Michael,

I've modeled your piping system in PIPENET and calculated force results in 3 legs as attached images.
As it's shown rise time is about 6 sec for a force to be developed in one leg. Then load duration is about 4 sec and fall time is about 0.3 sec. So you can define Force Spectrum trapezoid for each leg as follow:

0 ms 0 N
6000 ms 1500 N
10000 ms 1500 N
10300 ms 0 N

Anyway, this hammer load is not that significant to cause any failure because valve closure time is not that fast. I hope this hepls you.