In a hydrotreater I have normally seen shutdown valve provided on HC line exiting the HP separator. The shutdown valve will close upon sensing low level in the HP separator. Shutdown valve is provided between HP to LP interface. Level indication failure of HP separator will lead to gas break through from separator to stripper and stripper is not designed for such high pressure conditions.
But in a kerosene hydrotreater unit I have seen the shutdown valve tripping logic on high-high pressure in HC line exiting the HP separator instead of low level in HP separator ( this low level tripping is not present ). The tapping for pressure ( 2 out of 3 tripping logic ) is taken from downstream of angle valve (pressure reducing valve ). What can be the reason of changing the shutdown valve closing logic from low-low level in HP separator to high -high pressure in the HC line exiting the HP separator?