Heat recovery steam generators |
HP superheater thickness HP superheater headers also tend to be very thick components that experience a lot of fatigue stress during fast start operation. Vogt Power International aims to keep header thicknesses below 1.25in for fast start applications. One way to reduce such stress is to reduce the header OD and hence, the header thickness, by adopting a single row, steam side parallel superheater design in lieu of a 2-row superheater. See Figure 2.
The metallurgy of the superheater can also be improved to reduce header thickness and minimise the stress arising from fast start operation. The use of Grade 92 material has become increasingly commonplace especially after the recent reduction of allowable stress values for Grade 91 material.
Attemperators
Final stage attemperators are essential for combined cycle plants considering fast start. Final stage attemperators are needed for steam conditioning before rolling off the turbine or for temperature matching into a common steam header.
The interstage attemperator alone may not be able to control final steam temperature even if it is spraying down close to saturation temperature because with lower steam flows during start-up, the steam may significantly increase temperature in the final stage superheater.
The location of the interstage attemperator is also an important factor to consider. If the attemperator location is biased towards the back end of the superheater modules, it may not effectively control final steam temperature and adversely affect the operating temperatures of the front-end superheater tubes. A thermal study should be conducted on existing units to determine the suitability of interstage attemperators and the sizing of the final stage attemperators for fast start applications.
Figure 3 shows the damage that can happen to improperly designed attemperator piping.
Pre-warming the HRSG The pressures in the HRSG during start-up determine whether start should be classified as hot, warm or cold:
Vent
Figure 3. Cracks in attemperator piping and bowed tubes downstream of the attemperator can indicate an improperly designed system (photo: Vogt Power International)
HOT is when the HP drum pressure is above 500 psia WARM is when the HP drum pressure is above 200 psia COLD is when the HP drum is at ambient conditions
A cold start causes approximately 7 times more life usage than a warm start and a warm start causes 4 times more life usage than a hot start. As such, pre-warming the HRSG or maintaining the HRSG at a hot start condition can prolong the life of the HRSG during frequent fast start events. This can be achieved through the use of auxiliary boilers to create sparging steam that can be used to maintain the pressure in the HP drum. Ensuring that the drain valves in the unit are properly maintained and not leaking can also limit the pressure loss of the unit over time. Some plant owners/operators have also installed stack dampers in their units to retain the heat in the unit.
Emission compliance
Pre-warming the HRSG also has an added benefit of enabling the HRSG to achieve emission compliance quicker during a fast start. Since the catalyst section in an HRSG is typically located downstream of the HP evaporator section, pre-warming the HP drum and evaporators allows the catalyst to get up to operating temperature sooner and achieve emission compliance earlier during start-up. Some plants have also installed startup electric heaters for ammonia vaporisation. Since a
hot gas recirculation system, which takes a slipstream of exhaust gas, may not get hot enough during start-up, an electric heater can be used to vaporise the ammonia and achieve emission compliance.
NFPA purge credit
Another factor to consider is purging the HRSG, which can take several minutes and hence delay start-up. One way to mitigate this is to purge the HRSG during shut down and ensure that combustible gases in the fuel lines and ammonia system do not leak into the HRSG. This can be achieved by using a triple block and bleed system in the gas lines along with monitoring pressure and valve position validation (see Figure 4). With this system NFPA (US National Fire Protection Association) allows purging of the HRSG during shutdown, removing the need for a dedicated start-up purge and reducing start-up time.
Other ways to get power on the grid quickly
Power plant owners/operators are also considering other retrofits that allow them to get power on the grid quickly. For instance, a growing number of plants are considering installing a simple cycle bypass stack adjacent to the existing HRSG giving the plant the ability to operate in both simple and combined cycle configurations. The plant is operated in simple cycle configuration when the priority is getting power on the grid quickly and in combined cycle configuration when the priority is peak generation and efficiency.
Vent V4 PT Fuel supply V1 V2 V3 PT To
combustion turbine or duct burner
Figure 4. NFPA ‘System 1’: triple block and bleed system in a combined cycle fuel gas line, with pressure monitoring and valve position validation. PT = pressure transmitter
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Similarly, plants are also installing gas turbine retrofits that allow them to operate at very low loads. The plants use significant less fuel during such operation (25% GT loads) and the benefit is that they are able to ramp up to 100% load much quicker with less thermal stresses on the HRSG components.
Uninterrupted start-up In summary, when a combined cycle plant is being constructed or modified for fast start capability a good deal of consideration must be given to heat recovery steam generator design. However, HRSGs can be designed and existing HRSGs can be modified to enable uninterrupted gas turbine start-up and quick power delivery to the grid.
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