Pipe Freeze Analysis

[mechtimeng]

I have created a excel spreadsheet which simulates the time it takes to for both insulated and uninsulated pipes to freeze. The model is going to be used for an economic evaluation of whether or not a plant process water system will need to be electrically heat traced, and insulated. The environment design conditions are -10 °F and windspeed 70 MPH.

The results of my data are as follows:

Example 14" NPS Sch 40 Carbon Steel Line.

Fluid: Water

Uninsulated the time to freeze was 17.43 hrs

Insulated the time to freeze was 355.17 hrs.

It is important to note that the design conditions were considered constant, and the pipeline was assumed under stagnant flow conditions. In addition, I neglected radiation in my calculation as well.

The main problem I have now is interpretting the results of the analysis, I don't have a good feel on how a pipe will behave under these harsh design conditions.

It is important to note that these design conditions are worst case senarios only. The pipe system is actually going to be installed in Northern Alabama.

So I was wondering if any body had a good feel as to whether or not these times are reasonable, or perhaps
be able to point me to a reference that might contain
similar data for comparison purposes.

Thanks,

Tim

 

[athomas236]

Do you have any details of the insulation

athomas236

 

[mechtimeng]

The proposed insulation is Cellular Glass ASTM C552 (Foamglass)

 

[pmover]

for a -10°F, 100 mph design condition in northern AL seems a bit extreme. but that is not a problem to overcome.
you need to consider all operating scenarios for the process system as a whole. any stagnant/idle process conditions should then be evaluated for any freezing potential. if any exist, insulate the system(s). since the insulation material is known, be aware of the critical insulation thickness.
as far as the time values provided, they appear to be slightly high/excessive, but not by much. i'm in AK and freeze protection is a std practice and the data i remember reviewing for north slope piping systems indicates lower freeze times (due to lower temps!). i do not have my data file currently with me, so i cannot confirm.

-pmover

 

[mechtimeng]

Pmover,

Thanks for the info! you are absolutely right about those design parameters, unrealistic to say the least, considering the heating calculations for the plant buildings were based upon ASHRAE Dry Bulb Temps of 18°F and 23MPH. I am just looking for ball park, I have recrunched the numbers in my spreadsheet and made a comparison model, and the number and the times are holding, so for now I am going to trust that my calculations are correct. The power company that we are doing the plant design for does not have a internal freeze protection plan, I am pretty sure they were given those parameters probably from an engineer or firm that has done power plant ops further North.

Tim

 

[pmureiko]

mechtimeng,

Is the open & line flowing or blocked in? I'm guessing you calculated the time to completely freeze a blocked in line.

It make take 17 hours to freeze a line, but this is different than the pressure profile. The formation of a small amount of ice in a blocked, liquid full line will greatly increase the pressure to make room for the lower density ice. Because of this the pressure will rise and may burst the line before it is frozen solid.

Consider a freeze protection valve is this is clean water.

 

[mechtimeng]

Actually some of the lines, are slurry lines, while the rest of the lines are comprised of process water lines, being pulled from the retention ponds near the plant.

The design considered the lines to be fully stagnant.

Freeze Protection Valves are definitely an item that we
are considering to put in place on the non-slurry lines.

Thanks,

Tim