Tuesday, May 9, 2017

Knocking the Dust Off

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Pressure Wave Cleaning Offers Potential New Option for Power Plants

Two workers feed a hose-like lance capped with a plastic balloon into a power plant’s heat recovery steam generator (HRSG), maneuvering around tight spaces between tall bundles of tubes caked with yellow debris. Meanwhile, an engineer sitting at a computer at the other end of the lance has mapped out the HRSG on a grid. The computer controls the delivery of an ethane-oxygen mix through the lance to inflate the balloon. When the balloon is positioned at the proper grid coordinates, the engineer ignites the gases inside. The pressure wave rattles the HRSG components with a boom and dislodges debris, which settles to the bottom of the HRSG.

This new process, called pressure wave cleaning, shows promise for reducing plant operational costs and enabling less expensive, more reliable electricity.

Cleaning Tubes: An Important Job

People who like their music turned up loud have probably noticed their windows rattle with the beat. They’re enjoying the vibratory force of a pressure wave, which is similar to that caused by the ignited gases in the HRSG. In this case, the wave vibrates tubes just enough to rattle off debris.

Over time, power plant performance depends on keeping tube bundles clean in both coal-fired boilers and HRSGs. Tubes in HRSGs pick up heat from the plant’s combustion turbine for reuse in spinning steam turbines. When those tubes become fouled—by ammonia, sulfur, or rust—the resulting back pressure can damage the turbine and increase heat rate, leading to lost energy and higher generation costs.

According to EPRI Program Manager Bill Carson, tube bundles typically need cleaning after a few years in service, and traditional methods include chemical cleaning and groom ice cleaning. Many plants’ permits prohibit disposal of wastes from chemical cleaning, and groom ice cleaning requires significant work hours and resources to build scaffolding and complete the job. Considering that each approach can have drawbacks, EPRI is examining the effectiveness of pressure wave cleaning to provide utilities with another option to consider.

Pressure wave cleaning requires no labor-intensive scaffolding construction. Swiss company Bang & Clean developed the technology and in 2015 successfully tested it on an HRSG at ESB’s Dublin Bay Power plant in Ireland. GE has licensed the technology in the United States.

EPRI has played a key role in bringing the technology to the United States and facilitating industry collaboration. In 2015, ESB personnel presented on their field experiences at EPRI’s Boiler Reliability Interest Group meeting attended by dozens of utilities, and the group recommended that EPRI spearhead a project to test the technology in the United States. In subsequent tests in an HRSG at TVA’s Southaven Combined-Cycle Plant, inspections by TVA and EPRI indicated that no component damage occurred.

Successful Demonstrations in the Southeast

Jacob Pursley is an operations technician and HRSG system owner at a power plant in the U.S. Southeast. A few years ago, a round of groom ice cleaning helped the plant to reduce problems with back pressure. “We pulled out a few tons of debris from each unit, and we thought, ‘Hey, we’re good to go,’” recalled Pursley.

But after three months offline during a rainy fall and a major outage, fouling again accumulated on the HRSG tubes, and back pressure increased to unprecedented levels. “I was concerned that if we went back in there with ice cleaning, had all the people in there, put up all the scaffolding, racked up all the man hours, and banged up all the tubes again—would we get the results we needed?”

After reviewing EPRI data on pressure wave cleaning, Pursley and the plant’s management team decided to test GE’s PressureWave PlusTM technology. The previous round of groom ice cleaning required 20 work days (10 days, two shifts each day) to clean three modules in each HRSG. Using pressure wave cleaning, the team also cleaned a fourth module that could not be reached by scaffolding, completing the entire job in 14 work days (one shift each day)—including two days for a vacuum truck to remove debris.

“The safety side of it is that nobody is inside the HRSG when it’s going on, and you have none of the hazards with constructing scaffolding inside the HRSG,” said Pursley.

With chemical and ice cleaning, personnel must enter the HRSG to perform the cleaning, increasing risk of injury. (To EPRI’s knowledge, there have been no reported injuries as a result of these methods.)

With respect to efficacy, tests at Pursley’s plant demonstrated that pressure wave cleaning can reach deeper into tube bundles and clean sections unreachable by other methods.

“Our units are near design level now,” says Pursley. “It’s like we just put in two brand new HRSGs.”

The Work Ahead

A preliminary finding from these and other field tests since 2015: In the near term, it appears that pressure wave cleaning does not result in cracking or other adverse metallurgical impacts in tubes, liners, and other HRSG components. With the Colorado School of Mines and other partners, EPRI will follow the development of this technology and continue field testing to confirm that there are no such short-term effects. It will also examine potential long-term effects on component integrity and plant reliability.

Pressure wave cleaning must be done when the plant is offline, but that could change.

“There is potential for online cleaning in conventional boilers,” said Carson. “EPRI will be looking into that with Bang & Clean.”

EPRI also will examine pressure wave cleaning for air heater baskets, electrostatic precipitators, wires, and other boiler surfaces.

“It’s premature to say that pressure wave cleaning is superior to other methods,” said Carson. “EPRI is still researching the technology and its possibilities. But early results show that it could offer the industry new options.”

EPRI Technical Expert:

Bill Carson