In deciding on whether or not to make repairs, the following factors should also
be considered:
be considered:
The repair cost per pound of weld material will decrease as the amount
of material increases because of fixed set-up time. This favors a longer
period between repairs. However, if the cavitation pitting characteristics
are such that cavitation damage tends to accelerate, increasing the time
between repairs may substantially increase the amount and cost of
repairs which must be made.
of material increases because of fixed set-up time. This favors a longer
period between repairs. However, if the cavitation pitting characteristics
are such that cavitation damage tends to accelerate, increasing the time
between repairs may substantially increase the amount and cost of
repairs which must be made.
If the time between repairs is lengthened, additional care will be needed
to prevent distortion because of the increased amount of weld material to
be added. Blade distortion may adversely influence unit performance and
may reduce the life of the runner.
to prevent distortion because of the increased amount of weld material to
be added. Blade distortion may adversely influence unit performance and
may reduce the life of the runner.
If repairs are made on a more frequent basis, they may be completed
within the time frame required for other maintenance work on the unit. If
repairs are delayed, an extended outage or a specific outage for
cavitation repairs may be necessary, resulting in reduced availability and
of possible lost generation or capacity benefits.
within the time frame required for other maintenance work on the unit. If
repairs are delayed, an extended outage or a specific outage for
cavitation repairs may be necessary, resulting in reduced availability and
of possible lost generation or capacity benefits.
"An economic evaluation of the optimum repair frequency should consider the
following costs:
following costs:
Unwatering and dewatering the unit
Setting up and removing of the runner maintenance platform
Labor and supervision for repair
Material
Lost revenue due to downtime
4.3 "CAVITATION DAMAGE INSPECTION
"Cavitation damage inspection should
be made from both the draft tube area
below the runner and from the stay ring/wicket gate area in the spiral (or semi-
spiral) case.
spiral) case.
"Inspection from the draft tube area should normally be done from a temporary
maintenance platform installed below the runner. On units greater than about 12
foot (3.7 m) diameter when no repairs are planned, draft tube inspection
maintenance platform installed below the runner. On units greater than about 12
foot (3.7 m) diameter when no repairs are planned, draft tube inspection
8