Flowserve Vogt Forged Steel Gate - Manual (Page 37)

Flow Control
Vogt Valves
FCD VVENIM2000-02 Forged Steel Gate, Globe and Check Valves 37
5.18 Disc and Seat
Leakage through the seat and disc is not always easy to detect, but
when definitely known to exist, immediate repair is recommended
since delay may permanently damage the disc, seat or both.
The internal repair of gate valves 2" and smaller is usually found to
be uneconomical. However, if a gate valve is disassembled for in-
spection or cleaning, care should be taken when removing the wedge
to note and mark its orientation with respect to the valve body so
that when the wedge is reinserted in the valve, it will have precisely
the same relationship to the seats that it had in the original assembly.
The seat threads in angle, globe, and check valve bodies should be
carefully inspected to make sure they are in useable condition. When
installing new seats by using proper seat tools, the seats should be
screwed tightly into the valve body, then unscrewed and examined
to make sure they are making continuous contact for tight seal.
If seating surfaces show galling, slight pitting, grooving, or
indentations not deeper than 0.010", lapping will usually restore the
surfaces sufficiently to permit tight closure. Defects deeper than
0.010" can seldom be corrected by lapping, but seating surfaces
can be remachined or new parts installed. For relapping the seat and
disc of Vogt globe, angle, or check valves, use a fine and a coarse
grade emery base compound such as Clover Compound A and D.
Apply a light coat of fine, or A, compound to the seating surface of
the disc or one-piece stem, insert disc or one-piece stem into seat,
and lap using an oscillating motion. Lap a few minutes and then
wipe seating surfaces clean. This will clearly show the extent of
damage. If severe damage is noted, use the coarse, or D, compound
and lap until all defects are removed. Then finish with the fine, or A,
compound. It is recommended that the face of the disc be “blued” to
check for contact between seating surfaces after final lapping.
Globe and angle valves require a lapping guide fixture to maintain
alignment during the lapping operation. A fixture as shown in Figure
6 may be made or, for O.S. and Y. valves, a valve bonnet with yoke
nut removed may be used. For relapping loose disc globe valves,
place a washer between the disc and head of the stem and retighten
the disc nut, as shown in Figure 6.
Vogt piston check valves require an adapter to screw or mate into
the piston for lapping the piston seating surface to the seat. The
bore of the valve body serves as a guide for lapping, see Figure 7.
Ball check valves are primarily used for fluids of high viscosity and
the rolling action of the ball maintains seating surfaces in good
condition until ball size or ball guide is worn and replacement parts
are needed.
Figure 6
Stem
Bushing
Disc Nut
Disc
Washer
Seat
Body
Figure 7
Adapter
Disc
Body