Development of a Supersonic Ejector for Capturing Very Low-Pressure Vent Gases and Re-Injection Into a High-Pressure Gas Stream

The purpose of the ejector device is to capture the gas leakage from a dry-gas seal at low pressure, and re-inject it into the fuel gas line to the gas generator (without the use of compressors or rotating elements), hence providing a means to utilize the gas that would otherwise be vented to atmosphere. Implementation of this device will also have the benefit of reducing greenhouse gas emissions to the atmosphere. The primary challenge to achieve the above goal lies in the fact that the leakage gas pressure is in the range of 70–340 kPag, while the minimum pressure required upstream of the fuel gas regulator is in the range of 2400–3300 kPag. The device consists of a two-stage supersonic ejector. The first stage is highly supersonic (nozzle exit Mach number ≃ 2.54), while the second stage is moderately supersonic (nozzle exit Mach number ≃ 1.72). Several tests where conducted on various configurations of the two stages on natural gas in order to arrive at the optimum design and operating parameters. The optimum design gave an expansion pressure ratio (motive/suction) of the order of 14.0 and compression pressure ratio (discharge/suction) of around 8.1. These ratios would meet the requirement of the minimum suction and discharge pressure mentioned above. This paper presents the optimum configuration arrived at after several iterations of different geometries of the supersonic nozzles, particularly for the first stage ejector, and presents the performance test results of the integrated system. The results indicate that the device would meet the requirements of capturing the low pressure, low flow dry gas seal leakage and re-inject it into the fuel gas stream with an overall ejector efficiency (based on thermodynamic availability) of 80%.