Measuring Pipe Wall Thickness

The issue of when, and how, to measure wall thickness on a pipe has become a hot topic in recent months. Clearly, it’s important to maintain the mechanical integrity of refrigeration systems in order to avoid ammonia leaks. But there is debate in the industry over when pipes that display no visible damage should be examined.

“The question is, if you are doing an inspection to look for corrosion under insulation, are you introducing new hazards that didn’t exist before the inspection?” said Marty Timm, Process Safety Manager at Praxair, Inc. “We may be taking a perfectly good section of piping and insulation and introducing a hole through the vapor area that surrounds the insulation, thus creating a pathway for moisture to enter the insulation. And if we don’t do a good job of patching the hole, the moisture can migrate to the pipe and cause corrosion in a spot where it never would have happened.”

Accelerated corrosion has been reported at sites where insulation was removed for non-destructive examination, or NDE, and the ensuing repair to the insulation or vapor barrier was inadequate. In other cases, adhesive-backed labels attached to non-insulated pipes to mark measurements of wall thickness have led to problems. Residual heat has caused the label to bubble, trapping moisture and leading to corrosion.

In one recent case cited by Timm, an end-user hired a contractor to perform a non-destructive examination, or NDE, of wall thickness. After removing a “plug” from the jacket and insulation, the technician experienced resistance from ice when cutting the compromised insulation. When he pressed harder, the drill pierced the pipe, resulting in an ammonia release.

No one was hurt and the leak was quickly contained, but there are some important things to learn from this incident, said Timm.

“When the technician met resistance he should have backed off, moved away from the insulation and looked through the hole to see what was going on,” he said. “He shouldn’t have assumed it was ice. He should have looked into the hole to confirm it.”

The incident also was a lesson in evaluating and managing risk. All NDE methods should be carefully analyzed to determine if new risks could be introduced, said Timm. Those new risks should then be managed before an issue arises.

“If you’re working with a drill you need to ask yourself what happens if I cut into the pipe?” Timm said. “By asking ‘what-if’ questions you can look for new hazards that might suggest safeguards to avoid them. It might suggest different protective clothing, rubber gloves, a respirator, or other safeguards to protect the employee from exposure to the refrigerant.”

The ultrasonic method for measuring wall thickness is one of many non-invasive techniques. A probe generates sound waves that measure the reflection from the metal bounce off the side wall and back again, with the delay creating a reading corresponding to the wall thickness. But the probe must come in contact with the metal surface of the pipe, thus requiring cutting away the insulation. “It’s non-destructive as far as the piping is concerned, but not with the insulation,” Timm said.

IIAR guidelines recommend a careful examination of pipes every five years. It’s important that facilities continue to follow RAGAGEP, but be aware that there may be opportunities to be more selective in the future, said Timm.

There isn’t a consensus yet, but there’s an evolving thought that we’re doing too much cutting into the insulation and that we should fine-tune our methodology, he said. There’s an emerging viewpoint that says, Let’s be selective. Look for visible deterioration, a missing insulation jacket, a missing vapor barrier, frost or condensation on the pipe.

Timm said that visual inspection works well with non-insulated pipes, but it can be ineffective with insulated pipes. The insulation can hide damage, making a pipe that has been compromised appear fine. Still, Timm believes spot-checking visually undamaged pipes can offer a solution.

Instead of cutting into the insulation of every pipe every five years, inspect only a fraction of the pipes that don’t show visible damage. If you find a problem, then go on and inspect more, he said.

A plant could be doing everything right. They’ve got good insulation, a vapor barrier, they’ve put on an external jacket so you’re not poking holes in it. Everything is going great. No moisture is getting into the pipe, Timm said. And then you arbitrarily determine that you have to cut a hole and check for wall thickness every five years, so you go in, cut a bunch of holes and then run the risk that they’re not patched properly.

“Maybe we should be a bit more strategic in which pipes we select to inspect,” he added.