Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex 2507 in Flue Gas Desulfurization (FGD)
Super Duplex 2507 (UNS S32750 / EN 1.4410) is widely specified across wet flue gas desulfurization (WFGD) systems on coal-fired boilers, refinery and petrochemical sour gas, and waste-to-energy (WTE) plants. Wet absorber towers, quenchers, outlet ducts, mist eliminators, and slurry recirculation pumps see chloride well above 30,000 ppm, sulphate above 50,000 ppm, low pH (typically 5 to 6 in absorber, 1 to 2 in quencher transients), and process temperatures from 50 to 70 degrees Celsius. PREN above 41, CPT above 50 degrees Celsius per ASTM G48 Method E, and resistance to chloride stress corrosion cracking let 2507 displace Hastelloy C-276 / C-22 in the moderate end of the FGD envelope, with significant capital savings.
TorqBolt supplies 2507 plate, pipe, fittings, flanges, fasteners, and forgings to FGD EPCs and operators with EN 10204 Type 3.1 mill test certificates as standard, Type 3.2 third-party-witnessed available where the project specification calls for it.
Why Super Duplex 2507 in FGD
FGD material selection drivers stack three corrosive species:
- Chloride pitting and crevice resistance. Wet scrubber recirculation slurry concentrates chloride from coal and limestone feedstock to 30,000+ ppm. 2507 PREN above 41 stays clear of the pitting threshold; 316L (PREN around 25) and 317L (PREN around 30) pit within months.
- Resistance to mixed sulphate / chloride / low-pH chemistry. WFGD scrubber pH typically sits at 5 to 6 in the absorber, dropping to 1 to 2 in quencher transients during start-up or low-load operation. 2507 handles both regimes.
- Resistance to chloride stress corrosion cracking in hot scrubber slurry. The duplex microstructure resists CSCC much better than austenitic 304L / 316L / 317L.
- Mechanical strength margin at 550 MPa minimum yield. Absorber tower walls and ducting can be reduced in thickness vs austenitic CRA, with significant weight and cost savings.
- Cost vs Hastelloy C-276 / C-22. 2507 is 50 to 70 percent cheaper than nickel-base CRA. For moderate FGD service inside the 2507 envelope, the choice is straightforward economics.
Super Duplex 2507 FGD Design Data
| Parameter | Value | Reference |
|---|---|---|
| PREN | 41 to 43 | Cr + 3.3 times Mo + 16 times N |
| CPT (ASTM G48 Method E) | Greater than 50 degrees Celsius | ASTM G48-11 |
| WFGD absorber pH | 5 to 6 (normal); 1 to 2 (quencher transient) | Typical scrubber operating data |
| WFGD chloride concentration | 30,000 ppm and above | Recirculation slurry analysis |
| WFGD sulphate concentration | 50,000 ppm and above | Recirculation slurry analysis |
| WFGD process temperature | 50 to 70 degrees Celsius | Typical absorber operating range |
| Maximum continuous service temperature | 250 degrees Celsius (sigma-phase risk above 300) | NORSOK M-630 |
| Hardness limit (sour, where applicable) | 28 HRC max | NACE MR0175 / ISO 15156-3 |
| Ferrite content | 35 to 55 percent (base), 35 to 65 percent (weld) | NORSOK M-630 |
Where Super Duplex 2507 Is Specified in FGD
- Wet absorber tower vessel. Solid 2507 plate construction (ASTM A240) for smaller absorbers, roll-bond clad 2507 / carbon-steel for larger towers.
- Quencher and inlet gas duct at the wet-dry interface. Aggressive transient pH (down to 1 to 2 during low-load or start-up) drives the choice up to 2507 from austenitic stainless.
- Outlet gas duct and mist eliminator support after the absorber. Wet, chloride-laden post-absorber gas is a classic 2507 application.
- Slurry recirculation pumps and piping. Centrifugal pump impellers and casings in 2507 forgings or castings (analogous chemistry per ASTM A890 grade 5A).
- Slurry distribution headers and spray nozzles inside the absorber tower.
- Limestone feed and gypsum dewatering equipment: silos, slurry tanks, dewatering hydrocyclone bodies.
- Wastewater treatment equipment downstream of the absorber, where chloride concentrates further.
- Bolting on FGD flange connections, manways, and structural fasteners. ASTM A1082 / API 20F in matching S32750.
- Stack inlet duct and stack lining on smaller stacks. Larger stacks typically use Hastelloy C-276 / C-22 or Alloy 59 wallpaper lining.
Super Duplex 2507 vs Competing Alloys in FGD
| Alloy | PREN | FGD Verdict | Cost Index |
|---|---|---|---|
| 316L (austenitic) | ~25 | Pits in absorber within 6 to 18 months | 1.0 |
| 317LMN (austenitic) | ~33 | Used historically; pits at the high-chloride end of modern absorber service | 1.4 to 1.6 |
| 904L (austenitic) | ~35 | Marginal; pits in modern WFGD chloride concentration | 1.6 to 1.8 |
| 2205 duplex (S31803 / S32205) | 34 to 38 | Acceptable for cooler, lower-chloride FGD; insufficient for modern absorber | 1.5 to 1.8 |
| Super Duplex 2507 (S32750) | 41 to 43 | Standard pick for moderate WFGD absorber, ducting, slurry piping | 2.0 to 2.5 |
| Zeron 100 (S32760) | 41 to 44 | Equivalent envelope to 2507; choice often by mill spec or supplier | 2.2 to 2.7 |
| 6Mo (254 SMO, AL6XN) | 43 to 45 | Used at the more aggressive WFGD service (higher temperature or chloride) | 2.5 to 3.0 |
| Hastelloy C-276 / C-22 / Alloy 59 | 50+ | Reserved for severe WFGD: stack liner, hot-gas duct, very high chloride | 4.0 to 5.0 |
Super Duplex 2507 Installation Notes for FGD
- Welding per AWS D1.6 with filler ER2594 (overmatching, preferred for FGD) or ER2553 (matching). Heat input 0.5 to 2.5 kJ per millimetre, interpass below 150 degrees Celsius. Argon plus 2 to 5 percent nitrogen shielding for GTAW root.
- Pickling and passivation per ASTM A380 after fabrication: 20 percent nitric plus 5 percent HF, then citric or nitric passivation. Iron contamination from carbon-steel tools is the leading cause of pitting initiation in FGD service.
- Cladding for large absorber vessels: 2507 roll-bond clad to carbon-steel backer. Bond-strength qualification per ASTM A263 analogue, shear test on every plate, ultrasonic disbond inspection.
- Wallpaper lining alternative for retrofits: 2507 sheet welded to existing carbon-steel absorber. Strip seal welds with full-penetration filler per AWS D1.6.
- Crevice management at gasketed flanges and lining seams. Spiral-wound CG gaskets with PTFE or graphite fill. Continuous seal welds preferred over intermittent.
- Hydrotest with low-chloride water (less than 30 ppm chloride per ASTM A380), drain and dry within 24 hours.
- Bolting per ASTM A1082 / API 20F in matching S32750. PTFE thread lubricant on assembly.
- Inspection: ferrite count per ASTM E562, ASTM A923 Methods A and B for sigma-phase screening on critical heats. PMI per ASTM E1476 on every piece by handheld XRF or OES.
Recommended TorqBolt 2507 Products for FGD
- 2507 pipe: ASTM A790 seamless and welded for slurry recirculation
- 2507 fittings: ASTM A815 WP-S32750 buttweld
- 2507 flanges: ASTM A182 F53 weld neck and blind
- 2507 stud bolts: ASTM A1082 / API 20F for absorber manways and flange connections
- 2507 heavy hex nuts: ASTM A1082 in matching S32750
- 2507 plate, bar, forgings: ASTM A240 / A479 / A182 F53 for absorber towers, ducting, pump bodies
Related Application Pages
- Chemical processing: dilute HCl, H2SO4, HF service
- Pulp and paper: bleach plant ClO2 service
- Seawater applications: sea-chest, fire-water
- Desalination: MSF, MED, RO brine
- Oil and gas: wellheads, manifolds
Super Duplex 2507 FGD FAQ
Why does 316L fail in WFGD absorbers?
316L PREN sits around 25, well below the threshold for chloride pitting at modern WFGD chloride concentration (30,000+ ppm) and absorber temperature (50 to 70 degrees Celsius). Pinhole leaks and absorber-wall perforation typically appear within 6 to 18 months of commissioning. Low-pH excursions in the quencher region (down to pH 1 to 2 during start-up) accelerate the pitting initiation.
Where does 2507 displace Hastelloy C-276 in FGD?
2507 covers the moderate end of the WFGD envelope: absorber pH 5 to 6, chloride 30,000 to 50,000 ppm, temperature 50 to 70 degrees Celsius. Hastelloy C-276 is reserved for the more severe duty: stack liner where condensate concentrates further, hot-gas duct upstream of the quencher, and absorbers running on high-chloride coal where chloride climbs above 60,000 ppm. The choice is set by the iso-corrosion chart at the actual operating point.
Can 2507 handle quencher transients down to pH 1 to 2?
Yes, on a transient basis. Start-up and low-load operation can swing quencher pH down to 1 to 2 for short periods before steady-state buffering takes over. 2507 PREN above 41 holds up across this range, but continuous operation at pH below 2 with high chloride is outside the envelope and should drive the choice toward 6Mo or Hastelloy C-276 / C-22.
Is 2507 acceptable for slurry recirculation pumps?
Yes. 2507 forged or cast pump impellers and casings (analogous chemistry per ASTM A890 grade 5A for castings) handle WFGD slurry recirculation. Erosion-corrosion at the impeller tip is the controlling failure mode; 2507 holds up where 316L or 317L would fail by combined chloride pitting and erosion. Hard-faced inlays (Stellite 6 or equivalent) are optional for the highest-velocity wear zones.
Can 2507 be used as wallpaper lining for absorber retrofits?
Yes. 2507 sheet (typically 2 to 3 mm thickness) is strip-welded to the inside of an existing carbon-steel absorber as a corrosion barrier. Continuous seal welds (full penetration, ER2594 filler) are preferred over intermittent stitching to eliminate crevice geometry. The retrofit is significantly cheaper and faster than replacing the absorber shell.
What is the typical service life of 2507 in WFGD absorbers?
25 to 30 years where the operating envelope sits inside the 2507 corrosion limits and the fabrication followed pickling and passivation per ASTM A380. Pitting failures in 2507 absorbers are almost always traced to iron contamination during fabrication (carbon-steel tools, grinding swarf) or to operating excursions outside the design envelope (chloride spike, prolonged low pH).
What welding filler is correct for 2507 in FGD?
ER2594 (overmatching, slightly higher PREN than the base) per AWS A5.9 is the standard pick for FGD wet absorber service. ER2553 (matching) is acceptable for less aggressive applications such as outlet ducting downstream of the mist eliminator. Heat input 0.5 to 2.5 kJ per millimetre, interpass below 150 degrees Celsius. Argon plus 2 to 5 percent nitrogen shielding for GTAW root pass.
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