the bonnet and score the stem. Also, when a line is
subjected to freezing temperatures, liquid trapped in the
valve bonnet may freeze and rupture it.
Valves that have been in constant service over a
long period of time will eventually require gland
tightening, repacking, or a complete overhaul of all
parts. If a valve is not doing the job for which it is
intended, it should be dismantled and all parts inspected
for corrosion, scarring, and erosion of seating surface
and valve body. All defective parts must be repaired or
The repair of globe valves (other than routine
Figure 16-41.--Examples of spotted-in valve seats.
renewal of packing) is generally limited to refinishing
the seat and disk surfaces.
When refinishing the valve seat, do not remove any
more material than is necessary. Valves that do not have
replaceable valve seats can be refinished only a limited
The manual process used to remove small
number of times.
irregularities by grinding together the contact surfaces
Before you begin any repair work on the seat and
of the seat and disk is called grinding-in. Grinding-in
disk of a globe valve, check the valve disk to make
should not be confused with refacing processes in which
certain it is secured rigidly to and is square on the valve
lathes, valve reseating machines, or power grinders are
stem. Also, check to be sure the stem is straight. If the
used to recondition the seating surfaces.
stem is not straight, the valve disk cannot seat properly.
To grind-in a valve, first apply a small amount of
Carefully inspect the valve seat and valve disk for
grinding compound to the face of the disk. Then insert
evidence of wear, for cuts on the seating area, and for
the disk into the valve and rotate the disk back and forth
improper fit of the disk to the seat. Even if the disk and
about a quarter of a turn. Shift the disk-seat relationship
the seat appear to be in good condition, they should be
from time to time so that the disk will be moved
spotted-in to find out whether they actually are in good
gradually, in increments, through several rotations.
During the grinding-in process, the grinding compound
will gradually be displaced from between the seat and
disk surfaces. Therefore, you must stop every minute or
so to replenish the compound. When you do this, wipe
The method used to visually determine whether the
both the seat and the disk clean before applying the new
seat and the disk of a valve make good contact with each
compound to the disk face.
other is called spotting-in. To spot-in a valve seat, first
When it appears that the irregularities have been
apply a thin coating of prussian blue evenly over the
removed, spot-in the disk to the seat in the manner
entire machined face surface of the disk. Then insert the
disk into the valve and rotate it a quarter turn, using light
Grinding-in is also used to follow up all machining
downward pressure. The prussian blue will adhere to
work on the valve seats of disks. When the valve seat
the valve seat at those points where the disk makes
and disk are first spotted-in after they have been
contact. Figure 16-41 shows what correct and imperfect
machined, the seat contact will be very narrow and will
seats look like when they are spotted-in.
be located close to the bore. Grinding-in, using finer and
After you have noted the condition of the seat
finer compounds as the work progresses, causes the seat
surface, wipe all the prussian blue off the disk face
contact to become broader. The contact area should be
surface. Apply a thin, even coat of prussian blue to the
a perfect ring covering approximately one-third of the
contact face of the seat. Again place the disk on the
valve seat and rotate the disk a quarter of a turn.
Do not overgrind a valve seat or disk. Overgrinding
Examine the blue ring on the valve disk. The ring should
tends to produce a groove in the seating surface of the
be unbroken and of uniform width. If the blue ring is
broken in any way, the disk is not making a proper fit.
disk. It also tends to round off the straight, angular