In our previous post, we explored the critical role that axial seals play in hydro-turbines and the challenges of balancing leakage control with the demands of large-scale operations. We also touched on the limitations of traditional sealing materials like carbon, which can struggle with abrasive wear and brittle failure. Now, let’s take a closer look at how elastomeric polymer seals, specifically Thordon SXL, are addressing these challenges head-on—offering enhanced durability, cost savings, and improved performance in demanding turbine environments. 

Elastomeric polymer seals offer several key advantages over their traditional carbon counterparts. The abrasion resistance of elastomeric polymers is one of their strongest attributes. In the abrasive water environments of hydro-turbines, particles like silt and sand often become embedded in carbon seals, accelerating wear and damaging the shaft. By contrast, the elastomeric nature of polymer seals allows abrasive particles to deform the seal surface temporarily, rolling off without embedding and causing permanent damage. 

Moreover, polymers like Thordon SXL can be easily machined to large sizes without segmenting. This eliminates one of the major drawbacks of carbon seals, which often require segmentation for large shafts, leading to potential leakage at the segment joints. Elastomeric polymer seals also exhibit excellent tensile strength and elasticity, which help them withstand the dynamic conditions of a hydro-turbine environment, such as fluctuating water pressures and centrifugal forces. 

An axial polymer seal typically consists of a stationary seal holder, a polymer sealing ring, and a mating metal surface attached to the turbine shaft. Springs provide the necessary force to maintain contact between the sealing surfaces, while cooling water is injected into a central groove to regulate temperature and reduce friction. 

One of the key design elements in axial polymer seals is the water groove that divides the sealing surface into two concentric areas. This allows filtered injection water to flow across both sealing surfaces, preventing abrasive particles from entering the seal. The water injection also serves a critical function in controlling the leakage rate and dissipating heat generated by friction. 

There are several challenges when designing polymer axial seals, primarily related to maintaining the balance between leakage control and operational longevity. Some of these challenges include: 

A case study involving one of the largest hydroelectric plants in the world showcases the effectiveness of Thordon SXL seals. The plant had been using a segmented carbon seal for its 4000 mm diameter Francis-type vertical turbine. The carbon seal suffered from high leakage rates, averaging 140 L/min, and was prone to severe wear due to the abrasive sand present in the water. 

In 2005, the plant operators retrofitted the turbine with a Thordon SXL seal. The polymer seal, which weighed significantly less than the original carbon seal, was composed of only six segments compared to the numerous segments required for the carbon ring. Despite concerns over balancing the new seal, the existing springs provided sufficient force, and the system was tested under varying conditions. 

The results were impressive: leakage rates dropped to a fraction of the original, and the seal showed minimal wear even after a year of operation. Monitoring showed that the seal maintained stable temperatures and required no significant adjustments. After a year, the seal was disassembled for inspection, and the sealing face showed almost no signs of wear, with only a small area showing 0.1 mm of abrasion. The operators predicted that the seal would last for the required 20-year operational life of the turbine. 

And don’t miss our next blog post, where we’ll showcase a recent large-scale axial seal installation in India—highlighting how these advancements are being put into practice on a global scale. Stay tuned!