How to choose seal material when make ISO7-1 thread connection of stainless steel press fittings?
When choosing a seal material for ISO7-1 (also known as BSPT - British Standard Pipe Taper) thread connections of stainless steel press fittings, several factors should be considered to ensure compatibility, safety, and durability. The selection should account for material properties, the operating environment, fluid compatibility, pressure, temperature, and applicable standards.
Here's a detailed analysis of the considerations and common materials used for sealing such fittings:
1. Seal Material Properties
Chemical Compatibility: The seal material must be compatible with the fluid or gas passing through the stainless steel fittings. Incompatible materials may degrade, swell, or dissolve.
Temperature Resistance: Consider the range of temperatures the fittings will be exposed to, including any possible spikes or extremes.
Pressure Resistance: The material should withstand the pressure in the system without deforming or failing.
Durability: The material should be resistant to wear, aging, and fatigue, especially if subjected to dynamic or cyclic loading.
Elasticity: Sufficient flexibility to create a good seal when the fittings are tightened, yet firm enough to resist extrusion.
2. Common Seal Materials for Stainless Steel Press Fittings
| Material | Temperature Range (°C) | Pressure Resistance | Chemical Compatibility | Applications |
|---|---|---|---|---|
| PTFE (Teflon) | -200 to +260 | High | Excellent for most chemicals | High temperatures, aggressive chemicals, general-purpose plumbing |
| EPDM | -50 to +150 | Moderate | Good for water, steam, and alcohols | HVAC systems, water and steam applications, food processing |
| FKM (Viton) | -20 to +200 | High | Excellent for hydrocarbons, acids | High-temperature systems, fuel systems, oil and gas |
| NBR (Nitrile) | -30 to +100 | Moderate | Good for oils, fuels, and greases | Oil and gas, automotive, low-temperature applications |
| Silicone | -60 to +230 | Low to Moderate | Good for air, water, and some chemicals | Medical applications, food-grade, high-temperature applications |
3. Recommendations for ISO7-1 Thread Connections in Stainless Steel Press Fittings:
PTFE (Polytetrafluoroethylene):
Advantages:Excellent chemical resistance; almost inert to most chemicals.
Wide temperature range, suitable for extreme temperatures.
Non-reactive and non-contaminating, making it ideal for food-grade and medical applications.
Disadvantages:Low flexibility can make it harder to form a tight seal without enough compression.
Use Cases: Recommended for high-temperature, high-pressure, or chemically aggressive environments.
EPDM (Ethylene Propylene Diene Monomer):
Advantages:Good resistance to water, steam, and various polar substances (alcohols, ketones).
Good flexibility and excellent sealing properties in a wide temperature range.
Resistant to ozone and UV radiation, suitable for outdoor applications.
Disadvantages:Not compatible with hydrocarbons (oils, gasoline).
Use Cases: Ideal for HVAC systems, potable water, and steam applications.
FKM (Fluoroelastomer or Viton):
Advantages:Excellent resistance to high temperatures, chemicals, and oils.
Good flexibility, especially at elevated temperatures.
Disadvantages:More expensive than other materials like EPDM or NBR.
Use Cases: Ideal for high-temperature and chemically aggressive environments, including oil and gas applications.
NBR (Nitrile Butadiene Rubber):
Advantages:Good resistance to oils, fuels, and some acids.
Affordable and widely available.
Disadvantages:Limited temperature range compared to other materials.
Not suitable for strong acids or ozone-rich environments.
Use Cases: Suitable for oil and gas industry, automotive, and low-temperature applications.
Silicone:
Advantages:High flexibility and excellent performance over a wide range of temperatures.
Non-toxic, suitable for food-grade and medical applications.
Disadvantages:Lower resistance to pressure compared to other materials.
Not ideal for use with petroleum-based products.
Use Cases: Ideal for medical, food-grade, and high-temperature applications where chemical exposure is limited.
4. Selection Based on Application and Research
When choosing a seal material, refer to guidelines and research from authoritative sources like:
ISO Standards: ISO 8434-1 provides guidance on metallic tube connections and sealing methods.
ASTM Standards: ASTM F1387 provides test methods for performance of fittings, including seals.
Research and Industry Publications: Look for studies or white papers from materials science journals or industry publications related to pipe fitting performance, durability, and failure analysis.
5. Conclusion: Seal Material Choice for Stainless Steel Press Fittings
For general water or steam applications: Use EPDM seals.
For high-temperature or chemically aggressive applications: Choose PTFE or FKM seals.
For oil and fuel applications: Use FKM or NBR seals, depending on the temperature range.
For food-grade or medical applications: Opt for PTFE or Silicone seals.