Sewage force main freeze potential
Sewage force main freeze potential
(OP)
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?





RE: Sewage force main freeze potential
RE: Sewage force main freeze potential
Good luck...
BobPE
RE: Sewage force main freeze potential
Hope this helps.
saxon
RE: Sewage force main freeze potential
I'm with you that it isn't really needed for normal operations, but as others have said you should consider the worst case. Could the system be shut down for hours at time of maximum frost?
By the way, 4' frost depth in New Jersey sounds high. I checking some climate data at a Corps database. Most NJ locations had a design Freezing Index of about 400, which gives you about 2' of maximum frost penetration. However Sussex had a FI of 1060, which could cause about 4' of frost.
Now for some ammunition on your side;
Here in Anchorage the maximum frost penetration is 10', which has actually occured a few times in recent years under roads. The local water/sewer utility requires a minimum of 10' of cover over water mains and services, but 5.5 feet over sewer mains and services. They've apparently found that the heat from wastewater prevents freezing problems even if frost can extend around pipes. There are only a few instances of sewer lines freezing, usually in a low flow situation (individual service or upstream end of a main line).
The situation Gtrainor describes is more typical of very shallow lines in Arctic regions, often in permafrost. Those lines would also typically be heat traced.
Hope this helps,
Carl
Anchorage, Alaska
(40 degrees today)
RE: Sewage force main freeze potential
RE: Sewage force main freeze potential
I had the opportunity to talk to an operator from an Alaskan water system and hoe they get away with burrial in permafrost is to add an additional step to their water treatment which involves ading heat and recirc pumps to the water system. I believe this is how they get away with minimal freezing. I am sure you know heat trace in bigger designs serves as a thermal break. Without water movement and energy addition to the water, it will take a lot of heat trace to add enough energy to standing water to keep it from freezing.
I think one of the problems is that a lot of water pipe engineers are civil engineers and never had a heat and mass transfer class pertaining to pipe design. Cold weather pipe design all becomes very clear after a class like that, at least it did for me.
take care....
BobPE
RE: Sewage force main freeze potential
RE: Sewage force main freeze potential
http://www.flowlinealaska.com/insulation.htm
RE: Sewage force main freeze potential
How was that Alaska Engineering test? I have heard people talk about it and it sounds really interesting!!
BobPE
RE: Sewage force main freeze potential
RE: Sewage force main freeze potential
I designed and modified several water and sewer systems in the Yukon (Whitehorse), with various soil types and frost penetration. First of all, there is an excellent design manual called the Cold Climate Utilies Manual, and in it there is a section where heat loss in the pipe (including fluid) is calculated and should the fluid stop flowing, the time to complete freezeup.
Yes, adding heat is an option, but it is expensive and used sparingly. The use of recirculation pumps is more feasible because in reviewing the thermal analysis, flowing fluid does not lose heat as quickly as stagnant fluid, in fact, with an ambient tempurature af about -6 or -7, fluid moving through a pipe with a velicity of 6"/s will not freeeze.
Frost is an interesting phenomenon. Scientifically, it is the result of the soils giving up heat and the depth of frost penetration is proportional to the amount of cold degree days and the resistivity of the soils to heat loss. Freezing (nucleation) actualy occurs at about -3 Celcius. In a flowing pipe, freezing occurs as a result of the fluid losing heat and the inside of the pipe kind of resembles the fluvial sand deposits (waves) from a stream with different flow velocities (shallow vs. deeper x-sections).
2" Board insulation is an excellent form of frost protection for a buried pipe, provided the insulation extends beyond the pipe (at least one pipe diameter), because it allows the surrounding soils to retain heat (same pricipal as insulation on a concrete wall. 2" board insulation is equivalent to about 1.0 meters of clay.
I know of shallow buried transmission watermains (24" of cover), under a busy street, in Edmonton, Alberta where the amient tempurature surrounding the pipe on a worst case year could be as high as -15 Celcius. This pipe does not freeze due to the high velocities and relatively warm tempuratures of the fluid. The water does not lose much heat. Paulde, I suggest you undertake a thermal analysis and determine whether, at the worst case, the fluid loses suffiecient heat to warrant the costly undertaking of freeze protecting. Further, to balance your analysis, I would also suggest that you calculate the time to total freezeup of the main to determine how much time can transpire should a unforseen shutdown occur. I am assuming that in that case, the pipe will drain? (yes? no?)
To answer your question, yes, you can quantify the thermal losses in the unprotected pipe and whichever option is used for freeze protection. I hope this helps.
KRS Services
www.krs-services.com
RE: Sewage force main freeze potential
could you give us the formulas that you speak of concerning velocity and its ability to keeps systems from freezing. My thermodynamic equations for pipes do not include velocity because the heat generated is usually insignicicant for water since the velocity is limited by the fluid preperties. What ever heat is generated from friction is distributed uniformily throughtout the system surface area and is quite small, essentially adding no heat to the system. Modifying a system to have high velocities also would be more inefficient that designing a recirculation system with a heat source.
There really isnt any research out there on velocity induced insulation value since heat loss by water is independent of velocity of the water. The water will lose heat at the same rate, moving or still and will freeze, but the difference is that it will do so in bulk in recirculating systems losing heat since the system temperature will approach freezing. I would think that laminar flow (lower velocities would be the best since the flow in the pipe will allow the outside annulus of the fluid in the pipe to serve as an insulator and decrease thermal transmisivity since water is a good insulator.
Now that we are talking about moving fluid, if there is a system that has a net heat loss, then recirculation without heat addition will cause the entire system to freeze, not just a point in the system we were talking about before, unless heat is added.
Insulation board is still one of those misconceptions you spoke about in the beginning of your post. The insulation nor the soils retain heat, they only slow its movement from sources of high heat to lower heat sources. If the soil is permafrost, then it would have no insulating value from your target temperature of 32 degrees F and all you would have is your insulation board, which would do nothing really at that point to stop the flow of heat to the perma frost from a flowing of still water main.
I do agree though, people designing utilities in freeze prone areas should undertake a thermal analysis of the system. Justifying a boiler system to add heat for say a water main is a big step that not many people would even have a clue whay you are doing it and having a study by an engineer telling you why its needed would be invaluable.
Sorry I keep harping on this one, I have seen so may designes out there that were based on misconceptions and not heat transfer physics. I had the opportunity to see them becuase I was called there to fix them after they froze. Things like, "I put insulation board around the pipe and it was supposed to form a cup shape over the pipe and capture heat from below the pipe" were common statements, in addition to the general, "I put insulation around the pipe and the salesman told me that would keep it from freezing since it was insulation and thats what insulation does", (this was on a US Navy pier for a main that provided water and fire protection for ships)."
Take care
BobPE
RE: Sewage force main freeze potential
I can scan the refernce documents and email them to you. You can click on the link and email me directly if you wish, then I can reply. There are a few pages of formulas, coefficients, and piping options to go over.
With reference to recirculation, yes, in some instances such as shallower buried utilities heat was generally added because the velocities could not retain the heat. However, that being said, I know that earlier in my career when in the dead of winter, a waterline had to be isolated and residential homes were temporarily serviced with overland water hoses (1" diameter), even in -20 Celcius, if they kept the water flowing through the hoses, they would not freeze, although they must have been close to, if not at, supercooling tempuratures. Typical recirculation systems I designed utilized the water reservoir (buried) as the start and terminus point for recirculation. Returning water would warm up in the reservoir and the daily demand always brought warmer water into the system. We did however have backup heat exchangers, but to my knowledge have never been utilized at the three reservoirs I am referring to.
KRS Services
www.krs-services.com
RE: Sewage force main freeze potential
Take care,
BobPE
RE: Sewage force main freeze potential
RE: Sewage force main freeze potential
I can scan excerpts of the design manual is you wish. It is a very interesting read for those not accustomed to cold climate designs, yet are very useful for problems described by PaulLDE (forcemain query).
KRS Services
www.krs-services.com
RE: Sewage force main freeze potential
I thought we were talking about water in a pipe? The freezing pattern you describe would occur in laminar flow though krs. Yes post some exerpts if they are not too long....and who the author is along with credentials... I have read about the phenomea with streams but I understand the reduced temps are due to the purity of the water and the lack of nucli to allow water to freeze to, which we do not have the luxry of dealing with with drinking water
BobPE
RE: Sewage force main freeze potential
RE: Sewage force main freeze potential
Just to clarify something: the purity of water doesn't prevent freezing - exact the opposite happens. Pure water = distilled water freezes @ 0 Celsius, water containing salts and other free ions will freeze @ even lower temps.
RE: Sewage force main freeze potential
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
www.krs-services.com
RE: Sewage force main freeze potential
RE: Sewage force main freeze potential
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
www.krs-services.com
RE: Sewage force main freeze potential
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
RE: Sewage force main freeze potential
Have a great day folks!
KRS Services
www.krs-services.com