Compressors shrink in size, grow in power

Feb. 27, 2006
With pipeline construction set to grow in 2006 and beyond (OGJ, Feb. 13, p. 57), the need for a variety of different compression solutions will grow as well.

With pipeline construction set to grow in 2006 and beyond (OGJ, Feb. 13, p. 57), the need for a variety of different compression solutions will grow as well. Varied installation environments and job requirements have led to an equally varied array of compression technologies. Always sought, however, are increased power and efficiency, and increasingly important is reduced environmental impact, whether from emissions, noise, or physical footprint.

Urban settings

In recent decades natural gas pipelines and compression stations have expanded into increasingly high-population centers. The need for quiet, emission-free compression operations in such settings has led pipeline operators to pursue alternative compression options. Dresser-Rand developed its Datum Model C compressor electric motor-driven compressor as one such alternative.

The new pipeline compressor features an integrated, high-speed electric induction motor and modular design, allowing it to be placed near urban areas where electric power is readily available and environmental impact a concern.

Dresser-Rand developed the Datum-C with a major US pipeline client and the US Deptartment of Energy.

The client needed to build a pipeline station in an underground bunker, requiring little maintenance support and no effect on the surrounding landscape. The client also sought the security provided by an underground installation.

In 2003, Dresser-Rand had secured funding for the project as part of a larger DOE initiative for US manufacturers to improve infrastructure reliability in the natural gas industry.

Dresser-Rand selected Curtiss-Wright Corp. of Roseland, NJ, to provide the electric motors for the compressor, based in large part on the company’s work for the US Navy.

Curtiss-Wright supplied an integrated two-pole induction motor with magnetic bearings and a self-contained natural gas cooling system. The unit operates at 10 Mw, with 6,900 v available at the terminals and a maximum continuous speed of 12,000 rpm.

In-line flanges

The in-line piping flanges of the Datum-C allow the entire compressor to be installed directly into the line in both indoor and outdoor locations, greatly reducing infrastructure costs for new installations and for retrofit applications, according to Dresser-Rand..

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The small footprint-about 8 × 12 ft-and modular design also make installation and maintenance easier, Dresser-Rand says, with the compact size and quiet operation removing the need for a traditional pipeline station (Fig. 1).

The Datum-C is designed with a single-stage impeller (Fig. 2), producing an inlet flow of 600 MMcfd and inlet pressure of 650 psig at 60-80° F. The discharge pressure is 1,160 psig. A second-generation Datum-C will use up to five impellers to reach a discharge pressure as high as 2,500 psig, allowing for greater volumes to be transmitted through a given diameter.

Magnetic bearings

While magnetic bearings have been used for many years, previous compressor systems relied on older analog control systems, making the bearings difficult to tune and adjustment only possible through changing resistors and capacitors. Repeat adjustments on these bases could add significantly to downtime and expense.

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Digital controls, however, using computer-based algorithms make tuning the Datum-C relatively straightforward, according to Dresser-Rand.

Because of the magnetic bearings, there is no need for an oil lubrication system, oil film seals, or dry gas seals. The unit is a completely closed system, with no gas turbine or gearbox, making it extremely quiet and emission free.

The PGT25+G4 gas turbine was specifically designed for applications across the oil and gas industry, including large transport pipelines, and onshore-offshore production facilities, coupling a two-stage, high-speed module with the enhanced LM2500+G4 gas generator (Fig. 3).
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The Datum-C provides variable speed operation and can remain pressurized during shutdowns.

Additional frame sizes are planned, with operating powers of 5 Mw to 20 Mw.


Estimated cost details of a compression station using the Datum-C were not available, but Dresser-Rand asserts savings in both the installation and operations phases of using the compressor as opposed to a more conventional solution.

Estimated construction costs for similarly scaled compression (10 Mw = 13,410 hp) between July 2004 and June 2005 in Oil & Gas Journal’s exclusive annual Pipeline Economics Report (OGJ, Sept. 12, 2005, p. 50) ranged from $1,018/hp to $1,786/hp, with completed project costs filed during the same time period generally lying within the same range.

Second only to the cost of the unit itself, the single biggest expense in compression construction is labor. Dresser-Rand maintains that the Datum-C will also reduce installation time, thereby cutting this expense as well.

Additionally, Dresser-Rand expects Datum-C to provide much better reliability because it has fewer auxiliary support systems (e.g., speed increasing gearbox, lubricating oil system, shaft seal system) which can cause a shutdown if a they fail, thereby lowering maintenance costs.

Dresser-Rand expects the Datum-C to enter service later this year.

Bulk power

Also set to be in service in 2006 is GE Oil & Gas’ PGT25+G4 turbine (Fig. 3). The two-stage high-speed power turbine is designed to achieve efficiencies of greater than 40%, which GE says is the highest thermal efficiency level available in its power range (34 Mw = 45,594 hp).

Operating at 6,100 rpm, the power turbine can directly drive a compressor without the need of a gear in most oil and gas applications.

The PGT25+G4 generates its 34 Mw at ISO conditions, a 10% increase over its predecessor. It also offers a significant increase in power output at high ambient temperatures, with power rising 20% at 120° F. Given the typically linear nature between horsepower and transport rates, the value of this sort of power increase in the same footprint is clear

The turbine can operate on a range of fuels, including natural gas, various gas mixtures, and liquid fuel. Its dry low emissions version uses five valves instead of its predecessors three, providing more accurate combustion tuning. The DLE version operates with NOx and CO emissions of 25 ppmvd at 15% O2 in the 100-75% load range.