Electricity and Control November 2024

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: ƒ Unnecessary machine trips caused by surge and ex cessive recycle ƒ Split shaft compressor designs that are susceptible to cascading trips due to interdependency between refrigeration loops and rotating equipment ƒ Parallel compressors can create instability in the pro cess if not properly set up for load sharing ƒ Control loop interaction can drive adjacent compres sor sections towards surge ƒ 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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