Why is the vacuum tightness test not done at the rated load?
When doing the vacuum tightness test, it is always inevitable that the vacuum will drop, if the unit is running at rated load at this time, what will happen?
If the vacuum is poor, the coordination instruction is still the rated load, which will inevitably cause an increase in the steam flow, and if the vacuum becomes very poor, the steam flow will increase even more. Under the rated load, as long as the steam flow rate increases slightly, the steam turbine will be overloaded, which will cause a series of abnormal changes such as the increase of axial displacement, which will affect the safety of the unit.
Therefore, this test cannot be done at rated load.
Besides, why not do it at 50% load?
1. The lower the load, the more leakage points of the vacuum system, coupled with the low load vacuum, the steam flow of the low-pressure cylinder is not smooth, which will cause blast friction.
2. Another reason is that considering the convenience and operability of the test, imagine that if it is done at 50% load, for this test, the operator has to stop the coal mill, which is very troublesome and uneconomical to operate, and increases the safety risk. The general unit can be in the 70-100% load range without cutting, then the operation will be less, will also reduce the workload, but also one less danger point. In addition, in order to test the load to 50% load, it will not only affect the economy of the unit operation, but also affect the total power generation of the power plant.
3. It is not only the leakage point that affects the vacuum of the steam engine, but also the heat load of the condenser. Under the condition that the flow rate of circulating water is constant, if there are fewer leakage points, the vacuum at low load will be less than that at high load. We don't see a bad vacuum at high loads, nor a better vacuum at low loads, which is the result of a combination of these two factors. In addition, the working match between the vacuum pump and the condenser is also a problem.
The best working point of the condenser is 80% load, if the load is more than 80% load, the circulating water is fully opened, and the vacuum can not be guaranteed, then what to do, the general practice of the power plant is to open a vacuum pump, think about it, why do you want to do this, is it said that there is more air leakage when the load is high? No, there is more non-condensable gas, this part of the non-condensable gas is not air, but water vapor.
If I do a vacuum tightness test at 90% load, the results of the test do not accurately reflect the tightness of the vacuum system. When the load is too high, a part of the incomplete condensate gas, i.e. uncondensed water vapor, will also worsen the test results.
Therefore, the vacuum tightness test is required to be done at 80% load to ensure that the data can most truly reflect the tightness of the vacuum system of the unit, and the impact on the safety of the system is also minimal.
