Understanding Fired Pressure Vessels

“Fired pressure vessels” refer to a category of pressure vessels that are exposed directly to the combustion process or to hot flue gases. In other words, these are vessels where the heating medium is the direct product of combustion, rather than being indirectly heated using a secondary medium.

Some common examples and applications of fired pressure vessels include:

1. Boilers: One of the most common examples of a fired pressure vessel is a boiler. Boilers are used to produce steam or hot water for various industrial processes, space heating, or power generation. The fuel is burned, and the resulting hot gases are passed over tubes containing water. The water absorbs the heat, turns to steam, and then is typically used to drive turbines or for other purposes.
2. Direct Fired Heaters: These are often used in the oil and gas industry to heat crude oil, other hydrocarbons, or process fluids. The combustion gases come into direct contact with tubes that contain the fluid to be heated.
3. Thermal Oxidizers: These are used to burn off hazardous gases or volatile organic compounds (VOCs) as an environmental control measure. The harmful compounds are introduced into a combustion chamber where they are burned, converting them into less harmful products like carbon dioxide and water vapor.
4. Reformers: Used in the petrochemical industry, reformers change the molecular structure of hydrocarbons through a combination of heat and catalysts.

Fired pressure vessels are subject to rigorous design and operational standards due to the combination of high temperatures and pressures involved. This also means they are susceptible to unique challenges and failure modes not necessarily seen in other types of pressure vessels. For example:

• Creep: Due to sustained high temperatures, the vessel material may deform slowly over time.
• Thermal Fatigue: Repeated cycles of heating and cooling can cause stress and eventually lead to cracks.
• Corrosion from Combustion Products: Flue gases can be corrosive, especially if they contain compounds like sulfur.

Because of these risks, regular inspection, maintenance, and adherence to operational guidelines are especially important for fired pressure vessels. Design and manufacturing codes, such as the ASME Boiler and Pressure Vessel Code (BPVC), provide standards and guidelines to ensure the safe operation of these vessels.

Material Selection:

Given the challenging environment within fired pressure vessels—characterized by high temperatures, cyclic thermal stresses, and potentially corrosive combustion byproducts—the choice of material is paramount. Steels suited for high-temperature service, sometimes with special coatings or treatments to resist specific corrosive elements, are commonly employed.

Safety Devices:

Given the risks associated with fired pressure vessels, they are typically equipped with a variety of safety devices:

• Pressure Relief Valves (PRVs): These automatically release pressure if it exceeds a preset limit, preventing over-pressurization of the vessel.
• Safety or Blowdown Valves: Used especially in boilers to release water and accumulated sediments, thus maintaining efficient operation and preventing potential blockages or overheating.
• Flame Arrestors: These prevent the flame from traveling back into the system, which could cause explosions.
• Burner Management Systems (BMS): A control system that ensures the safe start-up, operation, and shutdown of burners, crucial for maintaining safe operating conditions and responding to any anomalies.

Operational Challenges:

• Slagging and Fouling: In boilers, the combustion of certain fuels can result in the accumulation of slag or fouling on the heat exchange surfaces. This not only reduces efficiency but can also lead to localized overheating.
• Erosion: The flow of combustion gases at high velocities can lead to the erosion of vessel surfaces, especially in areas with abrupt changes in flow direction or geometry.

Inspections and Maintenance:

Regular inspection of fired pressure vessels is not just a good practice—it’s often mandated by regulatory bodies. These inspections check for:

• Tube Leaks: Especially in boilers, the tubes are prone to leakage, which can reduce efficiency and lead to localized overheating.
• Refractory Condition: Many fired pressure vessels are lined with refractory materials to insulate the vessel and protect it from the heat. Over time, this refractory can degrade, necessitating repairs.
• General Wear and Tear: This includes checking for signs of corrosion, erosion, thermal fatigue, or other forms of degradation.

Environmental Considerations:

Modern fired pressure vessels, especially in industrial applications, are often paired with emission control systems to minimize the release of pollutants. These can include:

• Scrubbers: To remove particulates and certain gaseous pollutants from the flue gas.
• Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR): Systems designed to reduce nitrogen oxide emissions.

In conclusion, while fired pressure vessels play a pivotal role in many industries, their operation and maintenance demand a deep understanding of the associated challenges and risks. With rigorous design, inspection, and operational standards, these vessels can be operated safely and efficiently, delivering value while minimizing potential hazards.