Sewage force main freeze potential

[PaulLDE]

The existing condition is a 4 inch dia. ductile iron sanitary sewage force main with 20" to 24" of cover ( earth and pavement). Minimal cover occurs along a length of 60 feet in the middle of a 500 foot long force main. Standard cover to be below frost lines in our area (New Jersey) is 48". The pump station and force main operate frequently as the system services a fair sized area containing residential, business, medical, and hospital uses. The local MUA is requiring the section (approximately 60 feet) of minimal cover to be insulated. I believe the flow will maintain the pipe and contents above freezing even if ground frost surrounds the pipe but have no way to quantify my opinion. Is there data available to analyze frost protection?

 

[KRSServices]

gtrainor,

No, the forcemains were never recirculated, only water. The original thread implied that the latent heat in the flow of material would prevent freezing and therefore insulating the pipe was not necessary. He wanted some backup. I provided that backup.

A thermal analysis could be done on the section of forcemain, including a time to complete freezup. I worked in areas of frost penetrating down 12 feet or more so I have a rough idea of the ambient tempurature surrounding the ductile iron pipe in New Jerey, though i do not have acces to published values. 60 feet of pipe is not that long of a distance with flowing warm effluent. The thermal analysis will determine the rate of heat loss, and the designer can then prescribe an option, if one is required.

On the face of it, I don't think insulating the pipe and board insulation is necessary, but I'll leave that to the analysis. The designer can then recommend options. KRS Services

http://www.krs-services.com

 

[Chris73]

Here another logical thought: If water passes throught 60 ft of frost surrounded pipe quickly it won't have enough time to loose all its heat in that short passage time. If it moves only slowly through that section it might freeze. I think that's why the velocity is an important factor here in that case. Can you exclude any low flow conditions in your main you should be fine - especially with raw sewage, which usually has about 10 -15' celsius.

 

[KRSServices]

I think someone also referred to this aspect, but one must remember that we are concerning ourselves with 60 feet, or 20 metres of sewer forcemain. The velocities of the fluid, the chemical makeup, particularly chlorides (with lower the "freezeup" temp.), the cycling, the soil types (and their "R" factor and the worst case (reasonable) ambient tempurature surrounding the pipe will play a part in determining the freezeup.

Personnaly, having calculated many heat loss and time to freeze utility designs, I do not think there will be real issue here, even when the system free drains (gravity). The pipe run is simply too short. Now if the cycle times for the forcemain are once per day, the fluid cannot drain and the ambient temp somehow reaches -10 celcius, then there may be some cause for concern. Do the analysis and take it from there. KRS Services

http://www.krs-services.com

 

[BobPE]

KRS:
The original question stated that it was a force main, so there would be no gravity flow....


fluid will freeze in any section of the pipe exposed to freezing temps, espically a force main since diurnal flows are low at night if at all for long periods of time.

BobPE

 

[KRSServices]

Thanks folks....I understood the original question referenced that the flow was in a focemain, however, I posed a further thought that if, at some point the pipe will empty into a manhole, a chamber....something, does the pipe drain as well? If the line remains fully charged, then a set of calculations would need to be performed to determine the time to freeze. It could be four hours, it could be ten hours, it could be 24 hours, it could be 36 hours. I have calculated freeze times in excess of 80 hours on a shallow pipe. Again, there will be many considerations and factors concerning the R values of the fluid, soils, pipe, etc. If, for example, the complete freeze time was calculated at 12 hours, and the pip cycled every 8 hours, then a heat loss could be calculated and perhaps a different cycle time on the fluid would resolve the issue. At this point it is all speculation until the requisite calculations are undertaken.

Have a great day folks! KRS Services

http://www.krs-services.com