A check valve can leak through its seat for the following
reasons:
The valve is installed in an application in which it
is not suitable.
Most swing check valves cannot be installed vertical
with the flow passing though from above.
The valve was installed less than 10 pipe diameters
away from upstream pipe fitting or pump. This results in turbulent
flow, disc chatter and ultimately damage to the sealing surfaces.
The check valve is not closing fast enough allowing
water hammer to slam the disc into the seat. Continued slamming has
damaged the sealing surface preventing the valve from sealing fully.
