Hi All,
In a horizontal vessel we come across fixed saddle & sliding saddle.
What is the reason we have a fixed and sliding saddle support for a horizontal vessel?
Why not both fixed or both sliding?
On what basis do we fix this saddle?
Which end do we fix and which end sliding?

KARTHIK,
It is really very simple. The vessel is made of metal. Metal expands with any increase in temperature or contracts with any decrease in temperature.
Thus, One end of the vessel is anchored (fixed) and the other end is allowed to slide with the change in temperature. The temperature change might come from the process fluid or from solar radiation.
The determination of which end to anchor depends on the overall piping layout. Normally you would want to anchor the end nearest the pump suction nozzle so the full amount of the vessel thermal growth does not impact the stresses and forces on the pump suction piping.
Normally the pumps are located nearest the pipe rack (not always) so you would normally anchor the vessel saddle nearest the pipe rack and let the vessel grow away from the pipe rack.