Component design often overlooks the importance of well engineered seals. A properly engineered sealing system increases the mean time between component failures, reduces manufacturing costs associated with the use of exotic coatings, and reduces power consumption due to unnecessarily high friction.
Application data required to select a seal includes:
•Size – shaft, housing bore, available seal width
•Temperature – continuous and maximum
•Application parameters – equipment, sealing surface misalignment to housing bore, dynamic shaft run-out
•Media – type and level of lubricant
•Pressure – continuous and maximum
•Shaft surface speed – continuous and maximum.
From these, select either lip seals or isolators.
Lip seals are typically retained in a housing bore by a rubber-to-metal or metal-to-metal press fit, requiring considerable installation force. Press fits can let metal shavings enter a bore housing, leading to contaminated lube and premature bearing failure. Also, nicks, burrs, or scratches on a shaft surface can damage a seal lip and produce a leak. A mounting tool prevents damage, such as lip roll-over.
By comparison, isolators are easy to install. Isolator seals facilitate the installation and maintenance of sealing systems. They usually have O-rings on their inner and outer diameters to seal on the shaft and against the bore respectively. To prevent damage to O-rings, the sealing surfaces of the shaft and bore must be cleaned prior to installation. O-rings are not dynamic sealing elements so they are not subject to wear. Once the equipment is cleaned and inspected, the isolator can usually be installed by hand pressure alone.
There are many other sealing tasks in a nacelle. Hydraulic equipment, of course, needs them. Leak detectors (sensors) on some seals measure leakage. Onboard electronics then provide some analysis and can send results to a computer or telephone. This allows remotely monitoring a seal and scheduling an exchange when necessary in a normal maintenance interval.
German DIN 3760 standards describe function and lifespan for such seals. The sensor-seal combination is available in many different dimensions. The seals protect motors and machines in original equipment and provide options for maintenance personnel. Designs in special materials are available especially for wind turbines.
Where blades meet hubs also call for a seal. These junctions must all be weather tight yet allow rotation. Several designs in a range of materials are well suited for these applications. For instance, a form-pressed continuous ring also works well on the large dimensions encountered in wind turbines. Its good performance is due in part to a rust-proof tension spring which presses permanently against the seal rim. The seal comes in 200 to 1,700-mm diameters.
Profile rings for large seal areas are batch vulcanized for advantages over glued rings. Profile rings come in standard materials and others. The rings are said to ensure long working lives, low torques, high resistance, and security against press out. They are well suited for sealing pivoting large diameter bearings found on pitch and yaw mechanisms.
One of the more unusual sealing tasks on turbines deals with offshore foundations. Wind-power stations there are built using structures such as monopiles and tripods. How the turbine is installed depends on soil properties, water depth, and contractor experience. A monopile (single leg) provides an application example. After the pile is driven into the sea bed, its top will extend up to about 16-ft below the water surface. A transition piece, about 80-ft high, is lowered over the top of the pile and will clear the water by some 40 ft. The space between the pile and transition piece is sealed by an inflatable grout seal, much like an inner tube. This seal inflates with a few psi to close the substantial gap between monopole and extension. Grout is then pumped into the gap above the grout seal to produce a strong joint. A floating crane then installs the tower to the extension and the turbine.
Filed Under: Seals & slip rings