7.2 Guide Bush Removal
The guide bush (8) is press fit into the bonnet
and does not normally require replacement. If
necessary, it may be pulled or machined out.
When machining the bush out, care must be
taken to maintain proper dimensions and
tolerances in the bonnet. These will be
furnished upon request.
7.3 Lapping Seats
Lapping is the process of working the valve
plug against the seat ring with an abrasive to
produce a close fit. If valve leakage is excessive,
lapping becomes necessary. The plug and seat
ring seating surfaces should be free of large
scratches or other defects, and the contact
surfaces of the seats should be as narrow as
possible. This may require dressing both parts
in a lathe. A good grade of fine grinding
compound is required for the lapping
operation.The compound should be mixed with
a small quantity of lubricant such as graphite.
This will slow the cutting rate and prevent
tearing of the seating surfaces. The amount of
lapping required depends on the materials,
condition of seating surfaces, and accuracy of
machining. lf a short period of lapping does not
visibly improve seating, there is usually no
advantage in continuing as excessive lapping
may result in rough seats. The only, remedy is
replacement or re-machining of one or both
parts. When lapping new plugs and seat rings,
begin with a medium compound and finish with
fine compound.
Note : Lapping should produce a line
contact area, not the entire surface, due
to the difference in seat angles.The
seating surface angle of the plug is 28
degrees and the seat ring is 30 degrees
(relative to the centerline axis).
7.3.1 Threaded Trim (Figure 3 )
1. Clean body gasket surface areas.
2. When seat has been removed, ensure
that the sealing surface in the body
bridge and the threads are thoroughly
cleaned.
3. lnstall and tighten seat ring using
fabricated wrench used for removal.
4. Apply lapping compound on the plug at
several spots equally spaced around the
seating area.
5. lnsert the stem and plug sub-assembly
carefully into the body until it is seated.
6. Place bonnet (9B) on the body and fasten
the bonnet to the body using four body
stud nuts (11) spaced equally apart. Apply
slight pressure and tighten evenly.
Caution : Before lapping, the plug and
stem sub-assembly must be concentric.
(See pinning operation, section 7.5).
Note : A sealant compatible with the
process fluid should be applied
sparingly to the seat ring threads and
sealing shoulder.
Caution: Do not over-tighten. Do not
strike seat ring lugs directly. This could
distort the seat ring resulting in seat
leakage.
Caution : Do not tighten nuts to final
torque specifications at this time. The
bonnet is used temporarily for guiding
purposes only.
Figure 5
Seat Lapping Device
7. lnsert two or three pieces of packing (7)
into the packing box to assist in guiding
the stem and plug during lapping.
8. Screw a drilled and tapped rod with a T-
handle onto the plug stem and secure with
a locknut (see Figure 5).
7
