Understanding thermoplastic double

By Steven DolejsiAs its name suggests, a double-wall piping system is essentially one piping system within another. The inner pipe is referred to as the ‘primary’ pipe, while the outer is the ‘secondary’ or ‘containment’ pipe. The goal of these double-wall assemblies is to create a failsafe system where the secondary piping would contain any leaked fluid if a failure occurred in the primary piping. Available in various materials, double-wall piping systems are ideal for many applications.

The majority of double-wall piping systems installed in water and wastewater treatment facilities are mandated by the U.S. Environmental Protection Agency (EPA). While this body has oversight south of the border, this author has found regulations for double containment systems to be far better defined in the United States than in Canada. In other words, while the design/construction professional needs to be aware of all local rules and regulations, the EPA provides a good general guide.

Under the agency, all piping systems conveying hazardous wastes listed or identified under Subtitle C of the Solid Waste Disposal Act, or a mixture of such hazardous waste and other regulated substances, must comply with Section 280.42, "Requirements for Hazardous Substance Underground Storage Tank (UST) Systems." This section indicates underground piping must be equipped with secondary containment systems that:

Although EPA may not require the use of double-wall systems above ground, safety issues have become a serious concern in Canada and the United States. The combined cost of litigation, cleanup, and increasingly stringent safety guidelines has necessitated use of failsafe double-wall assemblies for above-ground hazardous piping.

Automatic leak detectionAccording to the EPA requirements, underground piping that conveys regulated substances under pressure must also be equipped with automatic leak detection. Such systems monitor the interstitial space in a double-wall piping system. Fully automated and failsafe, they can often be integrated into a plant's programmable logic controller (PLC) or supervisory control and data acquisition (SCADA) system for computer-based monitoring and control.

When a leak occurs in the primary pipe, the detection system alerts an operator by means of a visual and/or audible alarm. The two most common systems available are electronic low-point leak detection and continuous electronic leak detection.

The operating theory behind the first category involves creation of ‘zones’ in a double-wall system. This is accomplished by installing leak detection stations at specified locations. These are essentially a drip leg in the secondary containment piping where fluid accumulates if there has been a leak in the primary piping.

Each leak detection station is monitored with an electronic sensor, and various sensing systems are available. Most systems require the sensor to be in contact with the leaked fluid to send an alarm. There are also systems employing a proximity sensor that can detect fluid through the wall of the pipe, without coming into contact with the fluid. These sensors eliminate compatibility issues, optimize functionality, and render the sensors reusable and virtually maintenance-free.

Continuous electronic leak detection systems employ a sensing cable. The electrical properties of the cable are altered when it comes into contact with a liquid. The sensor cable is pulled through the piping's interstitial space after installation and rests on the bottom of the secondary piping. It is important to consider the secondary pipe's size when using continuous electronic leak detection systems—the interstitial space must be large enough to allow the sensing cable and associated connectors to be properly installed.

It is good practice to incorporate visual leak detection along with automatic leak detection. This allows an operator to verify alarms visually before taking corrective action. It is also a good idea to use such detection in above-ground systems. Clear polyvinyl chloride (PVC) can be employed for instant visual leak detection to ensure corrective action is taken immediately.

Material optionsWith double-wall piping systems, engineers do not design two separate single-wall systems, but rather a combination of the two. The primary and secondary piping of double-wall systems are interrelated, where changing conditions continually affect both pipes. It is therefore critical to have double-wall systems professionally designed by an experienced engineer and assembled at the factory by a credible manufacturer.

There are various materials available for thermoplastic double-wall piping systems, ranging from PVC and chlorinated polyvinyl chloride (CPVC) to polypropylene (PP), polyethylene (PE), and polyvinylidene fluoride (PVDF). Choosing the correct material for both the primary and secondary system depends on the pressure, temperature, and compatibility with the service fluid.

PVC and CPVC are frequently specified as primary and secondary containment piping materials in water and wastewater treatment facilities. Water treatment chemicals—such as sodium hypochlorite for disinfection—are often transported underground from storage or pump systems. Double-wall piping systems are required to prevent any possible leaks or spills into the surrounding environment.

Many applications may use the same material for both the primary and secondary pipe, but employ a thinner wall for the secondary pipe to provide material savings. One example would be a CPVC Schedule 80 x CPVC Schedule 40 double-wall system. This practice is often acceptable since the secondary pipe is not required to operate under pressure for a prolonged period.

It is also possible to use different materials for the primary and secondary piping for maximum performance while further minimizing material cost. For example, a primary CPVC Schedule 80 pipe may be contained in a secondary PVC Schedule 40 pipe where the CPVC is compatible with a chemical under pressure or at elevated temperatures, and the PVC is compatible only at drainage pressures and at ambient temperature.

Case studyOriginally built in the late 1950s to handle an average daily flow of 7.57 million L (2 million gal), the G.E. Booth (Lakeview) Wastewater Treatment Plant is a key facility for the Greater Toronto Area's (GTA's) Region of Peel. In 2009, the facility underwent a $260 million expansion to treat 518.6 million L (137 million gal) from the more than 1.3 million residents and 90,000 commercial businesses in the eastern section of Mississauga, Brampton, Bolton, and Caledon East.

Now expected to effectively meet the area's wastewater treatment needs until at least 2031, the expanded Lakeview facility includes a new headworks facility, enhanced nitrification, a new biosolids handling facility, and additional incinerator capacity. On completion of the project, Lakeview became the largest perforated-plate screening facility in North America, and the largest fluidized-bed biosolids incineration facility in the world.

At the facility, wastewater flowing through 11 primary sedimentation tanks is mixed with ferrous chloride at aeration tanks to solidify and remove excess phosphorus. After secondary clarifiers remove the additional solids from the wastewater, it is further disinfected before flowing over weirs and eventually discharging into Lake Ontario. The diffusion pipe system reaches more than 1.25 km (0.8 mi) from the shore to the lake bottom. To ensure safety and reliability, the chemical feed system transporting the ferrous chloride diluted with carrier water is double-contained to prevent any possible leaks or spills.

"The chemical feed system is a long system of pipes running through underground tunnels, and ferrous chloride is a very corrosive chemical," explains Vlad Petran, Peel's manager of wastewater treatment/capital works, and a former senior project engineer with AECOM Canada Ltd., which designed the system. "Any leaks can create a safety hazard, as well as potentially damage the concrete structure or other systems running through the tunnels. Using a double-containment system ensures a spill-free system for better reliability."

During the initial design phase of the Lakeview project, the piping manufacturer worked with designers on a double-containment system comprising 50-mm (2-in.) Schedule 80 PVC carrier pipe inside a 100-mm (4-in.) Schedule 40 PVC containment pipe. To reduce system installation and maintenance costs, it features a proprietary design allowing the system to be installed with full 6.1-m (20-ft) lengths, while keeping the carrier pipe perfectly centred inside the containment piping. The system is also available in spool-piece fabrications according to specific application designs.

"We chose the PVC double-containment system because it offered the corrosion resistance we needed, and it is was the most cost-effective option," says Petran. "The manufacturer also worked with us to find the most economical way to design the system with expansion joints to accommodate the seasonal temperature changes in the tunnels."

The system installation was performed by Maple Reinders Constructors Ltd.; project manager Lyndon Grovum co-ordinated onsite training for piping installers to emphasize proper solvent welding and other installation procedures before the job.

Visual leak detection stations were created using clear PVC S40 pipes.

"The plant operation staff was concerned about detecting any leaks in the system. To provide extra peace of mind and a worry-free system, the manufacturer provided clear tubes at visible low points in the system where potential leaks would be noticed," says Petran. "These detection points are inspected on a regular basis. The system is working as intended—we have not experienced any leaks since it was installed in 2009."

ConclusionSafety and environmental concerns are top priorities on today's global agenda. Reduction of emissions, energy conservation, and prevention of groundwater contamination are some of the areas where regulations are increasingly defining an important line between utilization and exploitation of our planet's resources. Where applications call for a fail-safe, leak-free piping system, a double-wall piping system could be ideal.

Steven Dolejsi is an applications engineer with IPEX USA LLC in Mississauga, Ont. He received a bachelor's degree in mechanical engineering from Ryerson University. Dolejsi has eight years of experience in thermoplastic piping system design and application; he has assisted in material selection and designed for multiple water and wastewater treatment facilities. He can be reached at [email protected].