Unique Compressor Design Allows Efficient Operation Over Wide Range

Compressor characteristics aligned with the client’s needs; emissions regulations addressed with electric motor driver.

For 60 years, natural gas pipeline companies relied primarily on reciprocating compressors to move gas along the line. Most of this compression was in the form of integral gas engine compressors, which combined the power cylinders and the compressor cylinders on the same frame in a compact configuration. These units operated at low speed (250 – 400 rpm, depending upon make and model) and were able to efficiently handle a wide range of operating pressures and compression ratios.

However, the increasingly wide-spread imposition of the EPA Clean Air Act requirements by the early 1990s forced pipeline operators to explore new solutions for the expanding network. As it turned out, however, these new solutions did not come without problems of their own.

el-paso2Technical Solution Translates into Optimum Reliability, Operational Flexibility and Efficiency

Ten years ago, El Paso Pipeline Partners (prior to its acquisition by Kinder Morgan, Inc.) initiated a study of its compression fleet to understand the life cycle costs and operational issues associated with the various driver/compressor technologies on its pipeline system.

When El Paso Pipeline Partners required additional compression at its Coudersport, PA Station 313 on the Tennessee Gas Pipeline in December 2009, they considered the tradeoffs associated with each of the potential technologies. Familiar with Dresser-Rand’s long tradition as a supplier to the pipeline industry and its comprehensive range of offerings – including centrifugal and both low- and high-speed reciprocating compressors – El Paso chose Dresser-Rand as a partner with whom to develop the optimum solution for this project.

As a clean air non-attainment area, an emissions permit at Station 313 would be very difficult for new stationary sources. Permitting was avoided by selecting an electric motor driver instead of a natural gas fired engine or turbine. To effectively cover the station’s wide operating range requirements a reciprocating compressor was requested. A centrifugal compressor has a limited range at fixed speed, and the compact build of the high-speed reciprocating compressor mitigates against easily-accessible, cylinder-mounted capacity control devices. The high-speed design is further disadvantaged by relatively poor compression efficiency at low compression ratios, and higher vibration at off-design conditions.

El Paso opted for the rugged design, high reliability and flexible operating range of Dresser-Rand’s process reciprocating compression technology, supplemented with an updated pipeline cylinder design that incorporated our legacy know-how with our latest advances in design practices, materials, valves, and capacity control.

The heavy-duty, slow-speed model HHE-VL reciprocating compressor frame has set the standard for reliability in the process compressor marketplace. These compressors are designed for critical services, and are often expected to operate uninterrupted for three to five years between service intervals. The Dresser-Rand process reciprocating compressor product line covers a range of models with up to 10 crank throws and can support driver powers from 200 to 45,000 horsepower (33.5MW) and discharge pressures to more than 60,000 psi (>4,000 bar).

El Paso purchased a 4-cylinder, single-stage HHE-VL compressor driven by a 6,500 horsepower synchronous motor at 360 rpm. Synchronous motors offer very high efficiency, but their primary advantage over induction types is the high power factor, which minimizes current draw and, therefore, power cost.

Given the daily and seasonal fluctuating flows and pressures at Station 313, the HHE-VL compressor solution was ideal, because it can handle all operating points at peak efficiency without wasting power due to gas recycle or operation far from the best efficiency point that would have occurred with a centrifugal compressor. Dresser-Rand estimates that the average operating efficiency with the reciprocating unit is at least 10 points higher than would have been achieved with a centrifugal unit.

Operational Flexibility with No Emissions

Reciprocating compression efficiency is fundamentally about pressure drops because of the orifice effect of compressor valves, capacity control devices, cylinder gas passages, and pulsation vessel nozzles and internals – any of which may be unnecessarily accentuated by poor pulsation control system design. These factors are especially important in natural gas pipeline services, which inherently involve very low compression ratios. Small design compromises can lead to large differences in power consumption per unit of gas compressed. The slow-speed process compressor design has significant advantages compared to the higher speed gas patch designs that have also been applied to this service, including at Station 313.

The longer stroke means larger cylinders, which enables more valve area and larger gas passages, both of which reduce pressure drop. Further, since much of the valve inefficiency occurs at opening and closing, it is logical that a machine operating at twice or three times the speed (and, therefore, has two to three times the number of events) will also have higher valve losses. The slow-speed compressor design also permits larger pulsation bottles, which provide better pulsation attenuation with less pressure drop over a broader operating range. And the extra physical space permits more capacity control devices, which enables closer load following without wasted power.

In addition to the efficiency benefits, the more massive HHE-VL frame provides greater rigidity that reduces vibrations transmitted to station piping. The fine-grain, cast iron HHE frame provides maximum stability through the use of internally ribbed walls and integral cross-member bearing saddle supports located between each crank throw.

The HHE-VL unit was the first low-speed reciprocating compressor installed in a natural gas pipeline application in more than a decade. Initial performance tests achieved 92-94 percent isentropic efficiency at Station 313. This translates into an estimated five to 10 percent savings in electrical power consumption when compared to other compression technologies – possibly more if the alternatives are driven by an induction motor instead of a synchronous one. Also, since these efficiencies were delivered over the whole operating range, it should not be necessary to revamp this unit in the future to restore high efficiency if station conditions stray from what they are today.

In summary, Dresser-Rand’s package combines the most efficient electric motor driver with the most efficient compressor. The unique design of this compressor allows efficient operation over a wide range of compression ratios. This will add flexibility to the entire transmission network, resulting in improved performance and reliability of the entire system – all with zero emissions and the lowest life cycle cost.

Tags: HHE Compressor, Reciprocating Compressors

other articles from insights issue Autumn / Winter 2013