11 minute read
Handling water supply cross connection and backflow prevention with ease
By Gilbert Welsford Jr.
Utilities and government agencies dealing with potable water supply must protect piping systems from contamination. The presence of pathogens, suspended debris, and foul water poses a health risk to consumers. Backflow at cross connection is one of the common sources of contamination in potable water piping systems.
As such, authorities and utilities need to carefully design and size piping systems and establish preventive measures against contamination. They also require robust maintenance programs to monitor the performance of different check valves, piping components and pipe connections.
A typical public water supply system contains several backflow prevention devices at consumer terminals. As part of maintenance, regulatory authorities conduct regular tests to verify the reliability of the devices. They recommend repairs and replacement of damaged contamination control devices and evaluate the hazard levels of existing backflow prevention devices.
UNDERSTANDING CROSS CONNECTIONS AND BACKFLOW
A cross connection can be a permanent or temporary connection between a potable public water supply line and a non-potable water source. It also refers to the potential connection between public water supply systems and pipes conveying wastewater, chemical products, sewage and stormwater drainage. Cross connections are available in heating, ventilation and cooling systems, fire suppression utilities, agricultural irrigation systems, chemical processing facilities and diverse factory floor equipment. A cross connection can cause undesirable backflows, which permit the flow of hazardous or pathogenic fluids into potable water systems.
Backflow happens when there is a reversed fluid flow. In the process, polluted substances get into the potable water system. Backflow is predominant at cross connections and occurs by back-siphonage, or backpressure backflow.
BACK-SIPHONAGE
A pressure difference must exist in the pipe systems for water to flow from the central distribution system to consumer premises. It could be from a water treatment plant to commercial or residential properties. The system maintains a positive pressure difference for the fluids to flow.
There are a few instances when the public water distribution system experiences negative pressure. The existence of negative pressure creates a full or partial vacuum. When this happens, the probability of backflow increases.
A negative pressure difference can occur if the supply is interrupted abruptly, experiences high demand, or if there is a break in the main water supply line.
High-demand flow applications that can cause a break in the water supply line include the activation of a fire hydrant or similar events. Back-siphonage allows water from the building’s plumbing system to freely flow back into the public water system to fill the existing vacuum.
BACKPRESSURE BACKFLOW
Non-potable water can flow into public water systems if there is a pressure imbalance in the system. Bad piping or poor pump selection and connection cause the downstream pressure to exceed upstream or the water supply pressure. When downstream pressure exceeds upstream pressure, fluids change their flow directions unless a suitable backflow prevention device is installed.
In industrial processes, there is a probability that equipment experiences abnormal temperature changes. An increase in boiler temperature affects the pressure conditions of fluids flowing through them. As a consequence, water pressure in the boiler drops below the supply line pressure level. Such conditions make it easy for non-potable fluids to flow into potable water supply systems.
Protecting public water supply from
A double-check valve backflow preventer. Credit: Simone, stock.adobe.com
backflow is a top priority for utilities, municipal authorities, and health and sanitation organizations. When backflow occurs, contaminated water and pollutants find their way into piping systems and it makes drinking water unsafe for human use. The presence of microorganisms, harmful chemicals, particulate matter and contaminants, increases the risk of illnesses.
HOW TO CONTROL CROSS CONNECTION AND BACKFLOW IN WATER SUPPLY SYSTEMS
Protecting consumers begins with the appropriate design of public water supply systems. Water suppliers may lack the capacity to repeatedly inspect every residential and commercial premise for cross connections and potential sources of backflow. However, they must provide sufficient protection devices to prevent backflows. These devices are installed at different sections of the pipeline. The utility company inspects and repairs backflow preventers at critical water service connection points in the public water supply system.
The utility company can install backflow preventers at the connection points of: • Dedicated fire suppression systems. • High demand irrigation systems. • Highrise buildings. • Facilities with reclaimed water systems and stormwater collection systems. • Industrial and chemical processing facilities with potential for backflows.
Here is a look at some of the practical backflow prevention mechanisms that utility companies can employ to protect their customers and maintain the potability of drinking water.
AIR GAPS
Air gaps are popular backflow prevention mechanisms used to create a physical separation between the end of a public water supply pipe and the overflow (flood-level) rim of the receiving vessel. The air gap eliminates a cross connection and provides sufficient protection against backflow. It is a simple back-
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flow prevention mechanism, but does not provide optimum protection against potential back-siphonage or backpressure backflow.
An air gap is a vertical separation and should be larger than one inch. The physical separation distance should be at least two times the inner diameter of the public water supply outlet. Air gaps are non-mechanical backflow prevention mechanisms and are easy to bypass.
REDUCED PRESSURE PRINCIPLE ASSEMBLY
The reduced pressure principle is a mechanical mechanism that utilizes two independently acting check valves, four well-spaced test cocks, a pair of isolation valves and a differential pressure relief valve. It is a reliable backflow preventer suitable for high- and low-hazard backflow prevention and isolation protection.
This mechanism is an assembly containing two loaded check valves and a relief valve between them. The relief valve maintains the differential pressure between the check valves. The reduced pressure assembly also has a pair of shut-off valves on the upstream and downstream of the piping system that close tightly to prevent back-siphonage and backpressure backflow.
A pressure drop is created when the water flows past the first check valve. The relief valve determines whether the supply pressure is higher. If that is the case, the relief valve remains closed. Once the pressure drop between the supply and the valve assembly drops, the relief valve opens. The loss of supply pressure keeps both check valves closed. When there is a total pressure loss, the pressure between the check valves is equivalent to the atmospheric pressure since the spring mechanism opens the relief valve.
PRESSURE VACUUM BREAKER ASSEMBLY
A pressure vacuum breaker assembly consists of an independent springloaded check valve and an independently operating air inlet at the discharge port of the check valve. It contains seated test cocks and tightly closing shut-off valves at both ends of the backflow preventer assembly. The spring-loaded check valve
is open, while the air inlet valve remains closed when the pressure vacuum breaker is in good working condition. There is no backflow in this instance.
When the check valve is damaged, the air inlet valve opens. It permits atmospheric air to pass through the check valve and enter the main water supply line. The presence of air breaks the partial or full vacuum created, thereby restricting back-siphonage.
Pressure vacuum breakers are effective for preventing backflow due to back-siphonage only. The backflow prevention mechanism is common in connections to irrigation systems.
DOUBLE CHECK VALVE ASSEMBLY
This is a mechanical backflow preventer, suitable for the protection of non-health backflow hazards. It features two independently acting check valves that are spring-loaded. The assembly contains test cocks and two shut-off valves located at both ends of the backflow preventer assembly. The double check valve assembly is vital for protecting public water supply systems from backflow due to back-siphonage and backpressure backflow.
Another alternative to the double check valve assembly is the residential double check valve assembly. The latter does not have shut-off valves and test cocks, which are less reliable and can isolate non-health hazards in single-family premises. Restaurants, commercial buildings and high-occupancy facil-
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ities, like apartment buildings, require backflow preventers. This is due to the high demand for potable water and equally voluminous discharge of effluent, wastewater and pollutants.
MAINTENANCE OF BACKFLOW PREVENTERS
Most backflow preventers rely on a mechanical action to protect public water supply mains. They contain movable parts that wear out over time. It is therefore important to pay attention to installation guidelines. Poor installation impedes the efficiency of the backflow preventers, rendering them unsuitable for long-term use and exposing customers to health hazards. Easy maintenance interventions include: • Utility companies need to inspect the backflow preventers regularly. Subjecting these systems to continuous fluid pressure weakens them. • Air gaps require visual inspections to evaluate their health. • Double check valve assemblies, pressure vacuum breakers and reduced pressure assemblies require advanced inspection tools. • Companies should calibrate backflow preventers often to ensure they operate as required. • Replace worn-out valve seals and damaged backflow preventers. • Conduct inspections at least once yearly.
CONCLUSION
The design of public water supply systems adheres to stringent health requirements and statutory plumbing codes. These measures aim to reduce contaminants in potable water systems. There are isolated cases when cross connections contaminate main water supply lines. Such contamination is due to the backflow of non-potable water and other pollutants.
Backflow preventers are vital for eliminating the potential for potable water contamination. These mechanisms require proper sizing and installation to operate reliably in the long term. Inspect and maintain backflow prevention mechanisms regularly for dependable potable water service.
Gilbert Welsford Jr. is with ValveMan. For more information, visit: www.valveman.com
Extreme heat in Canada should be fought with grey, green infrastructure, says report
By ES&E Staff
Anew report from the University of Waterloo is recommending a combination of grey and green infrastructure to combat the increasing frequency of extreme heat in Canada, which will only continue to soar to new levels.
The university’s Intact Centre on Climate Adaptation report notes that Canada has warmed at twice the global rate between 1948 and 2016, with annual mean temperatures increasing by 1.7°C. Much of Canada will experience extreme temperatures in the years 2051 – 2080, according to projections.
The report zeroes in on towns and cities, where urban heat islands can present some of the more daunting challenges. In these areas, daytime surface temperatures can be as much as 10 – 15°C higher than more remote regions.
“The risks of extreme heat are commonly considered in terms of health impacts, with the media focusing largely on heat-related deaths. However, extreme heat also has adverse effects on infrastructure and services, natural systems and ultimately, the economy, as exemplified by the range of impacts identified by the City of Montreal,” states the report, referencing the city’s Climate Change Adaptation Plan.
The Montreal plan is filled with warnings about extreme heat’s effects on road surfaces, as well as the increased demand on municipal services, such as drinking water. Heat can create shorter system idle time that can weaken water systems in case of problems, the plan notes.
Additionally, it can mean an increased presence of cyanobacteria in the water, requiring ozone treatment, as well as faster degradation of chlorine in the system, which will increase re-chlorination needs and associated operating costs.
“Extreme temperatures may increase community demand for water and wastewater treatment services at times when water levels are low,” states the University of Waterloo report.
Working with nature as green infrastructure, as well as improving buildings and public infrastructure as grey infrastructure, can help combat extreme heat, the report says. Green infrastructure can mean planting and maintaining trees in grounds and parking areas, expanding vegetated areas that can absorb water around buildings, or installing green walls.
Grey infrastructure improvements can consist of measures such as installing and maintaining backup power generation, or arranging for backup water supply during power outages. Additionally, buildings can enhance insulation and airtightness, use concrete, brick, stone and tile finishes that absorb heat, or install windows that reduce heat gain from the sun.
The report states that once a community has assessed infrastructure for vulnerabilities to extreme heat, it can work to reduce the risks to the infrastructure itself.
“When communities are designing new infrastructure or adapting existing infrastructure to be heat-resilient, they should consider future potential climatic conditions, including the risks from extreme heat, as a matter of routine,” the report suggests.
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