The periodic repair of Pelton runners manufactured by casting presents various problems, which it is worth investigating: in view of the large number of runners of this type stilt operating. These problems are linked to the characteristics of castings, as will be discussed here.
The traditional casting of the Pelton runner involves producing a sand mould, in which the bucket is positioned the required number ot times around the circumference of the central hub. The positioning of the buckets, which is done by hand, is typically not accurate, particularly as regards the angular and radial bucket position.
The complex geometry of buckets, and the considerable variation in thickness, make it difficult to obtain uniform cooling rates in the different parts of the runner during casting; therefore the material structure can incorporate a large number of defects (such as sand and gas inclusions and cracks), from the beginning.
Because of the complex shape of the buckets, particularly the curved surfaces, the evaluation of defects within the material by non-destructive tests such as radiography or ultrasonic methods is very difficult [ASTM A609, 1990]. 'Therefore during the finishing of runners it is normal thatonly a small proportion of defects that result in surface discontinuities can be repaired (that is, the surface defects, which can be detected by liquid penetrant or magnetic particle inspection).
Summarizing, cast Pelton runners have:
- imprecise angular position, which can often be out of tolerance
- the wrong bucket slope in the radial direction, so that the tangent circle of the bucket plane is not exactly the same for all buckets (influencing the discharge
- efficiency of buckets); and
- the presence of many defects, which remain hidden in the runner for all of its life and tend to grow under stress.