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?
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Sewage force main freeze potential
- Thread starter PaulLDE
- Start date
I would agree that an active force main should keep it from freezing, but you do know about Mr. Murphy? In Alaska, pipes come preinsulated and we also would put 2" thick "Blue Board" insulation in the trench (full width) at about 6" above the pipe. There are heat transfer calculations that can be done that would validate your lesser depth design. You'll have to know the thermal and physical properties of all the layers of cover and of the pipe itself. But it's very tedious and it's for ideal conditions, which never happens. I would insulate the 60', it's cheap and the MUA has already bought into the idea since it's their idea.
you wont protect the pipe from freezing, but I see a lot of people doing this and it seems to make them feel better about it. The pipe will be in a freeze zone and will make transitions to and from this zone from areas that the pipe is protected from the freeze zone from the bury depth. Insulation works by slowing the heat loss from the pipe, if there is no energy left as heat in the pipe, it will reach the temp surrounding the pipe which if it is in the freeze zone of the soil, will be 32 F or less. The transitions to and from the soil freeze zone will induce forces that may sheer the pipe, you should take this into account too.
Good luck...
BobPE
Good luck...
BobPE
Paul, Freeze protection is dependant on geographic location, burial depth, and medium. For water, fundamentally, to have freeze protection, you need to get below the frost penetration line. In the continental U.S. it runs from a minimum of 1" to a max. of 84", depending on geographic location. If you're inside the frost zone, freeze up will occur, necessitating insulation, heat tracing or both.
Hope this helps.
saxon
Hope this helps.
saxon
Paul,
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)
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)
CarlB, The piping I was referring was a force main by the Sullivan Arena. As designed, the force main dropped into the sewer at about 6' below grade. The force main was insulated and blue board laid over that as designed by Tryck-Nyman Engineers. The piping I've seen in Barrow and in other villages were above grade with super insulation and heat trace. But they also have buried pipe in the permafrost.
gtrainor:
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
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
If the MUA is requiring you to insulate all pipe above the 48" frost line, then you have to insulate. Use the heat transfer rate of uninsulated 4" pipe and you minimum ambient temperature, probably -40 deg F. Your will find that your pipe will freeze.
BobPE, I was responding to the original question concerning sewer force mains not potable water. I don't think they recirculate sewage. If you visit the village of Barrow you'll see instances where the force main piping is elevated with super insulation and heat trace. The piping comes pre-insulated with heat trace channels in the insulation. I was a PE in Alaska and also had to take the Artic Engineering course. Check out:
kewl site gtrainor, i liked the fluid glycol channels they can build in the insulation. Sorry, I got so cought up in the conversation that I reverted to my natural state....water...lol. I guess my concern is more general, it seems a lot of people think insulation without a heat trace thermal break or another source of heat like moving fluid, will protect the pipes from freezing. I had the opportunity to work on water systems in Russia and they are quite elaborate with recirculation and heat addition. They distribute recirculated hot water in Russia in lieu of domestic hot water heaters and use the heat from that system in parallel utility trenches to add heat. There are a lot of options, but all involve addition of heat and I guess the point I was making is that insulation alone will not work.
How was that Alaska Engineering test? I have heard people talk about it and it sounds really interesting!!
BobPE
How was that Alaska Engineering test? I have heard people talk about it and it sounds really interesting!!
BobPE
The Alaska PE test is no different from the National test. But in addition to the test, you have to take a college course in Arctic Engineering. It has it's own test in the class, but nothing about Arctic Engineering on the PE exam. I'm in Arizona now where I only see distant mountain tops of snow and that suits me just fine.
KRSServices
Civil/Environmental
I would like to clarify a few misconceptions regarding the freeze protection of infrastructure.
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
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
krss:
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
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
KRSServices
Civil/Environmental
BobPE
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
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
no problem krsservices, I see what you were saying. As long as you realize that velocity will not keep a pipe from freezing, it's the heat in the flow that does the trick. That is why a lot of plumbers say that flowing water will not freeze, although they really dont know why, they attribute it to velocity but its really the heat in the fluid from the distribution system. You are right about demands, this helps a lot of systems more than they know by moving more heat into the system. Most of the time the pipes are properly designed below frost lines and the ground will add or stabilize the heat in the system also.
Take care,
BobPE
Take care,
BobPE
dicksewerrat
Civil/Environmental
Maybe you would want to use HDD to place the force main under all the utilities in your way. It wouldn't take a long time to do and would give you more peace of mind. You could relplace the entire main with 4" HDPE and probably eliminate the repair that will be staring at the owner in 25 years. The force main activty will release a fair amount of H2S. This will allow the anerobic bacteria to attack the cemnet liner in the ductile pipe and eventually the metal.
KRSServices
Civil/Environmental
You know, it wasn't until I studied the water flows, particularly stream flows, when I found the velocity does indeed play a critical role in the prevention of water from freezing. Did you know that supercooled flowing water in a glacial stream for example can reach tempuratures of -5 to -7 celcius (or lower) without freezing. Supercooled liquid in a pipe, release heat in an absolutely interesting pattern....it is not uniform, rather, the pattern is like a cyclical (repeating) tapered pattern. As the taper narrows, the liquid loses further heat until a blockage occurs. Once this happens, flow stops and the liquid freezes solid. In reducing the heat loss by maintaining a higher velocity, this taper is not allowed to progress to a smaller appurature.
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
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
Kkrsservices:
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 LOL...although people may assume it is that pure....
BobPE
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 LOL...although people may assume it is that pure....
BobPE
KRRSServices, Got no misconception about buried utilites in frost prone soils. The original question pertained to sewage force mains in New Jersey, where there is no permafrost. Can you tell me that the sewer force mains systems you have designed are recirculating? The underlying ideal situation is to place the line below the frost line. But if that's not possible, then what do you do for a 60' length of sewer force main in New Jersey? That was the original question PaulLDE posted on this thread. I still say the simplest thing to do is to insulate the pipe and lay blue board down over the pipe. It's cheap and the regulating agency is telling PaulLDE to do this.
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