Northern water system innovations for freeze protection

Northern water system innovations for freeze protection D. Farrell McGovern, Independent Consultant Ken Johnson, Cryofront The Northwest Territories and Nunavut are a vast expanse of land lying mostly north of the Sixtieth parallel and making up about one third of Canada's total area. The population of this huge area is only 84,000, and the communities of Yellowknife and Iqaluit have 40 percent of the population. In both territories, the territorial governments are ultimately responsible for providing water distribution and sewage collection systems to the 58 communities, although, in the Northwest Territories a significant amount of this responsibility has been devolved to the communities, particularly the larger communities.

Of the 58 communities, only 10 of the communities have piped water and sewer, and the remainder of the communities are served with trucked water and sewer. Water supply and sewage collection systems of many types and materials have been installed over the past 60 years. While not always successful, these systems have provided invaluable experience and have all contributed to the development of the standards utilized today.

Many innovations have been made as part of the development and evolution of standards used today, and one of the most significant innovations has been the freeze protection of the water and sewer systems. Freezing is the inevitable outcome of placing water filled piping in an environment that is colder than 0â °C, and freezing usually leads to substantial damage to the piping, and therefore it is not a desirable situation.

Classic responses to freeze protection have been to avoid freezing environments, or provide freeze protection measures. In avoiding the freezing environment, the most common approach has been to bury the piping deeply enough to ensure that it operates in un-frozen soil, or provide insulation over the pipe to prevent the frost from penetrating to the depth of the pipeline.

Other common freeze protection measures applied in the far north have been insulation of the pipe itself by applying a factory installed layer of polyurethane foam to reduce heat loss. and heat tracing with a heating cable adjacent to the pipe to offset the heat loss from the pipe. Pipe insulation reduces the rate of heat loss and delays the time to freeze, however, freezing is still inevitable if you wait long enough. There are limited opportunities to optimize freeze portion by heat tracing, such as thermostatic control in small portion of the distribution system, however, heat tracing cables are expensive to operate, and have historically been unreliable and subject to failure after a period of time.

The fundamental practice for water systems, developed in southern Canadian environments, has been to design and construct water distribution systems as a grid to improve reliability and hydraulic performance. The hydraulic analysis often has the assumption that there are uniform water demands

applied across the entire system. The hydraulic design includes consideration for high water demands for fire flow plus maximum day demand, or maximum hour demand. Freeze concerns in this approach arise during low demand periods, and non-uniform distribution of demand, which may cause a change in direction, or stagnation of flow in the piping.

An innovation to freeze protection emerged with the consideration of the physics of water freezing, which included the principles of heat loss, mass flow and ambient temperature to estimate a temperature depression for the water. However, the thermal analysis requires ground temperature data, which was not easy to find. Some data was available from records of Canadian Climate Norms, and more site specific data was available from areas where agricultural research had taken place.

However, it was usually required to assume a value based on experience and make judgements that the ground temperature would be somewhat lower that the climatic mean, but warmer than surface temperature.

Another element to the analysis was the water temperature in the system itself, which could be obtained from system operating data. With groundwater supplied systems there is a freeze protection bonus because ground water is usually considerably warmer than surface water that is available during the winter months. Unfortunately, groundwater supplies are unusual in the north.

Surface water supplies can be challenging and may require tempering (heating). The most significant concern is the “temperature depression” which emerges from a calculation of heat loss. The volumetric flow rate is used to calculate mass flow rate, and by applying the specific heat the temperature depression may be determined. If all things are in order with this calculation, the mass of water leaves the section pipe under analysis with a temperature above 0⁰C. It is true that water can remain liquid at 0⁰C, but only the brave depends upon latent heat to avoid freeze.

Calculations alone may not be enough to finalize the system and developing a comfortable understanding about the flow patterns in the distribution system may be needed. This understanding includes a determination of the flow patterns during periods of low demand. The energy gradient may be very flat during low demands leading to challenges in reliably estimating flow directions and volume of flow. The available history of a water system can provide guidance about low demand rates, but if information is not available, assumptions may be necessary about the demands.

It should be noted that flowing water can freeze with the formation of frazil ice, which is “soft” or amorphous ice formed by the accumulation of ice crystals in water that is too turbulent to freeze solid.

Basically the water needs to be moved out of the system before the heated needed for freeze protection is lost, and newer, warmer water must be added to offset the heat loss. Freeze protection is best applied at point remote from the supply, which assures that system sees consumption plus freeze protection demands, and assures some energy gradient across the system.

The action to remove water out of the system before too much heat is lost is accomplished with two approaches. The traditional approach has been to “bleed” water from the water system into the sewer system, which is generally easy to do, but is wasteful and expensive. The second approach is to use pumps which create a recirculation flow, which demands that system does not include any local closed

loops, that could simply circulate the water until is uniformly freezes. This design provision often creates single loop, non-gridded water systems that are somewhat complex.

Associated with looped systems are some unique configurations of piping and some unique pieces of hardware. The unique configurations can include 3 pipe systems, where the third pipe is looping back on itself to overcome a dead-end system. The unique hardware includes the insluated access vault system, where the water and sewer pipe pass through a common access point.

In summary the development of a freeze protection strategy for a water system requires an reliable understanding of flow patterns that is independent of demand; a method to predict heat loss and temperature depression; and, sufficient flow that the returning water remains above 0â °C. It is also important to recognize that, despite our best efforts, things can go wrong. There are places where piping that resists damage due to freeze, such as HDPE, may be a good choice.

Figure 1. Design concept for recirculating Arctic water supply system with water reheat station.

Figure 2. Yellowknife water and sewer system with insulated piping, and recirculating (2 pipe) water service connections.

Figure 3. Rankin Inlet, Nunavut water and sewer system – three pipe system with one pipe used for water recirculation.

Figure 4. Access vault for Arctic water and sewer systems.