Pushing the envelope

Featured in the March 2019 issue of LNG Industry

Recent technological developments within the world of turboexpanders (TEXs) promise to make the utilisation of active magnetic bearings (AMBs) even more widespread across all gas-related industries, and to allow end-users and engineering, procurement and construction (EPC) companies the opportunity to benefit from the competitive advantages AMBs offer. For example, AMBs eliminate the need for lubricating oil.They also allow for a much smaller skid footprint in comparison to conventional oil-bearing skids and offer a significant reduction in scheduled maintenance costs.

Traditionally, safety concerns have placed the bearing controller panel outside of the skid and into a remote location (e.g.a control room). In many cases, plant designers pass on AMBs because of the costs and complications associated with the off-skid configuration.

In order to make the benefits of AMBs more accessible and appealing to the gas industry, L.A. Turbine (LAT), in collaboration with Waukesha Bearings, has developed the first TEX skid configuration that incorporates the AMB control panel entirely on the skid. This was accomplished by equipping the panel with a purge system that allows it to be rated for hazardous areas. This configuration removes several barriers for the application of AMBs in the industry and it offers the ability to ‘plug-and-play’, thereby removing some of the costs associated with installation and commissioning.

TEXs are standard in the natural gas industry for liquefaction and dew point control. They are also used in the petrochemical industry for ethylene plants, air separation, refrigeration and power generation. TEXs are also employed in small scale LNG plants, in which nitrogen is used as the refrigerant fluid to liquefy natural gas. Broadly speaking, the category of small scale LNG plants include those that produce anywhere from 50 000 to 500000 gpd. These provide LNG for high horsepower applications,including trucking, marine transportation, mining, locomotive and other industrial applications, to be transported via truck to end-user sites. TEXs are also found on floating production,storage and offloading (FPSO) barges for LNG rejection and gas injection applications.

The principle of operation of a turboexpander

TEXs were introduced in the mid 1930s when the first machine was designed and installed for air separation. The first TEX for a natural gas application was designed and installed in Texas in the early 1960s. Today, more than 5000 units are in operation globally.

A refrigeration cycle requires that the gas be greatly expanded in order to reduce its temperature until it reaches some level of liquefaction. This is referred to as the Joule-Thompson (J-T) effect and it can be accomplished with a valve. The J-T valve (or throttling valve) achieves a constant enthalpy expansion adiabatically with no work output. The expander is essentially a valve in that it accomplishes the sharp pressure drop but it also extracts work from the gas expansion via a turbine. By requiring the expanding gas to perform work, the resulting temperature can be further reduced and the efficiency of the refrigeration cycle improved.

The kinetic energy (work) produced by the turbine is consumed by a ‘loading’ element, which is mechanically coupled to the turbine via a spindle or shaft. This can be a dyno (oil-brake), an electric generator, or a centrifugal compressor stage. For the latter two, TEXs afford the opportunity to utilise energy which would otherwise not be available with a J-T valve. When coupled to a compressor stage, it can be used as a pressure booster to meet a need in the process gas. This additional pressure energy extracted by the expander from the process stream might otherwise be obtained by an electric or engine driven compressor, thus TEXs have the capacity to reduce front-end compression requirements.