Fusion for Fission
HDPE pipe use in mission-critical applicationsWith the nation’s nuclear power plants reaching scheduled water line pipe replacement deadlines, an economical solution has been found and approved. Pipe made from high-density polyethylene (HDPE) has been tested, evaluated and put to work in several nuclear plant non-safety areas, including the Catawba Nuclear Station in South Carolina as well as in the first nuclear safety-related application at AmerenUE’s Callaway Nuclear Plant in Missouri.
One of the key reasons behind the use of solid-wall HDPE pipe in a nuclear power plant is its ability to provide a durable, corrosion-free, leak-free pipeline regardless of the size of the pipe or the rigors of the application. Due to the material’s inherent composition, pipe sections and fittings can be fused together, often in complex geometries, to become a unified system. This fusion process is relatively easy to complete, especially when compared to the previous method of welding carbon or stainless steel pipe and fittings.
“For more than 50 years, HDPE pipe has performed under rigorous conditions such as very deep burial, earthquakes and in toxic soil where no other type of pipe could survive,” said Tony Radoszewski, executive director of the Plastics Pipe Institute Inc. (PPI). “Pipe manufacturers and the makers of the resin used for the pipe have traditionally worked together to develop new products. HDPE pipe use in nuclear plants is just a natural progression of this cooperative effort.”
According to Frank Schaaf, a nuclear utility consultant, HDPE pipe also is gaining popularity because of its favorable purchase and installation costs, which enable a plant to replace stainless steel water lines and even have a two-times redundancy, helping to increase operational safety.
Schaaf recently completed the development Code Case for the American Society of Mechanical Engineers (ASME), which has instructions for polyethylene pipe, including how the pipe is to be supplied, design parameters, installation rules, fusion joining requirements and pressure test criteria.
“Most of the nuclear plants in this country are 30-plus years old,” Schaaf said. “Stainless steel was the predominant material when these plants were built. It was readily available and the industry had the expertise to weld and install it. They knew and understood that material.”
Like all types of power generation that use a steam turbine, nuclear power plants require a large amount of water for cooling. Two-thirds of the energy produced by a nuclear power plant is excess heat that is carried away from the plant in water, which remains uncontaminated by radioactivity. The water is sent either into cooling towers where it is emitted as steam or is discharged into a large body of water such as a dedicated cooling pond, a nearby lake or river, or an ocean. Systems to carry this water, plus the pipelines for fire suppression systems, were the first to be converted to pipe made from HDPE.
Catawba Nuclear Station Replaces Non-Safety Pipe
“About 10 years ago, one of the earliest applications of HDPE pipe was at Catawba Nuclear Station in South Carolina for the replacement of non-safety pipe at the plant. Every plant is different, but the problem Catawba faced was tuberculation and corrosion of its steel pipelines,” Schaaf said. “Because this area doesn’t come under the Nuclear Regulatory Commission’s auspices, it was permissible for the plant to do that on its own. From this initial use, plant engineers gained a great deal of experience, which led them to decide to replace safety-related piping with HDPE pipe starting in the fall of 2009.”
At Catawba, about 60% of the piping is non-safety. “There’s a lot of piping under the plant’s control, so a strict quality program is maintained because of the volume of pipe. If something was to happen to the pipe—if it wore out or didn’t work correctly—it could shut down the plant, which would have an enormous negative economic impact,” Schaaf said.
The plant is on the swamp-like Catawba River. Over the years, a textile mill upstream discharged waste into it, which created a biological environment that eats steel pipe called microbiology-induced corrosion.
At the Catawba Plant, which is operated by Duke Energy Power Co., Schaaf and his team not only found the stainless steel pipeline corroded and constricted, but also that the inlet of the heat exchanger was clogged with the buildup of fine particles from rust, mineral and salt deposits in the water. Looking like a hornet’s nest inside, the heat exchanger’s screen is made up of tubes. If this screen becomes clogged, water flow is restricted.
“If water cannot flow in at the optimum rate, the plant will not be able to cool the heat load, which is the only mission of the heat exchanger and the pipeline,” Schaaf said.
Catawba started replacing pipe in late 1999 and has now replaced all non-safety service water piping with HDPE. This represents nearly 20,000 ln ft of buried and in-plant pipe in sizes up to a 32-in. diameter.
“The HDPE pipe has been in service at Catawba for nearly a decade, with no fouling or deterioration. What we found with polyethylene is that it doesn’t corrode or tuberculate like steel does,” Schaaf said. “Plus with HDPE pipe you don’t have all that buildup from corrosion product fluffing off the metal.”
Making the Switch to HDPE
In 2005, Duke Power met with the Nuclear Regulatory Commission to discuss the use of HDPE pipe in a safety-related service water system for its Catawba plant. About 40% of the plant’s piping is safety related. It was suggested that the ASME become involved to write a Code Case, and in October 2005, Schaaf put together a team that included representatives from Duke along with the PPI and its industry members.
“This was a true industry initiative,” Radoszewski said. “Experts in pipe, resin, fusion and fittings came together to take proven systems and tailor them to new surroundings. At first, using HDPE pipe in this application raised some eyebrows because many people in the nuclear power industry did not realize the strength, durability and track record of HDPE pipe.”
Under pressure, the pipe and its heat-fused joints can withstand temperatures up to 176°F. It is also chemically inert and will not rust. Found to be a cost-effective product in uses ranging from natural gas lines to potable water for more than 50 years, HDPE pipe is also practical to install, and because of its favorable weight, it can be handled by a reduced crew with minimal heavy equipment.
According to Stephen Boros, technical director for PPI, an important consideration when the HDPE pipe was being specified was its hydrostatic design stress (HDS), which defines the maximum design stress for a pipe made from a thermoplastic compound. This information as well as other important performance properties can be found in the Standard Pipe Material Designation Code.
“Multiple tests are done on plastic materials to determine the long-term strength properties for pressure piping applications. The results are published in PPI’s Technical Report TR-4 that lists the compound’s hydrostatic design basis and the HDS, which are typically used by codes and standards as well as engineers in designing a plastic piping system,” Boros said. “The pipe was made from a PE 4710-qualified material that would provide the long-term hydrostatic strength and the performance needed for this application, where temperatures could reach 140°F regularly and as high as 176°F on occasion. For example, the pressure requirement for the essential service water system at Callaway was 161 psi.”
The Hydrostatic Stress Board is a group of 20 to 25 engineers and chemists representing all facets of the industry, including resin manufacturing, pipe design engineers, manufacturing, certifications bodies and others under PPI’s auspices who issue design stress ratings and continually evaluate and update procedures for long-term strength forecasting of thermoplastic compounds.
The first use of polyethylene pipe in a nuclear safety-related application started with the selection of a high-performance PE 4710 resin made by Dow Chemical to make pipe that could replace carbon steel pipe at AmerenUE’s Callaway Nuclear Power Plant in Missouri for buried portions of the essential service water system. The resin grade used to extrude the pipe was specified as a PE 4710 material for use in making the 36-in.-diameter pipe for the plant.
Boros said: “A PE 4710 designation denotes a high-performance grade of polyethylene for pressure piping applications. These materials must surpass additional criteria, over and above that of other grades of PE to achieve this designation. It means that a piping system made from these materials can be operated at a higher design stress without sacrificing safety or service life.”
About 1,600 ft of the 36-in.-diameter HDPE pipwas used along with 120 ft of the 30-in.-diameter stainless steel.
“We’re still using stainless material that is very good but very expensive,” Schaaf said. “A vault was constructed to allow transition from stainless steel to HDPE.”
At Callaway, the DR 9.5 HDPE pipe from WL Plastics has a 36-in. diameter with a wall thickness of just less than 4 in. The previously used steel pipe had a wall thickness of nearly 3/8 in.
“The time and effort to fabricate a PE joint of this size is about an hour and a half with one operator, but a few hours with a crew of two to three people to make a steel weld,” Schaaf said. “So, instead of welding a couple of joints a day, you can fuse a dozen using HDPE pipe. And that comes back as a lower cost of installation. Labor is not cheap anymore.”
The replacement at Callaway required many changes of pipe directions and elevations. Strong fitting and elbows are essential components of the pipeline system. The HDPE pipe fittings and elbows were fabricated by Independent Pipe Products Inc.
For the system at Callaway, a 36-in. DR 9.5 HDPE pipe was used after the ASME Code Case wavier application was approved in March 2007.
“They wanted to use the 36-in. pipe on a big parabolic cooling tower as the flush line from the tower back to the Missouri River. The original pipeline had corrosion and fouling problems, and they needed to change it. In one year, they replaced about 40,000 ft of pipe throughout the facility. It’s unbelievable,” Schaaf said.
Fusing for Future Fission at Callaway
The predominate method for joining HDPE pipe sections and fittings is called heat fusion. The process actually melts an end of each length of pipe and then brings the two ends together to “fuse” the resin together. This creates an integral bond that is strong and leak-free—essentially a monolithic pipe string.
“With assistance from the PPI, we developed special hydrostatic test rules and added to the Code Case a whole instructional section on how to qualify the fusion machine operators,” Schaaf said.
According to Schaaf, one of the advantages of HDPE pipe is the fusion joining that delivers leak-free performance. “The nuclear power industry, however, is not generally familiar with the fusion process, at least not before these two projects,” Schaaf said. “So we had another PPI member company, ISCO, help train the contractors as well as the plant engineers.” McElroy fusion equipment was used.
“And I think that’s one of the hallmarks of the whole program—this joining operation where we took PPI’s Technical Report TR-33 and the PPI Polyethylene Generic Fusion Procedure and developed a whole joining program around it. This is a fusion process, not welding.”
Even in selecting the proper word, Schaaf and his team were meticulous. “I chose the word ‘fusing’ because it got people away from metallic welding. It’s the term the plastics industry developed and that clearly describes the process. Otherwise some would immediately think we’re borrowing ASME Boiler Code Section 9 rules for welding metallics, which we’re not.”
Schaaf is an unrepentant supporter of HDPE. “HDPE pipe is one-fifth the cost of metallic pipe, and it’s the material of choice. Aside from helping to control the bottom line, this cost reduction allows utilities to do more. Before a utility would have an A and B train. Now with HDPE pipe they’re able to have the A Train with A1 and A2 lines, plus the B Train with B1 and B2 lines. They’ve increased redundancy, installing twice as much piping, and that makes the plant safer. Design engineers for today’s nuclear power plants are reducing costs, creating longer life and making plants safer all because of the use of pipe made from HDPE,” Schaaf said.
Radoszewski summed up the importance of this milestone and how it can benefit the typical utility. “The nuclear energy industry’s realization that HDPE pipe is a smarter and much more cost-effective solution should be a wake-up call for cities looking to replace crumbling and degrading pipe systems. Here is a critical new application where the unmatched capabilities and benefits of the HDPE material are yielding savings and supplying a sustainable infrastructure with improved and reliable performance. HDPE pipe provides an optimum solution in wide-ranging industries for both above[-ground] and underground applications and in broad-ranging circumstances applicable to the majority of cities in our nation.”