Wind Energy Seals
A wind turbine may have up to 5,000 separate components, and some of the most important components are the seals. In this article we are going to look at wind turbine seals, including polymeric seals, and how they are chosen.
Need some information? Check out these educational articles keeping you ahead of the curve on the Advanced EMC Technologies blog:
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Types and Functions of Seals
There are five major areas where seals are used in wind turbine: rotor, blades, tower, alternator, and gear box.
The bearing rotor seal is considered the most crucial seal in a wind turbine, and assists with holding the main shaft in place, and prevent the entry of contaminants and debris. It would be a rotary shaft seals, possibly spring energized with an additional lip to keep dust out. The most common material is HNBR (hydrogenated nitrile butadiene rubber) to resist ageing, ozone, and damage from debris.
Pitch seals, also known as blade seals, help control the angle at which the rotor blades face the wind. They actually comprise a sealing system, with a dynamic seal, a sacrificial wear ring, and a static seal al working together. They are subject to lower rotary speeds than, say, the bearing rotor seal, which means they experience minimal thermal strain. Also like the bearing rotary seal, HBNR is often used.
Nacelle, or tower, also requires a double-lip seals where the inner lip holds lubricant in place and outer lip keeps debris out. The alternator seal is another seal whose job is to hold the lubricant in place.
Gear box seals usually take the form of a PTFE labyrinth seal, sometimes combined with a HNBR deflector seal. The gear box is often considered the heart of the wind turbine, and correct selection of seal here is vital. PTFE (polytretrafluoroethylene) is often chosen because it resists degradation in the presence of ozone, ageing, and is compatible with most (if not all) lubricants
There are also seals used to assist in locking the rotor blades during maintenance, which poses it own challenges including potential abrasion and poor lubrication.
Selecting Polymer Seals
Certain data is needed when it comes to selecting a good polymeric seal for a wind energy application. First is the size of the seal, which is a function of the diameter of the shaft, the diameter of the housing bore, and the available width for the seal.
From a purely engineering side, you need to know the expected pressures. Another vitally important detail is the surface speed of the shaft, which will tell you if you need a lip seal or an isolator. You also need to know application parameters such as an idea of the misalignment of the sealing surface relative to the housing bore. You will also need an idea of the dynamic shaft run-out.
To select the material, you need information on what the expected temperatures are, including the continuous environmental temperature and the maximum temperature. Temperatures can typically range from -30°C to +80°C depending on the region where the wind turbines will be used. The seal material can influence the type and level of lubricant used, which is yet another design detail needed when selecting a polymer seal for a wind turbine.
Elastomeric seal materials include rubber, NBR (nitrile butadiene rubber). NBR is known for low-compression set, resistance to ozone-related degradation, and good performance at low temperatures. HNBR (hydrogenated NBR) is another flavor of NBR that is commonly used. Molded cloth-impregnated nitrile is another widespread choice, but are known for issues with wear and are not especially reliable, causing frequent maintenance and associated revenue loss.
PTFE and Polyurethane are common polymeric choices. These materials offer very low friction, excellence performance in abrasive conditions, excellent tear strength, and good resistance to possible extrusion during service. They are usually extruded or machined to precise cross-section dimensions. Polymeric materials have demonstrated their ability to reduce maintenance requirements, extend the service life of the turbines, and reducing bearing failures associated with contaminants.
Issues with Seals
There are a variety of other issues to be considered when choosing the optimum seal for an application:
- How much heat will be generated by friction between the seal and the sealing surface?
- Will the seal increase potential cone wear?
- Is it easy to use retrofit them to existing seals in use?
- How easy is it to install the seal out in the field?
- How well does the seal account for misalignment and runout?
- Does the seal do a good job of accommodating bearing deflections?
- Are they available in split configurations?
Engineered Polymer Seals Option
No one would argue with the fact that seals are a vital part of a wind turbine. It is important to choose the correct wind turbine seal for each function required, and make sure that the material is appropriate. Polymeric seals are becoming increasing popular as a material option for wind turbine seals, and it is expected that this trend will continue as engineering polymers continue to evolve.
Need more in-depth information on sealing solutions for wind energy applications, then download the Advanced EMC Technologies Wind Energy Sealing System Solutions Guide.