Corroding and obsolete valves, adoption of stricter plumbing codes and the importance of protecting the public water supply from hazardous backflow all combined to present the City of North Las Vegas with a problem.
According to Stephen Gay, Cross Connection Control Specialist for the city, more than 250 fire sprinkler systems in the North Las Vegas water system are equipped with single-swing check valves. The valves - some installed more than 20 years ago -- no longer comply with the latest backflow protection standards outlined in Nevada Administrative Code 445-A. Wall mounted penstock
Designed for backflow prevention in tight spaces, Conbraco 4S Series assemblies are available in 2 1/2 in. to 10 in. diameters and come with an FDA approved epoxy coated ductile iron body and cover.Click here to enlarge image
Initial inspections have revealed other problems, Gay said. The old valves were never designed for routine testing and the city found many of the oldest single-check valves were corroded in the open position and wouldn't work even during a backflow condition.
Gay said the challenge for the city utilities department was to find a replacement double-check detector backflow preventer assembly that satisfied a number of specific needs. The new installation would have to comply with the revised codes for protection against backflow and fit within the limited space requirements of the existing below-grade units.
After a thorough review of the available products, North Las Vegas and Conbraco Industries agreed to conduct trial applications on the Conbraco 4S Series double-check detector backflow preventer assembly.
Designed specifically for fire sprinkler and irrigation applications, Conbraco 4S Series assemblies meet ASSE, AWWA, IAPMO and CSA, UL Classified and FM Approved standards. Available in 2 1/2 in. to 10 in. diameters, they come with an FDA approved epoxy coated ductile iron body and cover. Maximum working pressure is 175 psig; temperature rating is 33°F to 140°F.
The city decided to test the DCDAs at two fire sprinkler locations. Of all the products considered, the 4S Series, with its short lay length and ease of installation, showed the most promise for the least expense. Conbraco provided the assemblies; city crews installed them.
Gay said other conventional double-check assemblies considered by his department wouldn't fit in the existing below-grade vaults.
"The lay lengths of the single-check units are quite short. The Conbraco backflow assemblies had such a compact design, we found that they would fit below-grade with very limited modifications to the existing plumbing system," he said.
Moving the piping systems above-ground was considered but rejected. Cost was a concern. Gay said the additional materials required and adding a protective enclosure would boost the cost of the retrofitting substantially. The enclosure alone, he said, could add $3,000 to $5,000 to the installation. Adding enclosures also would damage the aesthetics and could hinder the sight lines for passing motorists.
In the two replacement projects, crews installed one 4-inch and one 6-inch Conbraco double check assembly. The three-person crew completed each installation within a few hours.
"Actually, the hardest part of the job was cutting the bolts from the old single-check assembly," Gay said. "The rest was easy... putting on transitioning couplings, tightening them up, recharging the line, flushing it out, testing and certifying operation of the assembly and they were done."
The new Conbraco assemblies are designed for easy maintenance. The top entry models feature a modular design and test cocks that allow for testing, repair and maintenance while the assembly remains in-line.
Since the Conbraco double-check backflow assemblies were installed nearly eight months ago, Gay's department has been checking them monthly -- something they couldn't do with the old valves.
"In that eight months, they have proven to be a very solid valve," he said. "They've maintained high differential pressures and haven't needed any maintenance."
When a North Carolina utility needs to protect commercial and residential property from back flows in its sanitary sewer lines, it uses an automatic valve system that closes off sewer lines serving a building whenever a backflow condition occurs.
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The utility district has a problem with occasional sewerage back flows into homes and businesses, primarily in drains installed below grade and connected to older sewer lines. To counter the problem, the district buys the valves and hires licensed plumbers to install them in affected homes and businesses.
Whenever a sewerage back flow occurs, trapped air in the line is forced into an expansion chamber through a port in the valve. As the expansion chamber fills with air and expands, a knife gate rises until the drain opening is sealed off. This usually occurs at 9 inches of head of water for a 4 inch valve, and 14 inches of head for a 6 inch valve.
Once the back flow subsides and the water level returns to normal, a counterweight atop the expansion chamber forces the trapped air out of the chamber and into the drainage line, allowing the knife gate to lower to a full open position.
Since the valves must be installed at least 13 inches below the lowest drain, most are installed in ventilated pits in the horizontal drain line between the building and sewer main. Although this provides protection from flooding, all plumbing fixtures in the building are out of service once the gate is activated.
An alternative installation is to locate the valve where only the fixtures installed below grade are connected on the upstream side of the valve. All fixtures installed above grade are connected on the sewer side of the valve. This allows continued use of the above grade fixtures while protecting those below grade.
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The Flood-Gate Automatic In-Line Backwater Valve is furnished with 4 inch or 6 inch no-hub connections. The unit is designed to be a full port valve and provided for in-line installation. The valves feature a coated cast iron housing, and a 14 gauge, type 304 stainless steel knife gate with neoprene O-rings seals. The expansion chamber is made of flexible PVC with a polyethylene cover and cast iron counterweight.
The valves are tested to 45 feet of head or 19.3 psi by the manufacturer. They conform to requirements of ASME A112.14.1-1975 for backwater valves.
Rotary Type Goggle Valve It has been three years since the first of the valves was installed by the district and there have been no reported problems.