4 replies to this topic

[Kwaku]

Per API-520, Q=21000*F*A^0.82 (taking credit for drainage or fire fighting equipment).

where:

Q=quantity of heat absorbed (btu/hr)

F=enviromental factor

A=wetted surface area

 

Will this calculation be the same or different for External Fire case calculation in the following 2 cases (same service)?

 

Case 1 (Jacketed vessel, liquid level in jacket=8ft):

Vertical vessel with OD=10ft (includes jacket); OD=8ft (w/o jacket). PSV sizing is for jacket side.

 

Case 2 (Unjacketed vessel, liquid level=8ft):

Vertical vessel with OD=10ft.

 

The source of my confusion:

The outside wetted surface area (A) for both cases will be the same due to the same overall OD, however, the jacket will contain significantly less volume of liquid as compared to the unjacketed vessel.

 

Can someone explain if Case 2 needs to be approached differently?

[fallah]

Kwako,

 

Yes, because the liquid in the jacket is certainly different than the one inside the vessel...Then different latent heat of vaporization...

[Kwaku]

Thanks for your response Fallah. The two cases are in the same service (same liquid). My question really is, will wetted surface area (A) of a jacketed vessel (jacket side) be calculated any different from an unjacketed vessel? The outside geometry looks the same, but there is significant difference in volume contained. Does anyone have an interpretation of the 0.82 exponent on A in the API-520 equation (above), and is there any chance this exponent could be different for external fire case psv sizing for a vessel jacket?

[fallah]

Kwako,

 

There is no difference in exponent value which is used to evaluate the heat absobtion rate in wetted wall fire case and because of the same geometry and fluid, 0.82 is consisered in both cases. The only difference beween jacketed and unjacketed cases is the time duration to reach the gas expansion case which is shorter for jacketed vessel; because the liquid volume inside the jacket is lower than that of the vessel without the jacket...

[Kwaku]

Fallah, thanks for this explanation!