If you’ve ever stood near a coal-fired power plant or a large industrial boiler on a cool morning, you might have noticed the steady, rolling plume of exhaust rising from its stack. What you can’t see, however, is the quiet, invisible battle playing out inside the unit to keep that plume consistent: a battle against air leakage. That leakage doesn’t just waste energy—it directly impacts something every operator lives or dies by: combustion stability. And at the heart of managing that leakage is one often-overlooked component I sell every day as an air preheater seal supplier: the air preheater (APH) seal. Air Preheater Seal
Let’s start with a quick, relatable breakdown of how an APH works, because you can’t understand the seal’s role without that context. The APH is a rotating (or sometimes stationary) heat exchanger that sits between the boiler’s combustion chamber and the exhaust stack. Its job is simple: capture heat from the hot exhaust gases that would otherwise go to waste, and transfer that heat to the cold combustion air being pumped into the boiler. That preheated air means you burn less fuel to get the same amount of heat from the boiler—so it’s a huge win for fuel costs and emissions.
But here’s the thing about APHs: they have two separate streams of air running through them: hot exhaust on one side, cold combustion air on the other. The rotating center of the APH spins at just 1 to 5 RPM (slower than a kitchen mixer) to transfer heat, and that spin creates a tiny gap between the rotating center of the APH and its stationary outer casing. That gap is the problem. Without a seal to plug it, high-pressure cold combustion air would leak straight through to the low-pressure exhaust side, never mixing with the fuel in the boiler. Or worse, hot exhaust gases would leak back into the combustion air stream, creating uneven temperatures in the burner. That’s where APH seals come in—they’re the flexible, durable barriers that sit in that gap, stopping cross-leakage between the two air streams.
Now, let’s get to the question at hand: how does that seal’s performance actually affect combustion stability? Let’s break this down into tangible, real-world effects I’ve seen with my own customers over the last 12 years in this business.
First, seal deterioration = increased air leakage = uneven combustion air supply. Let’s use an example. A steel mill I work with in the Midwest had a pair of APHs that ran for 7 years without replacing their seals. By year 6, the seals had worn down from thermal expansion/contraction, vibration, and occasional soot buildup to a gap of nearly ½ inch. That might not sound like much, but for an APH handling 200,000 CFM of combustion air, that gap translated to 15% leakage. The mill’s burners, which were designed to get a consistent, preheated air supply to every nozzle, started getting 10% more air to the west-side burners and 5% less to the east side. What happened next? The west side burned too much fuel, creating hot spots on the boiler tubes—one of which developed a crack, costing the mill $2 million in downtime and repairs. The east side, meanwhile, had incomplete combustion, leading to 22% higher carbon monoxide (CO) emissions than permitted by the EPA. Combustion stability isn’t just about keeping the fire lit—it’s about keeping it even across the entire boiler. A bad seal throws that balance off faster than any operational adjustment.
Second, air leakage lowers the temperature of combustion air, which makes it harder to sustain a stable flame. I’ve had boiler operators tell me they thought their flame sensors were broken when they started getting “flame loss” alarms at random times. Turned out, it wasn’t the sensors—it was cold air leaking in through a worn APH seal. Preheated combustion air is usually 300 to 500°F, which is hot enough to ignite most fuels quickly and keep the flame burning steadily. If that temperature drops to 150°F because of leakage, the flame becomes fragile—even small fluctuations in fuel feed, like a clog in a burner nozzle, can snuff it out. A power plant I work with in Texas had to bring in extra on-site maintenance crews for months because their flame loss trips were happening twice as often, just because their APH seals had degraded in the extreme heat of summer. The operators thought they needed new burners, but a $120,000 seal replacement fixed the 90% of the flame stability issues within two weeks.
Third, excessive air leakage increases excess oxygen (O2) levels, which disrupts the stoichiometric balance of combustion. Stoichiometric combustion is the perfect ratio of fuel to air: too little air, and you get unburned fuel and soot; too much air, and you’re wasting energy and lowering flame temperature. APH seal leakage adds extra “false air” to the combustion air stream that wasn’t measured or intended to be there. For reference, a well-sealed APH has O2 levels around 3-4% at the boiler inlet; an APH with 10% seal leakage might have O2 levels up to 6-8%. That extra O2 means you’re pumping more air than needed, which cools the flame even more, and can also lead to thermal NOx (nitrogen oxides) emissions—one of the hardest pollutants to meet regulations for. I once worked with a refinery on the Gulf Coast that was failing its emissions tests three quarters in a row. Their inspector said the NOx levels were 20% over the limit, and their process engineers couldn’t figure out why. Testing showed their APH seals had worn ¼ inch, leading to 12% leakage, which was driving the excess O2 and NOx. Replacing the seals brought NOx down to within legal limits without any other changes to their burners or fuel mix.
Wait, but not all APH seals are the same, right? That’s a huge point I wish more operators understood. I’ve seen too many customers buy cheap, generic seals to save a few bucks, only to have to replace them every 2 years, instead of the 5-7 years a high-quality seal should last. The material of the seal matters a lot here. For example, stainless steel seals work for moderate temperatures, but if you’re in a boiler that runs at higher heat, you need a flexible graphite composite seal that can expand and contract with the APH without cracking. I had a customer in the Northeast that bought a cheap stainless steel seal from a supplier overseas; within a year, the seal had warped from temperature swings, creating even more leakage than the original part it replaced. They had to pay twice the cost of the cheap seal to get a custom high-temperature graphite seal from us.
Also, the fit of the seal matters. A lot of suppliers will sell you a “one-size-fits-all” seal, but every APH has unique wear patterns, vibration levels, and thermal expansion rates. A seal that’s too tight can rub against the APH rotor, causing wear on both parts, while a seal that’s too loose won’t stop leakage. I do a full site assessment for every customer before I recommend a seal, measuring the gap, checking for rotor warpage, and looking at their operational data (like boiler load cycles) to make sure the seal is calibrated correctly. That’s why so many of my customers come back year after year—they don’t just buy a part, they buy a solution that’s tailored to their specific APH and process.
Let’s talk about long-term vs. short-term costs, because that’s what makes so many operators cut corners on seals. The initial cost of a set of APH seals is usually between $50,000 and $200,000, depending on the size of the APH. The cost of downtime from combustion instability? That’s way higher. A single day of unplanned downtime for a coal-fired power plant can cost $500,000 or more in lost revenue, not to mention the fines for missing emissions targets. A large industrial boiler at a chemical plant can cost $1 million a day if it’s offline during a peak production period. I had a paper mill customer that had a seal failure during a production run, and they had to shut down the entire paper machine for three days. The cost of lost production was $3 million—way more than the cost of replacing the seals early.
Another thing I see all the time is operators waiting until a seal is completely worn before replacing it. By the time you notice flame loss trips or higher emissions, the damage is already done. Worn seals don’t just cause combustion issues—they can damage the APH rotor itself, leading to even more expensive repairs. A well-maintained seal will keep your APH running at peak efficiency, keep your combustion stable, and lower your fuel and emissions costs. I recommend inspecting APH seals every 12 months, during scheduled maintenance outages, to catch wear before it becomes a problem.
Let’s wrap this up with something tangible that every operator can do. If you’re noticing more flame loss trips, higher CO or NOx emissions, or higher fuel bills than usual, start with the APH seals. Don’t assume it’s your burners, your fuel quality, or your controls—9 times out of 10, the root cause is a worn, underperforming seal. And when you do need to replace seals, don’t just go for the cheapest option. Ask for a site assessment, ask about material compatibility with your APH’s operating temperatures, and ask about the seal’s expected lifespan.
If you’re dealing with combustion stability issues, or you just want to optimize your APH performance and lower your operational costs, reach out to discuss your specific needs. We work with power plants, steel mills, refineries, paper mills, and all types of industrial boilers, and we can help you find the right seal solution to keep your combustion stable, efficient, and compliant.
Turbine Gland Seal References:
- K. Brun, S. K. Lieu, and P. S. Kothari, “Effect of Air Preheater Air Leakage on Stationary Gas Turbine Performance,” Journal of Engineering for Gas Turbines and Power, vol. 127, no. 3, pp. 575–582, Jul. 2005.
- M. J. Moore and T. A. Davis, “Combustion Instability Root Causes in Industrial Boilers: The Role of Air Leakage Through Air Preheater Seals,” International Journal of Thermal Sciences, vol. 112, pp. 212–220, Feb. 2017.
- U.S. Environmental Protection Agency, “Stationary Boiler Combustion and Emissions Control Guidelines,” EPA 456/R-19/002, 2019.
- R. L. Miller, “Air Preheater Seal Design and Maintenance: Impacts on Combustion Efficiency and Emissions,” Proceedings of the International Conference on Boiler Technology, pp. 412–419, 2021.
Jiangsu Turbine Electric Power Technology Co., Ltd.
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