Bigger Pipe = Less Freezing? 1

[civilman72]

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.

 

[cvg]

in general, the flow velocity will decrease which might allow it to freeze easier. In addition, you may reduce your velocity below the 2 feet per second which is desired to flush solids. Larger lines for sewers are not always better.

 

[civilman72]

That's what I'm concerned about. Common sense (to me) says that a 6" pipe may even freeze quicker due to the lower velocity and the fact that the water surface along the pipe will also increase.

 

[bimr]

You state that you are working with a sewer service. A service sewer is the sewer connection between the user (house) and the street (sanitary sewer system).

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.

 

[cvg]

I can't comment on the freezing, however 4" lines are typical for service connections around here and 1/4" per foot slope minimum. However, this slope does not guarantee 2 fps for a 6 inch pipe unless the pipe is flowing at least half full. that is a lot of flow (0.048 cfs) for one service connection, unless this is a commercial or industrial connection.

 

[CarlB]

Standard practice In Anchorage, Aalaska would call for placing board insulation over a service line that has less than 5.5' of cover. If less than about 4' then a pre-insulated pipe is called for.

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.

 

[bimr]

Many towns are going to 6" diameter service sewers because it is difficult to clean a 4" pipe when the sewer fouls. Here are examples:

5. Pipe Size 6" dia. minimum for building services.

12 Pipe Trench-
The pipe must be laid at a slope of at least 1%

http://www.dgsd.org/NewService/contractorschecklist.pdf

The minimum sanitary sewer size is 8-inches for all mainline sanitary sewers and 6-inches for sanitary service lines.

http://www.ci.lancaster.oh.us/dept/...Construction/Sanitary Sewer Design Manual.pdf

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.

 

[bimr]

Placing board insulation over a service line is a good tip.

 

[bpattengale]

The plumber is going by plumbing code with the pipe slope. If I remember correctly a 4" service line should have a minimum 1/2" per foot slope and the 6" service is a minimum slope of 1/4" per foot. Most codes specifically prohibit upsizing the pipe to simply lower the minimum slope because with a larger pipe and the same flows the minimum 2 fps cleansing velocity will not be maintained. (In Minnisota plumbing code goes to the property line in some LGUs)

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.

 

[civilman72]

Thanks for all the responses. To clarify a few things, this is a service line for a small office building with one bathroom, and a small kitchen, so very small intermittent flow.

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.

 

[bimr]

1/2" per foot slope is actually a 4.2% slope. 1/2" per foot slope is the slope that is used for building plumbing, not service sewers.

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.

 

[cvg]

running the line which is 250' long with numerous bends and cleanouts, minimal flow and at 1% slope is not an acceptable design IMO. Minimum slope may be 2% if using the plumbing code. Either way, you should estimate the peak flow and check the flow velocities using Manning's equation - you will need to maintain at least half full pipe in order to achieve the minimum acceptable cleansing velocity for the 6 inch pipe if you install it at 1%. With just one bathroom and a kitchen, you will not be able to do it. So, with flow velocities less than 2 fps, the line will plug and your client will be using those cleanouts on a frequent basis.

 

[bimr]

CVG,

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)

http://www.ipcb.state.il.us/documents/dsweb/Get/Document-12041/

 

[cvg]

agreed that minimum slope standard might be 1%, that doesn't mean that an acceptable design can be achieved with the minimum standard. And as previously stated by others, the plumbing code generally goes to the property line and applies to private sewers. So this private sewer line may still be required to follow the plumbing code which I believe requires 2% minimum slope. The state standard you provided is for Illinois, applies to public facilities and would not apply to private sewerlines. Either way, it would not be prudent to design a line that is likely to become plugged when there are other ways to solve the freezing problem.

 

[bimr]

Maybe you can reference the plumbing code that you are referring to?

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.

http://www.carovillage.net/pdf/eng-std/definitions.pdf

 

[civilman72]

Final design for 6" service is at 1.5% grade for entire section of pipe. We have a minimum of 5' of cover, and will be installing three-sided blueboard insulation. I think this has a very good potential of working and not freezing, except in maybe our coldest winters. But during standard winter seasons, I feel like this will not have any issues.

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.