What is critical to operators of thermal plants? Of primary importance is the delivery of process steam and electric power within the established environmental limits for the site. As long as the operator can meet those goals safely, subtle losses of efficiency are often ignored or not recognized as significant. As a result, tubular air heater performance is not recognized as critical until high level operational goals are affected or at risk.
Tubular air heater damage and leakage can affect many systems and cost centers. These include auxiliary loads, fuel consumption, emission control, consumables and, eventually, steam production. This article describes several physical damage mechanisms that can affect tubular air heater performance.
Heater Failure Causes: Erosion
Tubular air heater erosion primarily occurs in three locations:
- Tube internal surface.
- Tube external surface.
In many cases, fuels burned may have high ash content. Typically, 90 percent of the ash in the fuel is carried in the flue gas. Depending on the configuration of the tubular air heater, this fly ash may be on the inside or outside of the tube, and it can erode the tube material (figure 1). This causes the wall to become thinner, increasing the probability of leakage from the higher pressure air to the flue gas.
The air side is not immune to erosion either. Air may pick up dirt and even airborne fuel particles, depending on the location of the forced-draft fan inlet. Damage to the tubesheets provides another area for the air and flue gas to mix. Mixing of the hot and cold streams through leakage affects the plant performance in the several ways: reduced exit gas temperature, reduced temperature of combustion air, and increased load on the forced- and induced-draft fans.
Heater Failure Causes: Corrosion
Like erosion, corrosion attacks the tube surface and eventually leads to leakage. Corrosion develops because of chemical reactions of the flue-gas constituents. These reactions can generate acids in the flue gas. They can include:
- Sulfuric acid (H2SO4).
- Sulfurous acid (H2SO3).
- Hydrochloric acid (HCl).
- Nitric acid (HNO3).
These acids have a limited effect as long as the surface temperature remains above the acid dewpoint (ADP).
Unfortunately, leakage or gas distribution lowers operating temperature and creates cold areas where acids can condense. This initiates a reaction with the tube materials, causing corrosion similar to what is seen in figure 2.
Additionally, in many cases, the tubular air heater was designed prior to emissions limitations and installation of emission controls such as selective catalytic reduction (SCR) systems. The SCR may add chemicals into the flue gas not accounted for in the original design, and this can increase the acid production in the flue-gas stream. These changes all increase the probability of tube corrosion and, ultimately, leakage.
In some cases, the construction of the tubes themselves can add to the possibility of the corrosion. Seam-welded tubes (SWT) may be attacked by the acids preferentially along the weld. This can lead to a split along the seam and another leakage source.
Heater Failure Causes: Plugging
Like erosion, plugging can be caused by fly ash in the flue gas. When the fly ash mixes with moisture in the flue gas and condenses on tube surfaces, it becomes sticky. Once it becomes sticky, it can fill either the internal diameter of the tube or the spacing between the tubes, depending on the tubular air heater configuration. Two extreme cases of plugging are shown in figure 3. In both cases, this hinders the heat transfer by:
- Reducing the surface area.
- Altering the flow distribution.
- Increasing the pressure drop on the tubular air heater.
Another source of plugging comes in the form of a temporary repair to a tubular air heater. Often, erosion and corrosion can cause such extensive damage to tubes that they are rendered useless. Short of replacing the damaged section, the only way to try and minimize further damage is to physically plug the tube entrance and exit at the tubesheets. This provides a cost-effective repair and works well if the damage is in a limited area. However, as the damage spreads and the number of plugs increases, it becomes financially feasible to have a longer outage and repair the damaged tube sections.
Heater Has Operational Limitations
Similar to other plant equipment, the tubular air heater has a design operational lifespan. Normal operation results in changes that will eventually create defects. Each defect — if not monitored and controlled — has the potential to accelerate other issues. Common operation limitations are caused by limited tubesheet expansion, increased vibration and thermal cycling.
The design of the tubular air heater allows for the tube to grow in length with temperature increases. Often, this is accomplished by having one tubesheet fixed and allowing the second to float. Any limitation to the tubesheet motion can increase internal stresses that are seen along the length of the tube. This can cause bulging, bending, denting or splitting of the tubes.
Eventually, each of these damages will result in tube failures and — in different ways — limit the performance of the tubular air heater. For instance, bulging tubes reduce the separation between the tubes and increase the velocity and pressure drop on the outside of the tubes. Bending or denting of tubes changes the flow profile inside the tube and the heat transfer characteristics. These physical limitations lead to stress points in the metal that weaken the tube walls. This eventually leads to splits and holes that allow leakage to occur (figure 4).
Increased vibration in a tubular air heater can be caused by changes in the air- and flue-gas flows that generate turbulence. Operation of the forced- or induced-draft fans also can cause increased vibration. The resulting stresses in the tubes may not have been accounted for in the original design.
Because of fuel costs and load requirements, plants that were once designed to operate consistently are now forced to load cycle much more often. The load cycling forces the units to go through the low load turndown, resulting in acid dewpoint corrosion conditions almost daily. The cycling also makes it more difficult for the operator to observe subtle performance changes.
In addition, the use of emissions equipment encourages use of lower quality fuels — with higher sulfur and ash content — to reduce operating cost. The resulting impacts on the operating conditions are significant when air heater design parameters no longer cover the new operating profile.
Ultimately, any outside force that weakens the air heater tubes leads to leakage and performance degradations. Adding to the complexity, none of these damage mechanisms work alone. Erosion and corrosion can work together to thin the tube walls — in some cases, from both the inner and outer surfaces. The operating profile of the plant and the tubular air heater can accelerate the wall thinning that leads to leakage. Increased ash and sulfur content in the fuel accelerate the erosion and corrosion. Swings in the load can lead to operation below the acid dewpoint on nearly daily basis.
A tubular air heater that is not monitored and maintained can be a silent thief that slips in the back door of almost all types of thermal plants. With no moving parts, power or water consumption, and no unusual sound alerting of problems, it has the capability to empty your bank account or shut you down.
Poor air heater performance can affect almost every process in a plant whether the facility is a paper mill, biofuel or coal plant, or even a refuse processor. Most air heaters are designed for a specific, narrow operating range for both physical and chemical properties. The tubular air heater inlet and outlet temperature of combustion gas and air are important. Yet, it is the chemical composition of the gas entering the air heater that is the conduit for delivery of destructive mechanisms. Anything that alters gas-flow distribution affects all of the fault mechanisms. Often overlooked is the fact that air delivery also can bring in other destructive mechanisms.
The first step to minimizing the tubular air heater performance impacts is to schedule regular inspections. These inspections can be performed to suit operational schedule requirements. The inspections are focused entirely on the condition of the tubular air heater and include specific technical documentation of damage. They may require or include some minor emergency repairs.
As a part of the inspection process, it also is recommended to include a thermoeconomic analysis. This requires collection of operational data around the boiler and tubular air heater as well as selected auxiliaries. With this data, it is possible to evaluate the condition of tubular air heater and its impact on boiler efficiency. It also allows you to quantify the financial impacts. Tracking this information allows for the design and implementation of solutions that reduce damage mechanisms and financial impacts.