BOG Compressor Load Ratio Control for Startup Temperature Limits
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Solution Overview
Problem
BOG multistage displacement compressors face challenges in controlling the temperature of low pressure stage-side discharge gas, which can exceed allowable limits during startup, especially when the suction gas temperature is higher than in steady operation, leading to potential compressor failure.
Innovation Solution
The method involves detecting the suction and discharge temperatures of the low pressure stage compression section and adjusting the load ratio between the low and high pressure stage compression sections to reduce the compression ratio, thereby preventing the discharge gas temperature from exceeding safe limits by using capacity adjusting devices such as suction valve unloaders and slide valves.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the compression ratio of the low pressure stage is increased to achieve higher discharge pressure, then the productivity is improved, but the discharge gas temperature exceeds the allowable limit for compressor operation
Solution Approach 1:
The compression process is divided into multiple stages (low pressure stage and high pressure stage) with intermediate cooling. The low pressure stage compresses the gas to an intermediate pressure and temperature, then the gas is cooled in an intercooler before entering the high pressure stage. This segmentation allows achieving high final discharge pressure while keeping the discharge temperature of each stage within allowable limits.
Solution Approach 2:
An intercooler is introduced as an intermediary component between the low pressure stage and high pressure stage. This intercooler acts as a mediator that removes heat from the compressed gas, lowering its temperature before it enters the next compression stage, thereby preventing excessive temperature buildup while maintaining the required pressure increase.
2Reliability
If the suction gas temperature is high during startup, then the discharge gas temperature will exceed the allowable limit, but reducing the compression ratio decreases productivity
Solution Approach 1:
The compression system operates dynamically with adjustable load ratios between stages. During startup when suction temperature is high, the control system adjusts the load ratio to reduce the compression ratio of the low pressure stage, preventing excessive discharge temperature. As the system stabilizes and suction temperature decreases, the load ratio is adjusted to optimize compression efficiency, allowing the system to adapt to varying operating conditions.
Solution Approach 2:
The system changes operational parameters (load ratio and compression ratio) based on operating conditions. When suction gas temperature is high during startup, the compression ratio is reduced to maintain discharge temperature within limits. When suction temperature is normal during steady operation, the compression ratio is optimized for maximum efficiency. This parameter adjustment resolves the contradiction between safety and productivity.
3Temperature
If the load ratio between low pressure stage and high pressure stage is reduced during startup, then the discharge gas temperature is controlled within limits, but the compression capacity is reduced
Solution Approach 1:
The system employs periodic adjustment of the load ratio between stages. During the startup period when suction temperature is high, a lower load ratio is applied to control discharge temperature. During the steady operation period when suction temperature is normal, the load ratio is increased to maximize compression capacity. This periodic adjustment strategy allows the system to prioritize temperature control when necessary and productivity when conditions permit.
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 effectively prevents the low pressure stage-side discharge gas temperature from exceeding the temperature allowable for operation, even during startup when suction gas temperatures are higher, by reducing the compression ratio and maintaining stable operation.
Implementation Method 1
a low pressure stage compression section 9 for compressing the BOG
Implementation Method 2
Because the BOG is adiabatically compressed in the BOG compressor, the temperature of BOG becomes higher at higher compression rate
Implementation Method 3
a high pressure stage compression section 10 for compressing the gas discharged from the low pressure stage compression section 9
Implementation Method 4
an intercooler 15 for cooling the gas discharged from the low pressure stage compression section 9
Data Source
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AI summary
Method for controlling the operation of a BOG multistage compressor (8) including plural stages of compression sections (9,10), which are connected with each other, for compressing boil-off gas (BOG) generated from liquefied natural gas. The method comprises, in case of a predetermined condition, making a ratio of a load on a low pressure stage compression section to a load on a high pressure stage compression section (load ratio) smaller than load ratios in case of conditions other than the predetermined condition. The said predetermined condition is, either a condition where the suction temperature of the low pressure stage compression section (9) is not lower than a first preset temperature, or a condition where the discharge temperature of the low pressure stage compression section (9) is not lower than a second preset temperature. This control method prevents the temperature of the low pressure stage compression section (9) from exceeding a maximum allowable temperature.