Active Draft Control for HRSG Cooling and Restart Clearance
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Solution Overview
Problem
Combined cycle power plants face inefficiencies and operational challenges during shutdown due to tip clearance, ambient air infiltration, and undesirable rubs during restart, which are exacerbated by traditional draft control methods that fail to adequately manage airflow.
Innovation Solution
Active draft control system that measures turbomachine parameters to adjust exhaust gas temperature and draft flow temperature, using a control system to equalize pressure drops and provide sub-atmospheric pressure, thereby minimizing heat loss and preventing rubs during restart.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If tip clearance is designed into the turbomachine to accommodate expansion and contraction, then the risk of turbine damage is reduced, but the efficiency of the turbine is substantially reduced by permitting heated gas to escape
Solution Approach 1:
The patent actively controls and adjusts the tip clearance parameter during different operational phases. During shutdown and restart, the system dynamically modifies clearance conditions to maintain optimal values, preventing both excessive gas leakage during operation and blade-shroud contact during thermal transitions. This dynamic parameter control resolves the contradiction by allowing the system to have small clearance for efficiency during operation and adequate clearance for safety during thermal changes.
2Temperature
If ambient air infiltration is allowed to cool the turbomachine during shutdown, then the equipment is cooled down, but it is detrimental to quick restart due to temperature operational constraints
Solution Approach 1:
The system applies preliminary anti-action by using active draft control to prevent excessive cooling of the turbomachine during shutdown. By controlling the draft flow, the system maintains temperatures within operational constraints, preventing the need for lengthy preheating before restart. This resolves the contradiction by avoiding over-cooling while still providing necessary thermal management during shutdown periods.
3Object-generated harmful factors
If traditional draft control methods are used with closed inlet guide vanes and dampers, then induced draft is reduced, but substantial natural draft remains due to leakages
Solution Approach 1:
The patent extracts and addresses the leakage issue by implementing active draft control that specifically targets and compensates for leakage paths. Rather than relying solely on passive closure of dampers and vanes, the system actively measures and compensates for the draft caused by leakages, achieving effective draft control despite the presence of leakage paths. This resolves the contradiction by making the control system effective even with imperfect sealing.
4Use of energy by moving object
If the HRSG is retrofitted to the turbomachine, then heat recovery capability is improved, but undesirable rubs may occur due to expansion and contraction discrepancies not factored into the cooling schema
Solution Approach 1:
The patent implements feedback control by continuously monitoring the thermal states of both the turbomachine and HRSG components. The active draft control system uses this feedback to adjust operating parameters and cooling rates, ensuring that expansion and contraction of retrofitted HRSG components remain coordinated with the turbomachine. This prevents rubs while maintaining the heat recovery capability provided by the HRSG retrofit.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enhances turbomachine efficiency by maintaining larger clearances during restart, reducing heat loss, and preventing undesirable rubs, thus facilitating quicker and more efficient operation.
Implementation Method 1
The ambient air infiltration may be induced, for example, by at least one of natural convection of hot gas contained in the turbomachine, HRSG, and/or flue gas stacks
Implementation Method 2
pressure differences caused by wind speed and wind direction at the turbomachine inlet
Data Source
Figure 1~2
Figure 3
AI summary
A combined cycle power plant CCPP (10) includes one turbomachine (12, 12') and heat recovery steam generator HRSG (14, 14') that is shutdown (or offline) and a second turbomachine (12, 12') and heat recovery steam generator HRSG (14, 14') that is online. The HRSG supplied duct dampers can provide sealing air therein to ensure no back flow from the online turbomachine to the shutdown turbomachine of the plant. However, as cooling flow is desired, a controlled flow may be implemented to allow cooling flow to the shutdown turbomachine. This solution is valuable to cool CCPP configurations with multiple (e.g., 2) HRSGs discharging into a common exhaust stack without individual flues dedicated to each HRSG, regardless of turbomachine configuration.