Dear Mr Ohlinger

Thank you very much.

So, I believe that Joukowsky equation can be used for calculation of pressure difference downstream of the closing valve, if the pressure difference calculated by Joucowsky formula is less than, operating pressure in pipe.

So, I think force will be calculated as the lesser of:

(Pressure as per Joucowsky X area of pipe) or
(Operating pressure before valve closure X area of pipe)