I want to know if there’s a standard that requires the physical separation between the ESD (Emergency ShutDown System) and F&G system (Fire and Gas system), or it is just a good practice ? otherwise it is possible to use a single system (ESD/F&G) that incorporates both systems ESD and F&G
Separation of systems has nothing to do with the fact that one is fail safe and the other is non-fail safe. Nor does it have anything to do with alarm segregation.
There is no standard which specifies the requirement for independent ESD and F&G systems. However, if you read between the lines you will find that this is more often than not implied and common sense.
One of the important things to remember here is that if going for independent systems, you can take credit for these systems as independent layers of protection. So for instance, if the pressure in a vessel goes about High and into the HiHi range you can consider the ESD HiHi trip as a layer of protection. If this fails then a flange leak may lead to loss of containment which could be picked up by F&G detectors and prevent further escalation of events. If using an integrated system, you have to ensure that there are no common mode failures because of sharing the same PLC.
Generally it will be difficult to prove that no common mode failures can originate and this is why it is generally advisable to consider 2 independent systems up front, rather than changing things after orders have been placed.
If your F&G system is small, localized and has specific trip actions for electrical isolation then these may be treated as F&G executive actions and you need not implement these in a separate ESD system.
The design also depends on the nature of process risk associated with your operations. If risks are low then there is no need to go for independent systems. Always analyze risk before making decisions, this will make the answer clear to you.