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A labyrinth seal is essentially a series of annular orifices utilized to seal a region of high pressure from a region of low pressure. These are "clearance seals" and as such, leak. In a centrifugal compressor the impeller eye seals, the shaft seals between impellers, and the balance piston seal all are sealing a high pressure area from a low pressure area.
Labyrinth seals (labys) in centrifugal compressors can have a significant impact on compressor efficiency. In a centrifugal compressor, work is done to increase the gas pressure and this pressure is contained by labyrinth seals throughout the compressor. Compressor efficiency can be improved by reducing labyrinth seal leakage. However, the seal clearance cannot just be arbitrarily reduced, because reduced clearance metallic seals can rub and open up; resulting in a negation of the efficiency gain, potential rub related vibration problems, and possible damage to the rotating element.
This article contains excerpts from the paper, “POLYMER SEAL USE IN CENTRIFUGAL COMPRESSORS – TWO USERS’ EXPERIENCES OVER 15 YEARS” by John K. Whalen of John Crane Engineered Bearings, Jim Allen of Nova Chemicals Corporation, Jonathan D. Cardell of DuPont Packaging & Industrial Polymers, and John R. Dugas, a consultant, at the 2013 Middle East Turbomachinery Symposium.
Labyrinth seals made from high performance thermoplastics, when properly designed and installed, can be utilized because when they rub, during normal transients such as traversing a critical speed, they will "give" and then regain their original "pre-rub" geometry. This is the main driving force behind the use of reduced clearance thermoplastic labyrinth seals in centrifugal compressors. A typical metallic labyrinth is installed with clearance to the shaft in the "as-installed" case. In the "during rub" situation, which is commonly encountered, the rotor contacts the seal and causes permanent deformation of the seal tooth tip. Therefore, in the "after rub" case the tooth remains deformed and excessive clearance results, leading to increased leakage. It is also possible that galling can take place on the rotating surface as contact between the metallic seal and the rotor occurs. Also during the rub, enough energy may be imparted to the rotor to cause vibration problems and the associated reliability concerns.
With thermoplastics, the "as-installed" clearance is typically tighter than with the metallic seal. During the rub, the tooth deflects, moving with the rotor during this transient. After the rub the tooth regains its original "as installed" configuration, no damage occurs to the rotor, and the initially reduced clearances are regained.
The most common thermoplastics used to manufacture labyrinth seals for centrifugal compressors are DURATRON and PEEK. Other materials used include Fluorosint and Vespel products. All of these thermoplastics are supplied in various grades where the blending of the final product can influence mechanical properties and chemical compatibility. Fluorosint is considered an advanced engineering plastic. It is a crystalline high performance polymer with superior chemical resistance. Fluorosint is Teflon (PTFE, or polytetrafluoroethylene) filled with synthetic mica. This material, being Teflon based, lacks strength (750 to 1200 psi tensile strength), and needs to be carefully evaluated for each application. The best use of Fluorosint is smooth bore segments secured in a metal holder, which is then rolled as an assembly into the compressor. Here, rotating laby teeth can cut into this very abradable material. It is an excellent choice when the seal teeth are machined onto the rotating element. Because of its low strength and the availability of superior materials, it is rarely recommended for use when the teeth are machined into the stationary part.
PEEK, when used as a laby seal material, is typically either filled with 30% or 15% carbon fibers, 10% graphite powder and 2% PTFE. The carbon fibers are added for strength while the graphite and PTFE are added for lubricity. PEEK is highly resistant to chemical attack but will be attacked by concentrated, strong acids at high temperature. PEEK is sensitive to chromic, hydrofluoric, nitric, and sulfuric acids. It is unaffected by acetic acid, amines and hydrocarbons.
Duratron (PAI or polyamide-imide) is referred to as an imidized material, which is used in extreme service environments. Its properties usually classify this material as an amorphous thermoplastic. The grade typically used for compressor labyrinths is filled with 12% graphite powder and 5% PTFE, both added for lubricity. Note that no fillers are required for strength, making this material easier to machine than the PEEK products. Duratron is hygroscopic, which means it can absorb moisture (to 5% by weight and 2% by volume). Proper design, machining, storage, and installation prohibit this moisture absorption potential from being a problem. Duratron is less resistant to chemical attack then PEEK. It is sensitive to amines, ammonia, oxidizing acids, and strong bases. It is unaffected by aliphatic, aromatic and halogenated hydrocarbons.
For smooth bore polymer seals in centrifugal compressors, Fluorosint is often the recommended material. The material of choice for centrifugal compressor labyrinths is Duratron, assuming it will not be subjected to chemical attack and the temperature is low enough (usually below 350° F (177 °C)). If Duratron is susceptible to chemical attack, then PEEK can be used provided the temperature is low enough (below 250° F (121 °C)). Thermoplastic use in rotating equipment can increase efficiency and reliability. Understanding their properties is critical to successful applications.