calm, makes correct judgments based on the data and observations, and shuts off the outlet valve of pump A in a timely manner, it will have little impact on the boiler water level. 3. Any equipment defects identified during routine inspections and operation must be addressed promptly. Hidden dangers must not be allowed to exist; if the check valve of pump A had functioned properly, this accident could have been avoided. 4. We must prepare accident plans for every operation we carry out. Accident prevention plans can help avoid accidents. For us inexperienced trainees, it is crucial to become familiar with each procedure, anticipate potential dangerous situations, and conduct drills regularly. Accident cases serve as teaching materials for learning about accidents; only by analyzing and summarizing them carefully can we ensure that similar accidents do not occur in our future operations.
Cause analysis of the boiler MFT accident due to tripping during feedwater pump switching. The main reason for the tripping during feedwater pump switching was improper operation by the operator, which prevented the feedwater pump from delivering flow at the proper level, resulting in a signal indicating cavitation and thus triggering a trip. Pump A was in standby mode while pump B was operating. During the process of switching pumps, as the speed, pressure, and flow rate of pump A decreased, it would have been necessary to close the recirculation valve. However, the operator opened the recirculation valve of pump A. The low flow rate, combined with the open recirculation valve, resulted in even less water inside the pump. Continuing to operate the pump would have caused cavitation; a signal was sent to shut down the pump. Yet, the check valve at the outlet of pump A got stuck, preventing water from flowing through, which led to pressure buildup and caused pump A to trip. When the flow rate of pump B increases to 420 t/h, the recirculation valve of pump B opens, and water flows back automatically. Pump A is then in a tripped state with insufficient flow rate, resulting in a rapid drop in the boiler’s flow rate. The water level in the drum drops to -295 mm, triggering the low-level protection mechanism, which causes the boiler’s MFF system to activate in order to extinguish the fire. During the pump switching process, special attention is paid to the timing of opening and closing the recirculation valve: it should be closed when the flow rate drops to a certain value, and opened otherwise. It is essential to maintain communication with the site during the operation process; only by doing so, and by carrying out the relevant procedures, can everything be ensured to go smoothly, thereby enabling the backup pump to be put into use more accurately.
New insights from an incident where a trip occurred during the switching of the feed water pump, resulting in a boiler MFT accident. Accident sequence: During the process of switching the feed water pump, the operators contacted the turbine operator to first close the circulation valve of feed water pump A. The recirculation valve of the B feedwater pump was opened again, and the boiler side used the scoop tube of the A feedwater pump to adjust the drum water level. During this adjustment process, the speed of pump A dropped to 3,600 r/min, the outlet pressure was 10 MPa, and the flow rate was 120 t/h. The recirculation valve of the A feedwater pump was closed, triggering a protection mechanism that caused the A feedwater pump to trip. At the same time, the check valve at the outlet of pump A got stuck, resulting in backflow of feedwater. At 13:26, the flow rate of pump B increased to 420 t/h, while the flow rate to the boiler dropped rapidly; by 13:29 it had fallen to 130 t/h. At 13:31, the water level in the drum dropped to -295 mm. The low-water level protection activated, and the boiler’s MFF system was triggered to shut down the fire. Cause analysis of the accident: This accident was mainly caused by improper operation by the operators, which triggered the interlock protection of the feed water pump. As a result, insufficient water was supplied to the boiler, leading to a water shortage in the boiler and ultimately causing the boiler’s MFT to activate. During the switch-over to the feed water pump, improper operation resulted in a decrease in the water supply volume from pump A. At this point, the recirculation valve of the feed water pump should be opened to prevent it from tripping. However, the operator failed to open this recirculation valve. Additionally, the check valve at the outlet of the feed water pump became stuck, causing water to flow back into the pump; this led to water circulating inside the pump, resulting in a severe shortage of water in the feed water pipes. As a consequence, the boiler did not receive enough water supply, leading to a water shortage in the boiler. The lesson for us is that during operations, operators must strictly follow the operating procedures, closely monitor all key parameters throughout the process, observe their patterns of change, and analyze the reasons behind those changes ； At the same time, anticipate possible accidents so that one can remain calm in such situations and handle them promptly when they occur. During regular operation and maintenance, it is necessary to carry out proper maintenance tasks, carefully address any defects in the equipment, and ensure that it is ready for use as a backup.
Lessons learned from the boiler MFT accident caused by a trip during the switching of the feed water pumps. During this incident, a trip occurred while switching the feed water pumps, which led to a rapid decrease in the feed water flow rate; this in turn resulted in a low water level in the boiler, triggering the MFT mechanism and causing the boiler to shut down. Here is a brief overview of the accident: A certain power plant was equipped with a 125 MW ultra-high pressure steam turbine featuring primary intermediate reheating, a single shaft, two cylinders with dual exhausts, and an impulse condensing mechanism. The feed water pump set consists of a pre-pump, a hydraulic coupler, and a main feed water pump. It belongs to the hydraulic control type feed water pump. At 13:00 on a certain day, the pump switch-over was carried out as per normal procedures; at that time, A was the standby pump while B was the feed water pump. Specific steps for switching the feed pump into operation: First, conduct a thorough pre-start inspection of Pump A (including the pump body, auxiliary equipment, valves, and control panel). Next, open the inlet valve to fill the pump with fluid and remove air from it. Then, open the circulation valve to prevent the pump from operating under no-load conditions, which could damage the equipment. After that, disable the interlocks associated with the feed pump and start it up; monitor its operation by checking parameters such as current, vibration, and temperature. Finally, slowly open the outlet valve. Once the outlet valve is fully open and the pump is running stably, close the circulation valve. To stop Pump B, first open the circulation valve, then slowly close the pump outlet valve. Once the pump is running stably with the outlet valve fully closed, close the circulation valve to stop the pump’s operation, and finally close the inlet valve. Reconstruction of the accident: At 13:21, Pump A started up normally; its speed increased to 1000 r/min, and all parameters such as current, outlet pressure, and speed showed normal values. Open the electric valve at the outlet of Pump A for Feed Water in Unit 2, and leave it to the boiler operator to continue increasing the speed. After adjustment by the boiler crew, at 13:24, the speed of the A feed water pump was 3,485 r/min, the outlet pressure was 11.2 MPa, and the flow rate was 223 t/h ； The speed of pump B is 3490 r/min, with an outlet pressure of 11.2 MPa. Flow rate: 222 t/h. The boiler feedwater flow rate is normal. At 13:25, contact the turbine operator to first close the recirculation valve of pump A. The recirculation valve of pump B was opened again, and the boiler side used the scoop tube of pump A to adjust the drum water level. During this adjustment process, the speed of pump A dropped to 3,600 r/min (this value contradicts the previous value of 3,485 r/min); the outlet pressure was 10 Mpa and the flow rate was 120 t/h. The recirculation valve of pump A was then closed, triggering a protection mechanism that caused pump A to trip. At the same time, the check valve at the outlet of pump A got stuck. Feedwater backflow occurred. At 13:26, the flow rate of pump B increased to 420 t/h, while the flow rate to the boiler dropped rapidly, falling to 130 t/h at 13:29. At 13:31, the water level in the steam drum dropped to -295 mm. The low-water level protection activated, and the boiler’s MFF system was engaged to shut down the fire. Case analysis: 1. During normal water level regulation, the A pump suddenly malfunctioned, its speed decreased, and both the feedwater flow rate and pressure dropped rapidly. 2. Since the circulation valve is closed, when the feedwater flow rate drops to the low-pressure protection setting of the feedwater pump, pump A stops. 3. Due to the sudden shutdown of Pump A, the flow rate of Pump B increased instantly, reaching 420 t/h at 13:26. 4. Due to the jamming of the check valve at the outlet of pump A, most of the high-pressure water from pump B flowed back to the low-pressure main, resulting in a significant reduction in the flow rate of water entering the boiler; this rate dropped to 130 t/h at 13:29. This forced the boiler’s low water level protection to activate, resulting in the MFT triggering and the shutdown of the boiler. Accident summary: Before switching the pump, the operator failed to conduct a proper inspection of the equipment, which posed serious safety hazards. Due to an operational error, once pump A trips, it is necessary to quickly close the inlet and outlet valves of pump A in order to prevent high-pressure feedwater from flowing back, which could affect the stability of the high-pressure feedwater main. Safety management is not properly implemented; team members must conduct regular inspections of the equipment to identify potential safety hazards promptly.
Since the circulation valve is closed, when the feedwater flow rate drops to the low-pressure protection setting of the feedwater pump, pump A stops. Is there any problem? The pump is about to stop – should we close the recirculation valve? The original poster, please explain in detail