Rehabilitation of Hydrogen Grooving in Sulfuric Acid Line

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Courtesy of InduMar Products, Inc.

Around October 2001, one of the major petrochemical refineries located on the Texas Louisiana Gulf coast needed to rehabilitate 6' and 8” diameter “fresh acid” (98% sulfuric acid) lines.

Such lines tend to suffer from internal gouging due to formation of hydrogen gas in voids, which are caused by turbulence at directional changes in the pipe flow such as elbows, etc. This is a natural event, but results include “grooving” or “embrittlement” in those areas as the hydrogen gas attacks the pipe wall.

The “Sulfuric Acid Plant and Technology Training Manual,” states, “Hydrogen is generated when sulfuric acid corrodes carbon steel. The hydrogen gas formed rises towards the surface and the passage of these bubbles over the steel surface can result in the formation of grooves.”

“Corrosion by acid is an electrochemical reaction in which the acid (hydrogen ions) attacks the iron, resulting in hydrogen and metal ions as reaction products,' Dr. Ron Darby, Professor Emeritus of Chemical Engineering at Texas A&M University explained. 'This process is intensified when oxygen is present which accelerates the corrosion reaction by formation of metal oxide as the corrosion product. So the acid 'not filling the pipe' does not in itself cause corrosion or hydrogen generation unless there is oxygen (or air) in the gas space, which accelerates this reaction. However, if/when hydrogen is produced, it can cause further problems by way of hydrogen embrittlement of the metal.”

MEI-Charlton, Inc., a professional engineering and scientific consulting firm that specializes in solving materials-related problems, explains the phenomenon in more detail on their website. According to MEI's experts, carbon steel can be used for handling concentrated sulfuric acid because the acid reacts with the steel to form a protective film of iron sulfate that reduces the corrosion rate to a tolerable level (approximately 0.020-inch per year). Although the overall corrosion rate of carbon steel in concentrated sulfuric acid service is low, some corrosion still occurs.

In addition to iron sulfate, the other reaction product from the corrosion process is hydrogen gas. Hydrogen gas can be a problem with carbon steel in sulfuric acid environments because it can literally scrub off the mechanically weak iron sulfate film, which is the only thing protecting the steel from attack.

During periods of normal flow, the gas bubbles are very small and evenly distributed, and they are quickly carried downstream along with the acid, generally without causing a problem to the pipe wall. However, when the flow is periodically interrupted and the acid ceases flowing, the gas bubbles accumulate along the pipe wall in the upper half of the pipe. When the flow resumes, the hydrogen gas bubbles are dislodged and rise to the top of the pipe, scrubbing off the protective film along the way. With the film gone along the top of the pipe, the surface corrodes very quickly until the sulfate film reforms.

Repeating this process over and over again results in a pattern of curved grooves in the top half of the pipe. This is not an uncommon occurrence in sulfuric acid service, particularly under upset conditions.

The usual recommendation is to use very thick steel, in recognition of the possibility of localized corrosion, and to keep the average velocity in pipes below 3 ft/sec to reduce the scrubbing effect of any hydrogen bubbles that do form. Even under these conditions, however, rapid, localized attack can still occur if the fluid flow is disturbed. Features such as short-radius elbows or gaps at socket welds can cause trouble.

At this particular refinery, hydrogen grooving had formed 1-1/2' to 2' wide grooves in most of the 90° elbows, which were long radius sweeps. Between 50 to 60 of the plant’s sweeps were identified with internal hydrogen grooving and it was estimated that 8 to 10 linear feet around each elbow had been weakened.

There were no leaks, so management determined that a rehabilitation project was required to protect personnel and to prevent any potential future leaking that might result from the weakening of the line. The line was scheduled for replacement at some future date.

Prior to this project, two other 8” lines with similar problems were rehabilitated using a trowel on epoxy resin-based product combined with a fiberglass wrap. This product’s specifications required fairly extensive surface preparation and the application was labor intensive. Plant maintenance management personnel were looking for a better solution.

InduMar Products, Inc. recommended using the Stop It® Pipe Repair System for the reinforcement project. The Stop It® Pipe Repair System is resistant to 98% sulfuric acid and can withstand pressures up to 400 PSI.

Plant maintenance personnel found that among the advantages of the Stop It® Pipe Repair System was that minimal or no surface preparation was required. The product could be applied by one man with no special tools or hot work and no mixing or measuring of product was needed. The combined benefits of enhancing safety by avoiding potentially hazardous surface preparations on an operating sulfuric acid line and the ease of application made the Stop It® Pipe Repair System a better solution for the plant.

It was recommended that each 6” diameter 90-degree elbow be wrapped with 4” wide x 12’ long rolls of the Stop It® Pipe Repair System. Each elbow should be coated for 8 to 10 linear feet and wrapped to a 15-layer thickness sufficient to withstand 150 PSI.

Extensive surface preparation was not needed. The only surface preparation required was to simply remove any loose scale rust.

It was estimated that it should take no more than 20 minutes to apply the Stop It® Pipe Repair System to each elbow. The overhead piping above road and walkways, considered to be the most hazardous critical lines, would be reinforced first to minimize spill potential and any risk of employee exposure.

Plant management approved the project, and InduMar sent a factory employee to oversee the first few applications. Internal plant maintenance personnel applied the product throughout the project.

Plant management has been very pleased with the results of the job. A follow up in late October 2002, about one year after the application, found the pipeline and the Stop It® Pipe Repair System application to be intact and the line did not yet have to be replaced. Plant management now prefers the Stop It® Pipe Repair System product over the alternatives for most pinhole leaks as well as other rehabilitation jobs around the plant.

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