Thermo-hydraulic simulations of centrifugal compressors are usually based on a set of information derived from manufacturer design tools that are used for compressor modeling. When operating gas pipelines transportation companies use the simulation tool to predict and anticipate the behavior of the gas pipeline under different scenarios. The operation with different gas compositions from different supplies is not uncommon as well as the exposure to gas quality changes whenever contingencies happen to the gas processing plants. This paper address this important subject, presents a methodology of evaluation, a sensitivity study using different gas qualities and shows the impact on compression overall performance. A compressor is selected based on a set of conditions and then is checked for performance changes when working with different gas quality. The results are presented for comparison purposes and some suggestions are also presented on how to handle this situation to guarantee that the thermohydraulic simulation tools will always provide good results when dealing with natural gas quality changes.
It is not uncommon to operate a gas pipeline with different gas compositions and even in those pipelines where we expect a stable operating condition operating problems from gas supply may cause the gas to be out of specification. The purpose of this paper is to evaluate this scenario and check its influence on centrifugal compressor performance and then suggest a procedure that might help designers, operating companies and also vendors to deal with such a situation.
The development of centrifugal compressor and gas turbine design through the last 20 years has been impressive. Pipeline compressors now routinely achieve or exceed 85% isentropic efficiency, while exhibiting wide operating ranges. The study of operating scenarios suggests certain requirements for the compression system. Beyond the quest for higher compressor peak efficiencies, the operating requirements set forth in this study, as well as in other references (Kurz, 2001) require a compressor capable of operating over a wide operating range at high efficiency.
The proposed methodology assumes that the gas volumes to be delivered to the market (Gas Fired Power Plant or Local Distribution Company) are energy based. Different gas compositions have different LHV (Low Heat Value) and for the same amount of energy gas volumes may differ significantly impacting compressor performance, temperature and pressure drop across the pipeline.
The following steps were adopted:
Identify different gas compositions
Run thermo-hydraulic simulations using generic compressor
Pre-select compressors and drivers based on the thermo-hydraulic results
Check compressors efficiencies
Check the overall efficiencies for compressor units (Compressor + Gas turbine)
Perform technical and economic evaluation
Make the final decision on the equipment selection.
The thermo-hydraulic simulations were performed using generic compressors and drivers, in steady state, for three different gas compositions.