NIBCO Iron Gate Valves - Manual (Page 6)

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4.1 LEAKAGE THROUGH THE BONNET JOINT
1. Remove the handwheel from the stem.
2. Mark the body – bonnet flanges with a piece of chalk so they can be matched up
later.
3. Remove body-bonnet nuts and bolts.
4. Carefully remove the bonnet assembly. Be careful not to nick or scratch the wedge
seats.
5. Thoroughly clean the body and bonnet joint surfaces.
6. Check the body and bonnet gasket surfaces for any scratches or gouges or other
irregularities.
7. In the event the surfaces are marred, they must be filed or machine flat again. If the
damage is considerable, the body or bonnet may have to be replaced. A competent
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 again 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, and then carefully check to see that the
body and bonnet are in the same position as when they were separated. Follow the
bolt torquing procedure in Appendix I.
10. Pressure test the valve for leakage to see if a good joint has been made .
4.2 LEAKAGE THROUGH THE VALVE SEAT
Leakage across the valve seat most of the time is generally due to foreign matter lodged
in the seat. 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 valve 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 wedge and the seat surfaces within the body. (See section 4.1)
Minor scratches can be corrected by polishing the wedge face, but generally if there are
deep scratches in the body seats, the valve should be removed from the line and
replaced or repaired by a competent valve shop. If the wedge does have scratches it is
generally acceptable to take a piece of 400 grit sandpaper, lie it upon a flat plate, such
as an inspectors table (granite slab) (glass of modern manufacture is remarkably flat and
true) and slightly polish the surface of the wedge. This will remove some of the scratches