COVER ARTICLE
Optimising operation and profitability of compressor trains
A nti-surge control from Compressor Controls Corporation, and available locally from PREI Instrumentation, is designed to optimise the operation and profitability of multiple, multi-section compressor trains. The cooling process associated with large-scale LNG (liquified natural gas) production facilities involves some of the most highly sophisticated heat exchanger and re- frigeration compressor applications found anywhere in the world. The safe, reliable, and efficient operation of the mul- tiple, multi-section compressor trains found in these facili- ties is critical to the profitability of the operation as a whole. Specific process challenges include: Unnecessary machine trips caused by surge and ex- cessive 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 pro- cess if not properly set up for load sharing Control loop interaction can drive adjacent compres- sor sections towards surge Adjustment of gas composition on Mixed Refrigerant compressor trains can reduce the effectiveness of the surge control system. Solution Cascading trips – Multi-section trains in split shaft arrange- ments must work in highly interactive configurations to en- sure 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 anti-surge valves of the running string to open tempo- rarily to a predefined position when the other string has tripped or is shutdown, and the anti-surge valves on the string would fully open in an effort to protect the machine. Sudden full opening of the MR HP anti-surge valve will drive the MR MP stage and sequentially the MR LP stage into surge. The aggressive tuning needed to prevent surging results in the anti-surge valves opening almost fully, which leads to overloading of the compressor driver causing an under-speed trip. The new CCC control command opens the anti-surge valves of the MR LP and MP stages tempo- rarily, to a configurable predefined position, and holds the position for the disturbance to pass. The MR would contin- ue to run under full recycle and shutdown due to surge or under-speed would be prevented. Load sharing – High speed inter-controller communi- cations allow for decoupling of controller interactions. Par- allel refrigeration machines traditionally used in the LNG
process can be successfully operated by controlling suction header pressure of the individual refrigeration circuits and using load-sharing control to balance the par- allel compressor strings. Load sharing is accomplished by equalising the individual
compressors’ distance from surge parameter. In addition, by using proper load-sharing control, the individual com- pressor strings are easily and safely sequenced online and offline automatically. Results Avoiding surge of individual compressor stages is the end result – effectively preventing a shutdown of the compres- sor string due to excessive surging or speed reduction caused by driver power limitations. □
Schematic of a typical gas processing plant.
Schematic of a typical turboexpander train.
For more information contact PREI Instrumentation. Tel: +27 (0)11 867 5001 Email: sales@prei.co.za Visit: www.prei.co.za
NOVEMBER 2024 Electricity + Control
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