Check Valves for Pulp & Paper

Black liquor is one of the most challenging valve service fluids in the pulp and paper industry. The key properties of black liquor that drive valve material selection are: (1) Strongly alkaline pH of 12-14 (nearly as caustic as strong NaOH solution) from dissolved sodium hydroxide and sodium carbonate; (2) High temperature - weak black liquor is at 60-80°C; concentrated black liquor entering the recovery boiler is at 100-130°C; (3) High viscosity of concentrated black liquor (10-200 cP) that creates high pressure drop across partially open valves; (4) Contains dissolved organic compounds (lignin, hemi-cellulose degradation products) and sodium sulphide - both are corrosive to carbon steel and will cause stress corrosion cracking in unsuitable alloys; (5) Moderate-to-high scaling tendency from sodium sulphate and sodium carbonate crystallisation when cooled. For black liquor check valves: Recommended: Ductile iron (ASTM A536) body with EPDM lining - EPDM rubber has excellent resistance to NaOH and sulphide at temperatures to 120°C; the rubber lining protects the ductile iron body and provides sealing on the disc face. Design: full-bore swing check, flanged (ASME B16.1 or EN 1092-2), with replaceable rubber lining disc and seat insert. Alternative for temperatures above 120°C: Stainless steel SS 316L (A351 CF3M cast body) - SS 316L has adequate resistance to black liquor at elevated temperatures, though rubber-lined ductile iron is preferred for economy in standard service. Not suitable: Carbon steel, cast iron (without rubber lining) - carbon steel corrodes rapidly in strongly alkaline black liquor at elevated temperature; unlined cast iron rusts and introduces iron contamination into the black liquor circuit, which affects paper quality.

No - rubber-lined check valves are NOT suitable for chlorine dioxide (ClO2) service in the bleach plant. Chlorine dioxide is a powerful oxidising agent that rapidly degrades all common elastomers: EPDM, natural rubber (NR), nitrile (NBR), and neoprene are all attacked by ClO2 within hours to days of exposure at typical bleach plant concentrations (3-15 g/L ClO2). The oxidation causes the rubber to swell, soften, lose adhesion from the substrate, and ultimately disintegrate - causing both valve failure and contamination of the process. For chlorine dioxide (ClO2) service check valves, the material selection depends on concentration: High concentration ClO2 (above 5 g/L): Titanium Grade 2 (ASTM B367 cast Ti Gr.2) is the industry-standard material - titanium forms a protective oxide layer in ClO2 that is essentially immune to further oxidation. PVDF (polyvinylidene fluoride) lined or solid PVDF check valves are an alternative for smaller sizes. Medium concentration ClO2 (1-5 g/L): Alloy 20 (UNS N08020) cast check valve bodies with PTFE-lined disc and seat. Dilute ClO2 (below 1 g/L): SS 316L may be acceptable, but confirm with operating mill data - some mills have experienced localised corrosion on SS 316L even at low ClO2 levels at elevated temperature. PTFE is the correct seat and disc material for all ClO2 concentrations - PTFE has excellent chemical resistance to oxidising acids including ClO2, persulphates, and chlorine at normal bleach plant temperatures (40-70°C).