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Orbital welding gear fuses productivity to precision for solar power installation

66,000 welds join tubular assemblies to power a 250-MW electrical system

Solar power farm

When Lauren Engineers & Constructors, Abilene, Texas, needed welding equipment and a procedure for joining tubular assemblies for a challenging solar power project, it found that an alignment tool made by Centrator and a welding package supplied by Liburdi Dimetrics met all of its needs.

It wasn’t very long ago that converting the sun’s energy to electricity was mainly a matter of research and noncritical applications rather than a bona fide and reliable way to generate power. As recently as 2003, the net generation of solar power in the U. S. accounted for just 534,000 megawatt hours (MWh).

While the portion of power derived from solar sources is still small, just 1.6 percent of the total, the growth rate of solar power has been extraordinary. As of 2018 solar power provided 66 million MWh of power, having expanded more than 12,000% over 15 years. Of the two types, photovoltaic (PV) and concentrated solar power (CSP), the latter has grown the fastest.

As the names imply, the PV process converts light to voltage, whereas CSP gathers light from a relatively large surface area and focuses it onto a relatively small surface area. A PV system is the simpler of the two to install, requiring little more than panels, cables, and a power inverter. The downside is that the efficiency is less than 15 percent. A CSP system is much more complex, using a plumbing circuit, a heat exchanger, and turbine. Typical efficiency is around 35 percent.

Genesis Solar LLC uses CSP in an installation in southeastern California, roughly halfway between Los Angeles and Phoenix. At this installation, 1,840 solar collectors focus the sun’s light onto pipes that contain an oil that serves as a heat-conducting medium. As pumps push the oil through the system, the solar collectors increase its temperature to 750 degrees F. The oil goes through a heat exchanger where its heat is used to turn water into steam, which drives a turbine. The system has two generators that develop 125 MW each.

Capturing Sunlight in a Desert

The premise of a CSP system is simple. A long, parabolic-shaped trough of mirrors captures sunlight, concentrating it and focusing it onto a piping system that runs along its focus line; the pipe transfers the heat to the oil it contains. The trough’s orientation is controlled by a drive system that keeps the mirrors pointed toward the sun from dawn to dusk.

Using steam to drive turbines, building a motion control system that follows the sun’s position, and polishing a reflective panel to a mirrorlike finish—these are old hat. The most interesting portion of the system is the plumbing.

“The piping system consists of several layers,” said Ronnie Ward, former corporate quality control manager (now retired) for Lauren Engineers & Constructors, Abilene, Texas, the fabricator that built the pipe circuits.

“It uses a 23/4-inch stainless steel tube surrounded by a glass jacket,” he said. The stainless steel tube has a special coating that increases its ability to absorb solar energy and reduces its emissivity. The glass jacket comprises two concentric tubes fused to an end cap. This forms an enclosed volume held to a vacuum, which acts as a thermal insulator. This is the same principle used in some beverage containers to keep the contents hot or cold.

Finally, each assembly has a metal bellows welded to its end cap. The metal bellows provides enough elasticity to allow some movement, notably dimensional changes caused by thermal cycling, to prevent damage to the unit. The vacuum unit with the end cap and bellows is about 13 feet long. The stainless steel tube, which is in the center and bonded to the vacuum unit, is just a little longer. The stub that sticks out is just about 9/16 in. longer than the rest of the assembly.

Joining one tubular assembly to another was a monumental challenge for a couple of reasons. First, access to the stainless steel tubes was limited to twice the stub length, or 1.125 in. Second, the project called for a vast number of welds, approximately 66,000 in all. It would be repetitious, at best. Third, tube alignment was critical. Getting 26 ft. of tubing into a nearly perfect axial alignment would be necessary to have a consistent gap around the circumference. Finally, weld quality was a big concern. If just one percent of the welds were substandard, the project would have more than 600 poor welds.

stainless steel tube illustration

Two glass tubes insulate the central stainless steel tube, helping the unit to take advantage of every bit of solar energy possible.

While some welding equipment manufacturers might have looked at this project and determined that it was too much trouble—perhaps too much could go wrong—Liburdi Dimetrics figured that, although challenging, this project could be completed successfully by using an orbital welding unit. This application didn’t provide much clearance for the weld head, and clamping the unit to the end caps would be a challenge, but eventually the project came together. It used an alignment tool made by Centrator and a Liburdi welding package that comprised off-the-shelf weld heads and power supplies, custom collets, and an autogenous welding procedure.

Unloading, Handling, Fixturing, and Welding

The tubular assemblies arrived at the site in 13-ft. lengths covered with protective plastic sleeves and packed in specially made shipping containers. The unloading process was a strictly controlled regimen that dictated when to use a winch and when to use a crane, and the handlers were instructed to lift each tubular assembly only by the stainless steel stubs that protruded from each end. Workers were required to don clean, cut-resistant gloves; were instructed not to touch the glass; and had to get the tubes out of direct sunlight as quickly as possible. Because the metal and glass were fuse-bonded together, just a few minutes of direct sunlight without the heat-transfer medium would allow heat to build up quickly and weaken the bond. Finally, the welders had to use care in clamping the weld head into place and making the weld.

If this sounds like the way you pick up a porcupine, it is: You can do it, but you have to be very, very careful. Then, after you have taught your crew how to pick up a porcupine, get out of the way, because they have 65,999 more porcupines to go.

Present Tubes! “The damage rate was very low,” Ward said. “We had a good handling system. The workers’ motions were so precisely coordinated that they looked like an army drill team—each pair of workers coordinated lifting and carrying each tube assembly, always moving together, to transport each unit to the preparation area for cleaning, setup, purging, and welding.”

Each assembled unit, comprising three pipe lengths, was about 40 ft. long when finished. Because optimal flow required great care getting the pipes properly oriented for welding, the setup procedure minimized any discernable gap, offset, or misalignment.

The workers placed the pipe units end-to-end on stands that were sturdy and rigid—they had been sunk into concrete footings—and they were forbidden from estimating any of the various alignments and orientations. Workers established and secured the axial alignment of each pipe pair with the internal clamping tool. They used a bubble level to verify that the weld head was aligned vertically.

The welding hardware comprised an off-the-shelf orbital weld head, model L-4000, and a matching power supply, model P300. The weld head just met the space restrictions, clearing the gap between the bellows by a mere 0.25 in. Custom-designed collets gripped the end caps to provide process stability, keeping the electrode within 1/16 in. of the tube’s surface.

Capability and Efficiency. It wasn’t just a matter of getting the project done, but getting it done efficiently. This meant keeping the cycle time as short as possible.

In addition to aligning the tubes, the Centrator unit filled the tube’s ID, providing a localized purged area. The arc-on time itself was about 90 seconds. The entire cycle—clamping the weld head into place, purging the ID, welding, allowing the weld to cool, and unclamping the unit—was about three minutes, Ward said. Moving to the next butt joint took less than three minutes, so each welder performed an average of 84 welds per eight-hour shift.

By the Numbers

The project ran a little more than three years at a cost of $1.25 billion. It uses 500,000 collectors to harness the light captured over a mind-boggling 21 million square ft. of reflector surface area. That output powers twin generators, rated at 125 MW, that generate about 625 gigawatts annually. And, while PV panels degrade over time, a CSP’s reflectors are more robust, so the system is expected to provide a consistent output for many decades.

About the Author
FMA Communications Inc.

Eric Lundin

2135 Point Blvd

Elgin, IL 60123

815-227-8262

Eric Lundin worked on The Tube & Pipe Journal from 2000 to 2022.