Power station turns to composite bearings

London -- First Hydro Company’s Dinorwig hydroelectric power station in North Wales operates six 300MW pump storage turbines giving a total output of 1800MW and can generate at full capacity within 16 seconds, from a spinning in air condition.

In 1999, Dinorwig started to replace its top and intermediate bronze guide vane bearings with Orkot TXMM -- a fabric-reinforced thermoset composite from Orkot Hydro Bearings, part of Trelleborg Sealing Solutions -- which incorporate a low-friction wear surface and solid lubricants to greatly reduce friction.

However, after two years some of the self-lubricating bearings had become tight on the guide vane shaft due to a change in the inside dimensions. The problem was linked to Dinorwig’s bearing design specifications and was resolved by a manufacturing tweak that proved the material’s stability in these conditions.

First Hydro had specified the same running clearances for the Orkot bearings as used with bronze bearings because of concerns over guide vane misalignment creating higher wear rates and loads in other parts of the regulating system.

As this was outside Orkot’s design recommendations, they were supplied with correct outside dimensions but smaller than required inside dimensions to allow final machining by First Hydro after installation in the housings. The bearings were interference fitted in the housings using a hydraulic press and bored to the finished size before installation in the turbine.

The change in dimension caused the bearings to grip the guide vane shafts, which was unacceptable as it created higher working forces and loads on other parts of the regulating mechanism as well as making dismantling the guide vane assemblies more difficult. Further analysis showed that the bearings had reduced in diameter by up 0.05mm in some cases.

There were only three possible causes of this: a significant change in temperature between bearing machining at Dinorwig and installation in the turbine; a longer term post-curing process in the material that meant it continued to cure by negligible amounts after manufacture; or pressure caused by the interference fitting forcing the material to creep very slightly during such a residual curing process.

To solve the problem, a method of stabilising the material was required. The manufacturing process was examined and it was decided that having a post-curing process of the material would relax the material and stabilise it.

Some time before this issue even arose, Orkot had created a long-term quality plan to manage its material manufacture tightly, which included a post-curing process. But it had not been proven at the time that this would create the material stability that would overcome the current issue.

Now, to prove Orkot’s solution, bearings were manufactured using the new post-curing process on the quality plan, machined to size and fitted into a bearing housing supplied by First Hydro. Two-weekly measurements were then taken over a two-year period and no significant change in the material dimensions was found.

In addition to this, any guide vane bearings that require smaller running clearances are designed with interference fits as low as is safely possible.

First Hydro is owned by IPM Eagle LLP, a joint venture of International Power Plc and Mitsui & Co. Ltd.