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# AIR PREHEATER SEALING ADVANCES WITH ADAPTIVE BRUSH DESIGN

Pavan Kumar Ravulaparthy

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## To cite this version:

Pavan Kumar Ravulaparthy. AIR PREHEATER SEALING ADVANCES WITH ADAPTIVE BRUSH DESIGN. PowerGen International, Pennwell, Dec 2013, Orlando (FL), United States. hal-05108479

HAL Id: hal-05108479

https://hal.science/hal-05108479v1

Submitted on 11 Jun 2025

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Public Domain
TITLE

AIR PREHEATER SEALING ADVANCES WITH ADAPTIVE BRUSH DESIGN

AUTHOR

Pavan Kumar Ravulaparthy
## ABSTRACT

Addressing air prehater leakage has historically been a low priority maintenance outage issue for many fossil plants, petroleum refineries and pulp & paper plants. Axial, radial and circumferential metallic seals installed on rotary, regenerative air preheaters have not evolved from original metal strip designs dating back nearly a century ago. Metallic strip seals typically degrade following initial installation with wearing to the point of allowing excessive air-to-gas leakage which translates to increased fuel consumption and fan power draw. In June, 2007, Sealeze, a unit of Jason Inc, supplied a novel axial and radial brush seal design to both the hot and cold ends of the Unit 1 Ljungstrom APH at Bicent Power's 119 MW Hardin Generating Station, Hardin, Montana, operated by Colorado Energy Management Inc. With over 4 years in service, the high performance brush seals continue to outperform the original strip steel seals and expectations are those brush seals will remain in services through 2013. The brush seals have reduced air leakage considerably, and as a result, they have reduced operational costs through fuel savings, improved heat rate and lower parasitic losses. Since this installation, the product has been installed in over 5,000 MW of generating capacity in the United States and Asia.

The paper will evaluate different seal designs and quantifiable benefits of proper sealing on a power plant's operations.

## Description

Brush products are universally applied in virtually all industries where sealing, shielding, cleaning, and/or gap closing are required. Brush filament materials range from synthetic fibers to high alloy steels. Alloy brush seals, now regularly installed as an upgrade to conventional labyrinth seals on production steam turbines, reportedly can yield up to a 2% increase in unit efficiency and a 4% improvement in unit output. Some of these units have been in operation for eight years between inspections.¹
Brush seals are ideally suited for replacing radial, axial, circumferential/bypass and rotor post
strip seals on rotary, regenerative air preheaters (Figure 1). The brush seals products provide a
high degree of abrasion resistance, flex life and bend recovery not possible with rigid strip seals.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Ljungstrom Rotary Regenerative Air Preheater Seal Locations</figcaption></figure>

As shown in Figure 2, rigid strip seals rapidly wear as they are unable to conform to surface irregularities and varying gap sizes. The seals wear to the smallest gap size allowing leakage at the wider gaps. The strip seals are also vulnerable to damage at high differential pressures and expansion due to temperature increases where induced drag can shut down the rotor.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** A worn strip seal (on the left) compared to a new brush seal (right). Flexibility of the brush seal allows it to deflect at the smaller gaps and then rebound to ensure sealing at wider gaps. Degree of wear of the strip seal is evident.</figcaption></figure>
A brush seal, as shown in Figure 3, produces an extremely dense barrier as thousands of filaments nestle tightly together to create a high integrity seal. Each bristle is independent and flexible allowing deflection to conform to any irregularities and gap variations and recovery to its original position.

<figure><img src="image_5.png" /><figcaption>Figure 3. Patent-pending XtraSeal™ HT brush seal showing malleable alloy foil membrane nestled within brush filaments to provide an added 70-80% reduction in leakage without sacrificing overall seal flexibility.</figcaption></figure>

## Quantifiable Benefits to Improved Air Preheater Sealing

Air preheater leakage has historically been a low priority maintenance outage issue with many fossil plant engineers. Plants often experience leakage rates in excess of 15-20% and extreme leakage rates to 40% have been measured. These levels are often tolerated as they are often underestimated. As a result, plants can experience capacity losses, increased heat rates, higher parasitic losses associated with fan horsepower, and higher pressure losses for downstream air pollution control systems. A plant that has experienced “running out of fan” can conclude with a high degree of certainty that they have excess preheater leakage and are suffering from costly side effects.

As an example, a 500 MW plant firing coal and operating at an 85% annual capacity factor would consume 6000 tons of coal per day assuming an average heat rate of 10,000 Btu/kWh and an average coal heating value of 10,000 Btu/lb. If increases in boiler efficiency due to improved air preheater sealing reduce fuel consumption by 2%, the annual savings in fuel cost is nearly $3 million assuming a delivered coal cost of $80/ton.
Air preheater leakage can account for significant increases in parasitic power draw from the boiler fans and this translates into revenue losses from unsalable power. If a 500 MW plant has 15,000 hp installed fan power 25% of which is attributed to air preheater leakage, the plant essentially sacrifices 3750 hp, or 2.8 MW, that is unavailable for sale. If the sale value of one MWh is $30 off-peak and $150 peak, the plant operating on an 85% capacity factor running 6 hours/day peak and 18 hours/day off-peak will stand to lose $1.25 million per year.

Some plants may be able to reduce their total outage term if air preheater seal replacement is not required. The lost opportunity costs per day for a 500 MW unit offline can easily reach $800,000 - $1,000,000 or more.

A substantial benefit of reducing air leakage on a sustained basis translates to lower flue gas velocities and resultant pressure losses in downstream pollution control systems and a corresponding reduction in fan load. For plants with ESPs, increased velocities attributable to air preheater leakage may result in higher dust emissions at the stack. For plants with fabric filters, the higher air-to-cloth ratios due to air preheater leakage can affect the frequency of bag cleaning and possibly shorten bag life.

## Four Years and Running at Hardin Generating Station

In June, 2007, the Hardin Station engineering team and Sealeze (a Richmond, VA based unit of Jason, Inc) collaborated to design, manufacture and install radial and axial stainless steel brush seals on the plant's Unit A Ljungstrom air preheater (see Figure 4).

<figure><img src="image_6.png" /><figcaption>Figure 4. Ljungström rotor showing radial brush seals installed.</figcaption></figure>
Inspection of the brush seals in 2008 showed them to be in very good condition. Some splaying of the brush was evident on the cold end due to sootblower blasts of 400°F steam. To prevent direct sootblower impingement, the brush seals mounted in the path of sootblower blasts have been redesigned to incorporate an angled orientation and an integral protective shield as shown on Figure 5.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** New angle-mount design improves bend recovery and seal contact while deflecting direct impact of sootblower blasts. Additional shielding is provided by elongated holder flanges</figcaption></figure>

Now with close to five years in service, the brush seals continue to outperform the original strip seals, and expectations are that the brush seals will remain in service through 2012. The plant reports that the brush seals have reduced air leakage considerably providing reduced operational costs through fuel savings. Further, the plant has been able to postpone two scheduled air preheater outages.

For 2012, the plant is planning to install axial and radial brush seals on all rotor modules in its Unit B preheater. Also, the plant's target leakage goal is 6-8% and, to achieve this, the plant is now considering a complete set of circumferential brush seals.
## Continued Product Improvements

A new patent-pending Replaceable Seal Cartridge Quick-Lock Design, shown in Figure 6, allows plant operators to reduce outage time for seal replacement by 50%. As the XtraSeal HT with Quick-Lock is a two-part seal system, the seal holder is already set and locked down upon initial installation. During outages, the time-consuming process of seal realignment is eliminated as the brush itself can be removed and replaced quickly.

<figure><img src="image_8.png" /><figcaption>Figure 6. Easily bent tab for quick replacement of brush</figcaption></figure>

## A Growing Record of Commercial Installations

As of 2012, XtraSeal brush seals have been installed on air preheaters in power plants in USA and Asia as shown in Table 1.

<table><caption>Table 1. List of Installations</caption><thead><tr><td>Region</td><td>Capacities</td></tr></thead><tbody><tr><td>Eastern US</td><td>1440 MW</td></tr><tr><td>MidwestUS</td><td>3485 MW</td></tr><tr><td>Western US</td><td>3600 MW</td></tr><tr><td>Korea</td><td>560 MW</td></tr></tbody></table>

One eastern US plant reports measured air preheater leakage rates as low as 5-7%, a considerable improvement after the installation.
## Conclusion

Effective air preheater sealing with high performance brush seals can reduce fuel consumption, parasitic fan power losses, and variances in downstream air pollution control equipment due to increased control of flue gas velocities, temperatures and pressures. As air preheaters can deliver some of the highest returns in thermal efficiency to coal-fired power plants, operators can realize improved heat rates with a very small investment.

## Reference

<footnote n="1">¹ "Brushing Up On Brush Seals," *Power Engineering*, May 2010.</footnote>