What grades are used for ethylene cracker pipe fittings?

Ethylene crackers use a range of grades depending on the circuit: 321 (EN 1.4541, Ti-stabilised, max 700°C) for radiant coil transfer line fittings and quench headers exposed to cracked gas; P22 (EN 1.7380, 2.25Cr-1Mo) for steam cracker furnace charge pre-heaters and high-temperature transfer lines where hydrogen is present; P11 (EN 1.7335, 1.25Cr-0.5Mo) for lower-temperature steam and hydrocarbon circuits; 316L (EN 1.4404) for dilution steam and quench water circuits; WPB carbon steel for utilities and lower-temperature process streams. The specific grade selection depends on operating temperature, H₂ partial pressure, and the Nelson curve applicable to each circuit.

Petrochemical Pipe Fittings | P11 P22 P91 316L 321 Ethylene Cracker Reformer

Q: Which pipe fitting materials are needed for steam methane reforming (SMR)?

Steam methane reforming (SMR) for hydrogen production requires a range of materials by service zone: P22 or P91 (EN 1.7380 / 1.4903) for high-temperature reformer outlet fittings (800–900°C in the reformer; 400–600°C in pigtail transfer); P11 (EN 1.7335) for the pre-reformer and moderate-temperature H₂ circuits; 316L (EN 1.4404) for shift converter and gas purification (PSA) piping where H₂S or CO₂ may be present; 321 (EN 1.4541) for certain quench and heat recovery zones. Nelson curve compliance (API RP 941) is mandatory for all CrMo alloy steel in H₂ service within an SMR plant. PWHT is mandatory for P11, P22, and P91 fittings.

Material Selection by Petrochemical Process

Process / Service	Recommended Grade	EN W.Nr.	Key Reason	PWHT
Ethylene cracker radiant coil transfer	321 (or HP alloy)	1.4541	Ti-stabilised SS for cracked gas to 700°C without sensitisation	Not required
Ethylene cracker charge pre-heater	P22	1.7380	2.25Cr-1Mo for H₂ + HC at elevated temperature — Nelson curve	Mandatory 710–750°C
Ethylene cracker quench header	316L	1.4404	Quench water + dilution steam — mild corrosive, lower temperature	Not required
Catalytic reformer interheater / transfer	P22	1.7380	High H₂ partial pressure + high temperature — exceeds P11 Nelson curve	Mandatory 710–750°C
Catalytic reformer feed / product separator	P11	1.7335	Moderate H₂ — within P11 Nelson curve at operating conditions	Mandatory 680–720°C
SMR reformer outlet / pigtail transfer	P91 or P22	1.4903 / 1.7380	800–900°C reformer zone; 400–600°C pigtail — creep critical	Mandatory
SMR shift converter	316L	1.4404	CO + CO₂ + H₂O — moderate temperature, mild corrosion	Not required
Aromatics plant (BTX separation)	316L or WPB	1.4404 / —	Mild corrosion — 316L for chloride-bearing solvents; WPB for inert	Not required
PDH / dehydrogenation	P91 or 321	1.4903 / 1.4541	High-temperature dehydrogenation — creep-resistant or Ti-SS	Mandatory if P91
Polypropylene / polyethylene utility	WPB or 316L	— / 1.4404	Utility steam, catalyst handling — low severity	Not required

⚠ Nelson Curve (API RP 941) — Mandatory for H₂ Service

All CrMo alloy steel pipe fittings in hydrogen service must be selected against the current API RP 941 Nelson curves to prevent high-temperature hydrogen attack (HTHA). Each grade has a maximum allowable combination of temperature and H₂ partial pressure:

WPB (C steel)

Lowest Nelson curve — limited H₂ service

P11 (1.25Cr-0.5Mo)

Intermediate curve — moderate H₂ / T

P22 (2.25Cr-1Mo)

Higher curve — reformer and hydroprocessing

P91 (9Cr-1Mo-V)

Highest CrMo curve — highest H₂ severity

Always refer to the current edition of API RP 941 for applicable Nelson curves. Material condition (normalised, normalised + tempered) affects which curve applies. We can provide CMTR documentation confirming heat treatment condition to support your API RP 941 assessment.

Key Grades for Petrochemical

P11 / 1.7335

1.25Cr-0.5Mo — 13CrMo4-5

PWHT 680–720°C

Feed preheat, moderate-H₂ circuits, boiler feed at elevated temperature. PWHT mandatory. P-No. 4. Filler ER80S-B2.

P22 / 1.7380

2.25Cr-1Mo — 10CrMo9-10

PWHT 710–750°C

Catalytic reformer interheater, ethylene cracker charge heater, high-H₂ service above P11 Nelson curve. P-No. 5A. Filler ER90S-B3.

P91 / 1.4903

9Cr-1Mo-V — X10CrMoVNb9-1

PWHT 745–775°C

SMR reformer outlet, PDH dehydrogenation, supercritical boiler — highest creep resistance. Vanadium + Nb microalloying.

316L / 1.4404

X2CrNiMo17-12-2

Quench water, dilution steam, CO-shift, aromatics solvent circuits. No PWHT. Versatile Mo-bearing SS for moderate corrosion.

321 / 1.4541

X6CrNiTi18-10 — Ti-stabilised

Cracker radiant coil transfer line fittings, quench headers to 700°C. Ti-stabilisation prevents sensitisation without PWHT.

C-276 / 2.4819

NiMo16Cr15W

HCl-bearing service — catalyst regeneration vent, chloride-contaminated process streams. Used where 316L and 321 fail.

FAQ — Petrochemical Pipe Fittings

Ethylene crackers use multiple grades by circuit: 321 (EN 1.4541, Ti-stabilised, max 700°C) for radiant coil transfer line fittings and quench headers in cracked gas service; P22 (EN 1.7380, 2.25Cr-1Mo) for steam cracker furnace charge pre-heaters and high-temperature H₂ circuits (Nelson curve compliance required); P11 (EN 1.7335) for lower-temperature steam and hydrocarbon circuits within P11 Nelson curve limits; 316L (EN 1.4404) for dilution steam and quench water circuits; WPB carbon steel for utilities. PWHT (680–720°C for P11; 710–750°C for P22) is mandatory for all CrMo alloy steel fittings after welding.

Steam methane reforming (SMR) requires a range of materials by zone: P22 or P91 (EN 1.7380 / 1.4903) for reformer outlet and pigtail fittings (800–900°C reformer zone; 400–600°C in pigtail transfer); P11 (EN 1.7335) for the pre-reformer and moderate-temperature H₂ circuits; 316L (EN 1.4404) for shift converter and PSA gas purification piping where H₂S or CO₂ may be present; 321 (EN 1.4541) for certain quench and heat recovery zones. Nelson curve compliance (API RP 941) is mandatory for all CrMo alloy steel in H₂ service. PWHT is mandatory for P11, P22, and P91 fittings after welding.