Wetted Surface of S&T exchanger

[BaAn]

Recently I joined a discussion about safety considerations for external fire scenario. When calculating the wetted surface for protecting the tube side of an heat exchanger it was discussed whether to include the shell surface and the heads or whether to include just the head surfaces. There were basically two opinions:

1. The fluid in the tubes is not in direct contact to the shell side, therefore the shell surface does not have to be considered.
2. For heat exchangers with liquid (!) on the shell side, the heat transfer coefficient through the tube walls during a fire scenario becomes considerable high, therefore the surface of shell AND heads has to be considered to stay on the safer side.

The latter approach seems to be quite conservative but actually I have seen both approaches in different projects several times and couldn't find anything in API521 about it. I would appreciate your comments to this topic! Thanks!

 

[pleckner]

API does not consider the tubes in an external fire. Wetted surface of the vessel for a fire scenario is just that, a wetted surface exposed to the fire and this clearly does not include internals like the tubes of a S&T heat exchanger. Indeed, the tubes add nothing to the vapor rate generated and to be relieved by the relief device on the shell.

 

[MJCronin]

BaAn...

The API521 code mentions neither heat exchangers, pumps, piping strainers etc....etc..

It is my understanding that these components do not require a fire protection evaluation and the possible addition of relief devices. Whereas there is a real risk of an API-650 tank experiencing overpressure and rupture almost immediately during the first minutes of a fire, the same is not true for segments of piping in the same area.

Futhermore, the liquid volume contained is typically much, much smaller

The need for fire protection of all non-tank components should be addressed during a HAZOP

-MJC

 

[pleckner]

API 521 (which is a standard and note a code) does indeed mention heat exchangers. Only the shell of a S&T heat exchanger will get the ASME stamp and if appropriate, this must be evaluated for a fire relief scenario as well as all other credible scenarios. This of course implies the requirement for some type of ASME certified relief device.

 

[MJCronin]

I stand corrected, thank you

-MJC

 

[roker]

BaAn,

In case of fire you should consider the shell side, the tube channel surface is much smaller than the shell, also if the HE is insulated you reduce the relief flow.

In case of fire there will be no temp difference (thermodynamics...) between the shell and tube sides so if you protect the shell side it is protecting also indirectly the tube side.

regards,
roker

 

[BaAn]

Thanks for your valuable comments! I am tending to follow Roker's advice as this approach is certainly on the safer side and makes sense on a thermodynamical understanding, even if it's not required by API 521 as Pleckner mentioned!