What are boiler tubes?
With all boilers, heat is transferred from the inside of tubes to the outside (firetube boilers), or, from the outside of tubes to the inside (watertube boilers). The number of tubes within a boiler ranges from hundreds to thousands within a single boiler, depending upon the size and design of the boiler. The primary purpose of boiler tubes is to transfer the heat produced by the combustion of fuel into water or steam.
Fire Tube and Water Tube Boiler Tubes
Good to know – ‘watertube’ is also spelled ‘water-tube’, or ‘water tube’, but the different variations all mean the same thing.
Good to know – ‘firetube’ is also spelled ‘fire-tube’, or ‘fire tube’, but the different variations all mean the same thing.
Note - to aid understanding and familiarisation, this article uses a mixture of spellings for both firetube and watertube boilers.
Boiler Tube Types
A boiler may have multiple tube types to meet various heating requirements. The type of tube used depends upon the boiler design and the position of the tube within the boiler.
- Economizer tubes – for preheating feedwater.
- Superheater tubes – for superheating steam beyond its saturation point.
- Evaporator tubes – for conversion of water to steam (where evaporation takes place).
- Stay tubes – for heat transfer and to add structural strength to the boiler.
- Fin tubes – for increasing the heat transfer surface area.
Good to know – the term ‘superheat’ is the process of transferring sensible heat to saturated steam in order to increase its temperature and thus the amount of energy the steam contains.
Good to know – stay tubes are specially designed to not only transfer heat but also to strengthen the internal structure of the boiler. Stay tubes are thicker than regular tubes, providing additional structural support to the boiler.
Watertube Boiler Tube Designations
Boiler Tube Design
Boiler tubes are engineered to meet rigorous safety and efficiency standards, necessitating specific design and material requirements. Unlike ordinary pipes, boiler tubes often have thicker walls and lack longitudinal seams, enhancing their strength and integrity under high pressure and high temperature conditions. Boiler tubes are usually manufactured without seams to avoid weak points that could give rise to failures under stress. Boiler tubes are manufactured in strict compliance with specific international standards. For example, ASTM A192 provides standard specifications for high-pressure carbon steel tubes.
Water Tube and Fire Tube Boiler Designs
Boiler Tube Arrangement
The tube arrangement within a boiler is chosen based upon various factors, including heat transfer rate, pressure drop, manufacturing complexity, and desired efficiency.
Water-Tube Boiler Tube Layout
Listed below are some common boiler tube arrangements:
- Staggered Tube Arrangement – this configuration places the tubes in a staggered pattern, enhancing heat transfer rate due to increased turbulence in the fluid flow.
- Inline or Grid Arrangement – tubes are arranged in a regular grid pattern. While it offers ease of manufacturing and cleaning, it may result in lower heat transfer efficiency compared to staggered arrangements.
- Rotated Square Arrangement – tubes are placed in a square pattern but rotated at an angle. This arrangement can improve heat transfer by disrupting the fluid flow more effectively than a standard grid layout.
- Triangular Pitch Arrangement – tubes are arranged in a triangular pattern, which maximizes the number of tubes in a given area, leading to higher heat transfer rates.
Tube Pitch Examples
Boiler Tubes and Pipes Compared
Boiler tubes differ significantly from standard pipes in several ways:
- Wall Thickness – boiler tubes typically have thicker walls to withstand the large internal pressures to which they will be subjected (power station boilers typically operate at pressures over 160 bar (2,400 psi) and temperatures over 540⁰C (1,000⁰F).
- Measurement – unlike pipes, which are measured by their internal diameter, boiler tubes are measured by their external diameter.
Pipe With a Longitudinal Weld
Causes of Boiler Tube Failure
Boiler tube failures are among the primary causes of boiler shutdowns, with several underlying reasons:
- Overheating – the most common cause of tube failure. It can occur due to scale buildup inside the tubes, acting as an insulating layer, or, due to inadequate water or steam flow (inadequate cooling), or, from operational errors like over-firing, where excessive fuel raises the boiler's temperature beyond safe limits.
Scale Within A Boiler Tube
Good to know – scale within watertube boiler tubes causes the process fluid (water or steam) to flow at an increased velocity, whilst also reducing the heat transfer rate of the tube. A reduction in heat transfer rate causes a reduction in efficiency and increased likelihood of localised overheating (water- tube and fire-tube boiler tubes are cooled by the process fluid).
- Corrosion – can degrade tube metal both internally and externally, triggered by factors such as poor water treatment or environmental exposure to chemicals and gases.
- Erosion – wears away the tube material from continuous exposure to moving fluids or particulate matter, especially in high-ash coal-fired boilers.
- Mechanical Stress – caused by pressure, thermal expansion, and vibration, which can lead to fatigue and eventual failure of boiler tubes.
- Material Defects – manufacturing flaws or material inconsistencies can lead to premature boiler tube failure under normal operational stresses.
Tube Repair and Maintenance
The process of repairing boiler tubes typically involves welding (a technique used to fuse metal together through intense heat). Given the complexity and high skill required, welding is usually performed by certified welders.
Various Weld Types
Boiler Tube Repair Types
The four most common ways of repairing boiler tubes are:
- Cladding – this involves applying a layer of weld material to thicken and strengthen weakened areas.
- Rupture Closing – in cases of tube ruptures, the damaged area is often welded shut after realigning the edges.
- Window Repair – a severely damaged section of the tube may be cut out and replaced with a new piece, ensuring continuity and integrity.
- Plugging – performed in cases where damage is impossible to repair by other means. With this method, plugs made of suitable material are inserted into both ends of the damaged tube, thus hydraulically isolating it.
IMPORTANT: It is not recommended to operate a boiler with more than 10% of its tubes plugged.
Boiler Tubes Preventive Maintenance
Adhering to the boiler manufacturer’s recommended preventive maintenance plan is important to keep the boiler and its internal parts in optimum condition. Preventive maintenance plans usually include the following:
- Water and steam chemistry control (very important!)
- External visual inspections at scheduled intervals e.g. every 4 hours or daily etc.
- Internal visual inspections at scheduled intervals e.g. every 6 months.
- Boiler tube inspections and updating of boiler tube maps (these list the condition of each tube within the boiler).
- Fuel sampling.
- Exhaust flue gas sampling.
- Thermography inspections.
- Refractory inspections.
Please note that these are only some of the tasks required to keep an industrial boiler operating correctly!