Elliott’s New 140TCH Pipeline Compressor

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By Klaus Brun and Todd Omatick

Compressors are installed in natural gas pipelines to inject gas into the pipeline at its operating pressure and then recompress the gas at certain distances along the pipeline to compensate for its viscous pressure drop. Typical pipeline operating pressures range from several hundred psi to about 1500 psi (103 bar) for standard pipe diameters, which range from a few inches to greater than 60 in. (1524 mm). In the United States alone, there are currently more than 7000 compressors installed in natural gas pipeline compression services in more than 2.6 million miles (4.1 million km) of pipelines. Approximately 25% of these compressors are old, low-speed, integral reciprocating machines that were originally installed between 1940 and 1970, making them technically outdated and mostly obsolete.

Growing demands for natural gas driven by increasing liquefied natural gas (LNG) exports, new/refueled fossil power plants, and increasing domestic consumption, have triggered the need to rapidly increase the North American pipeline compression capacity, both through the installation of new units and the replacement of older, low-power units with modern, higher-power units. At the same time, environmental regulations require the reduction of carbon emissions and gas leakage from pipeline operations, which has resulted in the preferred installation of electric motors over gas engines or turbines as pipeline compressor drivers.

Most centrifugal compressors currently being installed in pipelines were originally derived from process gas, barrel beam-style compressors. For this design, a long, multistage impeller shaft is mounted on bearings and sealed at both ends of the casing, and a balance piston and thrust bearing are used to limit axial movement of the shaft. This design evolved from classical refrigeration, refinery, and chemical process compressors that were originally intended to handle high-pressure ratios and a wide range of different gases at high pressures.

However, this design style is suboptimal for basic pipeline service where the pressure ratios are relatively low and the gas composition is fairly constant, but the operating conditions are highly variable and the compressor must operate for long periods without scheduled or forced outages. Furthermore, because pipeline operators traditionally prefer to use gas turbine drivers, most current pipeline compressor designs are speed-matched to provide optimal performance at high, gas turbine speeds rather than at the lower speeds of electrical motors. Driving centrifugal compressors with electric motors requires the use of a gearbox in most pipeline applications.

Elliott’s new 140TCH pipeline compressor addresses some of these shortcomings with features that were specifically designed for natural gas pipeline service. It is also designed to minimize component count.

Elliott’s 140TCH is a single- or dual-stage, overhung design with an axial inlet that is electric motor driven without a gearbox (Figure 2). This design approach results in higher efficiency, reduced leakage from a single, dry gas seal, and the avoidance of a balance piston. The 140TCH comes with a direct connect, variable frequency drive (VFD) motor and a standard footprint, in addition to custom aerodynamics for optimum efficiency and extended operating time between scheduled overhauls. The gearless configuration provides a smaller footprint, reduced lube oil requirements, and higher net efficiency. The high-speed, VFD-powered motor significantly reduces carbon dioxide (CO2) and NOx emissions as compared to a gas turbine driver. The VFD addresses power-grid issues and allows for adjustable operation to match load/capacity requirements. The pipeline compressor’s single-lift plug-and-play design includes auxiliaries such as lube oil, a buffer gas panel, and integrated, customized controls. Piping and wiring are included on the skid. The Elliott 140TCH pipeline compressor is available from 10,000 to 42,000 hp (7460 to 31,332 kW) sizes to the full range of gas transport applications.

The 140TCH is designed to handle most modern, mid-sized, and large pipeline compression applications. The impeller aerodynamics are based on Elliott’s EDGE technology. With the axial inlet and flexible or customizable aerodynamic design, efficiencies of greater than 85% can be obtained. The direct drive compressor, with a variable speed electric motor, provides a wide operating range including efficient turndown operation. With three standard frame sizes, 15 MW, 25 MW, and 35 MW, the centrifugal compressor is designed for applications up to 7000 MMscf/d (198 × 106 m3/d) with a pressure ratio ranging from 1.15 to 1.8.

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