Yes fluid wt. should not be considered in that portion otherwise it will over-design your spring.

Now why the max. load will go to the pump support can be explained froma simple analogy. Imagine the pump support as a spring. This spring will be parallel with your elbow spring. Since in case of parallel springs the load distribution will be as per relative stiffnesses, the pump spring being much stiff in comparision to the pipe spring will share the max. load.

However ignoring the fuid wt. matters when the pipe size is big enough ( as is your case). Othwise the effect is very small.

Most consultancies follow this procedure.

Anindya Bhattacharya