Regular inspections can help reduce forced outages, cost of HRSG maintenance

It’s no secret: Turbine/generators—gas and steam—are the combined-cycle components that get the most attention from O&M personnel. Seemingly indestructible “big iron”—heat-recovery steam generators (HRSGs) and steam piping—often is an afterthought. Another observation: Plant owners normally don’t hesitate calling a service provider to keep turbines reliable and available, but they often hold the purse strings tight on other equipment vital to plant operation—sometimes until it fails.

Not a good strategy if this equipment is not spared, which is often the case in combined-cycle facilities. If you don’t run, revenue stops, even if the turbines are available. The O&M team is the owner’s eyes and ears in the plant and if properly trained and deployed can flag issues requiring attention before they cause a forced outage or run up maintenance costs unnecessarily.

That was the message conveyed by Scott Wambeke, PE, HRSG product manager for Structural Integrity Associates Inc, during his presentation on maintenance best practices at the 2014 meeting of the Combined Cycle Users Group (CCUG), last August, in San Antonio. Wambeke, a well-respected HRSG trouble-shooter who crawls through dozens of boilers annually, empowered attendees to conduct regular hot walkdowns and offline inspections of their units to maintain top performance.

Wambeke Fig 1,2

Hot walkdown. Starting point for the hot-walkdown portion of the presentation was penetration seals, which suffer wear and tear and likely will require replacement once or twice in the HRSG’s lifecycle. Wambeke said it’s much easier to find cracks and exhaust leaks—at least their approximate locations—when running (Fig 1); compressed bellows can hide cracks. Another indication of a problem is failing fabric. Here are more of the “pearls” he shared:

      • Inspect pipe hangers integral to the HRSG, the speaker noted, because attemperator steam piping and piping between modules often are omitted from the inspection program for high-energy piping (HEP).

      • You can get a good “feel” for the internal condition of the insulated duct delivering turbine exhaust gas to the HRSG by listening for audible liner rattling/flapping near the round-to-square transition. Recording and analyzing the dominant frequencies can help identify the component vibrating and gauge risk. Watch for casing vibration, especially at low loads.

      • When conducting a hot walkdown, take along an infrared scanner. A visual survey will find burned paint, but it does not quantify the hot spot, Wambeke cautioned. Having an IR signature will provide the temperature and enable you to track the progression of the problem. The size and shape of the IR signature, he continued, helps assess its cause—for example, missing insulation, conduction through structural members, exhaust leak, etc. Exhaust leaks typically are found at expansion joints, side doors (Fig 2), duct corners, and penetration seals.

      • Be sure to check the size and location of duct-burner flames, Wambeke told the group. Ideally, flames should be from 6 to 10 ft long and not visible within about 6 ft of any tubes. Flame color can tell you a lot about the combustion process, as it does for conventional boilers. Sagging of burner baffles, if found, should be addressed during next scheduled maintenance.

      • Check roof seals and inspect for standing water after a rainstorm. Regarding the latter, this is a situation you want to avoid, because water will leak into any opening and contribute to progressive deterioration of expansion joints, roof seals, and doors.

      • At the top of the unit, check for leaks at water-level indicators, safety valves, steam-valve packing glands, and around drum doors. Regarding the last, take no immediate corrective action without first consulting plant management and company safety personnel.

Wambeke Fig 3,4

Offline inspection affords the opportunity to document all findings—including the good ones. A complete condition assessment is valuable for comparison purposes during follow-on inspections. A primary goal of your effort is to identify and prioritize maintenance items—those requiring both immediate permanent repair and those having the luxury of time for corrective action. Generally, the more time available for engineering and procurement, the better and less costly the solution.

A rule of thumb is that it takes two days for an experienced engineer to inspect an F-class HRSG. Add-ons—such as tube-wall thickness measurements, borescoping, etc—extend the timeline. If plant personnel are conducting inspections, be sure to provide them checklists for the recording of key measurement and information. Good lighting and a high-quality camera with strong flash are required for both safety and documentation. Plus, basic tools for measurement and marking are recommended.

Start your walk-around at the transition duct with a liner inspection. Look for, and correct if found, spinning washers, loose liner sheets, failed washers or studs, and exposed insulation. Verify the integrity of the expansion-joint flashing at the round-to-square transition. Check for cracking and distortion of the perforated plate and for hang-ups at guides.

Wambeke then offered what he called a “starter” list for inspecting duct burners:

      • Sagging baffles or burner elements (Fig 3).

      • Baffle cracks.

      • Binding at wall pockets/supports.

      • Condition of intermediate supports.

      • Evidence of coking on burners (Fig 4).

      • Overheating of the liner in close proximity to the duct burner (Fig 5).

      • Fuel-nozzle cracking.

      • Visual hot spots on downstream tubes.

Wambeke Fig 5,6

On the speaker’s harp starter list for inspectors were the following:

      • Examine tube and fin corrosion. Do you see rust flakes or fouling? What percentage of the flow path do you think is blocked? Take high-resolution close-up pictures for the record.

      • Document condition of tube/header joints, if visible.

      • Note condition of bulkhead, sidewall, and center (Fig 6) baffles and guesstimate the total area of gaps—if any—to determine bypass gas flow.

      • Look for cracking and/or sagging of tube ties and any related fin damage (Fig 7).

      • Make a detailed map, supported by photos, of distorted tubes in each bundle.

Wambeke Fig 78

Starter list for the upper and lower crawl spaces had these items:

      • Wambeke Fig 9Characterize debris on the floor and on headers; take photos.

      • Examine liner material and determine if magnetic.

      • Check condition of flashing around liner penetrations.

      • Look for wet spots or drips (Fig 8); examine the spray signature, if any, from leaks.

      • Inspect for piping and header corrosion—especially at the liner interface (Fig 9).

In addition to inspecting for casing hotspots, standing water on the roof, and the condition of pipe hangers, all mentioned earlier, it is important to verify the condition of penetration seals. Local corrosion under the seals at the roof and floor can be problematic and often remains hidden unless the seal is removed. However, removing seals effectively demolishes them and makes replacement necessary.

Wambeke explained why you can expect to find corrosion of penetration seals:

1. Most HRSG penetration seals—typically bellows type on the hot end and fabric seals on the cold end—do not provide for drainage.

2. On stagnant lines (vents and drains), it is possible to have piping under the seal at a temperature lower than the exhaust dew point (usually 115F-120F).

3. Low-alloy steel corrodes, too. The HRSG expert cautioned about becoming overconfident with alloy piping if you have extended offline periods.

4. A systematic inspection approach was recommended. Nondestructive examination (NDE) technology can detect wall loss beneath seals without removal or demolition.

Material wastage from corrosion can be dramatic. Wambeke described a case where economizer vent piping on the roof of an 18-yr-old HRSG in a dry climate suffered a 45% loss in wall thickness and a 20% loss in diameter on adjacent hanger rods. For the first 10 years after COD the plant operated base-load; it cycled after that.

Regular steam-drum inspections are critical, Wambeke said, because they often provide the only direct access to the water-side of an HRSG. First thing to look at when you open the access door is the surface color and condition of the metal above and below the waterline (take photos). A red tint, such as that in the LP drum shown in Fig 10, is good.

Wambeke Fig 1011

Once inside, take a close look at the following:

      • Mechanical condition of steam separators, and chemical-feed, blowdown, and other internal piping. If plugging issues with injection and blowdown piping are identified, tube deposits may be lurking. Plugging of level-indication sensing lines is common (Fig 11) and may be difficult to see, depending on arrangement details. Also, check the location of the sensing line with regard to downcomers. If it is too close to a downcomer, water velocity may influence measurement.

      • Debris that has accumulated; note the location, type, and quantity.

      • Nozzles and seams for cracking at the welds.

      • IP and LP belly pans. Measure their thicknesses and trend over time; check for cracking at seams and ends.

      • Verify that the door sealing surface is pristine and hinge/davit condition is good.

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