Heading Off Disaster

A portable waterjetter proves to be an ideal tool for clearing a critical drainline beneath a large hydroelectric dam in Bulgaria
Heading Off Disaster

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Dimiter Dimitrov remembers his toughest cleaning job ever. The tool that saved the day was a waterjetter.

Dimitrov heads Double D Engineering Ltd. in Bulgaria, part of Russia-based Zevs Technology. He and his crew have been in the industrial drain-cleaning business since 1998, but they had never received a call like this.

The job involved blockage of a critical drainline beneath a large artificial lake at Dospat Dam in Bulgaria. The clogged conduit posed extreme danger to the dam wall’s integrity – and to local residents.

A crew from Double D used a portable waterjetter in a slow, methodical process to clear the drainline in a project that spanned three days.

 

A major installation

With 143 billion gallons of water behind it, Dospat ranks as Bulgaria’s third-largest dam. Officially managed by the country’s Dams and Cascades state company, it is also part of the National Electrical Company and a key link in the nation’s power grid.

Buried in the aging earthen dam lay 460 feet of 5-inch steel pipe that had not been cleaned since the dam was built 50 years ago. Adding to the problem’s complexity, the drainage pipe took four turns of 150 to 174 degrees, while descending only 11.5 feet from beginning to end. This shallow 2.5 percent grade, combined with the four bends, had allowed slow sediment buildup as the pipe drained lake water overflow.

By the time the Double D crew arrived, the line had nearly choked shut. That risked shutdown of the hydroelectric power plant, and possible catastrophic dam failure.

“The overflow was working, though very poorly, when we got there, and they vitally needed the pipe to work,” Dimitrov recalls. “If the pipe could not be cleared, it would be mortally dangerous to the dam wall.”

 

Gearing up

This was the company’s first experience with a new 3,000 psi/8 gpm waterjetter from General Pipe Cleaners.

Bringing a large trailer-mounted jetter to the remote site would have been exceptionally difficult; the dam seemed like the perfect application for the new jet machine. The Double D crew also knew they would need lots of water to clear a line of that size and length. All 8 gpm would be needed, but they didn’t know if they could get enough water for the pump.

They brought all the garden hose they could find, and two lengths of 3/8-inch jetting hose – 250 and 300 feet. The also brought a set of three high-performance nozzles and two standard nozzles.

They considered their possibilities and decided to attack the problem first from the downstream side. But the crew had considerable difficulty positioning the jetter close enough to the outflow through the dense forest and brush below the dam wall, yet also within range of a water supply. So they placed the machine under a tree midway between the water source and the outflow.

The water source was at least 130 feet away, and with a pressure of about 70 psi, the 1/2-inch garden hose proved insufficient to feed the pump. So they set up a second parallel hose directly feeding the jetter’s 12-gallon buffer tank. Although water from the second hose wasn’t clean, the machine’s inlet filter protected the pump from impurities.

 

Attacking the blockage

With the jet ready and supplied with water, the crew began work. It was tough going: after nearly 5 1/2 hours of grinding labor, they had cleared 295 feet of pipe. Dimitrov described the sediment as “very hard at the pipe wall and softer at the center.” In fact, his crew extracted slabs of sediment that resembled bricks.

Debris piled up next to the outflow as they toiled. It was hard going, and they were exhausted. At the end of day one, the job was barely half done. On day two, they returned to the same downstream outflow location and tried to get farther up the line. But the pipe length and multiple bends made progress slow and frustrating.

They worked another 5 1/2-hour day, yet only cleaned another 75 feet. At that distance, even with the high-performance nozzles, they couldn’t muster the pulling power to go farther. The last 90 feet proved the toughest, and the most dangerous.

“The only way to access the upstream end of the drainage pipe was through a tunnel,” Dimitrov says. “It was more like a cave.” The tunnel lay in the middle of the dam wall, the core of which was built of clay rather than concrete. Atop the clay was a pyramid of stones and earth.

Workers descended the steep steps of the access tunnel to a gallery 157 feet below the lake surface, and to the drainage conduit’s still-blocked upstream segment. “We worked carefully, using the less powerful nozzles to slowly break the debris into smaller pieces, and let them flush safely down the line,” Dimitrov says. “With 143 billion gallons of water over our heads, we obviously risked serious injury, at best!”

At any time, large chunks of sediment could unexpectedly loosen and block the previously cleared downstream pipe. The gallery could then quickly flood with the full force of billions of gallons of water, and endanger workers’ lives.

 

Saving the country

But the Double D crew gradually and methodically eroded the hardened sediment. Using the waterjetter, they slowly and cautiously increased the inside diameter of the drainage pipe, eliminating the risk of catastrophic flooding from dislodged debris.

They cleared the last 90 feet and continued out to 164 feet, re-cleaning the section of pipe that had been so difficult to reach the day before. Even with their slow progress, they finished the last day in less than 1 1/2 hours.

Thousands of dam, waterway and waste treatment pipelines potentially suffer from silt and debris blockages, but not all are accessible to larger trailer-mounted waterjetters, nor is larger wheeled equipment always economical to employ.

Double D blended perseverance, bravery, and a smaller waterjetter to solve a big problem – in the process perhaps saving their country from a catastrophic dam failure. “Everything worked very well,” Dimitrov concludes. “We could not have achieved those positive results without the right tools for the job.”

Marty Silverman is vice president of marketing with General Pipe Cleaners.



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