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XIV. Maintenance Instructions (Cont.)
F. Precautions and Hints for
Lapping Seats
To ensure a quality lapping process, observe the following
precautions and guidelines:
Keep work materials clean. Always use a fresh lap. If signs of
wear (out of flatness) are evident, recondition the lap.
Apply a very thin layer of lapping compound to the lap to
prevent rounding off the edges of the seat.
Keep the lap squarely on the flat surface, and avoid rocking the
lap, which causes rounding of the seat.
When lapping, keep a firm grip on the lapped part to prevent
dropping it and damaging the seat.
Lap in a circular motion while applying a uniform pressure.
Slowly rotate the lap to evenly distribute the lapping
compound. Wipe off the old compound and replace it with
new compound frequently. Apply more pressure to speed the
cutting action of the compound. To check the seating surfaces,
remove all compound from the seat and the lap. Then, shine
the seat with the same lap using the lapping method as
described above. Low sections on the seating surface show
up as shadow in contrast to the shiny portion. If shadows are
present, further lapping is necessary. Only laps known to be
flat can be used. It should take only a few minutes to remove
the shadows.
When lapping is complete, any lines appearing as cross-
scratches can be removed by rotating the lap on its axis
(which has been wiped clean of compound) on the seat.
Thoroughly clean the lapped seat using lint free cloth and a
cleansing fluid.
ATTENTION!!
Before assembly, grind the contact surfaces of the nozzle and
O-Ring retainer to provide metal-to-metal seat tightness in
the event of O-Ring failure.
G. Reconditioning of Laps
Ring laps are reconditioned by lapping them on a flat lapping
plate in a figure-eight motion (Figure 20). To ensure the best
results, recondition the ring laps after each use. Use an optical
flat to check the quality of the lap.
Nozzle laps must be re-machined to recondition the lapping
surfaces. Place the nozzle lap in a lathe between centers
(Figure 21). The surfaces marked A and B must be running
concentric.
34 | BHGE
LAPPING PLATE
Figure 20: Lapping Pattern
A
C
90°
Lathe Center
Figure 21: Nozzle Lap in a Lathe
H. Re-Machining Nozzle Seats
1.
Remove the nozzle from the valve to be remachined. If it
cannot be removed from the base, re-machine it inside
the base.
2.
Take the following steps to setup the lathe and nozzle:
a. Grip the nozzle in a four-jaw independent chuck (or
collet, if appropriate), using a piece of soft material
such as copper or fiber between the jaws and the
nozzle as shown at A (Figure 22).
b. True up the nozzle so that the surfaces marked B and
C run true within .001" (0.03 mm) on indicator (Figure
22).
© 2019 Baker Hughes, a GE company. All Rights Reserved.
RING LAP
D
B
85°
Lathe Center
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