Boiler feed pumps generally have speed control devices, such as hydraulic couplings. The function is to adjust the flow of the feed pump according to the change of the unit load, and reduce the throttling loss, thereby saving energy.
When the hydraulic coupling water supply pump (hereinafter referred to as the liquid adjustment pump) is running, it should be able to input automatic adjustment to reduce the operation of the operating personnel. The speed adjustment range should reach 25%~100%. However, some liquid regulating pumps have a large rate of change in the rotational speed, which causes a large fluctuation in the flow rate, which may cause a shaft breakage accident.
The pump of a power plant consists of a front pump, a main feed water pump, a hydraulic coupling and a speed increasing gear. The front pump is dragged by the motor main shaft, and the main feed water pump is driven by the electric motor through the speed increasing gear and the hydraulic coupling. The front pump model is QG50080, the main feed pump model is DG480180, and the hydraulic coupling model is C046, which is the product of Shanghai Electric Power Repair Factory.
Boiler feed pump failure
When the pump was put into operation after an overhaul, it was found that the automatic adjustment characteristics of the feed water deteriorated. When the injection is automatic, there is a large flow fluctuation, sometimes reaching about 80t/h, which seriously affects the normal operation of the unit. In order to analyze this problem, the control system of the normal pump was switched to the faulty pump, and the same fault problem occurred, thereby basically eliminating the cause of thermal control.
Fault treatment of boiler feed water pump: regulation of boiler feed pump hydraulic coupling
The adjustment of the hydraulic coupling is to adjust the speed of the feed water pump by adjusting the amount of oil discharged from the turbine through the spoon tube, so as to adjust the flow rate. The flow rate Q is proportional to the speed n.
According to the characteristic curve of the pump, a curve of the relative value g of the flow rate and the relative angular displacement L of the scoop tube can be made.
g=G/Gmax(1)
Where: G is the output flow of the pump, t/h; Gmax is the maximum output flow of the pump, t/h
μ=α/αmax(2)
Where: A is the angular displacement of the spoon; Amax is the maximum angular displacement of the spoon
K=dg/dμ=αmax/Gmax·dG/dα(3)
K is the slope of the curve, which is the amplification factor or the adjustment characteristic coefficient.
In the ideal state, K=1, that is, the inclination angle is 45b. During the adjustment of the speed of the hydraulic coupling, the flow rate changes smoothly, and does not cause a mismatch between the adjustment amount and the flow rate.
K "1, the flow changes slowly, which is not conducive to tracking the need for load adjustment.
When K"1, the flow rate changes too fast, which may cause the flow to fluctuate greatly, causing the drum water level to be unstable, and the pump adjustment is frequent, which may cause the pump shaft to break.
Fault treatment of boiler feed water pump: performance test of boiler feed water pump
In order to find out the cause of the failure, the horizontal feed pump was subjected to a characteristic test using a conventional test method. Check the relevant meter before the test.
During the test, the load of each working condition is shown in Table 1. The feed water after the pressure is raised by the liquid pump is passed through the flow orifice as much as possible to reduce the test error. Each desuperheating water should be used as little as possible and corrected accordingly. On this basis, the relevant data is copied.
Failure analysis of boiler feed water pump
During the test, the spoon was manually operated, the flow rate did not fluctuate, the current was stable, and the balance pressure was normal, indicating that there was no turbulence in the pump shaft. The motor works normally and the hydraulic coupling can operate normally at each load point. According to the needs of load regulation, the flow rate changes greatly every time the hand-held device is operated, and it is difficult to control the flow rate. When the scoop tube stroke changes from 61 mm to 71 mm, the flow rate changes from 284 t/h to 430 t/h. It can be seen from Fig. 1 that the linearity of the whole speed control system is good but the curve is steep. The change of the opening degree is 1%, and the flow rate changes by 11.23t/h. The flow rate of the feed pump is determined by the speed of the feed pump. The pump shaft is mounted on the turbine of the hydraulic coupling, and the speed of the turbine is determined by the amount of oil returned by the spoon. If the amount of oil change in the unit stroke is too large, it will seriously affect the stability of the feed pump flow. To this end, the rate of change in the amount of oil returned by the spoon should be reduced.
4.1 Magnification factor K
The maximum stroke of the spoon tube is 110mm, and the maximum flow rate is 440t/h. According to Table 1, the flow rate changes from 284t/h to 430t/h, and the stroke changes from 61mm to 71mm.
K=dg/dμ=αmax/Gmax·dG/dα=(110/440)·[(430-284)/(71-61)]=3.65
Then the inclination angle of the curve is 74.7b, which indicates that the curve is steep, which is the main reason for the instability of the hydraulic coupling. In order to improve the adjustment performance of the hydraulic coupling, it is necessary to solve the change amount of the oil return amount of the spoon tube, so that the oil return amount is reduced under the condition of the same opening degree variation.
Boiler feed pump fault handling 4.2 cam
After the old cam that was removed was reinstalled, it was found that the improvement of the adjustment performance of the hydraulic coupling was not obvious. For this reason, at the same time as the cam is added, the cam profile is changed to suit the needs of load regulation. That is, below 50% load, the cam profile is steep; in the interval where the load is frequently adjusted, the cam profile is relatively flat; the profile of the remaining load zone is also steep.
Boiler feed pump fault handling 4.3 reduce the rack and gear ratio
When the hydraulic coupling is adjusted, the cam drives the rack to move up and down, and the rack drives the gear, which in turn drives the sector gear, thereby causing the change of the opening degree of the spoon tube and adjusting the flow rate of the feed water. The adjustment of the spoon tube is driven by the rotation of the sector gear, and the rack drive is coaxial with the gear of the sector gear. Reduce the rack-to-gear ratio, that is, increase the gear diameter, extend the length of the rack, and ensure the opening and closing of the spoon. The rack-to-gear ratio is reduced, that is, the cam of the same stroke, the opening degree of the sector gear becomes smaller, and the oil return adjustment amount of the spoon tube becomes smaller, thereby reducing the fluctuation of the feed water flow rate.
Boiler feed pump fault handling method and result
Based on the above analysis, corresponding measures have been taken. First consider adding a cam. If the cam is not effective, consider using computer adjustments. The difficulty of adding a cam is relatively small, easy to implement, and the profile of the cam is improved. According to the load range of the normal operation of the unit, it is considered that the cam profile is steeper below 50% load; under 50%~100% load, the corresponding profile line is relatively flat. At low loads or during start-up, the pump can maintain a high flow rate due to the need to open the feed pump recirculation valve. See Table 2 for the flow and related parameters after adding the cam and improving the profile.
After the original cam is restored, it does not have much influence on the spoon itself. The stroke of the spoon is at (60~70) mm, and the slope is still 3.45. According to Table 2, after the cam profile is improved, the slope of the entire speed control system (including the hand-held device) is 100/440·[(416-274)/(71-48)]=1.4, the inclination angle is 540, and the inclination angle is large. The amplitude is reduced, which basically meets the needs of load regulation. The unit operated normally after the above transformation.
Boiler feed pump fault handling conclusion
The hydraulic couplings generally have cams, and sometimes the cams are eliminated to prevent the cams from being jammed. Since the adjustment characteristics of the spoons are unclear, the static and dynamic flow characteristic curves are not tested on the speed control system before commissioning. After commissioning, there may be poor regulation performance, which may also cause fatigue fracture of the pump shaft. Once the flow rate of the feed pump is found to be large, it is necessary to find out the cause of the fault in time and take corresponding preventive measures.
