Drones facilitate some HRSG inspections, maintenance decision-making

Commercially available drones have become much more than a weekend novelty. Some facility owner/operators use them to survey and inspect transmission and distribution systems, pipelines, plant boundaries, or the plant itself. And some have now moved indoors to inspect wall panels, burner systems, and critical heat-transfer surfaces of large fossil-fueled boilers.

Some have moved even further—to the more compact areas of an HRSG.

So these fleets that began as well-controlled eagles and hawks, those soaring surveyors with purpose, might now include a reflection of abnormally large, hovering hummingbirds.

The point is, powerplant engineers are tapping into these eyes in the sky to survey, record, analyze, revisit if necessary, and most importantly make critical decisions on the efficient use of personnel, time, and money.

These tools go by various names, including unmanned aerial vehicles (UAVs), and have proven their value in combined-cycle HRSGs. Field engineers have found that drones can reduce the overall time and expense of scaffolding, personnel baskets, and interior lighting systems, and can even reach areas that humans cannot easily inspect.

Take for example perforated distribution plates for the gas-turbine exhaust. In the words of one trained engineer (pilot) who has flown a number of missions in both HRSGs and large fossil boilers, “we looked at the distribution plate area as best we could from the ground, and we didn’t plan to scaffold the area for this particular outage. We saw up high a little bit of plate distortion, but nothing that seemed critical.”

The two-person crew then prepared a drone with new batteries and a new memory card. (If there’s a crash, you only risk losing one memory card.) “At about 40 feet off the floor,” he stated “we saw a six-foot crack in one of the seams. It was around the corner, and we could not have seen it from the floor.” The 4 × 8 ft perforated steel plate could have come off before the next outage and caused major damage. “So the crew erected scaffolding only where we needed it, got up there efficiently, and fixed it,” he explained.

Drones are useful for short outages and for spot checks of nuisance areas. For longer outages, they allow fast and economic inspection even before scaffolding is erected.

Targeting erosion/corrosion and exposed-insulation concerns seems obvious. But operators also have been able to inspect catalyst areas to identify slipping blocks or gas bypass. Another common flight path is duct burners, for baffle and nozzle investigation. Fuel nozzles get cracks in them, and you need to get within about 2 ft to see the cracks. Drones can do that.

It is important to note that the FAA has strict and evolving rules for drones, and the wise owner/operator will check with the FAA before any use. There could be some grey areas.

In one example discussion, this editor found an engineer who entered what is perhaps one of these areas, but left with an important discovery. Fully aware he could not exit the top of the stack, he flew to inspect the open stack damper. But with the camera on the bottom of this particular craft, he flew slightly above. The aerodynamic discovery was on the return.

Although open, this four-blade damper offered enough resistance to the drone’s rotor wash (and limited battery life) that the drone could not return. Think check valve. So the control room closed the damper to make a landing platform. The drone was retrieved through a stack door. Similarly, pilot engineers have experienced rotor wash pulling the units toward nearby surfaces (catalyst beds, for example).

Basic air currents are also significant, more so than outdoors. Inside a coal-fired boiler or HRSG, pilots lose the benefit of tools like GPS navigation that will let you stay in place even with a 10 to 15 mph wind. In a boiler none of that works. Even a slight draft, like an open stack damper or ventilation fan, can push the unit a few feet. Electronic interference can also occur within these steel enclosures.

Drones, ground robotics advance NDE

The relentless advancement of technology has impacted virtually every facet of electric power generation and delivery over the last decade or so. In the area of nondestructive examination (NDE), for example, there have been many developments in basic inspection tools—such as phased-array ultrasonics.

Recently, NDE developments have expanded to permit “hands-off” inspection by way of drones and ground robotics. For an industry with a focus on safety and cost, this is a big step forward. Unmanned aerial systems (UAS) and robotic crawlers enable inspection of hazardous areas and confined spaces with minimal risk to personnel. Plus, they can do these jobs faster and at less cost than traditional methods.

Aetos, a member company of Mistras Group, an organization known to many gas-turbine owner/operators for instrumentation able to remotely identify fluid leaks and the initiation of cracking in stationary engine components, is a leader in this emerging area. The company was the first to receive Federal Aviation Administration airspace approval to fly UAS in the petrochemical industry.

Today, Aetos’ team of licensed pilots is inspecting burners, igniters, stacks, etc, of heat-recovery steam generators (HRSGs) while they are operating. Only two people are required for such inspections. The benefits are obvious: Reduction in outage time, elimination of scaffolding and at-height work, cost saving.

So what’s next?

Keep in mind that most flights are initial surveys and inspections, but they do provide area focus and a high degree of visual precision. If hands-on personnel are needed, the playing field is narrowed and planning can be much more efficient. Safety and cost-effective mobilization of field personnel become measurable benefits. Drones are a valuable decision-making tool.

EPRI Journal (Jan/Feb 2016) discusses further initiatives like landing on and attaching to boiler tube walls (to conserve power during inspections) and rollers for tube wall climbing. But HRSG applications with finned tubes could present some additional challenges. EPRI and others also are looking at ultrasonic testing, dye penetrant, and even radiography, and similar tasks not needing extensive surface preparation.

And for confined space planning and possible hazardous conditions, think canaries in a coal mine.

If technology gets to where the drones can attack corrosion, perhaps our opening hawks will turn into vultures. Or in the case of HRSGs, martins. But all will carry that classic resonance.

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