Trading Risk for Reliability, Flexibility and Efficiency in NGL Plants

Gas processing plants need to cope with varying and often uncertain conditions. Operators of gas processing plants in North Dakota (USA) typically face challenges with the mode of operation (ethane rejection or ethane recovery) and feed gas uncertainty (rich gas or lean gas). In terms of equipment reliability, flexibility and efficiency, these circumstances also place significant requirements on the compressor technology used. This case study will discuss the deployment of a mechanical refrigeration cycle using commercial-grade propane (95 to 98.5% propane, with the rest being heavy hydrocarbons, or HD5, or higher-grade propane). In the context of different compressor technologies available for such applications (this includes oil flooded screw, integrally geared centrifugal or inline centrifugal, the authors examine the performance characteristics of integrally geared compressor technology applied in gas processing plants. Inherently, oil-flooded screw compressor systems require regular maintenance to ensure the availability of oil-free process gas. In turn, when seals or coalescing filters are not maintained or do not perform as expected, oil may carry over with the process gas and flows to downstream. These events require extensive cleaning and can lead to plant downtime. By comparison, integrally geared compressors and inline centrifugal compressors are 100% oil-free (no oil in compression chamber / process), providing increased reliability while requiring less maintenance. Also, integrally geared technology can be supplied with tilting pad thrust bearings which allow these propane refrigeration compressors to start at a higher suction pressure (i.e., settle out conditions on hot summer days), thus providing superior rotor stability while saving the propane because there is no need to flare the gas to reduce the system settle out pressure (Patel and Struck 2017). With regards to the parameter of flexibility, the authors will discuss how the variable diffuser guide vanes (vDGVs) are helping to provide the process flexibility, thereby extending compressor turndown up to 50% without recycle. vDGVs can maintain a required and subsequently designed discharge pressure that gives operators flexibility with varying mole weight and head requirements. vDGVs also help with start-up during high settle-out conditions like those in refrigeration processes. It was found that integrally geared compressors are about 10% more efficient than oil-flooded screw compressors. As each impeller has its own casing and seals, it will allow for easy accommodation of side streams. Also, integral gearing can match the impeller geometry to the required speed which results in higher compression efficiency, while dry gas seals reduce process gas leakage to improve plant reliability. Lastly, it was found that an integrally geared refrigerant compressor delivers more than USD 200 000 per year in OPEX savings, in addition to lower CAPEX of up to approximately 20% (based on the study done for a 200 MMSCFD plant).