Incoloy 800H Reformer Pigtails

Reformer pigtails carry the reformate gas from the outlet end of the primary steam-methane reformer (SMR) catalyst tubes to the outlet header. Each pigtail typically connects one HP-Modified or HK-40 centrifugally cast catalyst tube (running at 950 to 1000 deg C tube wall) to the manifolded outlet header (running at 850 to 900 deg C wall), with a hot-end pigtail temperature of 800 to 900 deg C and an outlet-end pigtail temperature of 750 to 850 deg C. The pigtail outer diameter is 50 to 100 mm with 4 to 10 mm wall thickness; the length runs 6 to 12 metres with 2 to 4 elbow bends to absorb the differential thermal expansion between the catalyst tube top end and the outlet header. Incoloy 800H and 800HT are the standard pigtail materials because the combination of creep-rupture strength, carburization resistance at the high-carbon-activity reformer atmosphere and thermal expansion compatibility with the catalyst tube + outlet header materials matches the SMR service envelope. Typical pigtail service life is 8 to 12 years between scheduled replacements.

Why Reformer Pigtail Service Specifies Incoloy 800H

The SMR pigtail is loaded with three simultaneous failure modes: (1) creep rupture under sustained 30 to 40 bar internal pressure at 800 to 900 deg C, (2) carburization from the reformate gas composition that runs CO + H2 + CH4 + H2O at carbon activity 0.2 to 0.8, and (3) thermal-shock cracking from start-up + shutdown + trip events. The 0.05 to 0.10 percent carbon window plus ASTM grain size 5 or coarser delivers the 100,000-hour creep-rupture stress of approximately 50 MPa at 705 deg C and 22 MPa at 815 deg C, which is what supports the 30 to 40 bar internal pressure in a 4 to 10 mm wall thickness. The 19 to 23 percent chromium content forms the Cr2O3 scale that holds the carbon ingress to approximately 0.25 mm at 1000 hours at 871 deg C. The moderate thermal expansion coefficient of 16.9 um/m/K at 650 deg C supports the bimetallic welds to the HP-Modified catalyst tube (alpha approximately 16.5 um/m/K) and the wrought 800H outlet header (same alpha).

Operating Envelope

ParameterHot-end pigtail (catalyst-tube weld)Cold-end pigtail (outlet-header weld)
Tube wall temperature800-900 deg C (1470-1650 deg F)750-850 deg C (1380-1560 deg F)
Internal pressure30-40 bar (3.0-4.0 MPa)30-40 bar (3.0-4.0 MPa)
Process gas compositionH2 60-75% / CO 8-15% / CO2 8-15% / CH4 2-8% / H2O balanceSame composition
Carbon activity (aC)0.2-0.80.2-0.6
Decoking required?Rarely (no coke deposition under H2-rich atmosphere)Rarely
Pigtail OD / wall50-100 mm OD x 4-10 mm wall50-100 mm OD x 4-10 mm wall
Typical service life8-12 years8-12 years

Pigtail Failure Modes (Documented)

  • Creep cavity coalescence at the hot-end weld: the heat-affected zone of the bimetallic weld between the cast catalyst tube and the wrought 800H pigtail is the typical creep cavity nucleation site. Cavities coalesce into a circumferential crack after approximately 80,000 to 120,000 hours of service at 880 deg C.
  • Carburization-induced embrittlement at the hot-end: the 0.2 to 0.8 carbon activity drives gradual carbon ingress into the hot-end pigtail. After approximately 60,000 hours the room-temperature elongation in the surface zone drops below 5 percent and thermal-shock cracking initiates.
  • Bend-radius cracking at the cold-end: the tight-radius elbows in the cold-end pigtail concentrate the differential thermal expansion stress. Cracks initiate at the elbow extrados after approximately 100 start-stop cycles.
  • Outlet-header attachment weld cracking: the bimetallic weld between the cold-end pigtail and the outlet header concentrates the residual stress from differential cool-down. Cracking initiates at the weld toe after approximately 50 to 100 start-stop cycles.

Documented Field References

  • 2024, Middle East hydrogen plant: 96 reformer pigtails of Incoloy 800H 65 mm OD x 7 mm wall to ASTM B407, EN 10204 type 3.2 with LR witness, full bank replacement during T+I.
  • 2023, North Africa ammonia plant: 240 reformer pigtails of Incoloy 800HT 75 mm OD x 8 mm wall to ASTM B407, partial-bank replacement on a 12-year-old SMR after carbon-ingress assessment.
  • 2025, South Asia methanol complex: 320 reformer pigtails of Incoloy 800H 50 mm OD x 5 mm wall to ASTM B407 with welded ASME B16.9 elbows, full bank replacement during scheduled turnaround.
  • 2024, Western Europe refinery hydrogen unit: 64 reformer pigtails of Incoloy 800HT 100 mm OD x 10 mm wall to ASME SB-407, EN 10204 type 3.2 with TUV witness.

Sizing + Design Notes

  • Pigtail OD: matched to the catalyst-tube outlet ID (typically 60 to 100 mm for HP-Modified catalyst tubes).
  • Pigtail wall thickness: sized to the ASME Section VIII Division 1 creep-rupture allowable stress at the hot-end design temperature plus corrosion allowance.
  • Corrosion allowance: 1.5 to 2.5 mm for carbon ingress + oxidation over the 10-year design life.
  • Bend radius: 3 to 5 x OD minimum to limit ovality + wall thinning at the extrados during cold bending. Hot bending is the preferred fabrication method above 50 mm OD.
  • Catalyst-tube-to-pigtail weld: typically GTAW with ENiCrFe-2 or ERNiCrFe-7 (Inconel 82 / 182) filler to bridge the cast-to-wrought metallurgy. PWHT not required.
  • Pigtail-to-outlet-header weld: typically GTAW with ERNiCrFe-7 filler for the wrought-to-wrought 800H connection.

Standards That Govern

  • ASTM B407: wrought Incoloy 800H seamless pipe + tube specification (primary pigtail material spec).
  • ASME Section VIII Division 1: pressure-vessel design basis for the outlet header.
  • ASME B31.3 Process Piping: pigtail design + weld procedure + NDT.
  • ASTM B564: forged Incoloy 800H components for the outlet-header tubesheet and reinforcement pads.
  • ASME B16.9 Buttweld Fittings: elbows + tees for the pigtail routing geometry.
  • ASME Section IX: welding procedure qualification + welder performance qualification.
  • API 530: design procedure for fired heater tubes (transferable to SMR catalyst tubes + pigtails).

Inspection + Documentation Expected

  • Mill test certificate to EN 10204 type 3.2 with Lloyds Register, DNV, BV, SGS or TUV witness.
  • Hydrostatic test report to ASTM B407 paragraph 11 at 1.5 x design pressure on every pigtail length.
  • 100 percent radiography on all pigtail butt welds and elbow welds per ASME B31.3 category M.
  • PT on socket-weld fillets and on dressed weld surfaces.
  • ASTM E112 grain size measurement on every heat showing grain size 5 or coarser.
  • Dimensional report covering OD, wall thickness, length and bend angle on every pigtail.
  • Photographic record of every completed pigtail with weld map referenced to the radiography records.

Companion Components

TorqBolt supplies the matched fastener range for this service from Incoloy 800H bar (ASTM B408) and forged blanks (ASTM B564), heat-treated to retain ASTM grain size 5 or coarser for ASME Section VIII design stress qualification.

Stud Bolts

Primary flange bolting form, M12-M64, both-end-threaded with matched heavy hex nuts.

Heavy Hex Bolts

ASME B18.2.1 heavy-pattern bolts, 1/2" to 2", petrochem flange service.

Heavy Hex Nuts

ASME B18.2.2 matched-grade nuts paired with heavy hex bolts and stud bolts.

Washers

Flat DIN 125 + spring DIN 127 + locking DIN 6798 in matched Incoloy 800H chemistry.

U-Bolts

Pipe-support clamps for high-temperature piping in petrochem and refinery service.

Threaded Rod

Continuous threaded rod M12-M48, cut to length for hanger and tie-rod assemblies.

Frequently Asked Questions

What is the typical operating temperature of a reformer pigtail?

800 to 900 deg C tube wall at the hot end (catalyst-tube weld), dropping to 750 to 850 deg C at the cold end (outlet-header weld). Process gas temperature is typically 50 to 100 deg C above the tube wall.

Why is decoking not required on reformer pigtails?

The SMR process atmosphere is hydrogen-rich (60 to 75 percent H2) with steam content high enough to suppress coke formation thermodynamically. Pigtails do not see the coke deposition that drives the 30 to 90 day decoking cycle on ethylene cracker pigtails.

What is the difference between SMR pigtails and ethylene-cracker pigtails?

SMR pigtails operate at higher pressure (30 to 40 bar vs 1.5 to 4 bar for ethylene cracker), lower carbon activity (0.2 to 0.8 vs over 1.0 for cracker), and no decoking cycle. The wall thickness on SMR pigtails is set by internal pressure (creep-rupture controlled); on cracker pigtails it is set by external thermal-shock cycle endurance.

Should I specify 800H or 800HT for reformer pigtails?

For hot-end temperatures above 870 deg C, specify 800HT for the additional creep-rupture strength and the continuous Al2O3 sublayer. For pigtails operating below 870 deg C hot-end, 800H is the cost-effective specification.

What is the typical replacement interval?

8 to 12 years of plant service. Replacement is triggered by UT wall thickness reduction below the design allowable, by visible carburization on metallographic sampling during T+I, or by crack initiation at the bimetallic welds.

Request a Quote on Incoloy 800H + 800HT reformer pigtails to ASTM B407 with EN 10204 type 3.2 certification

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