Dynamic balancing of 300MW steam turbine bent rotor with high and medium pressure cylinder

1 Introduction

A domestically produced 300MW steam turbine generator set had continuous shaft vibration amplitude exceeding the standard during cold start-up two years after it was put into operation. Measurements showed that the vibration amplitude of the No. 1 shaft of the high-pressure rotor of the turbine could reach 0.22-0.26 mm at rated speed, and the amplitude exceeded 0.30 mm at critical speed.

Although the corresponding No. 1 bearing vibration amplitude did not exceed 0.03 mm, obvious abnormal noise was heard in the bearing box many times during constant speed operation. The shaft system vibration state measurement test analysis diagnosis and external inspection were carried out, but it was difficult to rule out major equipment hazards such as the shedding of the internal cylinder blades or belts, dynamic and static local friction, and large shaft bending. It was decided to dismantle the cylinder for inspection.

2 Rotor bending shaft condition

The unit model N300-16.6(170)/537/537 is a subcritical intermediate reheat, high and medium pressure combined cylinder, single shaft double exhaust condensing steam turbine unit. The critical speed of the high and medium pressure rotor is 1680r/min, and the low pressure rotor is 1750r/min.

The main shafts of the high-pressure and medium-pressure rotors are integral alloy steel forgings. The high-pressure part has 11 levels of moving blades, and the medium-pressure part has 6 levels of moving blades. The shaft section between the high-pressure and medium-pressure is a "wall-type" steam seal groove, referred to as the "bridge steam seal". Eight dial indicators are arranged along the axial length within the two support spans of the rotor (L=5961mm). It is detected that the part with the largest bending deformation is the "bridge" in the middle of the shaft. According to the 12 screw holes of the coupling wheel, the circle is divided equally, which corresponds to the diameter direction of the #3~#9 screw holes. The maximum bending value is 0.12~0.135mm.

According to the design and manufacturing requirements, the process standard of the shaft bending amount should be less than 0.05mm. When the metal temperature of the shaft surface drops to the indoor ambient temperature 48 hours later, the bow bending value of the high-pressure-medium-pressure combined rotor is 0.13mm.

3 Calculation and analysis of the unbalance of the bent-axis rotor

The rotor weighs 20 tons, with a minimum diameter of Φ630 mm (at the bridge seal), a total length of 7351 mm, and a span of about 6000 mm between two supports. It is difficult for the manufacturer to ensure that the straight-axis correction reaches the standard of 0.05 mm. Many domestic technicians who are familiar with the straight-axis process do not have the confidence to correct it once, and the power plant is eager to solve the unit problem as soon as possible and put it into operation to meet the power generation task requirements.

Based on the theoretical analysis of the rotor dynamics of the misalignment and imbalance of the rotating body, combined with the engineering application and practical experience of the dynamic balance of the shaft system, the vibration of the unit can be reduced by reducing the deflection or imbalance of the rotating shaft. The operating speed of this high-pressure-medium-pressure combined rotor is between the critical speeds of the I and II vibration modes. The "bridge seal" section of the rotating shaft can be regarded as a rigid body, which is convenient for simplifying the quantitative calculation of the unbalance caused by the bent shaft.

Take the bridge shaft length L=750mm,
radius r=310mm,
rotor center hole radius r0=60mm, alloy steel specific gravity γ=7.8,
high pressure section speed regulating stage balance groove radius R0=360mm,
medium pressure section P11 stage balance groove radius R11=460mm,
bridge section bending value δ=0.13mm,
bridge section total mass M=π(rr0)2•L •γ=1148kg
Balance correction equivalent radius R=R0+R11/2=410mm
Balance correction weight m=M•δ/R=364g

If the mass of the high-pressure and medium-pressure impeller blades adjacent to the bridge and the proportional factor of the shaft bow bending effect are considered, the equivalent balance correction weight

Q=Σji=1kim

Proportional factor ki1, for this rotor, ki is preferably 3 to 5.

Since the high point of the shaft bending is in the direction of the #9 screw hole, the two planes of the speed stage and the medium-pressure P11 stage should have a relative weight of more than 1000g in the direction of the #3 screw hole.

4 Balance weight

It is well known that in the mechanism analysis of plastic bending caused by shaft friction, the high point of the bending shaft mostly occurs in the opposite position of the rotor friction part. If the shaft is subjected to the first-order vibration mode elastic deflection, the dynamic and static friction will cause the shaft to bend, and if the weight is not concentrated in the middle of the rotor properly, friction may be generated at the bridge steam seal at low speed, and the unit will be difficult to start and rush to the rated speed. The site conditions limit the balance correction weight test to only be completed once.

The implementation plan of weighting the bent shaft has been repeatedly demonstrated by relevant experts and leaders. It is recognized that it is completely possible in technology, but there is no precedent in the domestic 300MW high-pressure-medium-pressure combined cylinder steam turbine unit. For the needs of technical exploration and equipment production and operation, it is necessary to conduct a test.

First, the small shaft of the main oil pump connected to the high and medium pressure rotors was subjected to single-unit high-speed dynamic balancing on the test bench. Then, the speed regulating stage was weighted by 237g in the direction of the corresponding #3 screw hole (bending concave point), and the medium pressure P11 stage was weighted by 195g, with a total weight of 432g.

5 Conclusion

The unit was started in cold state, and the vibration amplitude of turbine shaft No. 1 was measured. There was no obvious increase or change during the warm-up at medium speed of 1200r/min. It was 0.20mm at critical speed, 0.24mm at fixed speed of 3000r/min, 0.18mm at grid-connected load of 200MW, and 0.15mm at 300MW. After two months of continuous operation, the rotor shaft amplitude changed normally under different load conditions at rated speed, and the maximum value decreased by 0.07~0.10mm. The unit shaft system vibration tended to be stable, and the application of balancing process control and vibration reduction achieved significant results. The unit operated continuously for 301 days, setting a record for the longest operating time of 300MW units in Central China Power Grid. (end)