Pipe Fittings for Amine Gas Treating: Amine SCC, Grade Selection, and PWHT Requirements
Amine gas treating — using MEA (monoethanolamine), DEA (diethanolamine), MDEA (methyldiethanolamine), or other amine solvents to remove H₂S and CO₂ from gas streams — involves two distinct material challenges that must be managed simultaneously. The first is amine stress corrosion cracking (amine SCC), a form of alkaline SCC that causes cracking of carbon steel in hot lean amine at residual welding stress levels. The second is wet H₂S damage (SSC, HIC, SOHIC) from the sour gas that the amine is absorbing. Both mechanisms can be controlled — but with different measures, and the amine SCC mitigation must be specified explicitly on every purchase order.
Amine Stress Corrosion Cracking
Amine SCC is an alkaline stress corrosion cracking mechanism that occurs in carbon steel in contact with lean amine (amine solution that has been regenerated and is returning to the absorber — low H₂S content, high amine concentration, elevated temperature). The cracking is intergranular and driven by residual tensile stress from welding. It occurs most frequently in the lean amine piping, reboiler circuits, and hot lean amine/rich amine exchangers — typically at temperatures above 60°C. The prevention method is straightforward: post-weld heat treatment (PWHT) of all carbon steel welds in amine service per NACE SP0472 eliminates the residual stress that drives SCC. PWHT is mandatory — not optional — for carbon steel in amine service, regardless of wall thickness or pressure class.
PWHT Specification for Amine Service
NACE SP0472 specifies PWHT at 620°C minimum for carbon steel in amine service — this is a stress relief treatment, not a normalise-and-temper cycle. The requirement applies to all field welds, branch connections, and shop-fabricated spools. For pipe fitting supply, the relevant requirement is: fittings themselves do not require PWHT before delivery (they are not welded) — but the PO should note "Amine service — all field welds to be PWHT per NACE SP0472" so the manufacturer's documentation references amine service and includes the correct alloy and hardness data. Carbon steel hardness must be ≤200 HBW throughout — the same limit as HF service — because the rich amine in the absorber contains absorbed H₂S, triggering NACE MR0175 requirements on the wet sour side.
Material Selection by Circuit
| Circuit | Condition | Fitting Grade | PWHT Required? |
|---|---|---|---|
| Absorber — rich amine outlet | Sour, ambient–50°C | WPB (hardness ≤200 HBW) | Yes — amine service |
| Lean amine return above 60°C | Hot lean amine, sweet | WPB (hardness ≤200 HBW) | Yes — amine SCC risk highest here |
| Regenerator overhead | H₂S/CO₂/H₂O vapour | 316L (dewpoint condensate corrosion) | No PWHT for SS — SSRT if spec requires |
| Reboiler / regenerator shell | Hot lean amine, boiling | WPB or 316L depending on amine type | Yes for CS — MDEA less aggressive than MEA |