Bigger Pipe = Less Freezing?
Bigger Pipe = Less Freezing?
(OP)
I designed a 4" PVC SDR 35 sanitary sewer service line in very cold country. Standard cover here is 7' over sanitary sewer lines, but based on recent potholing it was discovered that we will have anywhere from 3'-5' of cover. Under these circumstances I would have liked to spec a pre-fabbed insulated pipe with heat tracer wire, but we need to build in the next few days and cannot wait for the pre-fabbed pipe to be delivered.
A local plumber, whom I trust, stated that if we use a 6" pipe it will allow us to run the pipe at a lesser grade (1% for 6" vs 2% for 4"), which will help our overall cover. But he also stated that the 6" pipe will not freeze as quickly as a 4" pipe. While I realize that standard specs (in general) state that larger diameter pipes can be laid at lesser slopes, I believe this is more a mechanism of efficient flow and cleaning, not necessarily related to freezing.
So, anyone else know if a six-inch SS Service is less likely to freeze than a four-inch (assume they are laid at the same grade)? Any additional thoughts would also be appreciated.
A local plumber, whom I trust, stated that if we use a 6" pipe it will allow us to run the pipe at a lesser grade (1% for 6" vs 2% for 4"), which will help our overall cover. But he also stated that the 6" pipe will not freeze as quickly as a 4" pipe. While I realize that standard specs (in general) state that larger diameter pipes can be laid at lesser slopes, I believe this is more a mechanism of efficient flow and cleaning, not necessarily related to freezing.
So, anyone else know if a six-inch SS Service is less likely to freeze than a four-inch (assume they are laid at the same grade)? Any additional thoughts would also be appreciated.





RE: Bigger Pipe = Less Freezing?
RE: Bigger Pipe = Less Freezing?
RE: Bigger Pipe = Less Freezing?
Service sewers are usually run at a 1% slope which is higher than the minimum slope (for 2 ft/sec flow).
A 6" sewer is the standard connection in this part of the US and the slope is a minimum of 1% so the 6" sewer service at 1% slope should be no problem.
The 6 inch pipe also has 2.25 times the flow area of a 4 inch which makes it less likely to freeze solid. Since flowing water will not generally freeze, I would tend to agree with your plumber. The slower steadier flow of a 6 inch pipe is probably less likely to freeze.
RE: Bigger Pipe = Less Freezing?
RE: Bigger Pipe = Less Freezing?
I would say the 6" pipe is slightly more susceptible to freezing than 4", given the lower velocity, more surface area of a stream exposed to cold air.
RE: Bigger Pipe = Less Freezing?
5. Pipe Size 6" dia. minimum for building services.
12 Pipe Trench-
The pipe must be laid at a slope of at least 1%
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The minimum sanitary sewer size is 8-inches for all mainline sanitary sewers and 6-inches for sanitary service lines.
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I don't see that much difference in freezing between a 6" and 4" pipe assuming that they are at the same grade, slope, and flow. The big thing going for the 6" pipe is that the flow area is 30 square inches versus 13 square inches for the 4" pipe.
RE: Bigger Pipe = Less Freezing?
RE: Bigger Pipe = Less Freezing?
You can run the service with less than 7' minimum cover, its not a good standard practice but it is possible.
Extruded foam insulation board should keep the pipe from freezing. 4" of 2"x4'x8'foam board with the long dimension placed perpendicualer to and centered over the pipe and the second layer placed at 90 degrees to the first should provide enough protection on the 5' bury depth. On the shallower sections I'd extend the the foam board out farther from the pipe centerline.
Its not an ideal solution with a pipe so shallow, but it should prevent the pipe from freezing.
RE: Bigger Pipe = Less Freezing?
It's good to know the standard grade for a six-inch service is 1.0%. The proposed line is 250' (with numerous cleanouts and bends) and the elevation difference between the existing manhole connection and existing building is set, so going from a 2% to a 1% slope makes a big difference in the pipe cover. There is 5.0' of cover over the pipe at the manhole, and the existing grade along the pipe is about 1.3%. So, if I run the 6-inch line at 1.3%, I should be able to maintain 4.0'-5.0' of cover, which I am comfortable with. Running this line at a 2% minimum was leaving me with less than 3' of cover at the building, which would put it above the frost line during a large portion of the winter.
FWIW, I will also be requiring a three-sided "blueboard" insulation (three boards thick) along the entire length of the pipe.
RE: Bigger Pipe = Less Freezing?
Service sewers whether the sewer is 4" or 6" diameter should have a minimum slope of 1%.
1% slope in a 6 inch pipe will give you a velocity of 2.86 ft/sec assuming you have the flow volume. 1% slope in a 4 inch pipe will give you a velocity of 2.18 ft/sec assuming you have the flow volume.
RE: Bigger Pipe = Less Freezing?
RE: Bigger Pipe = Less Freezing?
The plumbing code only covers plumbing inside of buildings.
Here is an example of a state standard for service sewers:
i) Service Connections
Sewer service connections shall meet the same criteria as public sanitary sewers described elsewhere in this Subpart C except as noted in this subsection
(i). Roof and foundation drain connections to the sewer service connection are prohibited except as provided for in 35 Ill. Adm. Code 306.302. The service connection tap into the public sewer shall be watertight and shall not protrude into the public sewer. If a saddle type connection is used, it shall be a commercially available device designed to join with the types of pipe that are to be connected. All materials used to make service connections shall be compatible with one another and with the pipe materials to be joined, and shall be corrosion-proof.
1) Size
Service sewers and fittings shall be a minimum of 4 inches in diameter, but shall not be less than the diameter of the plumbing pipe from the building.
2) Slope
Service sewers shall have a minimum slope of 1%.
3) Alignment
When straight line alignment is not maintained on service connections, cleanouts or manholes shall be provided at points of changes in alignment.
(Source: Amended at 21 Ill. Reg. 12444, effective August 28, 1997)
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RE: Bigger Pipe = Less Freezing?
RE: Bigger Pipe = Less Freezing?
THe state standard above is for service sewers that go from the municipal sewer in the street ("service connection tap into the public sewer") to the building ("not be less than the diameter of the plumbing pipe from the building.").
The service sewer is typically on private property until it reaches the raodway R/W.
TERMS DEFINED
"Building Service Sewer (Wastewater)" shall mean the sewer extension from a building drain outlet point located five (5) feet outside of a building or a dwelling unit to a point of connection with a public sanitary sewer.
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RE: Bigger Pipe = Less Freezing?
As cvg stated, the number of bends and low velocities may prove to be an issue, but given the site constraints and existing tie-in point elevations, I'm not sure there are more viable options.