important that bridge, trolley, and hoist can be "jogged" to allow precise positioning
of the load. Any problems or deficiencies with the crane
must be corrected
before the overhaul is started.
Many times it is helpful to fabricate special lifting devices or install a trolley track to
facilitate disassembly. Any shop-fabricated lifting device or fixture must be designed
and certified by an engineer qualified in this field and tested at 125 percent of the
rated safe working load. All lifting devices and rigging hardware must be designed
with a safety factor of no less than 5:1 based on the tensile strength of the materials
Supervisors, craft personnel, and engineers should meet to discuss what work will be
required during the overhaul so that the schedule can be optimized. A critical path
flowchart can be very helpful in scheduling an overhaul. Typically, the flowchart
shows the order in which various tasks must take place, the time required for each
task, and which tasks can be accomplished concurrently. There are a number of
"critical path method" software packages for personal computers available to assist
in setting up a flowchart. This software allows the schedule to be easily adjusted
when unexpected delays occur. Many of these packages will also allow
manipulation of the schedule based on available personnel.
In setting up the schedule for a major overhaul, it is important to be realistic and
possibly a little pessimistic in regard to what work will have to be done and how
quickly it can be accomplished. On the first unit to be overhauled, extra time should
be allowed for unexpected problems.
A checklist should also be maintained to make sure all tasks are completed and to
record when they were completed. - As the overhaul progresses, the list should be
modified to include any items missed on the original list or remove any items not
required. The checklist should include any testing or adjustments that will be
required before the units are started or released for operation.
Hydraulic transient study
If the unit is being uprated, it is likely that the closure rate for the wicket gates will
have to be changed to prevent overpressuring the penstock following a load
rejection. To determine what the new wicket gate timing should be, a water hammer
or hydraulic transient study should be performed with the new parameters for the
uprated unit. If the overhaul is not an uprate, the original test data or records
showing the recommended wicket gate timing should be located. If this information
is not available, a hydraulic transient study should be performed. In the case of an
uprate or lost test data, load rejection tests must be performed following the
overhaul to verify the results of the hydraulic transient study.