Written by Tucker York | Power Plants

For any power plant, a key issue is minimizing heat losses to, in turn, maximize the energy available for power generation.  SVI Dynamics is proficient with the technique of thermography to identify locations of excessive heat loss in combined cycle power generating units.  Outlined below are results from a recent thermographic inspection of the heat recovery steam generator (Rankine cycle) portions of a two-unit combined cycle power plant in the western United States.

The Steam-Generating Cycle Components

The major components of each steam generating cycle from the combustion turbine exhaust forward include a diffuser duct that protrudes through the combustion turbine building, an expansion joint connection to the HRSG transition duct, a triple-pressure HRSG including a CO/SCR (carbon monoxide/selective catalytic reduction) section, and a final vertical main stack.  The ducts and casing typically have an internal insulating liner to funnel the exhaust gas energy to and through the HRSGs, but mechanical and thermal stress continually degrade liners, insulating materials and support structures.  In fact, prior to the SVI inspection and thermal survey, plant personnel discovered that excessive heat from the top of one diffuser duct had melted cable bundle jackets in a tray suspended above the duct.  The maintenance staff installed temporary exterior insulation to mitigate this problem until permanent repairs could be made.

Thermography Results

The technical objectives for this thermal evaluation were to characterize the general insulative performance of the expansion joints and lined wall construction of each HRSG exhaust system, and to identify any weaknesses due to liner fatigue, insulation evacuation, direct metal conductivity, or improper installation or repair work.  Besides efficiency degradation due to heat losses and the cable tray issue mentioned above, exceedance of steel temperatures above specified design conditions can lead to performance degradation over time.  Extremely high temperatures can potentially cause molecular grain growth, unwanted crystallization, and material phase transformations that may lower the tensile and shear strength characteristics of the duct and casing steel.  SVI Dynamics suggests the following temperature criteria for categorizing surface temperatures and service requirements.

  • CATEGORY 1: Apparent Temperatures, > 500o F. Requires immediate review by an SVI Dynamics structural engineering representative.  Upon further inspection, remediation may include repair of inner liners, insulative materials, outer casings, and adjustments to liner suspension (scallop plates, threaded standoffs, etc.), or full replacement.
  • CATEGORY 2: Apparent Temperatures, 300o – 500o F. Requires periodic inspection during service life of the equipment.  Over time, these conditions may progress towards Category 1 and require repair or replacement.
  • CATEGORY 3: Apparent Temperatures, < 300o F. Acceptable operating temperatures, no review requirements.

A Thermography Example from This Project

Below is a photo of the expansion joint between the diffuser duct and transition duct on one unit.  While some corrosion and degradation of the expansion joint flanges was evident, no accurate estimate of heat loss was possible from just a visual examination.

Figure 1.  Photo of the diffuser duct, expansion joint, flanges and downstream casing on one unit.

Thermography reveals the true story.

Figure 2.  Thermographic analysis of the same location with the unit at full load.

The analysis indicates temperatures greater than 500o F at the expansion joint.  As noted, SVI Dynamics recommends physical inspection and remediation by qualified personnel of such excessive heat loss locations as quickly as possible.  These steps may include repair of the inner liner and/or support structures, outer casing, and other equipment.  The complete thermographic analyses of these two units revealed numerous other weak points that allowed high energy loss; many of which had gone undetected.  Locations besides that shown above included HRSG casing, penetrations of small piping through ducts and casing, and other expansion joints and duct flanges.  Figure 3 shows a thermography scan of a transition duct section with a hot spot at the casing.

Figure 3.  Excessive heat loss at a spot on the transition duct of one unit.

Looking Towards the Future

Combined cycle power generation remains a large part of the U.S. energy production portfolio, and this trend is expected to continue, especially as many more data centers come into being.  Steady and efficient power plant operation will be keys for maintaining grid reliability.  Regular thermal inspections are a key component of reliability monitoring.  SVI BREMCO has a highly experienced staff to evaluate and offer solutions for many simple and combined cycle inspection and maintenance needs.