I have some doubts regarding calculation of piping loads on supports (for lines that do not qualify for stress analysis).

1. For calculating vertical load of piping on a particular rest support (structural steel member) the water filled weight of 50% pipe length upstream and 50% of pipe length downstream of the support is considered. Why effect of pressure of service fluid is not considered?

2. How to estimate loads on guides (horizontal direction loading)?

1. Use the greater of water-filled weight (assuming hydrotested in some form) or process fluid at max density. Generally water is heavier than most gases, and most liquids are incompressible, so this is why pressure is ignored.
2. Same as weight case, except instead of gravity, use wind and calculate an equivalent force per linear foot. Assume frictionless supports.