Fire Sizing for Air-Cooled Heat Exchanger Tube Bundle?

[KernOily]

Hi guys. Question has come up as to how to fire-size for an air-cooled heat exchanger tube bundle.

My initial thought is I would use the bare tube surface area as the wetted area in the 520/521 calculation, then add the area of the two header boxes. That total area would be used in the calc. What do you think?

Thank you!
Pete

 

[georgeverghese]

What service is this air cooler in? If the air cooler is in single phase gas cooling service, or if there is no liquid inventory held up in the tubes ( cooler is seld draining) , then an RV is not required for firecase only relief - see API 521, section on external fire, subsection on air coolers. The same position is taken in my previous OpCo company engineering standards.

 

[EmmanuelTop]

Also consider the actual elevation of the air cooler. If significantly higher than 25 ft, you may not need to consider any exposure to fire.

From API 521:

To determine vapor generation, it is necessary to recognize only that portion of the vessel that is wetted by its internal liquid and is equal to or less than 7.6 m (25 ft) above the source of flame.
NOTE: Hydrocarbon fires can exceed 7.6 m (25 ft) in height; however, experience has shown that it is necessary only to size relief devices on the basis of the averaged heat input up to a height of 7.6 m (25 ft) above the base of a pool fire.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Firecase relief loads at 2phase or single phase liquid air coolers can be very large, and the preferred option would be to elevate the air cooler so the bottom of the tube bank is at 25ft or higher above grade, if the cooler can hold up a liquid inventory. The exposed fire case area would then be only be the liquid wetted portion of the external surface of the exposed tubes, plus the area of the corresponding liquid wetted portion of the header boxes.

 

[KernOily]

Hi George and Dejan - Thank you for the very informative replies. Very helpful.

Thank you!
Pete