Draft Tube Liners. The pitting of the turbine runner and in the upper part of draft tube has
occurred in almost all types of runners and kinds of metals used. It is even reported that the runner
blades of 5 cm thick have been eaten in less than two years. In avoiding the pitting, the draft head is of
greatest importance. A poorly designed runner may be set with a low height above tailrace level and
show no pitting, while a properly designed runner may be set high enough above tailrace level but
pitting is unavoidable.