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6
loosened, it is absolutely necessary to make sure that no nicks or scratches be put on the
body or bonnet seating surfaces.
When removing the hanger assembly, especially for brazing installation, and for models
valves which have PTFE or Buna-N discs, when soldering, it is first necessary to remove
the bonnet then loosen the side plug and take it out. After the hinge pin is removed the
hanger and disc assembly can be taken out of the bonnet area of the valve. While the
PTFE and Buna-N disc are not very susceptible to nicks, it is still wise to handle them with
care so that they do not get damaged before they are put back into the valve for
reassembly.
When reassembling the valve, it is important to check the interior of the valve to see that
seats are totally clean and have not been damaged due to high temperature heat applied
during the brazing operation. If there is doubt about how much temperature has been
applied to the valve, it is a wise practice to take a flat disc and put blueing on it so that it
can be determined if there are any high spots on the seat. If there are high spots, then a
lapping operation should be done to bring the valve back into functional position. After the
hanger and disc are put in to place, the hinge pin should then be pushed into position.
The side plug should be cleaned and reinstalled into its original position. Before putting
the bonnet onto the valve, a check should be made to assure that the disc and hanger
move freely.
When assembling the bonnet to the valve body, a slight amount of lubricant equivalent to
about 10 weight oil should be put on the bonnet joint sealing surface of the body and the
bonnet. (Some lubricants can be a serious contaminant to the line fluid.
Consideration must be given to the application when any lubrication is used). This
gives lubrication so that good tightening pressures can be applied. The wrench should be
given a slight impact for the final tightening.
After the valve has been installed, the valve should be tested to make sure that it opens
and closes easily and that the bonnet joints or pipe joints are not leaking.
For more details on proper check valve selection and installation, see NIBCO Technical
Bulletin NTB-0703 Design and Use of Check Valves.
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 allowable leakage rate for gate valves is 10ml per nominal
inch of diameter per hour; a check valve has an allowable leakage rate of 40ml per nominal
inch of diameter per hour. This is in accordance with MSS Standard Practices for check
valves.
(1 ML=.035 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.
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