by James Schultz

Travelers crossing unfamiliar or exotic terrain are often advised to stick together. At Jefferson Lab, where explorers chart the frontiers of nuclear particle physics, similar counsel applies. Though at JLab, it's the welding that literally holds things in one piece.

Danny Forehand stick-welds a bracket.

While not all of the Lab's 14-member corps of welders work in cryogenics, all are well aware that welding is crucial to the smooth operation of JLab superconducting technology. Even the smallest seepage could allow ambient air to infiltrate and contaminate the system, which would shut down cryogenic cooling. Experiments would thus be impossible to complete.

"When we say something leaks, we're talking molecules of helium," says Brian Murphy, a senior mechanical technologist and a certified welding inspector. "Even that can be enough to contaminate the system. Anything threaded will leak. The safest way to make everything leak-proof is to weld. Once [fittings or pipe sections are] welded and leak-tested to our stringent standards, leakage is not much of a problem."

Even though welding may convey longevity and strength, a welder's job at JLab is never done. To guarantee a leaktight system, an estimated 10,000-plus welds have been completed on the components and subcomponents comprising the Lab's cryogenics network. System improvements, increased capacity, upgrades, even work requested by other laboratories ends up on the welders' to-do lists, ensuring there's little down time.

"There's plenty to do and the work is not always simple," says Cryogenics Deputy Bill Chronis. "We have multiple pipe systems, for instance, and inner lines within one main line. In summer the sun can heat the surface of that outer pipe, but we have to keep the inside pipes at about two degrees Kelvin (minus 426 degrees Fahrenheit)."

Brian Murphy mends a hole in stainless steel piping.

Of the three welding materials commonly used at the Lab " aluminum, copper and stainless steel " stainless steel is preferred because it expands or contracts the least over the operating-temperature range. Although Lab engineers typically take such variations into account " designing and building anchors, supports and contraction joints that allow the colder interior pipes to contract without over stressing those inner lines " one 380-foot section of helium-supply pipe can contract more than a foot as the line is cooled to operating temperature.

Before being allowed to weld in the Lab's Cryogenic Group, welders must master a three-level certification process, which includes welding tests and X-ray examination of their work's integrity and strength. Any welder's goal is to keep joined materials as strong, or stronger than the sum of the individual parts. Therefore, as they connect pipe or other structures, welders must take pains to ensure that the molecular structure of both parent and daughter materials don't substantially change when heat is applied.

"We try to keep it simple," Murphy says. "Ninety percent of our welding is gas/tungsten-arc welding. Maybe 10 percent of the time we use silver solder or hand brazing. We do use stick welding, but that's for a stair or a railing."

The Lab thus far has recorded no weld-related mishaps or equipment breakdown related to faulty welds. Chronis says that's no guarantee that something can't go wrong. But JLab's twin emphases on excellence and expertise so far appears to have forestalled any serious difficulty.

"Our welders are good," Chronis contends. "Our cryogenics availability confirms it."

maintained by webmaster@jlab.org