Gate Valves for Pulp & Paper

Black liquor (dissolved lignin + NaOH + Na₂S at pH 12–14, 80–110°C) is moderately corrosive. SS 316L (CF8M cast, 1.4404 equivalent) performs well in weak-to-medium black liquor (up to ~50% solids). For strong black liquor (50–75% solids) above 100°C, SS 316L may suffer from stress corrosion cracking (SCC) at elevated temperatures if chloride contamination is present — duplex 2205 (UNS S31803) offers better SCC resistance. NACE MR0103 compliance is specified by some mills for equipment handling sulphide-containing streams, analogous to sour service in refinery applications. Carbon steel is not recommended for black liquor contact — it corrodes at unacceptable rates.

Standard API 600 gate valves with SS 316L trim are not suitable for chlorine dioxide (ClO₂) bleach stage service. ClO₂ is a powerful oxidiser that aggressively attacks stainless steel (including SS 316L and 317L) through oxidative pitting and crevice corrosion. Bleach stage isolation requires valves with titanium (ASTM B265 Grade 2) or Hastelloy C-276 (N10276) trim, or FRP (fibre-reinforced plastic) lined valves. For hydrogen peroxide (H₂O₂) brightening stages, SS 316L is generally acceptable at concentrations below 35% — above 35% H₂O₂, titanium or PVDF-lined alternatives are preferred. Always confirm the bleach chemical concentration and temperature with the mill's process engineer before specifying trim material.

Recovery boiler steam systems in kraft pulp mills fall under ASME B31.1 (Power Piping Code) for the steam drum, steam headers, and feedwater piping. Gate valves on ASME B31.1 steam lines must be: ASME B16.34 rated for the operating pressure and temperature; manufactured by a certified valve manufacturer with documented quality management (ISO 9001 or equivalent); hydrostatically pressure tested per API 598; provided with ASME B31.1-required documentation (MTCs, test records, weld procedure records if applicable). Recovery boilers typically operate at 60–80 bar / 480–510°C — Class 600 (PN 110) gate valves with A217 WC9 body (1.25Cr-0.5Mo alloy steel, NACE-compliant) and pressure-seal bonnet design are standard.