The Benefits of Floating HRSG Duct Liners

Brad Buecker – Editor
Buecker & Associates, LLC

Posted 1/30/2024

Introduction

The heat recovery steam generators (HRSGs) of combined cycle power units utilize the exhaust gas from the combustion turbine(s) to generate steam for additional power production.  CT exhaust gas temperatures may reach or exceed 1,000o F, and can be even higher if direct fire burners are placed in the HRSG inlet duct.  Accordingly, HRSGs have inlet duct liners to keep most of the heat within the duct to maximize HRSG efficiency and to protect plant personnel from what otherwise would be extremely hot duct surfaces.  Obviously, duct liners are subject to very harsh conditions, including large temperature swings.  The floating liner design allows thermal expansion and helps to give liners greater longevity and performance over rigid liners, as this article explains.

Duct Liner Materials

The three major components of a duct liner are the insulating material, mounting brackets (either studs or scallop plates), and internal liner sheets to protect the insulation from the harsh environment of the hot, rapidly flowing gas stream.

Insulation

The two most common insulating materials are ceramic fiber and fiberglass, with the former being most prominent.  

installed insulation without duct liner sheet
Figure 1.  A section of installed insulation without the liner sheet.

Two primary factors parameters influence insulation selection, thickness and density.  Of course, the major objective is to restrict heat loss.  However, acoustic mitigation often also comes into play, where the liner design can reduce the noise emitted by the flowing gas through the duct.  Figure 2 below shows the basic design details of a 6-inch layer of insulation selected for a 1,200o F gas temperature.

Liner system diagram for a 6” layer of insulation
Figure 2.  Liner system diagram for a 6” layer of insulation.

Liner Sheets

Without protection, the insulation would rapidly be damaged by the flowing exhaust gas in the inlet duct.  Liner sheets offer the needed protection.  As shown in Figure 2, a durable material for the sheets is 409 stainless steel, with 11- or 12-gauge thickness being common.  Also common are 4’ x 4’ or 4’ x 6’ liner sheets, installed in an overlapping pattern similar to shingles on roofs.  Overlap protects the leading edge of every downstream liner sheet from direct impact of the flue gas.

Mounting Brackets

Extremely important is correct design/installation of studs or scallop plates for insulation and liner sheet support.  A primary feature of the floating liner concept is to allow expansion and contraction per thermal changes induced by cycling operation.  Depending on operating needs, temperatures within the duct can range from 1,300o F to ambient.  Figure 2 is based on a computer-designed stud arrangement.  Compare that to the haphazard arrangement shown below of a system in need of repair. 

bad stud arrangement for insulation and liner support
Figure 3.  A very haphazard stud arrangement for insulation and liner support.

Figure 4 shows the precise stud alignment in a SVI Bremco relining project.

relining project
Figure 4.  Photo of a relining project underway at the time.

Stud diameters may be adjusted per the support strength needed.  Diameters can range from ½” to ¾” or perhaps even 1”, as necessary.

Computational Design Assistance

Modern computer and analytical technologies have greatly enhanced design capabilities.  Computational fluid dynamics (CFD) software can predict gas flow patterns through the inlet duct, HRSG, and outlet stack.

CFD analysis of flue gas flow from the inlet duct through the HRSG and exhaust stack
Figure 5.  Example CFD analysis of flue gas flow from the inlet duct through the HRSG and exhaust stack.

CFD data helps to pinpoint locations of maximum flow, which in turn may require adjustments to design parameters that are otherwise acceptable for other locations in the system.

Another valuable technique is thermal imaging of the duct from all sides.

two thermal images from HRSG inlet duct
Figure 6.  Two thermal images from an existing HRSG inlet duct.  The bright areas indicate locations of excessive heat loss.  Seams in sectional components are particularly noticeable.

The images can then be utilized to install or repair insulation at “weak” locations.  Also, support studs and scallop plates offer a direct path for conductive heat to escape.  High temperatures in localized spots can cause metallurgical degradation of the carbon steel outer duct.  

Weak spots and some liner failures may be difficult to observe visually.  Consider the typical HRSG design as shown below.

heat recovery steam generator
Figure 7.  A heat recovery steam generator.  Note how the inlet duct greatly enlarges vertically to allow the exhaust gas to contact the HRSG water/steam circuits. (1)

As can be imagined from this photo, accurate visual observation of upper duct and roof areas may be quite difficult.  And, it is often not possible or timely to erect scaffolds in these locations for visual inspections or even to make repairs. 

Roof liner failure
Figure 8.  Roof liner failure.

Thermal imaging may be the only viable method to find failed liner locations or other spots of excessive heat loss.  Creative methods, including repairs made from the external side of the duct, may be required in difficult-to-access spots.

Conclusion

The harsh environment that exists in HRSG inlet ducts requires careful design/installation of insulation and liner plates.  Expansion and contraction issues play a great role in proper design.  SVI Bremco personnel can assist with all phases of inspection, repair, and installation of duct insulation, utilizing advanced analytical tools and years of past experience.


Contributing editor for this SVI Bremco article is Brad Buecker.

  1. B. Buecker (Tech. Ed.), “Water Essentials Handbook”; 2023. ChemTreat, Inc., Glen Allen, VA.  Currently being released in digital format at www.chemtreat.com.

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Brad Buecker

Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as a senior technical publicist with ChemTreat, Inc. He has many years of experience in or supporting the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, IL, USA) and Kansas City Power & Light Company's (now Evergy) La Cygne, KS, USA, station. His work has also included eleven years with two engineering firms, Burns & McDonnell and Kiewit, and he spent two years as acting water/wastewater supervisor at a chemical plant.

Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, NACE (now AMPP), the Electric Utility Chemistry Workshop planning committee, and he is active with the International Water Conference and Power-Gen International.

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