Recycle Valve Control for Compressor-Expander Critical Speed Avoidance
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Solution Overview
Problem
Turbomachinery compressor-expander sets face instability and potential damage due to operation in critical speed zones, which can lead to surge and disrupt the gas refrigeration process, especially when a compressor-expander set trips or is shut down.
Innovation Solution
Implementing a control system that uses adjustable nozzles and recycle valves to manage the rotational speed of compressor-expander sets, avoiding critical speed zones by increasing flow rate through the recycle valve when entering a critical speed zone, and employing a feed-forward signal to prevent surge during shutdowns.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the compressor-expander set operates through critical speed zones during startup or shutdown, then the operational flexibility is maintained, but the turbomachinery experiences instability and potential damage from prolonged operation in critical speed zones
Solution Approach 1:
The control system performs preliminary action by detecting entry into a critical speed zone and automatically initiating avoidance maneuvers before damage can occur. The system monitors rotational speed, identifies critical zones, and preemptively adjusts operational parameters to prevent prolonged operation in unstable conditions.
Solution Approach 2:
The system dynamically adjusts operational parameters such as rotational speed and flow rate in real-time based on the current operating state. When approaching a critical speed zone, the control system modifies these parameters to rapidly traverse through the unstable region, minimizing exposure time and preventing sustained operation at critical speeds.
2Loss of time
If high angular accelerations are used to traverse critical speed zones quickly during startup, then the time spent in critical zones is minimized, but the mechanical stress and potential damage to the turbomachinery increases
Solution Approach 1:
The control system employs periodic monitoring and adjustment cycles to manage traversal through critical speed zones. Rather than applying continuous high acceleration, the system uses controlled periodic adjustments to rotational speed and flow rate, creating a rhythm of acceleration and stabilization that reduces mechanical stress while maintaining efficient traversal.
Solution Approach 2:
The system changes multiple operational parameters simultaneously and in a coordinated manner during critical zone traversal. By adjusting rotational speed, flow rate, and other parameters in combination, the system achieves rapid traversal through critical zones while distributing mechanical stress across different system aspects, preventing concentrated damage.
3Device complexity
If the compressor-expander set is shut down by simply stopping the driver, then the shutdown procedure is simplified, but surge may occur causing large thrust loads and potential damage to thrust bearings and vanes
Solution Approach 1:
The control system performs preliminary action before shutdown by detecting the shutdown condition and preemptively adjusting operational parameters to prevent surge. The system reduces flow rate and adjusts rotational speed in advance of the complete shutdown, ensuring the compressor operates in a stable region throughout the deceleration process.
Solution Approach 2:
The system uses feedback from sensors monitoring flow rate, pressure, and rotational speed to continuously adjust shutdown parameters. The control system responds to real-time measurements, modifying the shutdown trajectory to maintain stable operation and prevent surge conditions, thereby protecting the turbomachinery during shutdown.
4Reliability
If recycle valves are opened to increase flow rate and avoid critical speed zones, then the turbomachinery operates more stably, but the energy consumption and process efficiency may be reduced
Solution Approach 1:
The control system employs skipping by rapidly traversing through critical speed zones rather than operating within them. By using temporary flow rate increases via recycle valves only long enough to pass through the unstable region, the system minimizes energy loss while achieving the goal of avoiding prolonged operation in critical zones.
Solution Approach 2:
The recycle valve operation follows a periodic pattern of opening and closing based on the operational state. The valve opens temporarily when approaching a critical zone to provide additional flow and stability, then closes once the system has traversed through the unstable region, thereby minimizing the duration of reduced efficiency while maintaining operational stability.
Data Source
AI summary
A control method and apparatus for critical rotational speed avoidance in a compressor-expander set in a gas refrigeration system. By varying an opening of an antisurge or recycle valve, a shaft power used by the compressor in the compressor-expander set may be varied, thereby varying the rotational speed of the compressor-expander set to move it away from its critical speed zone. Additionally, a feedforward signal may be provided by a compressor-expander set control system to cause an antisurge valve for a recycle compressor to open upon a trip or shutdown of one compressor-expander set.