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For the same reason we do not include actual pressure thrust in the nozzle loads, we should not incorporate hydrostatic pressure thrust in the pump nozzle. This load is exclusively taken up by the bolts.

With respect to the original question, if you want the most realistic analysis, first analyze with the vertical pipe full. Then analyze with the vertical pipe empty. Then subtract the vertical loads and apply it as a force at the pump center.

If you were to replace the vertical column of liquid with solid metal, it's clear to see how the load pans out. When we convert solid interior metal to liquid metal, the liquid metal fails to hold itself up, and then exerts pressure laterally to its container.