Steam System Turbine Trip Control Stability
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Solution Overview
Problem
The steam system in chemical plants experiences instability during turbine trips, leading to abnormal pressure drops and hunting in the high and low-pressure headers, due to rapid changes in steam flow and control valve operations.
Innovation Solution
Implementing an after-trip control mechanism in the bypass valve controller that increases the opening of the bypass valve beyond normal control settings when the turbine trips, and adjusts the pressure set values to prevent excessive pressure drops, ensuring stable steam flow by prioritizing steam supply from the high-pressure header to the low-pressure header.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the bypass valve opening is rapidly increased during turbine trip to maintain steam flow, then steam supply stability is improved, but pressure fluctuation and hunting in headers worsen
Solution Approach 1:
The control system dynamically adjusts the bypass valve opening based on real-time pressure conditions. During turbine trip, the system transitions from normal control to trip-time control, where the pressure setpoint is temporarily raised to prevent excessive pressure drops while maintaining adequate steam flow to the low-pressure header.
Solution Approach 2:
The control method changes the pressure setpoint parameter during turbine trip conditions. The pressure setvalue is temporarily increased to a higher value during the trip period, allowing the system to maintain stable pressure despite the sudden change in turbine demand, then gradually returns to normal setpoint after the trip.
2Stress or pressure
If the discharge valve opens rapidly to release excess steam pressure, then pressure control is improved, but system stability and hunting worsen
Solution Approach 1:
The control system anticipates pressure increases during turbine trip and preemptively adjusts the discharge valve control parameters. By raising the pressure setpoint during trip conditions, the system prevents excessive pressure buildup before it occurs, avoiding the need for rapid discharge valve operations that would cause hunting and instability.
3Speed
If control valve openings are rapidly adjusted to respond to turbine trip, then response speed is improved, but control stability and pressure fluctuations worsen
Solution Approach 1:
The system prepares control parameters in advance for turbine trip conditions. The trip-time control section pre-configures appropriate pressure setpoints and valve control strategies before the trip occurs, enabling smooth transition to trip-mode control without rapid, destabilizing adjustments. This preliminary preparation maintains both response effectiveness and stability.
Data Source
AI summary
In a steam system having a turbine driven by steam supplied from a high-pressure header to a low-pressure header, when the pressure in the low-pressure header drops, a turbine bypass valve is opened and the high-pressure side steam is supplied to the low-pressure side header in a normal control. When the turbine is tripped, steam is rapidly flow into the low-pressure side header and its pressure temporally increases. the steam in the low-pressure header is discharged through a discharge valve. After that, if a steam supply from the low-pressure header to another process increases, the discharge valve is closed. After the discharge valve is fully closed, an after-trip control is performed in which the opening of the turbine bypass valve is increased at an earlier timing than the normal control for preventing the steam amount in the low-pressure header to be too small. The control stability of the steam system when the turbine is tripped can be enhanced.