NIBCO Check Valve Bronze Swing - Manual (Page 6)

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3.0 OPERATION
Check valves seal adequately when the back pressure is high, but some leakage is
expected when the reverse pressure is relatively low. In testing procedures, a check
valve is allowed to leak 4 times as much in the reverse flow (check) position as a metal
seated gate valve. In other words, the normal leakage rate for gate valves is 10cc per
hour per inch of diameter; a check valve is allowed a leakage rate of 40ML per inch of
diameter per hour. This is in accordance with MSS SP-80 standard for check valves.
(1 ML=.35 oz.).
The check valves described in this manual are not to be used as foot valves on a pump
or pressure regulating devices. Their construction is not suited to these types of
requirements.
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.