Coal, Gas, O&M

Drone Inspections

Issue 3 and Volume 122.

HRSG Maintenance from a Bird’s Eye View

Power plants are using drones to inspect critical power generation equipment, including firing ducts, catalysts, silencers, tube panels, stack dampers, baffles, heat recovery steam generators and pretty much anything else they can reach. Photo courtesy: HRST Inc.

Battery powered unmanned aerial vehicles (UAVs), or drones are finding applications in an increasing number of commercial industries and the power industry is no exception. Originally more commonly associated with military and recreational applications, drones have expanded their resume to include HRSG maintenance inspections. With increased affordability, advanced maneuverability and some proper lighting, drones have enabled a rapid evolution in inspection capability.

Drone Technology

Along with the reduction in cost, drones, or unmanned aerial vehicles (UAVs), are now being equipped with a growing number of features that makes them increasingly autonomous and easier to operate. By means of sensors and intricate programming, drones can now hover in place in 20+ mph winds, avoid obstacles, fly to a pre-established “home” point, and land with pin point accuracy at a location designated by the operator. Automatic flight paths can even be pre-populated onto a tablet or phone so that the pilot’s role is relegated to simply telling the drone to take off and the drone then does the work all by itself.

View looking down on silencer shows internals in proper position with support intact. Photo courtesy: HRST Inc.

View looking down on stack damper allows blade alignment and mechanical stop position checks. Photo courtesy: HRST Inc.

Camera costs have come down, all while their quality has improved to surpass 4K resolution. A new variety of gimbal attachments allow a larger field of view than previously possible. In some cases, drones can be fitted with multiple cameras to allow the drone to capture views that encompass the whole area surrounding the aircraft.

The improved portability and development of smaller models allow for drone use in tighter locations, particularly useful for HRSG inspections. With proper application and piloting, drones can be a safe and reliable inspection tool that can be used to take the guesswork out of what lies above.

HRSG Maintenance Planning

Some areas of the HRSG are rarely inspected because they are visually inaccessible from the ground which adds a degree of risk to maintenance planning. If there are no problems visible in my bottom burner elements, is it safe to assume that is also the case further up? Is it worth spending tens of thousands of dollars scaffolding with the chance of not finding anything? Situations like these often fall onto a maintenance manager’s desk. Decisions that should be easy become difficult when the cost of scaffolding, manpower and down time get factored in. In these situations, drones are a game changer. Guesswork can be eliminated by utilizing a drone to fly up and assess suspect areas. A 15-minute flight can fill in the blanks and stamp a yes or no on a wide variety of issues. If a concern area is seen when “looking up,” then the drone can help make the “repair now or later” decision. If the drone flies up and nothing is found, then the cost of scaffolding was saved and a good decision was still made to ensure peace of mind.

If the drone finds an issue, the drone can also optimize the cost of repair and decrease the unit’s downtime. If it turns out that the problem is about 1/3 up the height of the duct, scaffolding will still have to be erected to perform repairs, but it can be configured to meet the precise need, for example scaffold can be installed to span the needed ~40ft in height rather than the entire duct. Repair plans can be thought out and materials can be procured while the scaffolding is being erected rather than after. For these reasons, drone inspections can potentially reduce a multiday job to a one-day job with the problem positively identified in 15 minutes.

With equipment aging and maintenance budgets tightening, the ability to reduce inspection times, increase maintenance planning abilities, and having the proper information to know whether or not to spend large sums of money is vital.

History Condition Assessment

Drones continue to prove their worth in one-off unexpected uses, but where they are starting to add even more value is in history assessments. Many plants have opted to perform drone inspections annually or biannually to compare the condition of their equipment over time. The most common location for a condition assessment is the firing duct. Baffle and burner nozzle cracking can be inspected for crack size, location, and any changes from previous inspections. This can help determine when maintenance should be scheduled ahead of time rather than “right now”. If a crack propagated from 1” to 3” in a year, then plans can be made for a repair schedule. If there is no change, the decision can be made to continue monitoring.

“A 15-minute flight can fill in the blanks and stamp a yes or no on a wide variety of issues.”

Other HRSG areas benefit from history assessments as well. Tube bundles downstream of a duct burner can be monitored for any changes in coloration indicative of overheat. Operators can then adjust duct firing accordingly and extend the equipment life. Repairs can be monitored over time to make sure they are holding steady or need to be readdressed. Catalyst fouling can be inspected at all heights to determine when to clean. (Sometimes what appears nice and clean from the floor is in fact heavily fouled as you approach the roof level.) Stack damper blades and shafts can also be better assessed for cracking or deviation in position relative to years past.

The ability to track and plan maintenance based on solid historical information is much better than guessing what’s to come and scrambling to address the unexpected.

HRSG Drone Inspection areas

When HRST piloted our way into the drone business, the main focus was on the firing duct, but plants soon began asking about other sections of the HRSG. The list of areas that could benefit from a drone inspection soon grew. Found below is a sampling of the issues identified in 2017 drone inspections.

Duct burners:

  • Cracked and damaged burner baffles
  • Coked elements
  • Bent or warped igniters
  • Missing fuel tips
  • Failed flame holder castings
  • Debris stuck in the burner face

Tube harps:

  • Tube overheat indications at the top of the bundle
  • Center baffle damage
  • Failing tube ties nestled in the bundle
  • Fin/tube damage at tube ties and bumpers
  • Tube leak indications

Catalysts:

  • Heavy fouling near upper level (appeared clean from the floor)
  • Water damage from roof leaking rain water
  • Guide pin failures

Others

  • Exhaust Flow Turning Vane and Flow distribution plate cracking and support problems
  • Stack damper angle iron failed
  • Stack damper not against stops
  • Outside casing condition

Many of the issues identified required repairs or a monitoring schedule. The majority of the findings would not have been properly identified without the use of UAVs.

Firing duct drone Inspection. Photo courtesy: HRST Inc.

In the case of fouling SCR catalysts, the first catalyst we inspected was just going to be a test. From the floor, the catalyst looked perfectly clean as far up the duct as we could see. After flying about three quarters of the way up, heavy debris was found caking the catalyst. Old liner failures that caused insulation to liberate had been fouling the catalyst without anyone being able to tell from the ground level. It was completely unexpected.

In aging HRSGs with duct burners, downstream tube bundles have begun showing signs of oxide exfoliation that reduces the creep life of the material. What might be minor exfoliation at the floor level can be much worse at the top of the tube bundle due to the upward steam flow having higher temperatures near the roof. Several units have been found with severe damage at the roof level undetectable from the floor. This is an area very rarely inspected that drones can easily access.

Pilot in Command

Flying a drone inside an HRSG requires experience and skill. In the United States, flying a drone outside requires the pilot to be certified by the FAA and know how to follow FAA regulations. While locations inside the HRSG are not regulated by the FAA, in the case of stack dampers, the airspace enters a grey zone. Does the stack damper count as inside the unit or outside? In most cases, drones have cameras on the bottom of their rig which prevents them from looking at what is above. For this reason, typically, the easiest way to inspect a stack damper is by flying from the outside of the stack into the top and peering down at the damper blades and shafts. Since outside airspace does fall under FAA regulations, it’s crucial for the drone pilot in command to have a firm grasp of what’s required to operate. Several parts of the United States are located in unregulated airspace zones that do not require additional authorization, but if your plant falls close to an airport or other regulated zones, permission has to be requested from the FAA. In short, for your drone inspection to be successful, it is essential that your commercial drone pilot knows how to fly and the rules of the sky.

Duct burner nozzle Condition from drone inspection. Photo courtesy: HRST Inc.

Much like obtaining a driving learner’s permit, a drone license is acquired by studying a variety of airspace and operation rules and then taking a written test at an authorized testing center. With a passed result the pilot receives a remote pilot certification card. This ensures that each commercial pilot or remote pilot in command (PIC) knows where they can and cannot legally fly as well as the rules of operation.

Since it is a written test, it does not guarantee a pilot knows how to operate a drone. Each company is left to their own to determine training requirements for flying. This can include a certain number of logged practice flight hours, in-house testing, or training classes with outside companies. The FAA is strict on knowing the rules, but not so much in how pilot flight capabilities are determined.

Most companies are in the early stages of building their drone programs and establishing what they consider a qualified pilot. As far as HRSGs are concerned, flying inside a giant metal box in the dark introduces flight challenges not faced in most non-HRSG drone flights.

When flying outside, GPS location and positioning sensors can keep a drone hovering in an exact spot, regardless of fluctuating wind streams and conditions. Inside an HRSG, those same sensors often fail to function. A GPS signal cannot be obtained through the insulated HRSG casing, and the drone positioning sensors reflect off of the liner metal. If the sensors are left on, the drone may fluctuate positions rapidly as the GPS recalibrates its location during brief moments it’s able to connect. For this reason, the sensors are turned off for most indoor HRSG flights and flying is done manually, leaving the pilot to have to react to drafts and other factors which can impact the flight plan.

Tube condition at 15’ (bottom) and tube condition at 50’ (top) showing possible long term overheat. Photo courtesy: HRST Inc.

When training pilots for indoor flights, pilots should have several hours logged without the use of GPS or added sensors. Manual flying skills are a must.

Along with the FAA rules and tips, a few HRST HRSG flying tips are as follows:

  • Practice flying without the sensors and GPS outdoors before attempting indoors
  • Get used to flying in a weak to moderate wind without any sensors
  • Wear a dust mask indoors so you don’t inhale whatever the drone kicks up off the floor
  • Wear safety glasses, preferably spoggles to keep the debris out of your eyes
  • As always, wear proper PPE in the event of a crash
  • Use propeller guards so small bumps into the liner or HRSG components won’t cause a crash
  • Find the proper lighting for the situation. You don’t want a washed out view for inspection or a muddled line of sight

Following these tips, along with having the proper training and knowledge of flight guidelines will allow for safe operation and a successful inspection.

Summary

As strapped maintenance budgets become more common with aging equipment, the need for time and cost-efficient inspections increases. Adding a drone to your regular inspection scope of visual inspection from the floor, targeted NDE and borescope inspection of the water-side components, fills in the inspection void of identifying taller issues. Drone inspections offer a safe, reliable, and affordable way to understand and track the condition of HRSG components in areas unable to be visually inspected from the floor level inspection. Drones are continuously improving. Extended flight times, improved sensors, and quicker outdoor flight FAA approvals are just a few improvements coming down the line. Offering reduced inspection costs, reduced downtime, quicker inspection times, and historical trending data, the drone “boom” isn’t landing anytime soon so now is the time to get on board.

Natalie Teuchert is a mechanical engineer, HRSG inspector and drone pilot at HRST Inc.