Testing the new design
The next round of tests investigated the concurrent use of MSP and control of steam temperature sprays and the number of burners in service and their tilts to reduce temperature changes in the HP rotor. The testing was conducted in two phases. First, engineers quantified the potential increase in HP rotor temperature from running in sliding-pressure mode and the feasibility of operating at lower throttle pressure at low loads. During the second phase, engineers verified that the unit can maintain a high ramp rate at low loads while in sliding-pressure mode. The testing, which combined burner transitions, burner tilts, and spray control to effectively regulate steam temperature and turbine metal temperatures, used ramp rates of 18 MW/minute from minimum load to approximately 40% load.
The focus of this testing was on the low-load range, where sliding pressure can be effective at raising turbine metal temperatures and thereby reducing temperature changes during ramping. The tests were conducted during the early morning hours of October 28 and 29, 2004. Data acquired on October 28 included low-load numbers, the values for fast ramping to high load, and high-load steady-steady figures. This data provided additional information on turbine metal temperatures and steam temperature control requirements.
A Wonderware system (www.wonderware.com) was used to collect the data at 10-second intervals. The operator ably performed manual control on several loops affecting steam temperature to provide more effective control and data suitable for modeling. Foxboro's Connoisseur system was used to analyze the data.
First-day tests
The first tests, conducted at 60 MW, determined the relationship between HP rotor temperature and sliding-pressure and superheat steam temperature. The pressure was lowered via a series of ramps that applied several rates and amplitudes to achieve an end point pressure of approximately 1,525 psig
.
Lower and upper burner pairs were removed from service and returned to service sequentially, to observe the effect on superheat steam temperatures, superheat spray flows, and gas header pressure. Then the burner tilts were moved to quantify the impact on steam temperatures.
The burners, tilts, and sprays were then controlled during a fast-pressure ramp test to regulate steam temperature. Pressure was ramped from 1,525 psig to full pressure and then returned to a low value of 1,595 psig prior to returning the unit to dispatch control.
Figure 5 illustrates the results of the throttle-pressure ramp tests in sliding-pressure mode. The trend lines represent load, throttle pressure, first-stage metal temperature, superheat temperature, burner tilt position, spray valve positions, and the number of burners in service. The values in white correspond to the white vertical timeline near the left axis and are the conditions at the start of the test: a 760F first-stage metal temperature at full pressure and low load. The values in color represent the low and high range values for the individual variables, corresponding to the tick marks on the left side of the trend.
5. Lowering the pressure. Throttle pressure ramp tests are illustrated here. The values in white correspond to the white vertical timeline near the left axis and are the conditions at the start of the test. Source: LCRA
The trend lines of Figure 6 present the key results of the first series of tests. Lowering the throttle pressure increases the temperature in the first-stage shell, subject to variations in superheat temperature. The white timeline indicates a first-stage temperature of 783F at 1,525 psig and a throttle temperature of 992F. The automatic spray control was challenged to regulate steam temperature during pressure ramping.
6. First-day test results. Lowering the throttle pressure increases the first-stage metal temperature, subject to variations in superheat temperature. Source: LCRA
During the fast pressure ramp test (from low pressure to full pressure and back again), manipulating the number of burners in service, and their tilts and sprays, helped reduce steam temperature variations. The steam temperature effects demonstrated during the tests were applied during the second night's load ramp testing to reduce steam temperature variations.
No major control issues arose during this test. Drum level and deaerator level were effectively controlled. There was an occasional alarm for a feedwater heater level, but that did not appear to be cause for concern.
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