NIBCO Bronze Gate Valves - Manual (Page 6)

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3.0 OPERATION
Gate valves should never be used for throttling purposes. They should be fully opened
or fully closed. If the valves were only slightly opened, wire drawing and erosion of the
wedge and seats would result. It is not recommended valves be tightly back-seated as
line contamination over a long period of time may lock up the valve causing problems
when the valve has to be closed..
4.0 TROUBLESHOOTING
4.1
LEAKAGE THROUGH THE BONNET JOINT
If the bonnet joint leaks, the valve should be isolated and depressurized. The bonnet
should be removed according to instructions in Section 2. The seating surfaces of the
bonnet and body should be closely inspected. If there is any scratch or mark on these
surfaces then it is necessary that a flat surface sanding plate of 400 grit be used on the
body to remove all the scratches. After the scratches are removed, blueing should be
applied to a flat plate and put onto the bonnet surface, if there are no high spots on the
bonnet surface, then it is prepared for reassembly. On the bonnet surface, if there are
scratches or nicks that are evident it is necessary that it be indicated in a lathe and a
very slight cut taken off of the bonnet sealing surface at an angle of approximately 2-1/2”
tapered outward with a very fine 32 RMS finish. The two body bonnet surfaces then can
be reassembled using procedures in Section 2.
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. 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.
Minor scratches can be corrected by polishing the wedge face, but generally on bronze
valves if there are deep scratches in the body seats, the valve should be replaced
because it is extremely difficult to repair the surfaces in the small interior of the valve
body. If the wedge does have scratches it is generally acceptable to take a piece of 400
grit sandpaper, lay it upon a flat piece of glass, and slightly polish the surface of the
wedge. This will remove some of the scratches and possibly reduce the amount of
leakage.
4.3 LEAKAGE THROUGH THE STUFFING BOX
Leakage through the stuffing box packing may be stopped by tightening the packing nut.
Do not over tighten as excessive tightening may cause difficult operation of the valve
and could cause damage to the stem or packing nut.
If the gland has run out of travel, isolate and depressurize the valve for repacking or the
addition of more packing. Repacking of valves under pressure is not a recommended
practice. This is dangerous and could result in serious injury even if the valve appears to
be back-seated.