The solid titanium construction gives the reactor vessel lighter weight, compared to thin titanium cladding over a layer of much heavier steel.

The vessel, which will contain HOCL, or hypochlorite, measures 150 feet long and 20 feet in diameter, and has a capacity of 285,150 gallons. The vessel's external structure, itself weighing in at 242,500 pounds, and various internal components that facilitate the physical transfer of chemicals inside the vessel combine for a total weight of 345,000 pounds. The vessel's wall has an average thickness of 0.75 inch, with the top measuring 0.728 inch and the bottom measuring 0.843 inch.

Those are daunting numbers, but in fact the vessel is significantly lighter than it would be if more common metals had been used, according to Brent Willey, president of TiFab.

Larger pressure vessels have been constructed of other, more common materials, such as carbon steel, he said. Although TiFab has built other vessels of solid titanium, this is the largest that the company has ever built. According to Willey, it may be "the largest vessel of solid titanium that's ever been built."

(If the tank is of record size, its status must remain unofficial. Mechanical Engineering was unable to find independent corroboration that the vessel is the largest of its kind.)

After reviewing cost estimates for a number of different materials, TiFab and its customer chose solid titanium. The fabricator compared the cost of using three "solid" options—grades 2, 3, and 12 titanium—and a clad construction of titanium over carbon steel.

"We had to come up with the optimum design that would give the best price for the customer," said Willey. Titanium grades 2 and 3 are "commercially pure," while grade 12 is an alloy containing small amounts of nickel and molybdenum, and is priced about 15 percent higher than the pure grades. Titanium was attractive to the customer because of its corrosion resistance, Willey added.

According to the Titanium Metals Corp. of Denver, which supplied the material, titanium forms a tenacious surface oxide layer, which serves as an effective corrosion inhibitor. The company claims that titanium can outlast competing materials in harsh environments by as much as five to one.

"On a life cycle analysis basis, it can compare favorably with much less expensive materials," said Michael Metz, the company's worldwide marketing director. He noted that titanium is priced competitively with other high-performance, corrosion-resistant metals such as some advanced stainless steels and high nickel alloys. The company points to titanium's high strength-to-weight ratio, so that less titanium is required to do the same job, based on strength. Still, the cost of using a solid titanium vessel versus the titanium-clad carbon steel construction was close, Willey said.

In discussing the options with the customer, TiFab suggested building the vessel to ASME Code Section VIII Division 2.

ASME Division 2 is a rigorous set of criteria for the design and manufacture of pressure vessels. Division 2 mandates that finite element analysis should be used in the design of the vessel. It also lists a narrower range of construction materials than in Division 1, which contains more generic criteria for building pressure vessels.

The lighter weight of the titanium meant easier and less expensive handling and transportation from the fabrication plant to the customer's site in Texas.

Building to Division 2 saved material costs. "It allowed us to thin the wall down, because of the three-to-one safety factor," Willey said. (Division 1 has a four-to-one safety factor.) The savings from building the vessel to ASME Division 2 code, plus discounts from the material supplier, helped convince the customer to opt for the solid titanium design. The main cylinder and heads are constructed of grade 3 titanium; various attachments on the vessel's outside wall are made of titanium grade 2.

"Division 2 recognizes higher stress values, if certain requirements are met, which allow thinner solid titanium construction," Willey explained. "The customer pays less and gets a solid titanium vessel, which can be more forgiving than a clad vessel. In addition, clad vessels require extra fabrication steps and can weigh almost twice as much, meaning that there are added handling and transportation considerations. In effect, the code allows more options for reducing costs than are generally recognized."

Willey acknowledged that Division 2 requires rigorous inspection and FEA. He estimated that the requirements added about two weeks to making the proposal. "However, the cost saving of going to a thinner material often outweighs the added labor, as it did in this case," he added.

The solid titanium tank was welded with the TIG welding process, which uses constant inert gas. The key in welding titanium is to keep oxygen out of the weld, because oxygen can make the weld brittle at high temperature, explained Willey. The weld has to be purged with argon gas on both sides. "You are really making a weld by blowing a gas onto the weld while you are welding. The key is to keep oxygen out of the weld until it cools below 600°F."

It's also necessary to cover the weld on the inside wall, to keep it from absorbing oxygen, he added. It may require another person on the other side of the weld, following it on the opposite side of the wall.

Willey noted that TIG welding "is a little more expensive, because you've got the argon gas expense, and you are really doubling your labor because you've got a guy on the back side."

On the other hand, welding clad constructions is more labor intensive than welding solid material, Willey said. "Titanium doesn't weld to steel," he explained. "You've got to weld the steel first, to make sure you don't contaminate the titanium. Then you come back and weld a titanium batten strip to the titanium." He estimates that there are about 10 percent more man-hours involved in welding clad constructions versus solid.

The vessel took 14 months to build, including six months for material delivery and eight months actual fabrication time. TiFab subcontracted the work of forming the flat sheets into the body of the vessel and the heads, or the closures at either end. A head-former shaped flat sheets into hemispherical forms. For the body of the vessel, sheets were formed into cylindrical halves. TiFab welded the halves into "cans."

A full cylinder section was roughly 8 feet long and 20 feet in diameter, Willey said. "You have a lot of these sections. You put a head on one end and you start building the cans, until you have a long vessel. Then you put a head on the other end. It looks like a giant submarine."

After the main section was completed, various attachments such as nozzles and valves were added. Internal components, which facilitate the physical transfer of chemicals, were welded to the inside of the vessel. Rings and braces were also welded to the inside of the vessel to hold the internals in place.

The vessel was transported by truck from the Harrison, N.J., facility to a waiting barge at a Port Newark pier, and then it was shipped by barge to its destination in Texas.

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