Ball Valves for Pulp & Paper

White liquor is the active cooking chemical in the kraft pulping process - a solution of sodium hydroxide (NaOH) and sodium sulphide (Na2S) at pH 13-14, operating temperature 80-170°C (from storage tank temperature to digester liquor fill temperature). Ball valve specification for white liquor digester isolation service requires: (1) Body material: ASTM A351 CF8M (cast SS 316) or A182 F316 forged is the industry standard for white liquor service. CF8M provides adequate corrosion resistance to the combined alkaline + sodium sulphide environment. Duplex 2205 (A890 4A) is specified where lime mud abrasion is severe (white liquor from causticiser plant with suspended CaCO3). Rubber-lined ductile iron is used for cooler white liquor storage outlet lines (below 80°C); (2) Ball and stem: SS 316L or ASTM A276 Type 316 ground and polished stainless steel ball; 17-4PH stainless stem for strength; (3) Seats: Modified PTFE (25% glass-filled or carbon-filled PTFE) for white liquor temperatures to 160°C - standard PTFE creeps at digester temperatures; for primary digester fill liquor valves at 160-170°C, specify metal seats (SS 316 Stellite 6 overlay) to prevent seat relaxation; (4) End connections: API 6D flanged (ASME B16.5 Class 150 or 300 RF) for process isolation valves; SW (socket weld) or BW (butt weld) for trim points and instrument connections; (5) Testing: API 6D hydrostatic shell (1.5x rated) and seat test (API 598 Class VI or better); factory witnessed; (6) In Indian mills: IBR certification for white liquor lines that cross-connect to steam-heated jacketed pipes or vessels above 3.5 bar is required for the circuit as a whole - confirm with mill engineering whether the specific white liquor ball valve falls under IBR scope.

Chlorine dioxide (ClO2) in the bleach plant is one of the most aggressive chemical environments for ball valves in the entire pulp and paper industry. ClO2 is a powerful oxidising acid that attacks most metals and all common elastomers. The ball valve specification for ClO2 service depends on the ClO2 concentration: For high-concentration ClO2 (D-stage bleaching filtrate, concentrations above 3 g/L at pH 2-4): Body, ball, and stem: Titanium Grade 2 (ASTM B367 Grade C-2) - titanium forms a self-healing, impermeable TiO2 oxide layer in the presence of ClO2 and oxidising acids that makes it essentially immune to ClO2 corrosion. This is the dominant material choice in Scandinavian and North American bleach plants; Seats and seals: PTFE (100% pure, unfilled grade) - PTFE is the only elastomer/seat material with acceptable resistance to concentrated ClO2; all rubber compounds (EPDM, NBR, neoprene) and even FKM (Viton) are degraded by concentrated ClO2 within days; For medium-concentration ClO2 (1-3 g/L, bleaching stage dilution water): SS 317L (higher molybdenum than SS 316L, UNS S31703) or Alloy 20 (N08020, CN7M cast) body with PTFE seats is acceptable and significantly less expensive than titanium. Confirm with materials corrosion data for the specific ClO2 concentration and temperature at the valve installation point; For ClO2-related oxidising rinse water (below 1 g/L ClO2 equivalent): SS 316L with PTFE seats may be acceptable - confirm with plant corrosion engineer. Assembly requirement: All ball valves for ClO2 service must be assembled clean and degreased - grease or oil contamination will react violently with concentrated ClO2 (risk of fire or explosion). Specify clean assembly and individual valve bagging per ASTM G93 or equivalent for all oxidising service ball valves.