Waste Heat Steam Generator Evaporator Flow Stabilization
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Solution Overview
Problem
Combined cycle power plants face challenges in maintaining stable flow through the evaporator of the medium-pressure stage in waste heat steam generators designed for the forced flow principle, which complicates fuel preheating and increases costs due to the need for high-pressure components and reduced flexibility.
Innovation Solution
A method that overfeeds the evaporator heating surface of the medium-pressure stage with a water-steam separator to discharge excess water to a heat exchanger circuit for fuel preheating, ensuring stable flow and eliminating the need for large-volume drums and additional pressure equalization components, allowing for flexible operation and cost savings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If a forced-flow waste heat steam generator is used to eliminate large-volume drums, then start-up time is reduced, but stable flow through the evaporator heating surfaces becomes difficult to maintain
Solution Approach 1:
The patent applies preliminary action by pre-heating feedwater in an economizer section before it enters the evaporator heating surfaces. This preliminary heating ensures that water enters the evaporator at a controlled temperature, preventing thermal shock and maintaining stable flow conditions from start-up through operation, thus resolving the contradiction between fast start-up and stable flow maintenance.
2Use of energy by moving object
If feedwater is extracted from the intermediate-pressure stage for fuel preheating, then thermal efficiency increases, but flow stability through the evaporator is compromised
Solution Approach 1:
The patent introduces an intermediary feedwater line that branches from the feedwater supply to the intermediate-pressure evaporator and directs a portion of the feedwater to the fuel preheater. This intermediary connection allows selective extraction of feedwater for fuel preheating while maintaining sufficient flow through the evaporator, thus enabling both thermal efficiency improvement and flow stability.
Solution Approach 2:
The patent applies parameter changes by controlling the mass flow rate of feedwater supplied to the waste heat steam generator. By adjusting this parameter, the system ensures that even with extraction for fuel preheating, the evaporator receives adequate water flow to maintain stable operation across the entire load range, balancing thermal efficiency and flow stability.
3Device complexity
If the preheater and evaporator are tubed in a single pass without additional pressure equalization, then device complexity is reduced, but achieving stable flow requires very small inner diameters that eliminate the outlet manifold
Solution Approach 1:
The patent applies universality by designing the preheater outlet to serve dual functions: as the outlet for the preheater section and as the inlet for the evaporator section, eliminating the need for separate outlet manifolds and inlet distributors. This multi-functional design reduces component count while maintaining flow stability through careful pressure drop management in the preheating section.
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
This approach stabilizes the flow through the evaporator and economizer surfaces, reduces material and operational costs, and enhances system flexibility by maintaining consistent boiling temperatures across evaporator tubes, enabling efficient fuel preheating and reducing condensate recirculation pump requirements.
Implementation Method 1
a water-steam separator is arranged between the outlet of the evaporator heating surface and the inlet of the superheater heating surface of the intermediate-pressure stage, in which excess water can be separated from the steam
Implementation Method 2
Heat transfer occurs via a number of heating surfaces arranged in the HRSG in the form of tubes or tube bundles
Implementation Method 3
the evaporator heating surface of the intermediate pressure stage is overloaded, and thus a defined quantity of excess water, heated but not evaporated in the evaporator heating surface
Implementation Method 4
the forced-flow principle is applied, meaning that a high-pressure pump in the water-steam cycle delivers precisely the amount of water
Implementation Method 5
a heat exchanger circuit for fuel preheating, in which a defined quantity of excess water separated in the water-steam separator is introduced into the heat exchanger circuit
Data Source
Figure 1
Figure 2
AI summary
The invention relates to a combined cycle gas turbine plant and to a corresponding method for operating such a combined cycle gas turbine plant, in which, during load operation of the combined cycle gas turbine plant, a water mass flow which is supplied according to the forced-flow principle to a waste heat steam generator is adjusted such that the evaporator heating surface (6) of the medium pressure stage is oversupplied and thus a defined amount of excess water, which is heated in the evaporator heating surface (6) but not vaporized, is discharged via the water-steam separator (11) to a heat exchanger circuit for preheating fuel for the gas turbine.