Using A Damper Seal To Eliminate Subsynchronous Vibrations In Three Back-To-Back Compressors.

Abstract

A new type of labyrinth seal that reduces cross coupled rotor forces and produces a remarkable amount of damping has been invented at Texas A&M University. Laboratory tests have shown complete elimination of critical speeds under some conditions and orders of magnitude more damping than conventional labyrinth seals. The new seal acts as a damper by dynamic variations of gas pressure in large pockets around the shaft that always oppose the rotor vibratory motion. The pocket walls also serve to block the gas swirl that produces the cross coupling in conventional seals. The background is described of the invention of the new seal along with two case histories of its design, installation, and use for solving subsynchronous vibration problems in back-to-hack centrifugal compressors. In Case 1, the seal construction is of conventional metallic materials, while in Case 2, the seal is made of an amorphous copolymer with engineered properties to produce a better tolerance of shaft rubs during surge events. The subsynchronous vibration problems were solved in both cases by retrofitting the new type of seal. In Case 2, a small number of seal blades was used in order to produce large pockets with a very large damping value, and the use of engineered plastic as a seal material allowed the machine to tolerate surge and remain stable, which had not been possible with conventional labyrinth seals.