Analysis and Control of Differential Expansion Changes During Unit Startup

During the start-up and shutdown process of the steam turbine, the heat exchange conditions of the rotor and the cylinder are different. Therefore, their axial expansion is inconsistent, that is, relative expansion occurs. Relative expansion is usually also called differential expansion. The size of the differential expansion indicates the change of the axial dynamic and static clearance of the steam turbine. Monitoring the differential expansion is an important task during the start-up and shutdown process of the unit. In order to avoid friction between the dynamic and static parts due to the change of the axial clearance, not only should the differential expansion be strictly monitored, but also the impact of the differential expansion on the operation of the steam turbine should be fully understood.
After being heated, the cylinder expands from the "dead point" to the head direction, so the differential expansion signal generator is generally installed at the "dead point" position relative to the cylinder base. Compared with the 5#6# machine, the differential expansion transmitter is installed on the bearing box seat near the three bearings on the lower side of the turning motor.
The start-up of the unit is divided into cold start (metal temperature 150-180 degrees), warm start (180 degrees-350 degrees), hot start (350 degrees-450 degrees) and extremely hot start (above 450 degrees) according to the metal temperature level of the steam turbine before starting. Now we will simply analyze the change and control of the expansion difference of the unit during the common cold start and hot start:
During the cold start of the unit, the change of the expansion difference and the control of the expansion difference can be roughly divided into the following stages:
1. The stage of steam seal supply and vacuum pumping. The expansion difference value has been changing in the positive direction from the steam seal supply and vacuum pumping to the rotor rushing. Because in the heating or cooling process, the speed of the rotor temperature increase or decrease is faster than the cylinder, and the corresponding expansion or contraction speed is also faster than the cylinder. When we put the equalizing pressure box into the steam seal supply, the steam seal sleeve expands to both sides after being heated, which has little effect on the expansion of the entire cylinder. The rotor main shaft section corresponding to the steam seal is heated, which makes the rotor elongate. The effect of steam seal heating on the rotor elongation value is determined by the steam supply temperature, but the heating time also has an effect. Therefore, the pressure of the equalizing pressure box of the 5# unit should not be too high during cold start, and should generally be kept below 0.1MPA, and the temperature should be around 200 degrees Celsius. After the exhaust system is put into operation and begins to evacuate, if the expansion difference changes to a positive value too quickly, the pressure of the equalizing box can be reduced or the vacuum of the condenser can be appropriately increased, because the residence time of steam in the steam seal can be reduced by increasing the vacuum. Generally speaking, when the machine is started in a cold state, the temperature and pressure of the steam coming from the steam seal should be lower, and the vacuum should be increased faster, so as to achieve the conditions for the start-up as soon as possible while ensuring safety.
2. Warm-up and speed-up stage. From the start-up to the constant speed, the expansion difference basically continues to rise. At this stage, the steam flow is small, and the steam mainly works in the regulating stage. After the medium-speed warm-up, when the speed is increased again, the expansion difference will tend to decrease. This is mainly because as the speed increases, the centrifugal force increases, and the axial force component also increases, which makes the rotor thicker and shorter. At the same time, the cylinder temperature gradually rises, and the expansion rate of the cylinder also increases, which relatively delays the expansion value of the rotor. For the 5# machine, during the start-up, the pressure and temperature of the steam should be appropriately lower, but the temperature should maintain a certain degree of superheat, and the start-up rate should be low. During the run-up process, we should pay close attention to the change of cylinder temperature. If the differential expansion is too high, we should stabilize the speed or reduce the vacuum to allow the steam to stay in the cylinder for a longer time and fully warm up the machine. Sometimes during the warm-up and speed-up process, if the cylinder body drain is improperly adjusted, it will also affect the differential expansion. Therefore, we should pay attention to controlling the cylinder body drain when starting the machine. In order to prevent the distortion of the differential expansion table data, we should also closely observe the changes in the thermal expansion and axial displacement of the unit, and further judge the changes in the differential expansion by comparing the thermal expansion and axial displacement. At the same time, we should closely monitor the vibration of the unit, especially when crossing the critical speed.
3. Constant speed and parallel load stage. Since the time from speed increase to constant speed is short, the steam temperature and flow rate hardly change, and the impact on the differential expansion can only be reflected after the constant speed. After the constant speed, the differential expansion increases by a large margin and lasts for a long time, especially after the generator is connected to the grid. In the low-load warm-up stage, the steam heats the rotor and cylinder more violently. After the connection to the grid, as the regulating steam valve opens, the temperature of the regulating stage rises faster. The opening speed of the regulating steam valve has a greater impact on the expansion difference. Therefore, the regulating steam valve of the 5# unit should be opened slowly after the connection to the grid, and attention should be paid to the temperature change of the regulating stage. In other words, in order to prevent the expansion difference from changing too fast, the unit should be warmed up for a period of time under low load after the connection to the grid. The specific low-load warm-up time is determined by the temperature of the upper and lower walls of the cylinder, the temperature of the regulating stage and the change trend of the expansion difference. Only when the expansion difference value shows a downward trend and drops by 10% compared with the value at the time of connection to the grid can the load be gradually increased. Once the expansion difference rises again and reaches the value at the time of connection to the grid, the load increase speed should be appropriately slowed down or even stopped to continue warming up. This continues until the unit load rises to the rated value.
In general, the factors that affect the expansion difference of the unit are mainly the following: the length of the warm-up time, the change of the condenser vacuum, the height of the shaft seal steam supply temperature and the length of the steam supply time, the temperature rise and temperature drop rate of the main steam, the impact of load changes, etc. To put it simply, starting a unit in a cold state requires: "Adjust the vacuum, maintain a steady steam supply, increase the speed slowly, and warm up the unit slowly. At low load, don't rush, and wait until the cylinder temperature rises before starting again."