Should water hammer be considered as an occasional load case such as wind or seismic? Combine OCC (waterhammer)+ Sustained less than 1.33 Sh?

If seismic is considered with waterhammer in the same run should these two occasional cases be SRSS combined and added to sustained and compared against the occasional allowable? This seems like a very conservative approach for B31.3 however I'm wondering if there is anything incorrect in this method? Theoretically an earthquake could cause valves to close and cause waterhammer so combining the two cases is a possibility.

If you accept the static approximation of dynamic events, then it really doesn't matter what the event is (I'm not saying you can't or shouldn't do this). Typically, only one occasional event is considered at a time. However, if you have a scenario that is in the realm of plausable possibilities, then it may be worth a look. You won't run into trouble unless something fails or is overloaded, then you'll have a small battle on your hands.