– What we saw was that the teflon layer was worn down after 55 years, but because of the bronze, the plain bearing still worked. It's a good idea to use bronze in the backing as a safeguard in case the bearing wears out completely. It is normal for such ball valves to be overhauled every 25-30 years. 55 years is a lot, says Bjørn.
The original drawing from 1969 specified DU segments with steel backing. Upon opening, it turned out that DU-B segments with bronze backing had been used.
Tribology is the study of how surfaces in relative motion and contact act on each other. To find the right plain bearing, issues such as force, movement, friction, lubrication and wear are considered.
- In this bearing, the movement is smooth and the load is not so great. Then DU-B is enough. It's proven to be fit for purpose - so that's why it will work and replace it with equivalent DU-B segments, explains Bjørn.
If the load had been greater and the movements different, a different type of plain bearing might have been appropriate. We have alternatives such as stainless steel backing or composite bearings.
In this case, the deviation from the drawing has been useful as the bronze has functioned as a sliding bearing after the sliding layer was worn out, which is not unnatural after 55 years.
The recommendation to the customer is to replace the old segments with new ones in DU-B and at the same time switch at slightly shorter intervals next time.
