Shared Variable Frequency Drive for Sequential LNG Compressor Start-Up
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Solution Overview
Problem
Existing LNG liquefaction plants require multiple variable frequency drives for each high-power motor, leading to increased complexity, cost, and space requirements due to the need for separate start-up systems for each motor, which is impractical for large-scale operations.
Innovation Solution
A method where a single variable frequency drive is used to sequentially start and regulate the speed of multiple machine strings in an LNG liquefaction plant, connected to the power supply grid, allowing for efficient start-up and operation with reduced infrastructure and costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple variable frequency drives are used for each high-power motor in LNG liquefaction plants, then reliable motor control and start-up is achieved, but device complexity, cost, and space requirements increase significantly
Solution Approach 1:
The patent combines multiple variable frequency drives into a single shared unit that sequentially controls multiple motors in an LNG liquefaction plant. Instead of having separate VFDs for each compressor motor, one VFD is used to start and control motors in sequence, reducing the number of VFD units from multiple to one, thereby simplifying the overall system architecture
Solution Approach 2:
The single variable frequency drive is designed to perform multiple functions by controlling different motors at different times. The same VFD unit is reused across multiple motor start-up sequences, making it a universal control device that serves multiple purposes throughout the plant operation cycle
2Ease of operation
If multiple variable frequency drives are installed for each motor, then precise speed regulation is achieved, but plant space requirements and construction costs increase
Solution Approach 1:
Multiple VFD units are merged into a single shared VFD that provides speed regulation for multiple motors sequentially. This consolidation reduces the total equipment footprint and eliminates the need for multiple separate control cabinets, directly addressing the space reduction goal while preserving speed control functionality
3Reliability
If separate start-up systems are provided for each motor, then reliable motor starting is achieved, but device complexity and cost increase
Solution Approach 1:
The system implements preliminary sequencing logic that prepares and controls motor start-ups in a predetermined sequence. The single VFD follows a programmed start-up sequence, bringing motors online one by one in advance of full operation, ensuring reliable starting while avoiding the complexity of multiple simultaneous start-up systems
Solution Approach 2:
A sequential control mechanism acts as an intermediary between the single VFD and multiple motors. This control logic mediates the start-up process, managing the transition of the VFD from controlling one motor to the next, ensuring reliable operation without requiring direct separate connections for each motor
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 enables the construction of smaller, more efficient LNG plants by eliminating the need for multiple variable frequency drives, reducing costs and space requirements while optimizing motor speed for efficient operation.
Implementation Method 1
a variable frequency drive connected to a power supply grid having an its own frequency. The variable frequency drive is also associated to the electrical motors
Implementation Method 2
the plant comprises several heat exchangers in which the natural gas transfers heat to a refrigerating fluid
Implementation Method 3
these are 'condensing-type' processes, where the refrigerant used for the liquefaction makes use of its latent heat of vaporization to cool the natural gas
Implementation Method 4
a compressor or a set of compressors (for the vapor phase)
Implementation Method 5
a string is powered by a turbine and, optionally, by an electrical helper motor
Data Source
AI summary
A method for starting and operating a plant for the liquefaction of a gaseous product comprising the steps of electrically connecting a variable frequency drive to a motor of a first machine string; increasing the speed of the motor of the first machine string up until a first predefined threshold; electrically disconnecting the variable frequency drive from the motor of the first machine string; electrically connecting the variable frequency drive to a motor of a second machine string; the first predefined threshold is function of said frequency of the power supply grid. The variable frequency drive can be switched during operation of the plant among the strings according to process requirements.