Flowserve Edward Cast Steel Bolted Bonnet Valves - Manual (Page 8)

8
Flow Control Division
Edward Valves
Repair Procedures
(cont.)
oil to a molasses consistency is recom-
mended for finish lapping. For rough lap-
ping, Carborundum H20 coarse is recom-
mended.
In the lapping operation, lap against the
seat with a small quantity of the lapping
compound placed between the mating sur-
faces. It is important that not too much
pressure be applied on the lap or disk
against the seat. With the lapping com-
pound in place between the mating sur-
face, the lap or disk should be reciprocally
rotated as far as arm movements will per-
mit while standing in one position: the
strokes should be light, and the lap or disk
should be lifted frequently and turned to a
new position circularly around the valve
body so that lapping will be rotated over a
new area. To make certain the pressure
strokes are light, it is necessary on large
valves to suspend the disk and stem assem-
bly from a coil spring in such a manner as
to allow the disk to bear, but lightly,
against the seat. See Fig. A pg. 10.
For smaller size valves, a driving handle
can be easily made of 3/8” diameter wire
bent as per Fig. B. These small assemblies,
being much lighter, do not require a sup-
porting spring. Hard surfaced seating
faces are hard and lapping time is vari-
able, depending on the extent of flaws on
the surface and the position of the valve in
the line. If a seat requires machining prior
to lapping, portable boring machines are
available from Edward Valves.
The disk of stop valves may also require
refinishing. When the only defects that can
be found on the disk-stem assembly occur
on the seating surface, it becomes very convenient to push the stem into a lathe
spindle and chuck on the disk nut diameter
without taking the assembly apart.
(However, if the stem is too large to fit
through the lathe spindle, it will have to be
taken apart as described in the following
paragraph.) Hold the disk using a four jaw
chuck so that the large O.D. and seating
surface run true. Grind the seating surface
using a tool post grinder. Just go deep
enough to clean the surface. Polish the
seating surface with fine emery cloth.
If when checking the disk stem assembly, it
is found that the assembly is tight or does
not swivel freely, it will be necessary to
disassemble.
The lock key weld and locking key can be
drilled out. After the key has been
removed, the disk nut can be readily
unscrewed. Repair any damage surfaces
on the stem, disk nut, stem collars or disk.
Then proceed to repair the disk seating
surface as described above. When finish-
ing the disk in this manner, it will not be
necessary to lap it to the seat.
BODY WALL REPAIRS
There are five basic steps in repairing a
casting defect:
1. Cut out to sound metal. Attempting to
weld over the defect will only leave a
notch that may re-introduce the defect.
Cutting may be done by chipping,
grinding or flame gouging. The amount
of metal removed should be held to a
minimum to avoid distortion during sub-
sequent welding.
2. Preheat, using the minimum temperature
specified by the material specification
and/or the design code. Use at least
400F on WC9 or C5 material, 300F
on WC6. Although cast carbon steels
such as WCB or WCC do not require
preheat, it may be advantageous to
remove any moisture or other contami-
nants from the area to be welded. This
may also identify any leak paths. There
are also disadvantages to preheat,
especially localized preheat, that must
be considered when working in areas
of the casting with finish machined
dimensions. Distortion may result in
more damaging problems than those
concerns created by the original defect.
Lower preheats and the control of inter-
pass temperature are two methods used
to minimize distortion.
3. Welding should be done by qualified
welders, using qualified procedures and
weld material of a chemistry matching
the casting (see Table B for welding rod
recommendations). The final weld
should be blended into the contour of
the casting.
4. Stress relieving is generally recommend-
ed. Decisions to not stress relieve should
factor in piping code rules. The temper-
atures must be based on material speci-
fication and piping code recommenda-
tions. Again, since temperatures are
much higher than those experienced in
welding, there are also disadvantages
that must be considered. Distortion may
result in more damaging problems.
Lower temperature postweld heat treat-
ment is sometimes an option for carbon
steels.