6. Clean the packing box thoroughly before replacing
packing.
7. Bonnet, plug, bushing, cage and seat ring may now be
inspected for wear and service damage. After deter-
mining the maintenance required, proceed to the
appropriate section of this IMO.
3.5
Repair
1. The purpose of this section is to assist maintenance
personnel by suggesting methods of component
maintenance which may be largely dependent on the
tools and machine shop equipment available. Each sec-
tion should be read and understood before proceed-
ing.
3.6
Lapping of Seats
NOTE:
Lapping is the process of working the valve plug
against the seat ring with an abrasive to produce a close fit.
When valve leakage becomes excessive, lapping becomes
necessary. The plug and seat ring seating surfaces should
be free of large scratches or dents, and the contact surfaces
of the seats should be as narrow as possible. Lapping can
correct shallow scratches or slight roughness only.
Excessive lapping produces a groove in the plug, therefore
plugs or seats having relatively deep scratches must be re-
machined.
1. Lapping compound should be grade “A” or finer. 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 seat surfaces.
2. Apply lapping compound to the seating surface of the
seat ring only. Compound on the characterized portion
of the plug could increase the clearance between plug
and seat, thus changing the flow characteristic and
rangeability of the valve.
3. A film of light machine oil on the valve plug stem will
facilitate stem movement during the lapping proce-
dure.
4. Carefully insert the valve plug and stem assembly into
the bonnet from the bottom. Avoid damaging the stem
threads or the walls of the guide bushing.
5. Position the bonnet and the plug assembly on the
body and install and tighten two or three body stud
nuts to secure the bonnet during lapping.
6. Install temporary packing so that the valve plug stem
will be aligned during the lapping operation. Any rope
packing may be used for this purpose.
7. Screw a lock nut onto the valve plug stem, then screw
the lapping tool onto the stem and lock it in position.
NOTE:
A “T” handle for the valve plug stem can be made by
welding a nut (with threading to match the plug stem
threads) to the center of a rod. (See Figure 2.)
8. Lap with short, oscillating strokes. The weight of the
plug, stem, and lapping tool provides ample pressure
for lapping - do not bear down.
9. Raise the plug occasionally, lower it to another position
and resume the oscillating strokes. This ensures an
even lap over the entire seating surfaces of plug and
seat.
10. Approximately 5 to 10 minutes lapping time, with
grade A compound, is usually required to obtain a sat-
isfactory fit between a new plug and seat.
11. Clean the plug and seat, then, holding the plug on the
seat by hand, apply compressed air to the inlet side of
the valve to check the tightness of the lapped parts.
Repeat the lapping procedure if necessary.
12. Disassemble the valve and clean all parts thoroughly.
Remove all traces of lapping compound. Remove the
temporary packing and clean the packing box.
3.7
Replacement of Soft Seat
1. The soft seat insert is captured between the upper sur-
face of the seat ring and the lower surface of the cage.
No special parts or tools are required.
3.8
Plug/Stem Disassembly
1. Using a drift punch, drive out the pin, a drill bit some-
what smaller than the pin should be used to remove
the remainder of the pin.
2. Unscrew the plug from the stem.
3.9
Packing Box
Packing box maintenance is one of the principle chores of
routine servicing. Tightness of the packing is maintained
by packing compression. Compression is achieved by
evenly tightening the packing flange nuts against the
packing flange. Care must be taken not to over tighten as
this could prevent smooth operation of the valve. If all
compression is used up and the valve leaks, new packing is
required.
Packing box maintenance may be performed as a part of
major disassembly and repair or as a separate routine with
the valve installed in the process line. In either case the fol-
lowing steps should be closely adhered to, otherwise seri-
ous injury or equipment damage could result.
CAUTION: Valve must be isolated and the pressure vent-
ed before performing packing box maintenance.
1. Loosen and remove packing flange nuts.
2. Raise packing flange and packing follower up the valve
stem.
NOTE:
If the new packing rings are solid endless rings (i.e.:
PTFE V-ring, grafoil, etc.), the actuator stem coupling will
have to be disconnected. Refer to the actuator removal
section of these instructions for details of coupling
5
Figure 2 Lapping Process
