Check Valves for Chemical Processing

Sulphuric acid is one of the most complex corrosion environments because the optimal material depends strongly on acid concentration: (1) Dilute H₂SO₄ (below 80% w/w) at ambient temperature: Hastelloy C-276 provides excellent resistance — it resists both dilute H₂SO₄ and the reducing conditions that exist at lower concentrations. (2) Concentrated H₂SO₄ (80–98%+): Alloy 20 (CN7M, UNS N08020) is the industry standard — concentrated H₂SO₄ is actually less corrosive to carbon steel and many alloys due to the formation of a passive iron sulphate film, but Alloy 20 provides reliable service across the full concentration range. (3) Oleum (fuming H₂SO₄, >100% by SO₃ dissolved): Hastelloy B-3 or cast high-silicon iron are required. (4) For dilute H₂SO₄ at elevated temperature (above 60°C): Hastelloy C-276 or PTFE-lined valves — SS 316 is not resistant to dilute hot H₂SO₄. Never use standard SS 316 in direct H₂SO₄ service above trace concentrations — the austenitic stainless will suffer rapid corrosion in dilute acid and stress corrosion cracking in concentrated acid.

Standard carbon steel (WCB) check valves are commonly used for cooling water in chemical plants, but with important caveats: (1) Closed-loop cooling water: carbon steel WCB is generally acceptable if the cooling water chemistry is properly controlled (pH 6.5–9.5, chloride below 200 ppm, inhibitor dosing for corrosion and scale). (2) Open-loop (once-through) seawater or river water: carbon steel corrodes rapidly in seawater (chloride concentration ~19,000 ppm) — duplex stainless (2205) or bronze check valves are required. (3) For chlorinated cooling tower water: SS 316 (not SS 304) is minimum standard — chlorides above 200 ppm cause pitting of SS 304; SS 316 is more resistant but also not ideal for high chloride. (4) For brine cooling in chlor-alkali or salt processing: duplex 2205 or Hastelloy C-276 are required for chloride concentrations above 1,000 ppm at elevated temperature. In cooling water systems that draw from rivers, lakes, or the sea and have variable water chemistry, specify SS 316 or duplex stainless check valves as the standard for corrosion reliability.

Yes — a foot valve is a type of check valve installed at the submerged inlet (foot) of a pump suction pipe in a sump, tank, or river intake. The foot valve allows flow from the water source into the suction pipe when the pump is running, and closes under gravity or spring pressure when the pump stops — this keeps the suction pipe primed (filled with liquid) so the pump does not lose prime when it restarts. Without a foot valve, a centrifugal pump would lose its suction prime every time it stopped (if the suction pipe discharges above the water level), requiring manual re-priming before each restart. Foot valves are essentially swing check valves or flap check valves with a strainer basket on the inlet side to prevent debris from entering the suction pipe. For chemical process sump pits with corrosive liquids, foot valves in SS 316 or PVDF are used. In wastewater and sewage sumps, foot valves in WCB with rubber-lined discs are standard.

For chemical dosing pump discharge check valves, the selection depends primarily on pump type and flow characteristics: (1) Diaphragm metering pumps: produce pulsating flow (reciprocating) at low flow rates and high differential pressure. Spring-loaded ball check valves (small DN6–DN25) with PTFE balls and PVDF or PTFE bodies are the standard — they open on each pump stroke and close rapidly on the return stroke, providing high leak-tightness between strokes. (2) Peristaltic pumps: similarly pulsating — spring-loaded ball or disc check valves in PTFE or Hastelloy. (3) Centrifugal pumps on chemical service (larger DN25–DN200): dual plate wafer check valves (spring-loaded) provide fast closure to prevent water hammer on pump stop, with good corrosion resistance when specified in SS 316 or Hastelloy C-276. Swing check valves are generally not the best choice for dosing pump discharge because: (a) the pulsating flow of dosing pumps causes swing check discs to chatter and wear; (b) swing check valves have higher cracking pressure and may not open fully on each dosing pulse. Always confirm the cracking pressure (minimum differential pressure to open) of the spring-loaded check valve is lower than the pump's minimum discharge pressure.