Supersonic Ejectors

Capturing and Reusing Leaked Natural Gas from Pipeline Compressors

Supersonic ejectors (SSEs) are devices that capture low-pressure gas leakage from the gas seal vents of a compressor and then re-inject the captured gas into the fuel gas line of a gas turbine. By reclaiming gases normally vented into the atmosphere, SSEs help to decrease hydrocarbon emissions and reduce operating costs.

The Technology

SSEs were first developed in 2005 and have already experienced several rounds of modification and optimization under a collaboration agreement between TransCanada and Dresser-Rand. Dresser-Rand has been offering the technology to the market since 2009. For example, a family of three different ejector sizes was developed and tested in 2010. The different sizes allow the SSEs to cover a greater range of operating conditions associated with gas turbinedriven pipeline compressors. 

SSEs are devices that capture and boost the pressure of leaks from gas seals so that they can be re-injected into gas turbine fuel systems. They contain no moving parts and do not require compressor modifications or a power source. To re-inject the emissions, the seal gas pressure must be increased from about 50 pounds per square inch (psi) to 500 psi. 

This pressure increase can be achieved using a two-stage system with two ejectors working in tandem. In the first stage, a high-pressure motive gas from the compressor discharge accelerates through a supersonic nozzle into a mixing chamber. There, the motive fluid combines with the low-pressure fugitive emissions before going through a diffuser to create a higher (intermediate) pressure combined stream. In the second stage, another stream of motive gas accelerates through a second supersonic nozzle and into a mixing chamber where it mixes with the intermediate pressure combined stream. Finally, the combined gas enters a supersonic diffuser to achieve the pressure level required for re-injection. The ability of SSEs to increase the pressure of leaking gas by such a large degree, in order for it to be injected back into the fuel line, makes them unique in the industry

The Benefits

• Improved environmental performance: By reclaiming gases that would otherwise leak into the atmosphere, SSEs reduce hydrocarbon-related emissions.
• Cost savings: SSEs get their energy from the pressure of the gas in the fuel line, so there is no cost associated with powering them. Captured leakage gas is consumed by the gas turbine, therefore operating and maintenance costs are reduced due to lower fuel use.

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