19
V.
FIELD MACHINING PROCEDURES
5.1
Seal rings
One of the most common field machining operations performed on hydroelectric
units is the boring of the stationary seal rings. In most cases, the operation involves
boring new seal rings, but it may also include the machining of the discharge ring,
throat ring, and/or headcover to accept the new rings.
units is the boring of the stationary seal rings. In most cases, the operation involves
boring new seal rings, but it may also include the machining of the discharge ring,
throat ring, and/or headcover to accept the new rings.
The boring bar must be precisely set up so that the rings are bored concentric to
each other and to the unit guide bearings and generator stator. To accomplish this,
the desired center of the unit with reference to the seal rings must be determined.
To find the unit's center, the upper bridge and headcover should be installed after all
the rotating parts have been removed. A single tight wire should be hung through
the unit and reference measurements taken from the stator, the turbine bearing
housing, the packing box bore, and to trammel points in the draft tube. The trammel
points may be bolts welded to the draft tube liner with the heads machined to match
an inside micrometer or any marked point where measurements can be repeated
exactly.
each other and to the unit guide bearings and generator stator. To accomplish this,
the desired center of the unit with reference to the seal rings must be determined.
To find the unit's center, the upper bridge and headcover should be installed after all
the rotating parts have been removed. A single tight wire should be hung through
the unit and reference measurements taken from the stator, the turbine bearing
housing, the packing box bore, and to trammel points in the draft tube. The trammel
points may be bolts welded to the draft tube liner with the heads machined to match
an inside micrometer or any marked point where measurements can be repeated
exactly.
If the generator guide bearings are not adjustable, the lower bridge should also be
installed and reference measurements taken from the lower and upper bridges. Once
the measurements have been taken and the centers plotted, it can be determined
where the center of the seal rings should be. Ideally, the generator stator, all of the
guide bearings, and the seal rings should be perfectly concentric, but some
compromise within the recommended tolerances may be required.
installed and reference measurements taken from the lower and upper bridges. Once
the measurements have been taken and the centers plotted, it can be determined
where the center of the seal rings should be. Ideally, the generator stator, all of the
guide bearings, and the seal rings should be perfectly concentric, but some
compromise within the recommended tolerances may be required.
In determining the center for boring the seal rings, several things should be taken
into consideration. The center of the turbine bearing housing should be within the
allowable tolerances for center of the stator
into consideration. The center of the turbine bearing housing should be within the
allowable tolerances for center of the stator
Although it is
not a simple job, the turbine bearing housing, in many cases, can be moved to center
and redoweled. If the seal rings must be bored significantly off center to make them
concentric to the unit center, there must be sufficient "meat" to allow boring to the
required diameter. In some cases, the packing box is not movable and will be the
limiting factor in how far the seal ring centers can be moved. If the calculations
show that after moving the seal ring centers as far as they can be moved, the stator
will still be excessively off center, it may be necessary to move the stator. As
moving the stator can be difficult, it should only be attempted if absolutely necessary
and after careful planning.
and redoweled. If the seal rings must be bored significantly off center to make them
concentric to the unit center, there must be sufficient "meat" to allow boring to the
required diameter. In some cases, the packing box is not movable and will be the
limiting factor in how far the seal ring centers can be moved. If the calculations
show that after moving the seal ring centers as far as they can be moved, the stator
will still be excessively off center, it may be necessary to move the stator. As
moving the stator can be difficult, it should only be attempted if absolutely necessary
and after careful planning.
Once the center for the seal rings has been determined, it can be helpful to move the
plumb wire to the center point and take the final reference measurements. While
this is not necessary, it will simplify the calculations required during the installation
of the boring bar. The boring bar is positioned and plumbed by centering it at the
reference points at
plumb wire to the center point and take the final reference measurements. While
this is not necessary, it will simplify the calculations required during the installation
of the boring bar. The boring bar is positioned and plumbed by centering it at the
reference points at