Sacrificial PTFE Boosts Compressor Efficiency

Dec. 9, 2009
Fluorosint 500 enhanced-PTFE combines sacrificial, abradable sealing with a coefficient of linear expansion approximating that of aluminum for use in a high-speed compressor shroud.

Quadrant Engineering Plastic Products, (800) 729-0101,

UGI Energy Services,

Fundamentals of a turboexpander,

A high-tech compressor/expander for liquefied natural gas spins at over 60,000 rpm while cranking out nearly 950 hp, thanks partly to abradable PTFE on the rotor shrouds that lets fast-moving blade tips lightly touch the shroud without destroying either the rotor or shrouds.

The expander, an enhanced version of an older model, has better rotors that efficiently handle higher speeds and larger flow volumes. Built by Simms Machinery International, Santa Maria, Calif., and dubbed the XXM design, it uses relatively tight rotor-blade-to-shroud clearances to boost efficiency. This increases the risk of rotor-to-shroud rubs that could cause catastrophic failures. To reduce this risk, Simms sought an abradable material for the shrouds that would let fast-moving blade tips make light contact without harm.

Simms Engineers eventually settled on Fluorosint 500-enhanced PTFE from Quadrant Engineering Plastic Products, Reading, Pa. This grade of Fluorosint combines sacrificial, abradable sealing with a coefficient of linear expansion about equal to that of aluminum. Quadrant fabricated a set of prototype inserts for Simms to install and finish.

The incremental horsepower made possible partly by the Fluorosint increased liquefaction at the original plant from 2.5 to 4.6 mmscfd without adding fuel or power. Fluorosint 500 on the rotor shrouds of the expander and compressor also reduced risks of mechanical failure.

The XXM turbo-expander/compressor sits in a UGI Energy Services plant near Reading, Pa. It works in a refrigeration cycle that condenses high-pressure gas from an interstate pipeline into LNG for storage at low pressures. A stream of gas, dried and precooled to 20°F, goes into the expander-end of the machine at 840 psia. It exits at 80 psia and –158°F. The cold gas from the expander flows through a heat exchanger where it condenses and subcools a second gas stream that then flows through three stages of flash separation to reach the equilibrium temperature of –256°F in the LNG storage tank.

On the warm side of the exchanger, the expander discharge returns to the compressor-end of the expander/compressor where it is boosted to 160 psia and delivered into a pipeline that feeds a gas-distribution network. Energy extracted from the gas by the expander would otherwise be wasted in a pressure-regulating station where pressure would normally be reduced across a regulator or control valve.

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