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COMPRESORS HOW TO USE TURBOEXPANDERS IN ENERGY RECOVERY NEW DESIGN FOR NATURAL LETDOWN STATIONS BY FREDDIE SARHAN

atural gas from producing wells and storage facilities is typically The turboexpander generator reduces pressurized to facilitate efficient Ntransportation in pipelines across gas pressure and converts the resulting long distances. The pressure is stepped down at pressure letdown (PLD) stations kinetic energy into electrical energy. at various stages across the network. The PLD station depressurizes the gas to lower levels for safe delivery through Traditionally, the turboexpander uses the overall installation, entailing ongoing local distribution networks such as local a large gear box to adjust the speed of the maintenance. gas mains, smaller-diameter service lines expander , which is coupled to a These drawbacks can be largely over- and individual meters for industrial, com- synchronous generator. It requires a lubri- come through the use of new magnetic mercial and residential end users. cation system that includes an oil reser- bearing technologies that eliminate the Pressure reduction is most commonly voir, pump cooler, filters and piping. lubrication subsystem. A cutaway model accomplished through the installation of Seals are needed to separate the lubri- of an alternate turboexpander system is Joule-Thomson (JT) regulating valves, cant from the high-pressure to shown in Figure 2, known as the FreeSpin which reduce the gas pressure to the prevent contamination. The lubrication Inline Turboexpander (FIT). The hermet- desired level. A simplified JT valve is system is the least reliable component in ically sealed unit consists of an integrated shown in Figure 1. It consists of a high-speed turboexpander and a mechanical valve, which is throt- permanent-magnet generator, both tled to create the desired pressure driven by a single spinning rotor differential between the inlet and levitated on non-contact magnetic outlet The JT PLD device is a sim- bearings. ple and generally effective tool for The direct-drive, perma- pressure reduction, but a by-prod- nent-magnet generator has vari- uct is waste . able speed and load capabilities A turboexpander and generator and can run at rated speeds up to can be used to capture this waste Figure 1: Simplified model of a Joule-Thomson pressure 30,000 rpm. The flow of the natu- energy. The turboexpander gener- regulating valve ral gas is used to cool the stator, ator reduces gas pressure and con- rotor and magnetic bearings. verts the resulting kinetic energy Since moving parts do not into electrical energy, which can come into direct contact, there are be sold to the grid. no friction losses. No auxiliary Turboexpanders have been lubrication system is needed. This successfully deployed for energy eliminates risk of oil contamina- recovery in , refiner- tion, provides a smaller footprint ies, geothermal power recovery, and lower installation and mainte- steam pressure letdown stations, nance costs. turbochargers and gas turbine The FIT works as follows: recovery, as well as in 1. The pre-heated high-pres- , and hydrogen sure gas flows into the turboex- liquefaction. However, such sys- pander tems have had difficulty in attain- 2. The gas expands through the ing viability for natural gas PLD radial turbine wheel applications due to capital costs Figure 2: Cutaway model of the Calnetix FreeSpin Inline 3. The low-pressure gas exits and operating expenses. Turboexpander the turboexpander and is ready for

32 Turbomachinery International • November/December 2019 www.turbomachinerymag.com flow into the distribution and delivery net- Thomson pressure regulator, which is vapor condensation. As a result, the FIT work for end users installed in parallel with the turboex- system can include an inlet heating source 4. The energy wasted during the pres- pander (i.e., waste heat recovery). The flow sure reduction process is recovered by the 6. The produced electricity enters the through architecture of this system is turboexpander and converted to electrical Vericycle Bi-directional Power Electron- scalable to approximately 500 kW. power ics Unit, which can be programmed to 5. Any excess high-pressure gas not specific power requirements. Initial prototype directed through the turboexpander is Pressure drop is often accompanied by Collaborating with Baker Hughes, a pro- expanded by the conventional Joule a drop, and this can cause totype 300 kW unit was built and tested at a PLD station in Bologna, Italy. The unit is designed for a life of over 10 years without maintenance. Based on prelimi- nary data, Baker Hughes projects an esti- mated potential energy recovery of over 1.6 million kWh per year. ■

Freddie Sarhan is Vice President of Business Development at Calnetix Technologies, the devel- oper of the FreeSpin In-line Turboexpander with many years of expe- Figure 3: FIT rience in turbomachinery and industrial flow diagram ORC heat recovery systems. For more infor- mation, visit calnetix.com

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