1.Introduction
Stainless steel tubes are tubular materials made from iron-based alloys with a chromium content of no less than 10.5%, featuring unique performance advantages. The most notable characteristic is the formation of a dense chromium oxide passivation film, which endows the material with excellent corrosion resistance. Depending on the alloy composition, the tensile strength of stainless steel tubes can range from 520 to 1035 MPa, combining good mechanical strength and plasticity. In terms of temperature adaptability, different types of stainless steel tubes can meet the application requirements from -196 cryogenic environments to 1100 high-temperature conditions. Additionally, their smooth surface and absence of precipitates make them fully compliant with food-grade (FDA) and medical-grade (ISO 13485) hygiene standards.
1.1 Scope
This guide is applicable to a wide range of stainless steel pipe types, including austenitic grades (such as 304, 316), ferritic grades (such as 430), and duplex stainless steel pipes (such as 2205). It provides detailed and specific instructions and considerations for each of these grades to ensure their effective and efficient application in various industrial and practical scenarios.
1.2 Safety Precautions
When handling stainless steel pipes, it is of utmost importance to wear appropriate protective gear, including gloves and goggles, to safeguard against potential injuries. Additionally, during welding or cutting operations, it is essential to ensure proper and adequate ventilation to prevent the accumulation of harmful fumes and to maintain a safe working environment.
1.2.1 Eye protection:
Impact-resistant safety goggles (ANSI Z87.1 standard) must be worn.
When performing welding operations, use an auto-darkening welding helmet (shade number ≥ 10).
1.2.2 Respiratory protection:
Use N95 grade dust-proof masks in dusty environments.
The pickling operation is equipped with a dual-effect filter gas mask for organic vapors and acidic gases.
1.2.3 Body Protection:
Cut-resistant gloves (EN 388 standard, cut resistance grade ≥ 3)
Acid and alkali resistant apron (made of PVC or rubber material)
Steel-toe safety shoes (in accordance with ASTM F2413 standard)
2.Principles for Selecting Materials
Selecting the most suitable stainless steel grade is of paramount significance for achieving optimal performance in specific environmental conditions and operational requirements.
2.1 Principle of Adaptability to Corrosive Environments
Acidic medium (pH < 7): Preferentially choose steel grades containing molybdenum (316L/2205)
Chlorine-containing environment: PREN value > 35 (PREN = %Cr + 3.3 × %Mo + 16 × %N)
Oxidizing medium: Chromium content ≥ 18% (such as 304/310S)
High-temperature oxidation (>800): Select 310S (Cr25Ni20)
Stress corrosion sensitive zone: Avoid using 304 in environments with chlorine and temperatures above 60.
Low-temperature operating conditions (< -50): Austenitic steel needs to be ultra-low carbonized (304L/316L)
2.2 Principle of Mechanical Performance Matching
Ordinary pressure-bearing: 304 (σb ≥ 515 MPa)
High-pressure pipeline: 2205 duplex steel (σb ≥ 620 MPa)
Wear-resistant working condition: Surface hardening treatment (HV ≥ 800)
Cyclic stress: Fatigue strength coefficient ≥ 0.35 (316LN is more preferable)
Impact load: -196, Akv ≥ 100J (Ultra-low carbon and nitrogen alloyed)
2.3 Principle of Process Adaptability
Thin-walled tubes (δ< 3mm): Select 304L/316L (low-carbon non-sensitizing)
Thick-walled tube: For 2205, the heat input needs to be controlled (15 - 25 kJ/cm)
Post-weld treatment: pickling and passivation (nitric acid 20% + hydrofluoric acid 3%)
Cold bending forming: 304 is superior to 430 (elongation ≥ 40%)
Expansion tube processing: 316Ti (Titanium stabilized for anti-intergranular corrosion)
2.4 Principle of Economic Optimization for Cold Bending
Material cost: 2205 ≈ 2 × 304,904L ≈ 4 × 304
Life cycle: For chemical pipelines, the annualized cost is calculated based on a 20-year period.
Grade-based usage: main pipe 316L + branch pipe 304
Wall thickness optimization: ASME B31.3 allows for a reduction of 15% in wall thickness.
Alternative solution: Composite pipe (carbon steel matrix + stainless steel lining)
2.5 The principle of conformity to standards
Food grade: ASTM A270 (316L)
Pharmaceutical industry: ASME BPE (Ra ≤ 0.5 μm)
Pressure piping: GB/T 14976 (Intergranular corrosion test)
3.Proper installation prevents leaks, stress fractures, and premature failure
3.1 Check of materials
Confirm that the material of the stainless steel pipe (such as 304, 316L, etc.) complies with the design requirements.
Check whether there are scratches, deformations or oxide layers on the surface. If necessary, perform acid pickling and passivation treatment.
3.2 Tools and Accessories
Cutting tools: Specialized stainless steel cutting machine (to avoid contamination by carbon steel).
Beveling machine: Ensure that the welding interface is flat (with a bevel angle of 30° to 45°).
Cleaning tools: acetone or alcohol (for removing grease and impurities).
3.3 Environmental requirement
Avoid direct welding in damp or high-chlorine environments (such as near the seaside).
Ensure that the working area is kept clean and prevent contaminants such as iron filings from adhering.
3.4 Cutting Specification
Use plasma cutting or stainless steel-specific saw blades instead of ordinary grinding wheel discs (which may cause carbon pollution).
The deviation of the incision's verticality should be no more than 1°. The burrs need to be smoothed out by using a file or sandpaper.
3.5 Edge preparation
The welding interfaces need to be machined into single V-shaped or double V-shaped grooves (with an angle of 30° to 45°and a root edge of 1 to 2 mm).
The surface roughness of the groove is Ra ≤ 12.5 μm.
3.6 Weld inspection
Visual inspection (no cracks, pores)
Penetrant testing (PT) or radiographic testing (RT)
4.Technical Specifications for Operating Conditions of Stainless Steel Pipes
4.1 Regular working scope
Austenitic stainless steel (304/316): -196 ~ 800
Duplex stainless steel (2205): -50 ~ 300
Ferritic stainless steel (430): -20 ~ 600
4.2 Critical temperature caution
Sensitization range: 450 - 850 (risk of carbides precipitation)
Low-temperature embrittlement: For 304L, impact toughness needs to be verified when the temperature is lower than -196.
4.3 Prohibit the use of media
Hydrochloric acid (any concentration)
Hydrofluoric acid (>1% concentration)
High-temperature concentrated alkali (NaOH > 40%, > 80)
4.4 Special medium treatment
Chlorinated solution: For 316L, the concentration of Cl should be controlled below 1000 ppm at 25.
Hydrogen sulfide environment: Requires ultra-low carbon grade (316L, C ≤ 0.03%)
4.5 Prevention of cavitation
The inlet pressure of the pump is more than 1.3 times the saturated steam pressure of the medium.
Avoiding the design of 90-degree sharp turns.
4.6 Limit on the number of cycles
304: > 5000 fatigue analyses are required.
2205:> 10,000 tests are required to be conducted.
4.7 CIP cleaning parameters
Temperature: 80 ± 5
Cleaning agent: 1-2% nitric acid solution
Time: 30 - 60 minutes
4.8 Key detection indicators
Wall thickness monitoring: Annual UT thickness measurement (warning required when corrosion rate > 0.1mm/year)
Surface inspection: PT flaw detection every six months (with a focus on weld seams)
5.Maintenance and Inspection
5.1 Periodic inspection system
Daily inspection: Leakage/Abnormal noise/Vibration inspection (High-risk areas)
Monthly inspection: Tightening status of flange bolts (torque verification ±10%)
Annual inspection: Comprehensive wall thickness measurement (UT thickness gauge accuracy ± 0.1mm)
5.2 Cleaning and Maintenance Standards
Surface dirt: Neutral cleaning agent (pH 6-8) - wipe with soft cloth
Welding area: Quarterly vinegar test (to detect iron contamination)
Drainage requirements: The system must be completely drained when it is out of use (to prevent freezing and cracking).
5.3 Corrosion Monitoring Technology
Hang Plate Method: Install monitoring plates at typical positions (weigh every quarter)
ER probe: Online monitoring of corrosion rate (data uploaded in real time)
Endoscopic examination: Internal corrosion at the elbow/branching point (high-definition video)
6.Troubleshooting Common Issues
6.1 Emergency repair for leakage
Pressure-retaining leak sealing fixture (maximum pressure ≤ 80% of the design value)
High molecular sealant (with temperature resistance ≤ 150)
6.2 Mechanical Failures
Remove the damaged section (the incision should be ≥ 50mm away from the defect)
The welding repair requires local heat treatment (sulfuric acid pickling treatment is needed for 316L).
6.3 Removal of corrosion defects
Pitting Corrosion Repair welding polishing When the depth exceeds 20 of the wall thickness the pipe needs to be replaced
Gap Corrosion Replace the flange gasket Use PTFE wrapped gasket instead
Stress Corrosion Replace the entire pipe Prohibit local repair welding
6.4 Cathodic protection system
Sacrificial anode: Magnesium alloy (-1.75V) is used for seawater pipelines.
Additional current: Rectifier output ≤ 10A (Regular calibration required)
6.5 Coating maintenance
Repair of damage: Sandblasting Sa2.5 grade + Epoxy primer (dry film ≥ 150μm)
Insulation layer: Chloride ion content test (≤ 50 ppm)
7.Conclusion
Stainless steel pipes, as an important material in modern industry, can fully exert their performance advantages only when the whole life cycle is managed in a standardized manner. It is suggested that users establish a complete technical archive covering "material selection - installation - operation - maintenance", and focus on monitoring the changes in the working conditions of the corrosion-sensitive areas. For key pipeline systems, a management model combining online monitoring technology with regular professional assessment is recommended. At the same time, technical communication with material suppliers should be maintained to obtain the latest solutions.
The technical parameters provided in this guideline need to be adjusted in accordance with specific working conditions. For special application scenarios (such as nuclear grade, ultra-high purity, etc.), special technical specifications should be followed. It is recommended to entrust professional institutions to conduct a system health assessment every three years to ensure the safe and economic operation of the pipeline system.
