NIBCO Technical Services • Phone: 1.888.446.4226 • Fax: 1.888.336.4226
7
For more details on check valves, see NIBCO Technical Bulletin NTB-0596 Check Valve
Cracking Pressure.
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 on 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 is generally due to foreign matter lodged in the seat.
Occasionally, such foreign material can be washed away by allowing flow through the
valve. 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 lapping the bronze valve disc while the disc is in the
valve body. The bonnet is removed, lapping compound is put on the seating surfaces of
the disc and the valve body, and on top of the disc is a screwdriver slot which can be used
to twist the disc while holding a mild amount of pressure on it and lapping the surfaces.
This lapping will generally remove small scratches in the seat areas and make the valve
function normally. It is normally advisable to remove the hanger assembly to clean off the
lapping compound and blue the surfaces to make sure there are no high spots. After this
lapping operation is done, the valve can be reassembled.
In the case of rubber or PTFE seated valves, it is best to get new replacement discs and
install them.
5.0 MAINTENANCE
5.1 TOOLS AND EQUIPMENT
Standard wrenches and tools are suitable for servicing valves as follows:
A. For removing bonnet, an open end wrench of the proper size to fit the bonnet nut is
required. Small wrenches are also required of the socket type to remove the side
plugs. Needle nose pliers are recommended to pull the hinge pin out of the side of the
valve body. Never use a pipe wrench to remove or reassemble the valve bonnet.
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