5 | Baker Hughes
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5.3 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. The
seating surface angle of the plug is 30 degrees and the seat ring
is 40 degrees (relative to the centerline axis). 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 fine (240 grit) and
finish with a finer grade (600 grit).
Note: Lapping should produce a line contact area, not the
entire surface, due to the difference in seat angles.
CAUTION
CAUTION
WARNING
DANGER
Before lapping, the plug and stem sub-assembly
must be concentric. (See pinning operation, section
5.4).
5.3.1 Threaded Trim
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.
Note:
A sealant compatible with the process should be
applied sparingly
to the seat ring threads and sealing
shoulder.
3.
Install seat ring (B002), followed by seat ring retainer (B001)
and tighten seat ring retainer using fabricated wrench used
for removal. If the seat ring is exceptionally resistant to
removal, the application of heat or penetrating oil can be
helpful.
CAUTION
CAUTION
WARNING
DANGER
Ensure proper safety practices are observed when
using heating devices. Flammability and toxicity of
the process fluid must be considered, and proper
precautions must be taken.
Do not over-tighten. Do not strike seat ring lugs
directly. This could distort the seat ring resulting in
seat leakage.
4.
Apply lapping compound on the plug at several spots equally
spaced around the seating area.
5.
Insert the stem and plug sub-assembly carefully into the
body until it is seated.
6.
Place bonnet (B034) on the body and fasten the bonnet to the
body using the body stud nuts (B032) spaced equally apart.
Apply slight pressure and tighten evenly.
7.
Insert two or three pieces of packing rings (B011) 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.
Note:
As an alternative, drill a hole through a flat steel
plate and fasten to the plug stem using two locknuts.
9.
Apply slight pressure on the stem, and rotate the stem in
short oscillating strokes (around 8 to 10 times). Repeat this
step as necessary.
Note: The plug should be lifted and turned 90° each
time before repeating Step (9). This intermittent lifting is
required to keep the plug and seat ring concentric during
lapping.
CAUTION
CAUTION
WARNING
DANGER
Avoid over-lapping as this can cause damage to
the seating surface rather than improve leakage
performance.
10.
After completion of the lapping operation, remove bonnet
and plug. The seating area of the seat ring and the plug
must be cleaned of all lapping compound in preparation for
reassembly.
Do not remove seat ring.
CAUTION
CAUTION
WARNING
DANGER
Maintenance of the plug should be limited to
cleaning of the ports and the procedures defined
under Sections 5.3 (Lapping) and 5.4 (Pinning) as
required.
5.4 Plug Stem Pinning
Plug stem pinning in the field may be required for the following:
- Replacing existing plug and stem, or
- Replacing existing stem only
Note: If it is necessary to replace the plug, then the plug stem
must be replaced at the same time. The original pin hole in
an existing stem will not provide the necessary fit, and might
seriously impair the strength of the assembly.
5.4.1 Replacing Plug and Stem
CAUTION
CAUTION
WARNING
DANGER
Do not tighten nuts to final torque specifications at
this time. The bonnet is used temporarily for guiding
purposes only.
CAUTION
CAUTION
WARNING
DANGER
Use care to avoid damage to the guiding surfaces of
the valve plug during disassembly, assembly, and
pinning operations.
