In areas with acid and alkali corrosion, instrument air supply lines (compressed air pipelines) are prone to damage due to acid mist, splashes, or volatile gases, leading to gas leaks, contamination, and affecting the normal operation of instruments. Protective measures must be taken in terms of material selection, pipeline protection, installation layout, and maintenance. These measures are outlined as follows:
1. Material Selection for Air Supply Lines (Core Protection)
Use Corrosion-Resistant Materials:
Stainless Steel Pipes: Choose 316L stainless steel (which contains molybdenum for better acid and alkali resistance than 304 stainless steel), suitable for medium to high concentrations of acids and alkalis (e.g., sulfuric acid, nitric acid, sodium hydroxide).
Plastic Pipes: For strong corrosive environments (e.g., hydrofluoric acid, high-concentration hydrochloric acid), choose PTFE (polytetrafluoroethylene) or PP (polypropylene) pipes, which offer excellent chemical stability and resistance to most acids and alkalis.
Avoid Carbon Steel/Galvanized Pipes: These materials are easily corroded by acids and alkalis, leading to rust contamination and potential leaks.
Valve and Fitting Compatibility:
Ensure that valves (e.g., gate valves, ball valves) and fittings (e.g., tees, elbows) match the pipe material (e.g., 316L stainless steel valves, PTFE fittings) to prevent galvanic corrosion.
Use corrosion-resistant sealing materials (e.g., fluororubber O-rings) to prevent seal failure due to acid or alkali exposure.
2. Pipeline Surface Protection
Coating Protection (for Stainless Steel Pipes):
For 316L stainless steel pipes, apply corrosion-resistant coatings such as epoxy glass flake coatings or polyurea coatings to form a protective layer and prevent direct contact between acid mist and the pipe surface.
Ensure the coating is uniformly applied on the pipe’s outer surface and welded joints (which are more prone to corrosion due to oxidation). The coating thickness should be ≥0.3mm, and it should be inspected for integrity every 1-2 years, with damaged areas promptly repaired.
Protective Sleeving:
For pipes exposed to acid or alkali splashes, install protective sleeves (e.g., rigid PVC or fiberglass) to prevent direct contact with splashes. The sleeve should be sealed at both ends with small vent holes to balance pressure.
Leave a 5-10mm gap between the pipe and the protective sleeve for regular inspection of potential leaks or corrosion.
3. Optimizing Installation Layout
Distance from Corrosion Sources:
Install the air supply lines away from acid/alkali storage tanks, reactors, and leakage points. The pipeline should be placed above or to the side of potential spill points, with a minimum distance of ≥1.5m.
If the pipeline must cross corrosive areas, minimize the pipe length and prioritize overhead installation (above 1.8m) to avoid immersion in liquid.
Slope and Drainage:
Install the pipeline with a slope of 1:100 to 1:50 and set up a drain valve at the low point (made of the same material as the pipe). Regularly discharge condensed water from the air supply to prevent corrosive liquids from forming inside the pipe.
Avoid Dead Zones and Liquid Accumulation:
Use welded or flanged connections (with corrosion-resistant gaskets) instead of threaded connections to minimize crevices where corrosive substances can accumulate. Use elbows with a large radius (R≥3D) to avoid local corrosion caused by turbulence.
4. Air Supply Purification and Terminal Protection
Enhanced Air Supply Purification:
Install multi-stage filters at the air supply inlet:
Pre-filters (for water and oil removal): Prevent moisture and acid/alkali gases from reacting to form corrosive liquids.
Precision filters (with ≤5μm filtration accuracy): Prevent particulate contaminants from entering the instruments (e.g., positioners, pneumatic valves), avoiding jamming.
Protection for Instrument Connections:
Use corrosion-resistant materials (e.g., PTFE hoses, stainless steel braided hoses) for connecting the air supply to instruments (e.g., pneumatic control valves, transmitters). The hose length should not exceed 1.5m to prevent prolonged contact with corrosive media.
Install protective covers (e.g., fluoroplastic sleeves) on hose connections and secure them with brackets to prevent splashes from corroding the joints.
5. Routine Maintenance Measures
Regular Inspections:
Inspect the pipeline surface weekly for corrosion spots or coating damage. Check valve/flange seals for leaks (use pH test strips to detect any acid or alkali residues).
Monthly, drain the condensation valve and test the pH of the condensed water (which should be neutral). If it’s acidic or alkaline, it indicates pipeline leakage or worsening corrosion.
Corrosion Monitoring:
For critical pipeline sections, attach corrosion coupons (made of the same material as the pipe) and weigh them periodically to calculate corrosion rates. Replace the pipe if the corrosion rate exceeds 0.1mm/year.
Emergency Response:
Keep corrosion-resistant repair materials (e.g., PTFE tapes, emergency sealant) on hand. If local corrosion leaks occur, cut off the air supply, clean the affected area, and temporarily repair it. Then, plan for a permanent replacement.
Conclusion
The core of protective measures for air supply lines in acid and alkali corrosive areas is “blocking corrosive agents + enhancing material resistance.” By selecting corrosion-resistant materials such as 316L stainless steel and PTFE, optimizing the installation layout, and improving air purification and maintenance, the lifespan of the pipeline can be effectively extended, ensuring the stable and clean supply of air to the instruments.