Natural gas is a major source of energy; however, users are located far from gas fields making transmission through pipelines impossible. By super-cooling natural gas to -160 degC (-260 degF), the gas (primarily methane) becomes a clear and odorless liquid occupying 1/600th of its volume in its gaseous state. The liquefied gas can then be efficiently stored & shipped to the various users around the world.
The cooling process associated with large-scale LNG production facilities involves some of the most highly sophisticated heat exchanger and refrigeration compressor applications found anywhere in the world. The safe, reliable, and efficient operation of the multiple, multi-section compressor trains found in these facilities is critical to the profitability of the operation as a whole. Specific process challenges include:
- Unnecessary machine trips caused by surge and excessive recycle
- Split shaft compressor designs that are susceptible to cascading trips due to interdependency between refrigeration loops and rotating equipment
- Parallel compressors can create instability in the process if not properly set up for load sharing
- Control loop interaction can drive adjacent compressor sections towards surge
- Adjustment of gas composition on Mixed Refrigerant compressor trains can reduce the effectiveness of the surge control system
Cascading Trips – Multi-section trains in split shaft arrangements must work in highly interactive configurations to ensure prevention of compressor surge and cascading trips. To avoid tripping without over aggressive tuning, CCC developed a modification to the control system to prevent surging as well as driver overload of a running compressor when the other has tripped. This is achieved by allowing the antisurge valves of the running string to temporarily open to a predefined position when the other string has tripped or is shutdown. For example, in a split shaft arrangement where the MR HP compressor is located on the same shaft as the Propane compressor, if the propane compressor – and subsequently the MR HP stage, shutdown, the antisurge valves on the string fully open in an effort to protect the machine. Sudden full opening of the MR HP antisurge valve will drive the MR MP stage and sequentially the MR LP stage into surge. The needed aggressive tuning to prevent surging results in the antisurge valves to go nearly full open which leads to overloading the compressor driver causing an under-speed trip. The new CCC control command temporarily opens the MR LP and MP stages’ antisurge valves to a configurable predefined position and holds the position for the disturbance to pass. The MR would continue to run under full recycle and shutdown due to surge or under-speed would be prevented.
Load Sharing – High speed inter controller communications allow for decoupling of controller interactions. Parallel refrigeration machines traditionally used in the Phillips Cascade LNG process and more recently the APCI LNG process can be successfully operated by controlling suction header pressure of the individual refrigeration circuits and using load-sharing control to balance the parallel compressor strings. Load-sharing is accomplished by equalizing the individual compressors’ distance from surge parameter. In addition, by the use of proper load-sharing control, the individual compressor strings are easily and safely sequenced online and offline automatically.
Avoiding surge of individual compressor stages is the end result. Ultimately, preventing a shutdown of the compressor string due to excessive surging or speed reduction caused by driver power limitations.