Presentations by owner/operators get high marks from HRSG Forum attendees

Four user presentations followed Scott Wambeke’s detailed review of Xcel Energy’s experience with unmanned aerial systems (drones) at the HRSG Forum with Bob Anderson. That material is summarized below.

Converting from simple to combined cycle. Yogesh Patel, Tampa Electric (TECO), discussed the recently commissioned Polk Unit 2 Expansion. The project began in 2012 with four existing 7FA simple-cycle gas turbines. The result would be a 4 × 1 combined cycle with new diverter dampers, auxiliary firing for an additional 30 MW per HRSG, and one 500-MW steam turbine. Cooling towers were added to support the new auxiliary equipment.

Patel clearly outlined TECO’s planning and assessment factors for all major project participants. The common theme was what he called a “blended team arrangement,” including hands-on participation by the owner/operator.

Also of interest, HRSG supplier bid criteria included all standard items, then added adaptability to cycling service, and ease of inspection and maintenance. Cycling design elements were therefore critical, as was full-system accessibility. For example, inspection ports, doors, and platforms would be installed in both the front and rear of the boilers.

Wide maintenance bays (23 versus 18 in.) were specified. Lower headers also required good access and room was specified under all modules for blowdown and drain inspection. As Chairman Anderson clearly acknowledged, “The industry is changing. We will have more cycling and inspections; we need more room.”

Discussion also took place (in general terms) on the cost impact of the added features. Methods of vendor quality control also were noted.

For resource selection, weaving a thread with Jeff Henry’s discussion on steelmaking, raw materials originating in certain countries were not allowed. Patel noted that the entire supplier list was scrutinized and evaluated. Weld qualification tests were mandated for P91 tube-to-header welds.

He then reviewed the construction management team setup, organized in part to strengthen owner/operator personnel for future involvement in field engineering and construction.

Tube cleaning. Jacob Pursley, Southern Power, gave a new perspective on HRSG tube cleaning by discussing both CO2 and an alternative, GE PressureWave Plus™ (developed by Bang & Clean Technologies AG of Switzerland).

Scaffolding is not required; after setup, pressure waves penetrate the tube bundles. With the unit offline, a lance is placed between the modules and a bag is inflated with a combustible mixture that is then remotely ignited. The pressure waves and tube vibrations remove the deposits. This is performed at various locations.

Pursley explained the process, site history with CO2 cleaning, and the decision to use this technology.

The HRSGs at Southern’s Plant Rowan F-class 2 × 1 Unit 4 were cleaned by ice blasting in 2015. Corrosion at the time was visible with excessive bridging of rust between the fins. Tubes are in-line. Backpressure was approaching the GT trip point of 24 in. H2O (original design backpressure was 15 in.). Three tons of debris were removed from each unit. Backpressure decreased by 3 in. for Train A and 2.5 in. for Train B.

But soon after the outage, Train-B backpressure increased to 28 in. Potential reasons were found in areas that had no personnel access (18 in. between modules). Pursley did point out that “previous ice blasting showed good results on the surface but various concerns were raised in the ability to access hard-to-reach areas, as well as depth of penetration.” He added, “We did not use a spreading or deep-clean method previously.”

Also, during an 86-day outage in 2015, Unit 4 had no dehumidification or layup, and there was rain “almost every week.”

For PressureWave Plus, cleaning locations were set in five horizontal and nine vertical spots. There would be at least four bangs at each location in the Train B HRSG. The primary target was the middle of Module 4, between HP Economizer 2 and the IP superheater section. Roof-access sky climber ports were installed for the lance rigging. The results: 14.5 tons of debris removed; back pressure decreased by 8 in.

Both units then were cleaned in fall 2016. The results: Train-A debris 24.8 tons and backpressure decrease of 9 in.; Train-B debris another 3.3 tons, backpressure decrease another 2 in.

Pursley then gave comparative results (both cleaning methods) for personnel required, backpressures, stack temperatures, and tons of debris. There were several questions—including the specific type of fouling, potential impact on seals and baffles, potential impact on catalysts, and theoretical depth of cleaning. Experience with this technology is growing and under review. An EPRI program or paper is being considered.

Penetrations and seals. In another site-specific example, the OG&E/Dekomte presentation focused on the 500-MW McClain Power Plant commissioned in 2001. Thermal and visual HRSG inspections in 2014 and 2015 showed several bellows and penetration-seal concerns.

McClain’s Benn Privett explained the inspection findings, ranging from sediment and debris at the bottom of an expansion joint to severely leaking penetration seals (essentially overheating the surrounding casing). There are 103 seals per HRSG at this plant. After initial inspection, Dekomte performed a complete thermal survey.

Side-wall penetration technology was reviewed first, noting various OEM installation types and retrofit options. Discussion then centered on converting OEM bellows to fabric (to increase flexibility), mechanical seals to fabric, and labyrinths to fabric.

Specific and detailed examples followed, supporting the conclusion that “each site and application requires a tailored solution.” These details also showed that “monitoring and annual inspection are critical,” to address small issues early rather than major problems later.

The McClain replacement project was 80% complete at the time of the meeting.

Dekomte’s Jake Waterhouse then discussed pumpable fiber insulation, defined as a mix of short fibers dispersed in high temperature binders which, upon drying, produces a strong insulation structure with low thermal conductivity. It can be pumped or troweled into place for use in insulation, sealing and duct repairs while the unit is operating.

PI dashboard. Madeline Dean, Exelon Generation, discussed the PI Dashboard, “Using PI to mitigate thermal transients.” PI (plant information) performance equations, with real-time sensors, offer thermal transient calculations for:

      • Condensate detection.

      • Attemperator leakage.

      • Valve instability.

Condensate: The system identifies times when undrained condensate travels across selected thermocouples, identifying large and sudden temperature drops by looking at slope and standard deviation. This helps focus corrective actions on the specific portion of the HRSG.

Attemperator: These calculations identify when water is exiting the attemperator with the valve closed, allowing detection of valve degradation.

Valve instability: Thermal transient calculations will count the number of times a valve changes from open to closed over a specific time interval, providing a specific maintenance alert.

“The Transpara LLC interface,” she added, “is the visual end product that can be used on computer, phone or tablet.” She also stressed the value of having historical data in an easily searchable and adaptable format. Other applications discussed were system chemistry and catalyst monitoring for pressure drop and ammonia use.

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