Could someone refresh my memory as to the the reference for the 2.0 DLF? I have typically used this factor for calculating the forces on PSV's. I'm wondering however whether this 2.0 factor should also be used for calculating waterhammer forces when the valve closure time is slow,3 sec.?

For suddenly applied loading DLF = (1-COSWNT)where symbols have their usual meaning.The maximum value for this is 2.

The above expression for DLF can be easily derived and you should be able to do it yourself using the concept of Dirac Delta Function.

Read any good book (Dynamics of sructures by Anil Chopra or the one by Mario Paz for e.g) to properly understand the concept of DLF.

Regards