| Organic Rankine cycle
Above: Figure 4. Overall aerial-view visualisation of the ORC system, gas turbines and heat recovery units at the Dahshour gas compressor station
The capacity of the new gas turbine driven compressor is equal to that of the existing gas turbine driven compressor trains and that of the electric motor driven compressor trains is half that of the existing GT driven compressor trains. Hence, the gas compression installed capacity will be expanded from actual 979 MMSCFD (4 x 326.3 MMSCFD compressors in N+1 arrangement) to 1632 MMSCF. The additional 653 MMSCFD capacity consists of 1 x 326.3 GT driven and 2 x 163.15 MMSCFD electric motor driven compressors, with one GT driven compressor as standby spare. The goal is achieved thanks to the waste heat recovery system employing ORC. The waste heat recovery units (WHRU) employ heat exchangers installed in the exhaust gas systems of the existing and new gas turbines, so that the sensible heat from the hot flue gas streams (with exhaust gas temperatures of up to 560°C) can be recovered into a hot oil loop
that transfers heat to the ORC system. The ORC system produces electricity to power the motor-driven compressors and other electrical consumers in the gas compressor station. Figure 6 shows a visualisation of the waste heat recovery units and oil heat transfer circuit of the Dahshour gas compressor station. Moreover, since an electric motor is consistently smaller than a gas turbine, this solution allows best use of space and optimal layout, as well as reduced gas consumption of the compressor station and shorter start-up times. Furthermore, electric motors operated in partial load conditions achieve higher efficiency compared with the traditional gas turbine.
This novel project ultimately increases the compression station´s gas processing capacity while at the same reducing the amount of natural gas consumed to compress natural gas transported in the pipeline system.
Island mode and control system As already mentioned, the ORC system will operate in island mode, ie, it will continuously feed the compression station’s internal electrical loads (motors, new compressors, existing equipment, HVAC, light etc) without importing electricity from the grid.
This mode of operation requires a very flexible and responsive power plant, able to keep frequency and voltage within very strict limits. In order to achieve this goal, the electricity generated and the electricity consumed within the localised small grid must be regulated with a dedicated control automation system, a “power management system” (PMS).
The PMS prioritises the power requirements for all available operating gas turbine driven compressors, and continues operation of the electric motor driven compressors based on power available from operating ORC units. The system is designed to guarantee operation under all operating scenarios (transient and normal), taking into account that the connection with the national grid is not possible.
The ORC control system / power management system provides the frequency signal (50 Hz ± 2%) as a set point to the compressor suction flow controller, which, in turn, sets the electric motor speed controller, achieving a flow corresponding to the maximum available power level, aiming at continuously keeping the frequency at 50 Hz, plus acceptable frequency tolerance. The new motor driven compressors are configured in a master/follower arrangement floating over the same suction/discharge conditions as other parallel turbo compressor units.
Above: Figure 5. Visualisation of ORC units at the Dahshour gas compressor station
The gas pumping capacity of the EMD will therefore be decided by the electrical frequency
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