Thermal Oil Expansion Tank in Fire Case

[Armen75]

I have a fairly large (100m3) expansion tank in a closed loop hot oil system.
the cases looked at include:
Gas blow through due to blanketing failure,
Gas blow through due to user failure - tube rupture
Thermal expansion of the liquid (ambient to working pressure)
they are all relatively small loads.

My issue is how to address the FIRE Case with vapor.
tank design temperature is 290oC
tank design pressure is 3.5 bar
the boiling point at 3.5 bar is roughly 400oC
the tank is uninsulated but protected by a sprinkler system.
We estimate close to 10 hours to get from ambient to 400oC.
The tank has a vent connected to flare via a fail-open pressure control valve.
IS FIRE with vaporisation a viable case or should I stick with liquid thermal expansion?

 

[JoeWong88]

If hot oil can reach 400 degC, probably the tank steel already reach > 500 degC... You may check the vessel strength at this temperature and see if the vessel collapse before it reach this temperature. Check and tell us...

There will be vapor space in the tank. Check if the liquid expansion can cause entire tank filled with hot oil. If not, there is not thermal expansion case.

I would think the blanketing gas would experience gas expansion due to heat input causing internal pressure increase to PRV set pressure.

Another stuff is any light HC or water trapped in hot oil (due to HX leakage) would vaporize and contribute to overpressure in hot oil tank.




JoeWong
Chemical & Process Technology

 

[Chance17]

It is well known that heavy-end fluids may have boiling temperatures above the vessel metal failure temperature.
A safety valve has little value if the metal fails before valve activation.
The main question will 400-C (752-F)boiling cause the tank metal to fail.
Only you and your vessel expert can answer that question. If the vessel is carbon steel, my guess is "no" the vessel does not fail at 400-C and 3.5 bar (50-psig).

Yes, external Fire is a viable case.

Instrument systems such as control valves to the Flare header can substitute as safety valves
but the substitution requires rigorous expert design (not casual)