To Pumpvlvguy and Abeltio,
Regarding swing check valve pivot hardware degradation from too low flowrates to backseat, there's an obscure report that deals with the influence of upstream pipebends on valve disk dynamics. It's Kalsi etal (1988), "Prediction of Check Valve Performance and Degradation in Nuclear Power Plant Systems", Kalsi Eng. Rpt. #1559 (NUREG/CR-5159). Among the findings:
"At proximities of 3 to 5 pipe diameters, upstraem elbows require an increase in flow velocity over baseline of 10 or 15% to fully open the valve disk. Proximities of 0 to 1 diameter will require up to 50% higher flow velocity over baseline to fully open the disk, except for clearway designs which will require velocities more than 100% higher than baseline [clearway disks lift nearly totally out of the flowstream and may never achieve stability under some flow conditions]."
"The amplitude of disk motion which occurs before fully seating the disk increases as the flow disturbance is brought closer to the valve. The maximum disk fluctuations in the case of a severe turbulence source can reach as high as 16 degrees and, for elbows, up to 9 degrees. The reducers have negligible effect on disk fluctuations."
"The onset of tapping begins at lower flow velocities when the elbow is oriented up than when oriented down. This results in wider tapping zones for elbow-up installations compared to elbow-down. This is particularly evident at 0d and 1d and less so at 3d and 5d (d is upstream pipe diameter distance)."
Kalsi etal appear to be talking about single upstream bends. Multiple, closely spaced bends upstream of swing check valves could have considerably worse consequences.