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PipePak Helps Engineers Keep $90 Million Sewage Treatment Plant Upgrade on Budget 

A $90 million AUS upgrade ($49 million USD) to the Cronulla Sewage Treatment Plant in Southern Sydney, Australia was undertaken to meet the requirements of a growing population and to add advanced sewage treatment processes including ultra-violet disinfection. Projecting that the population served by the plant would increase from 216,000 in 2001 to 224,000 by 2010, Sydney Water Corporation contracted Bovis Lend Lease to design, construct and operate the plant during the upgrade period. During the design phase, Design, Detail and Development, a division of Blenray Pty. Ltd., was hired to find a cost-effective engineering solution for a new aeration system that provides air to the biological reactors. Design, Detail and Development used ALGOR's PipePak to analyze modifications to the proposed aeration system design to ensure that it could withstand thermal strains.

Aerial photo of the upgraded Cronulla Sewage Treatment Plant. 

Upgrading for Greater Capacity and a Cleaner Environment

The upgrade to the Cronulla Sewage Treatment Plant was designed to accommodate projected population growth and a small volume of flow from nearby communities, without any need for further amplification before 2010. The plant originally provided a primary level of treatment, meaning that it used sedimentation and the addition of ferric chloride and polymers to help remove the suspended solids and grease. The upgrade added three more steps to the sewage treatment process: 1) aeration and further sedimentation (secondary treatment); 2) filtration through sand filters (tertiary treatment); and 3) ultra-violet disinfection. 

The upgrade benefited the environment as well. The area around the plant is environmentally sensitive because it is bordered by internationally recognized wetlands and sensitive sand dunes. It is also home to remnants of littoral rainforest and a threatened species of frog, the Green & Golden Bell frog. The out-fall pipeline travels through Botany Bay National Park and opens into Bate Bay and the Cronulla beaches on the Pacific Ocean. The advanced treatment processes produce cleaner out-fall and, therefore, improve the quality of water in the Pacific Ocean near the plant. The award-winning environmental processes introduced by Bovis Lend Lease both during construction and on an ongoing basis have improved the local habitat and left the nearby Cronulla beaches sparkling clean for swimmers. 

Aeration System Needed for Secondary Treatment 

The aeration system, a key component for the secondary treatment process, provides air to a series of submerged diffuser grids in five biological reactors. The air encourages the growth of specific types of bacteria which break down the organic matter. The oxygen demand varies daily, seasonally and with the flow volume through the plant, and is controlled by three blowers.

Aeration piping system

The aeration piping system, shown here, carries hot air to the biological reactors. Since the air blowing in from the Pacific Ocean can be as little as 5 °C, thermal strains were an engineering concern in the design of the piping system. Because of the low pressure in the system, they were not required to follow any specific piping code. However, the design was undertaken to meet AS 4041 of the Australian Standard for Pressure Piping. Although the aeration piping system does not experience significant pressure loads, thermal strain was a concern because the discharge air temperature is about 100 °C whereas the ambient temperature ranges from 5 to 50 °C. The first system design contained numerous stainless steel bellows to account for thermal expansion and contraction. However, the bellows significantly added to the cost of the system. 

The analysis of the piping system was undertaken to verify that the number of bellows could safely be reduced by using lightweight, spiral-wound stainless steel. In addition, some of the piping lines needed to be rerouted in order to take advantage of available concrete structures for mounting, while maintaining straight lengths both upstream and downstream that were required for the proper functioning of flow meters. 

Validating a Cost-Effective Design with PipePak 

The proposed redesign of the system consisted of four high-speed turbo-blowers, discharge and start-up silencers, 316 spiral-wound stainless steel headers and branches. Geoffrey Stone of Design, Detail and Development modeled a complete header with one of four branches running the length of the biological reactors. Only one branch was modeled because all four branches are similar. 

"PipePak's built-in spreadsheet modeling option makes it easy to define piping systems," said Stone. "The ability to add various standard types of supports with manufacturer's data on spring hangers is a real time-saver."

The upper left image shows Stone's complete piping model. The lower right image shows the hoop stress results. 

The model included spiral-wound stainless steel piping of 3-mm wall thickness, reinforcing pads at many of the branch connections and a one-way limit stop representing a doubler plate at each pipe support to prevent local buckling. Bellows were placed at the blower discharge nozzle to prevent loads from being transmitted to the piping system. In addition, bellow elements were used to represent connecting hoses between the stainless steel pipes and the uPVC aeration grids. Where the piping system was constrained local to the silencers, bellows were also employed to accommodate the thermal strain. 

A number of load combinations were analyzed in the design including thermal strain, dead weight and pressure loads. The results demonstrated that the redesign was able to accommodate the thermal strains without imposing excessive stress in the piping system. 

"The analysis results demonstrated that a simpler, less expensive design could withstand the expected thermal strains," said Stone. "PipePak's dynamic graphics were useful in explaining to my client what would actually happen in the system."

Geoffrey Stone of Design, Detail and Development analyzed the aeration piping system using ALGOR's PipePak to ensure that the new design could withstand expected thermal strains. 

Replacing bellows with spiral-wound stainless steel saved $150,000 AUS ($81,800 USD) and helped to keep the project on budget. The system has been installed and is functioning successfully. 

Geoffrey Stone is a consultant and piping engineer based in Sydney, Australia. He has been using PipePak since 1985 for clients in the steel, coal, power, mineral processing and water and wastewater industries in southeast Asia. Stone is currently working for Bovis Lend Lease to analyze piping design and water-hammer phenomena for an upgrade to 330 pumping stations in the Sydney Water system.

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