Outsize components help clean up San Diego
The San Diego Sewer Authority in the US hit a particular problem with valves for its large sewage outfall pipe. David Snodgrass, UK and Ireland country manager for UK manufacturer Victaulic, explains how bespoke couplings helped the challenge of restricted space in the valve vault.For many years the sewage treatment plant in Tijuana, Mexico, had been unable to process all the raw sewage it received, diverting the excess into the Tijuana River, which flows into the Pacific Ocean. From there the prevailing current carried it northwards to the US city of San Diego's South Bay, forcing the closure of 24km of prime beach at one point.
To solve this unpleasant problem, San Diego and the International Boundary and Water Commission decided to build a primary treatment plant to supplement the Tijuana facility and a US$140 million outfall pipeline to carry the treated effluent over 4km out to sea. Effluent from the treatment plant will enter the South Bayland outfall through a 3600mm diameter pipeline capable of handling 1.3 million m3/d.
The reinforced-concrete cylinder pipeline was built by US manufacturers Colich & Sons and Ameron International Corporation. Colich had used double-gasketed, spigot-and-bell joints to connect the more than 3.6km of pipe, according to Thomas Bensfield, the project manager. However, the company encountered an extraordinary challenge when it came to making the final connections in a four-storey deep, underground valve vault, situated 800m from the treatment plant.
The San Diego Sewer Authority wanted the ability to remove the two 1200mm bonneted knife-gate valves for cleaning and maintenance. It was determined that the flanged valves would not come out even after loosening; nor was it feasible to use sleeve couplings because of the space constraints of the vault. What was required were special couplings that could be taken apart to facilitate removal of the valves.
Solving the problem
The 3650mm couplings, the largest ever manufactured by UK manufacturer Victaulic, were cast of ductile iron in 24 segments and machined to extremely close tolerances to assure proper alignment during installation. The couplings were supplied with continuous elastomer gaskets and two coats of paint to resist corrosion.
Under normal circumstances, using the Victaulic system simply involves grooving the pipe to receive a bolted coupling with a synthentic rubber gasket that seals on the outside diameter. The resilient C-shaped gasket provides a leak-tight seal that is further strengthened by compression when the coupling is tightened as well as by line pressure. The groove where the pipe and fitting engage forms a self-restraining joint capable of withstanding loads of up to 1,000psi.
However, the circumstances of the South Bay project were anything but normal.
"Usually we just cast our couplings," said Phillip Thomas, the Victaulic engineer who designed them, "but in this case, there were so many mating surfaces that we had to machine the inside diameters and bolt pads of each segment. If you're off just minutes, the pieces won't align properly. A further complicaton was the fact that the most precise measuring machines require more than 15 degrees of arc length to give you a good reading."
It took the company only five months to design and produce the couplings. Not withstanding the precision of the finished product, Colich & Sons had an installation challenge.
Pipe alignment required the use of jacks and a 12-ton crane. Workmen could not use the couplings to align the pipe because the stresses were too great, and pipe concentricity had to be controlled.
In assembling the huge couplings, they tightened eight bolts on the top and bottom sections of the pipe, leaving a gap of about 25mm between the bolt pads on the side segments. Tightening the bolts on these segments drew the bottom segments snugly into place.
"The Victaulic engineers did a great job," said Bensfield. "We were able to install the couplings in a single day. I was surprised how well everything went together considering the pipe and the couplings came from two different manufacturers. All we had to do on the second day was torque everything down".
At the end of the land outfall, the effluent will enter a 610mm thick, 12m diameter reinforced concrete drop shaft that will take it 50m below sea level. From the bottom of the shaft, a tunnel with a 3070mm internal diameter will carry the effluent 1.5km underground and another 4km offshore. At that point it will join a 2745mm diameter, 45m long riser that will carry the effluent upward to a 3050mm, 1.5km pipeline on the seabed that ends in a 'y' with two 610m diffuser legs - a long way from the beaches of San Diego.