Boiler tubes operate over a wide range of temperatures, pressures and thermal loads. Different sections of a boiler serve different needs of heat-to-water transfer and steam production at the desired operating temperature and pressure. Superheaters are required in order to bring saturated steam to a temperature higher than its saturation temperature at a given pressure. Boiler tubes for superheaters must be selected according to the specific operating conditions and requirements of the application.
The choice of suitable boiler tubes for superheaters must consider the operating temperatures, pressure, oxidation, and applicable construction standards. The same grade of material could have different needs depending on location in a boiler and resulting temperatures at the metal surface.
What Do Superheater Tubes Do?
Superheater tubes contain steam flowing through a heat-transfer section, where it gains more energy from the hot combustion gases to increase temperature. This happens without the need for steam to go through an additional phase change from liquid to vapour.
Superheating has applications in steam-power generation and some industrial processes where the produced steam will have a certain design pressure and temperature.
Why Is Superheating Important?
Depending on the steam-cycle or process needs, superheated steam could allow for the desired power-generation conditions in a turbine or the necessary process temperature in an industrial steam application.
The design of superheaters and materials used for tubes depend on the required outlet temperature and pressure.
How Do Superheater Tubes Experience Different Conditions?
Superheater tubes are exposed to hot combustion gases on the outside and steam flow on the inside. The temperature of the tube metal will depend on a combination of gas temperature and steam, the heat-transfer coefficient, the tube geometry and load.
The metal temperature could be higher than the steam temperature, and selection of a suitable material grade for tubes must reflect the intended use and not just the steam outlet temperature.
Factors in Selection of High-Temperature Boiler Tubes
High temperature boiler tubes must have the strength and properties needed for the intended service. The selection process must consider the following aspects:
Operating Temperature and Pressure
Temperature has an effect on a material’s strength characteristics. As metal temperature increases, the resulting stress allowed in a material might decrease. Time-dependent deformation mechanisms could also be an issue under high temperature conditions.
Pressure imposes demands on the tube wall thickness. Requirements for tube size and material grade must be determined based on the applicable design rules, operating conditions, and material corrosion resistance.
Oxidation and Corrosion
Tube surfaces are exposed to the effects of combustion gases containing compounds that contribute to oxidation or corrosion mechanisms. The degree of exposure will depend on the fuel type, combustion process, deposits and deposits, gas chemistry, and temperature.
On the steam side of a tube, water chemistry and operating conditions could also affect corrosion on the inside wall surfaces. Care must be taken during the selection process in order to account for the intended service.
Thermal Cycling and Local Stresses
Boiler operation involves numerous start-ups and shutdowns as well as changes in load. Such conditions could subject tubes to thermal cycling and mechanical stresses caused by differential thermal expansion and sustained loads.
Such factors as tube geometry, location, and support, operational practices, and material properties will affect the susceptibility to stress and need to be considered during the selection process.
When Are Alloy Steel Boiler Tubes Used?
Alloy steel boiler tubes are used when a simple carbon steel grade will not have the properties needed under the operating conditions.
Alloying elements have various effects on steel material, including: impact strength, hardenability, oxidation resistance or high temperature strength depending on the content. The addition of elements could have different effects depending on the material grade. Not all grades of alloy steel are suitable for all high temperature applications, and the name “alloy steel” does not necessarily apply to all grades.
How Should the Alloy Grade Be Selected?
Alloy steel grade selection should be in accordance with the applicable boiler code and material specification. Consideration should be given to the expected tube metal temperature during operation, design pressure, service lifetime, exposure to corrosion environments, fabrication and weldability, and applicable mechanical properties.
The selected grade should also be compatible with the applicable welding, heat-treatment and inspection requirements. The grade should be described by the relevant material specification and documentation should be provided as part of purchase requirements, including material test certificates and traceability records.
Why Is Grade Verification Important?
Two materials with the same name could have differences in composition limits, mechanical properties, and permissible operating conditions. Using one grade in place of another without approval could create non-compliance or service issues.
The material specification should include grade description (not just a reference to heat-resistant steel) and the applicable standard.
How Are Steam Generation Tubes Selected?
Steam generation tubes are used in different sections of a boiler and each has different needs for operating conditions. Evaporator tubes transfer heat to water and generate steam. Superheater tubes increase steam temperature further. Other sections, such as economiser tubes, have other needs, such as utilising heat from flue gases to preheat feedwater in a suitable boiler design.
Since evaporators, superheaters, economisers and other boiler sections have differing needs for pressure, temperature, fluid conditions, and exposure, their requirements for tubes could be different, even within the same boiler.
What Dimensions Should Be Checked?
Outside diameter, wall thickness, length, tolerance, and material composition requirements should be checked in order to ensure that all specified dimensions are in accordance with the applicable pressure and thermal needs. Wall thickness should not be selected based on a comparison to another boiler but by using the applicable design method and operating conditions.
What Standards and Documentation Are Required?
The applicable boiler construction code, product material standard and project specification should be identified prior to purchase. There are a range of requirements that will apply to tubes depending on the project and location, including chemical and mechanical properties, heat treatment, testing, identification and inspection documentation. Exact requirements will vary depending on the project and jurisdiction. The buyer should ensure that the tube manufacturer is aware of the intended application and documents that will apply to the supplied tubes.
How Do Inspection and Quality Control Support Tube Reliability?
Inspection ensures tubes supplied meet the specification requirements. Depending on the product and applicable specification, the inspections could involve dimensional checks, visual examination, material verification, mechanical examination, hydrostatic testing, eddy current and ultrasonic examination.
Each method will not necessarily be used for all tubes. The inspection plan should identify methods, scope, acceptance criteria, and records.
Traceability is important for boiler components as it provides evidence that the supplied material has met the specification requirements. Deviations should be addressed through the quality control process prior to acceptance of the material.
Sainest Tubes supplies boiler tubes for industrial applications. Its product and quality information can help buyers review the available tube range and understand the importance of matching material and dimensional requirements to their application.
Conclusion
Boiler tubes for superheaters and steam generation must be selected according to the operating needs and temperature conditions. Alloy steel could be a consideration under high temperature conditions but the grade must be clearly specified in accordance with the applicable design code and material standard. Specification, traceability, and suitable inspection support the selection process in order to ensure tubes will meet the needs of the boiler system.