NIBCO Flanged End Outside Screw and Yoke Cast and Ductile Iron Globe and Angle Valves - Manual (Page 6)

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valve shop or machine shop may have to be consulted or contact NIBCO Technical
Services or your nearest NIBCO representative for help. In many cases, it is less costly
to replace the entire valve rather than have it repaired.
8. With the surfaces now clean and flat, place the gasket on the body and replace the
bonnet. Be careful to use the chalk marks to put everything back together the same way
it came apart. Always use a new, clean, fresh gasket. It does not pay to take a chance
using the old gasket over again.
9. Reinstall the body-bonnet nuts and bolts then carefully check to see that the body and
bonnet are in the same position as when they were separated. Make absolutely sure
there is no seat contact before tightening. This could damage the seats. Follow the bolt
tightening procedure in Appendix I.
10. Prior to use, pressure test the valve for leakage to see if a good joint has been made
and the seats are holding.
4.2 LEAKAGE THROUGH THE VALVE SEAT
Most of the time, leakage across the valve seat is due to foreign matter lodged in the
seat area or damage to the seat by passing objects. Occasionally, such foreign material
can be washed away by allowing flow through the valve. In valves that are used with
fluids that are contaminated and known to cause buildup, a drain can be installed at the
bottom of valves so the seats can then be flushed prior to closing. At times, the leakage
may be overcome by tightening the valve further. If the leakage persists, disassemble
the valve and examine the seat surface on the disc and the seat surface in the body.
Minor scratches can be corrected by polishing the disc face, but generally, if there are
deep scratches in either of the sealing surfaces, the valve should be removed from the
line and replaced or repaired by a competent valve shop. An acceptable method for seat
renewal is as follows.
Mark the position of the disc nut in the disc with a marker. Note: the disc nut will be
staked. Carefully remove the disc nut from the disc to release the stem from the disc.
The disc nut unscrews by turning counter-clockwise. The disc can now be removed from
the stem. Cut strips of 120 grit emery paper about 1-1/2” long and 3/8" or ½” wide. Using
double-backed adhesive tape, stick strips on the body seat ring seating surface. The
strips should be evenly spaced around the seat. Place the disc in the body and rotate
the disc back and forth with very light pressure. The further around you can rotate, the
better surface you will get. Finish up with full rotation. Examine surface periodically and
the new, clean, lighter surface will be seen developing. When complete, the new surface
on the disc should be all the way around and across equal to the width of the seat ring
seating surface.
Repeat the previous step for renewing the body seat by placing the strips of emery and
double-backed tape on the disc.
Clean thoroughly then rotate the disc once or twice lightly in the seat. A shiny line should
appear all the way around on the disc and the seat ring. Repeat the above steps until
this shiny line can be clearly seen. This line should be unbroken all the way around if a
good seal is to be achieved. A break in the line is a place where the valve will leak.