Stainless Steel -Water Filtration

Benefits of Stainless Steel in Water Applications such as Water Filtration and Waste Water Projects

Stainless Steel Pressure Vessel Fabrication
Stainless Steel Pressure Vessels and Tanks

Introduction

Stainless steel offers economic benefits to the water industry by reducing both initial and operating costs due to its durability and low maintenance requirements. Known for its corrosion resistance, strength, and toughness, stainless steel has a proven track record in water treatment. It is also hygienic, visually appealing, and available in various forms, making it a versatile choice for long-lasting infrastructure.

Applications

Mechanical Systems 

  • Screening systems, sieves
  • Grit chambers
  • Aeration tanks
  • Sedimentation tanks
  • Scrapers
  • Screening Drums
  • Pre-treatment tanks
  • Syphons and lifting devices
  • Weirs and overflows

Biological Systems

  • Sedimentation tanks
  • Inlet and outlet construction
  • Aeration installations
  • Ozone treatment
  • Sludge separator installations
  • Anaerobic waste water treatment
  • Disinfection – UV systems
  • Tank covers

Sludge Treatment

  • Tanks

  • Mixing containers
  • Thickening Containers
  • Sludge Dewatering Containers
  • Sludge Circulation Equipment
  • Filter-presses
  • Stop logs
  • Valves

Other

  • Linings for concrete tanks
  • Manholes and covers
  • Shaft covers
  • Climbing rungs
  • Ladders
  • Railing and platforms
  • Firedoors, safety doors, pressure doors
  • Wellheads
  • Ventilation stacks

Stainless Steel’s mechanical properties, including strength, ductility, and toughness, offer several benefits:

  • No need for coatings for corrosion protection, eliminating the risk of coating wear, water contamination, and coating maintenance.

  • High strength and ductility allow for lighter components and greater impact resistance.

  • Corrosion and wear resistance are essential for systems like sludge treatment, while non-toxicity is crucial for potable water.

General Design Principles of Stainless Steel Equipment

 

Principles Description
Ensure Continuous Flow Design systems to maintain liquid flow, avoiding areas where water may stagnate or sediment can accumulate.
Agitation in Wastewater Keep wastewater agitated to reduce sludge buildup
Optimal Flow Rates Achieve at least 1 m/s for raw water and 0.5 m/s for treated water to minimize sediment deposits.
Avoid Deadlegs Avoid pipe sections where stagnant water and air may trap deposits. For pipes with intermittent flow, slope horizontal pipes and tank bottoms to drain completely.
Regular Wetting For parts of the system that can’t be fully drained, design so they can be wetted periodically, which helps prevent salt or deposit buildup.
Consider Acoustic Damping For thin stainless steel pipes, account for any potential noise from pressure pulses.

Stainless Steel Grade Selection

  • 316 Grades with Molybdenum: These stainless steels have a higher resistance to corrosion than 304 grades, making them suitable for environments with higher levels of chloride and chlorine. 316 is commonly used in water and wastewater treatment and preferred for building water systems that may experience stagnation or have numerous fittings.

  • 304 Grades: These grades work well in most ambient water systems with chloride levels below 200 ppm. They are suitable for areas where abrasion or erosion resistance is needed, like in screens or grids. Chlorine or other oxidizers reduce this threshold, making 304 more prone to corrosion at lower chloride levels.

  • 316 Grades for High Chloride Levels: The 316 grades, resistant to pitting and crevice corrosion, are ideal for waters with chloride levels up to around 1000 ppm.

  • Impact of Oxidizing Agents: Chlorine and other oxidizing agents can increase the risk of crevice corrosion even at a given chloride level. Trials in the U.S. found 304 grades suitable for up to 2 ppm chlorine in waters with chloride levels below 23 ppm, with 316 types offering a larger safety margin.

  • Areas with Chlorine Vapours: In areas where moist chlorine vapours may build up, it may be necessary to improve ventilation or choose a more corrosion-resistant grade.

  • Low-Carbon and Stabilized Grades: For welded structures, the low-carbon “L” grades or those with a maximum of 0.05% carbon are recommended. For heavy-duty structures that involve multiple welding passes, stabilized grades may be necessary; consulting a welding engineer or steel supplier is advised.

Check our COMPARISON OF STAINLESS STEEL GRADES Page

Behaviour of Stainless Steel in Water

Stainless steels, especially grades 304 and 316, are extensively used in water handling and treatment due to their superior corrosion resistance, durability, and low maintenance needs. Their passive oxide film provides a protective layer that self-repairs if damaged, effectively preventing corrosion under most conditions. Below are the specific factors influencing their performance:

Stainless Steel in Water
Chrome pipe manufacturing

1-Corrosion Resistance and the Passive Oxide Film

The passive oxide film formed on stainless steel surfaces is key to its corrosion resistance. Grades 304 and 316 require no additional corrosion allowance, which is necessary for materials like cast iron or carbon steel. This film regenerates quickly in water if damaged, preventing long-term corrosion issues. However, localized corrosion can occur under specific conditions, primarily as crevice corrosion or pitting, especially in areas with low flow or sediment buildup.

2- Influence of Flow on Stainless Steel

Stainless steel’s erosion-corrosion resistance allows it to handle turbulent flow and high flow velocities up to 30 m/s, unlike ductile cast iron or copper alloys. However, maintaining minimum flow rates is essential to prevent sedimentation and ensure optimal performance:

  • Clear Water: Minimum velocity of 0.5 m/s.
  • Raw Water: Minimum velocity of 1 m/s.

High flow reduces the risk of sediment buildup in crevices, which can initiate localized corrosion.

Skid modules, stainless steel fabrication
Novelty Structures Polished Stainless Steel

3- Effects of Aeration on Stainless Steel

Increased dissolved oxygen from aeration processes, which typically causes corrosion in carbon steels and cast iron, does not harm stainless steel. Stainless steels remain unaffected under normal variations in dissolved oxygen levels, making them highly suitable for aerated environments and oxidation processes.

4- Resistance to Chlorides and Pitting/Crevice Corrosion

Chloride levels are a critical factor in determining the risk of pitting and crevice corrosion:

  • 304 Stainless Steel: Typically safe for chloride levels up to 200 ppm.
  • 316 Stainless Steel: Generally resistant to chloride levels up to 1000 ppm due to added molybdenum.

In highly localized or challenging environments, such as with occasional drying or concentrated chlorides, corrosion may still occur. In such cases, more conservative limits—e.g., 50 ppm for 304 and 250 ppm for 316—are advisable. For wastewater treatment, grades with 2-3% molybdenum are preferred for increased resistance.

 

Types of Crevices and Mitigation

Crevices in stainless steel systems arise from:

  • Natural Causes: Sediment deposits and sludge buildup.
  • Man-Made Causes: Fabrication imperfections like incomplete welds, surface contamination, or poorly aligned flange joints.

To minimize these risks:

  • Design practices should aim to avoid crevices.
  • High flow rates help reduce sediment buildup.
  • Regular cleaning may be necessary to clear sediments.
  • Fabrication should ensure complete, high-quality welds without excessive projections and that gaskets used in joints are inert, chloride-free, and well-aligned.

5- Galvanic Behavior and Corrosion Prevention

Galvanic corrosion occurs when two metals of different reactivity are in contact in a conductive environment. Stainless steel, as a more noble metal, can accelerate the corrosion of less noble metals like carbon steel, especially in freshwater.

  • Mitigation Measures:
    • Use insulation between dissimilar metals.
    • Apply protective coatings around joints, ensuring the coverage extends slightly beyond the joint.
    • If using sacrificial anodes, select materials like zinc or magnesium to protect the stainless steel from corrosion.
    • For example, fusion-bonded epoxy-coated carbon steel flanges can prevent contact between stainless steel pipes and other metals, reducing the risk of galvanic corrosion.
  • Practical Applications: Stainless steel’s position on the galvanic series is passive when its oxide film is intact, but under conditions like crevice corrosion or local breakdown of the oxide layer, it can become active. Careful material selection, insulation, and coating techniques can protect both stainless steel and adjacent metals from galvanic corrosion.
Aluminium Welding

6- Wrap Up

  • Flow Management: Maintain recommended flow rates to avoid sediment deposition.

  • Design and Fabrication: Limit crevices, ensure high-quality welds, and avoid sharp corners where deposits may accumulate.

  • Galvanic Prevention: Insulate dissimilar metals, use sacrificial anodes as needed, and avoid using less noble fasteners in stainless steel assemblies.

  • Surface Treatments: Remove heat tint from welds to prevent crevice corrosion, and protect against chloride contamination under insulation with barrier paints.

  • Regular Maintenance: Periodically flush systems to remove deposits, especially in areas susceptible to sediment buildup.

Skid modules, stainless steel fabrication

Cleaning of Stainless Steel

  • Thorough Draining After Testing: Fully drain systems after hydrostatic testing.

  • Cleaning Ports and Flush Schedules: Where deposits are inevitable, include ports for cleaning and schedule regular flushing. For instance, in raw water lines prone to manganese or iron deposits, plan for periodic flushing or hydroblast cleaning.

  • Long Shutdown Preparation: For extended shutdowns, plan to prevent corrosive deposits from drying out. Options include keeping pipework wet by circulating water for at least an hour every two days or fully flushing, draining, and drying the system.

Stainless Steel Fabrication Tips

  • Minimize Traps and Crevices: Design to reduce areas that may trap deposits, such as sealing any overlapping plates.

  • Welding Protocols: Choose welding methods that are best for the specific steel grade.

  • Accessible Weld Design: Ensure the design allows easy access for welding to create the best weld shape, reducing heat tint buildup.

  • Smooth, Full Penetration Welds: Aim for smooth, full-penetration welds. For instance, use a return edge on plates for a smooth corner, allowing easy welding and cleaning.

stainless steel welding
Aluminium Welding

Key Take Away

 

Stainless steel is a highly effective material for water industry systems, especially when managed with care.

By following key guidelines—like designing for steady water flow, choosing the right type of stainless steel, using proper stainless steeel fabrication and machining techniques, and setting up easy cleaning methods—the risk of corrosion can be greatly reduced, helping equipment last longer.

Selecting the right steel grade based on factors like chloride levels and exposure to chemicals, along with smart design choices, ensures that stainless steel parts hold up well, even in tough conditions.

Together, these practices help create durable, cost-effective stainless steel systems for the water industry.

How can we help you ?

Stainless steel provides a durable, low-maintenance solution for water systems with long service life and low life-cycle costs.

Novelty Structures stands out as a reliable partner with stringent quality control and compliance with global standards in providing

high-quality stainless steel fabrications tailored to Water Industry needs.

Contact our team to further discuss your needs.

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