Fire case thermal relief - Advantage of unheated fluid in a long pipe

[ginsoakedboy]

I am trying to calculate pressure rise and excess (relief) volume for a fire scenario on a long pipe. The fire load is limited to a small portion of the pipe. The pressure increase and excess volume are dependent on the volume that gets heated up.

But, there is a much larger volume that does not get heated to the same extent. Will the pressure increase be smaller since there is a much larger volume in the pipe which does not get heated up? How to perform this calculation?

This will also reduce the excess volume to be relieved. How to get the new relief volume?

Any insights on this will be helpful. Also please let me know if my logic is flawed.

Thank you in advance.

 

[dcasto]

think thermospyphoning. Given an infinite long pipe and a not so big fire exposure, the pressure may not go up at all.

BUT, error on the safe side and use the API recommended fire equation and set a resonable lenght of a piece of line as the exposure area.

Here's on to tackle while you are at it. Thermo expansion from the sun on a buried line that comes above ground for a few feet and then back under ground.

 

[ginsoakedboy]

dcasto, good point.

However, I am dealing with a very well defined fire heat flux which is quite high, I mean really large like 200 kW/m^2. But, there is a layer of fire protection material applied on the "exposed" length.

Thermosiphoning effect does not have a chance to take place, as the temperature rise will simply overwhelm the setting up of a convection cell.

Is there some way I can use the "cool" portion of fluid to my advantage? As in reduce the pressure rise. Which equations may have to be used?

Please help!!

 

[lizking]

1. You may take credit for the protection (insulation that meets certain criteria) with an appropriate factor per API 521.
2. If you can assume that the fluid is being heated somewhat uniformly, then yes you can look at how long the fluid would have to be heated to get to the design pressure of the piping. See API 521 Eq. 3 for the relationship between temperature rise and pressure rise.

 

[ginsoakedboy]

lizking, I have been using equations 3, 4 and 5 for my calculations of pressure rise etc.

Which equation should I use for taking advantage of the cooler fluid (fluid not exposed to the fire)?

I have a hunch that all that cooler fluid has gotta provide some sort of buffer in reducing pressure rise and excess volume. Can anyone inform if this hunch is accurate?

 

[lizking]

(Heat input) x (specific heat) / (mass of fluid) will give you temperature rise per time. You have your "allowable" temperature rise from the API formula. Now determine if the resulting time is sufficient enough for you to take mitigating action.

 

[ginsoakedboy]

lizking, my concern is totally different from your response.