Optimal Valve Material Temperatures

Overview

In petrochemical, power, metallurgy, and defense industries, valves are often subjected to extreme conditions, such as high temperatures, high pressures, and corrosive environments. Proper selection of valve materials, including primary and auxiliary materials, is crucial to prevent failures and ensure reliable operation. The working temperature limits of auxiliary materials are typically lower than those of the main materials, meaning auxiliary materials often determine the operational temperature range of valves.

Common Materials and Their Temperature Limits

Non-ferrous Metals

• Babbitt Alloy (for ammonia valves): Contains 10-12% Sb, 5.5-6.5% Cu, with the rest being Sn. It has a hardness of ≥27HB. It starts to fail at 135℃, so it should be used at ≤120℃.
• Copper Alloys (Aluminum bronze or silicon brass): Used in water, seawater, and oxygen. In high-temperature valves, they can handle up to 550℃.

Cast Iron

• Gray Cast Iron (HT): Used in low-pressure valves for water and non-corrosive media at room temperature. ASME B16.1-1998 specifies a range of -29 to 232℃.
• Ductile Iron (QT): Used in medium-pressure valves. ASME B16.42 specifies a range of -29 to 343℃.

Polymers

• PTFE (Polytetrafluoroethylene): Works between -180 and 250℃. Pure PTFE is suitable from -196 to 180℃. Enhanced PTFE (FZ), reinforced with glass, graphite, or copper, works from 150 to 200℃ but is not for oxygen valves with graphite.

• PPL (Polyphenylenes): Used where PTFE is unsuitable, with a range of -110 to 325℃.

• Nylon: Rarely used due to its narrow temperature range of -73 to 93℃.

Rubber

• Natural Rubber (NR): Elastic and wear-resistant but not heat-resistant. Suitable from -50 to 80℃.
• Butyl Rubber (IIR): Good air tightness, heat, and aging resistance. Suitable from -40 to 100℃.
• Chlorosulfonated Polyethylene (CHC, CIIR): Works from -45 to 100℃.
• Nitrile Rubber (NBR): Excellent oil resistance and better heat resistance than natural rubber. Suitable from -20 to 93℃.
• Chloroprene Rubber (CR): Good aging and heat resistance. Suitable from 0 to 80℃.
• Ethylene Propylene Rubber (EPT): Suitable from -40 to 160℃.
• Silicone Rubber (SI): Good heat and thermal stability. Suitable from -70 to 200℃.
• Fluorine Rubber (FPM): Suitable from -15 to 200℃.

Conclusion

Choosing suitable auxiliary materials for valves ensures they work well under specified conditions. Each material has unique properties and temperature limits, essential in valve design to prevent failures.

FAQs

1. Why is the temperature limit for auxiliary materials lower than main materials?
   • Auxiliary materials are more sensitive to temperature changes compared to main materials.
2. Can PTFE be used in all valves?
   • No, PTFE and reinforced variants are unsuitable for oxygen valves and high-pressure conditions due to safety risks.
3. What role do ASME standards play?
   • ASME standards provide guidelines for safe and reliable material use under specified conditions.
4. Why isn't natural rubber used in high-temperature applications?
   • Its poor heat resistance and aging limit its use to low-temperature conditions.
5. How do reinforcement materials improve PTFE?
   • Glass, graphite, and copper improve PTFE's strength and wear resistance, making it suitable for higher temperatures.