+30–50 °C
Metal above steam temperature (convective banks)
Boiler pressure parts · Steam side
Boiler superheater: design, materials and remaining life
The superheater raises saturated steam to final temperature — typically 400–540 °C on biomass and sugar co-generation units — and runs the hottest pressure-part metal in the boiler: tube metal sits roughly 30–50 °C above steam temperature in convective banks. Material selection, attemperation control and creep-life management decide whether it lasts 25 years or fails in 5.
≤ 425 °C
Carbon steel (SA 210) metal limit
≤ 580 °C
SA 213 T22 metal limit
400–540 °C
Typical final steam, biomass co-gen
01 — Function and behaviour
What the superheater does, and why radiant and convective banks behave oppositely
The hottest metal in the boiler, controlled by where you hang it in the gas path.
The superheater takes saturated steam from the drum — 257 °C at 45 barg on the recurring design basis used across this site — and raises it to the final temperature the turbine is bought for, typically 400–540 °C on biomass and sugar co-generation units. It is the hottest pressure part in the boiler: unlike water-wall or economizer tubes, superheater tubes are cooled by steam, a far poorer coolant than boiling water, so tube metal runs roughly 30–50 °C above the steam inside it in convective banks, and more where gas-side heat flux is high. The superheater is specified together with the rest of the boiler pressure parts because its position fixes the gas temperature entering every surface downstream.
Placement determines the characteristic curve — how outlet steam temperature responds to boiler load. A radiant superheater, seeing furnace flame, receives nearly constant radiant flux while steam flow rises with load: its outlet temperature falls as load increases. A convective bank, buried in the gas stream, sees gas mass flow rise faster than steam flow: its outlet temperature rises with load. A well-proportioned superheater combines the two so the curves partially cancel, leaving spray attemperation to trim only the residual — which is why a boiler that "runs out of spray" at high load usually has a proportioning problem, not a spray-valve problem.
02 — Material selection
How is superheater tube material selected?
By metal temperature, not steam temperature. Add 30–50 °C to local steam temperature for convective banks, take the hottest tube in the hottest row (not the average), then pick the grade whose oxidation and creep limits cover it: SA 210 to 425 °C metal, SA 213 T11/T12 to ~540–550 °C, T22 to 580 °C, T91 above that.
| Material (tube / header) | Type | Metal temp. limit | Typical duty (capacity · pressure · final steam) |
|---|---|---|---|
| SA 210 Gr A1 / SA 106 Gr B | Carbon steel | ≤ 425 °C | 30–80 t/h · 25–45 barg · steam ≤ 400 °C (primary stages) |
| SA 213 T11 / SA 335 P11 | 1¼Cr–½Mo | ≤ ~540 °C | 60–170 t/h · 45–68 barg · steam 440–480 °C |
| SA 213 T12 / SA 335 P12 | 1Cr–½Mo | ≤ ~550 °C | 60–170 t/h · 45–68 barg · steam 440–490 °C |
| SA 213 T22 / SA 335 P22 | 2¼Cr–1Mo | ≤ 580 °C | 100–250 t/h · 65–110 barg · steam 490–540 °C |
| SA 213 T91 / SA 335 P91 | 9Cr–1Mo–V | ≤ ~620 °C | Final stages · ≥ 87 barg · steam ≥ 520 °C |
Design pressure, minimum wall and weld qualification follow ASME Section I with ASME IX procedures, or EN 12952-3 on European-code plants; JIS practice applies on Japanese-code boilers. Dissimilar-metal welds between grades (T22 to T91, carbon steel to T11) are qualified transitions, made in the shop wherever the coil geometry allows, because a shop butt weld with full NDT is worth more than any field weld made overhead in a penthouse.
03 — Temperature control
Spray attemperation: holding ±5 °C without quenching the header
The spray station protects the turbine — and can damage the superheater if misapplied.
Final steam temperature is controlled by spray attemperation: feedwater-quality water injected into the steam line between superheater stages, typically at 2–5 % of steam flow, holding final temperature within about ±5 °C of setpoint across the control range. The station sits between stages deliberately — the downstream bank evens out the temperature profile and evaporates any residual droplets before steam reaches the turbine. Two disciplines keep the attemperator from becoming a failure source: the injected water must be low-solids (silica and sodium in spray water plate out on downstream tube bores as insulating deposits that raise metal temperature), and the spray control must never drive the interstage temperature below saturation, or water slugs quench the downstream header and thermal-fatigue its bore. During commissioning, the characteristic curves are trimmed against measured tube-leg temperatures so the spray range covers the whole load line with margin at both ends.
04 — Failure modes
Why do superheater tubes and headers fail?
Three dominant modes: long-term overheating creep, where metal running 10–20 °C over design quietly spheroidises and ruptures after years; cracking at welds where the wrong material grade was installed; and gas-side erosion on abrasive biomass ash. All three are detectable years before failure — but only with the right inspection technique.
Long-term overheating creep. A tube whose metal temperature sits persistently above its design value — from internal deposits, gas lane channelling, or a misproportioned bank — accumulates creep damage continuously. The microstructure tells the story: pearlite spheroidises, carbides coarsen, creep voids nucleate at grain boundaries, and the tube finally fails as a thick-lipped longitudinal rupture with heavy external oxide. Because creep life halves for roughly every 10–20 °C of sustained overtemperature, an early-stage spheroidisation finding is worth an entire retube budget in warning time. In-situ metallographic replicas — a polished, etched spot on the live component, read like a lab micrograph — grade the spheroidisation stage without cutting a sample.
Wrong material in service. A finding from an Arrow remaining-life assessment on an operating unit: a superheater header specified as SA 335 P11 (1¼Cr–½Mo) had been supplied and installed in SA 106 Gr B carbon steel. At header metal temperature the carbon steel was already cracking — and the governing threat was cracking, not the gradual strength shortfall. Hardness testing alone had missed it: hardness numbers for the two materials overlap enough to pass a spot check. Metallographic replication identified the microstructure, and therefore the actual material, unambiguously. The lesson is procedural: on any boiler whose material certificates are incomplete — which includes most units more than 20 years old — positive material identification and replicas on hot headers are mandatory scope, and hardness surveys alone are not an acceptable substitute.
Erosion on biomass ash. Bagasse and rice-husk fly ash is high in abrasive silica (rice-husk ash 85–90 % amorphous silica). Protection follows the same rules as for the economizer: limit local gas velocity, seal bypass lanes, and fit replaceable tube shields on leading rows and soot-blower lanes so wear lands on hardware that unbolts.
05 — Remaining life and scope
Remaining-life assessment and Arrow's fabrication scope
Measure the damage state, then decide between reinspection interval, partial replacement and full retube.
Arrow's remaining-life assessment (RLA) for superheaters combines ultrasonic wall-thickness mapping, oxide-scale thickness measurement (a proxy for time-at-temperature), hardness surveys, positive material identification, and metallographic replicas on headers, stub welds and the hottest identified tube legs. Inspection is performed by ASNT Level II NDT personnel with welding inspection to CSWIP 3.1, and the deliverable is per-component: estimated remaining creep life, reinspection interval, and where replacement is due, the material grade that should have been there. The same assessment feeds retrofit engineering — a mill raising steam temperature for a new turbine needs the existing bank re-evaluated at the new metal temperatures, not just a hotter final stage bolted on.
Fabrication scope covers replacement coils bent to existing drawings, complete superheater banks with headers, and dissimilar-metal transition pieces, produced at the Samut Sakhon factory to ASME/JIS practice with procedures and welders qualified to ASME IX. ISO 9001:2015 certification (TÜV Rheinland) covers spare parts, installation and maintenance services; code-stamp scope is stated per project. Superheater work is coordinated with the adjacent water wall and pressure parts scope so header locations, sootblower lanes and support attachments are engineered as one system. Reference classes for this duty range include a 170 t/h bagasse-fired sugar-mill boiler in Thailand, a 250 t/h biomass power unit in Thailand and a 230 t/h sugar-mill unit in Colombia. Guaranteed performance figures are stated per project after the technical assessment, on a stated basis.
FAQ
Engineering questions, answered
What does a superheater do in a boiler?
It heats saturated steam from the drum — 257 °C at 45 barg, for example — to the final temperature the turbine needs, typically 400–540 °C on biomass and sugar co-generation units. Dry, superheated steam prevents blade erosion and carries more work per kilogram through the turbine.
Which materials are used for superheater tubes?
Selection follows metal temperature, which runs 30–50 °C above steam temperature in convective banks: SA 210 carbon steel to 425 °C metal, SA 213 T11/T12 to about 540–550 °C, T22 to 580 °C, and T91 for hotter final stages. Specifying by steam temperature alone under-specifies the material.
How is superheater steam temperature controlled?
By spray attemperation: feedwater-quality water is injected between superheater stages, typically holding final steam within ±5 °C of setpoint across the load range. The spray station sits between stages so downstream surface evens out temperature; injected water must be low-solids, or deposits form on downstream tube bores.
Why do superheater tubes fail by creep?
Tube metal operating near or above its design limit accumulates creep damage continuously; a sustained 10–20 °C metal-temperature excursion above design can cut remaining creep life by half or more. Long-term overheating shows as spheroidised microstructure and fine external oxide before rupture, which metallographic replicas detect in time to act.
What is a superheater remaining-life assessment?
A remaining-life assessment combines wall-thickness surveys, hardness testing and in-situ metallographic replicas on headers and hottest tube rows, evaluated against operating hours and temperature records. Arrow performs RLA with ASNT Level II NDT personnel and CSWIP 3.1 welding inspectors, reporting per-component reinspection or replacement intervals.
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