Q: What are the main challenges associated with valve stem packings, and how do these impact valve performance and safety?
A. Valve stem packings present two significant challenges for control valve designers:
1. Leakage prevention: The primary purpose of stem packings is to prevent stem leakage, but this is not always achieved. Such leakage can have serious consequences, including:
- Fire hazards when handling flammable liquids
- Environmental damage from chemical leaks
- Process inefficiencies and product loss
2. Friction-induced dead band: Stem packings are a source of friction, which creates a ‘dead band’ in valve response. This dead band can cause instability in process control loops, leading to:
- Reduced control accuracy
- Increased process variability
- Potential damage to valve components due to rapid oscillations
The paradox lies in the fact that tightening the packing to reduce leaks simultaneously increases friction, exacerbating the dead band issue. This trade-off between leak prevention and smooth operation is at the heart of the stem packing challenge.
Q: How have valve designers traditionally attempted to address these challenges, and what are the drawbacks of these approaches?
A. One common approach has been to use highly polished valve stems in an attempt to reduce packing friction. However, this strategy may be counterproductive.
Standard centreless-ground stem material has microscopic cracks and voids on its surface. When using a polytetrafluoroethylene (PTFE) chevron packing, parts of the PTFE material rub off against these ground stems and fill the small voids. As the stem travels, there is sliding contact between the embedded PTFE and the inner packing rings. This PTFE-on-PTFE contact has an average coefficient of friction of 0.12.
In contrast, a highly polished stem results in metal rubbing directly against PTFE, with a higher average coefficient of friction of 0.4. Therefore, maintaining a standard surface finish for valve stems is actually more effective in minimising friction.
It’s worth noting that the microscopic cracks and voids in centreless-ground stems result from the grinding process itself. As the abrasive wheel removes material, it can create tiny imperfections on the surface. These imperfections, while potentially beneficial for PTFE packings, may not be desirable for all applications.
Q: What is the recommended solution for control valve packing, and how does it address the challenges discussed earlier?
A. The most effective solution for control valve packing is a spring-loaded chevron PTFE packing. This design offers several advantages:
1. Constant friction: The spring maintains a consistent force on the packing, ensuring that friction remains constant throughout the valve’s operation. This consistency helps to:
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Reduce dead band
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Improve control loop stability
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Minimise wear on the packing and stem
2. Improved sealing: The spring-loaded design allows the packing to adjust to small changes in stem position or wear, maintaining a tight seal over time.
3. Longevity: PTFE’s low friction properties and chemical resistance contribute to a longer service life for the packing. To accommodate the friction from this packing design in demanding applications, valve designers have two options:
a) Use a larger actuator: This ensures sufficient force to overcome the packing friction and provides precise control.
b) Specify a gain-adjustable positioner: This allows fine-tuning of the valve’s response to input signals, compensating for the effects of friction.
While this solution effectively addresses both leakage and friction challenges, users should be aware that PTFE packings may have temperature limitations and may not be suitable for all process fluids. In such cases, alternative materials or packing designs may be necessary.
Q: Are there any industry standards for stem surface finish that valve designers should be aware of?
A. Yes, there are industry standards that specify surface finish requirements for valve stems. The most commonly referenced standard is the Manufacturers Standardization Society (MSS) SP-55, which provides guidelines for valve stem finishes. Typically, a surface roughness of 0.4 to 0.8 micrometres (16 to 32 microinches) Ra (Roughness average) is recommended for most applications. However, specific requirements may vary depending on the valve type, size and intended service.
It’s crucial for valve designers and users to consult relevant standards and manufacturer recommendations to ensure the appropriate stem finish for their specific application.
Conclusion
Stem packings remain a critical component in valve design, directly impacting both safety and performance. While challenges persist, the use of spring-loaded chevron PTFE packings, combined with appropriate stem surface finishes, offers an effective solution for many applications. As the industry continues to evolve, ongoing research into new materials and designs may yield further improvements in stem packing technology, potentially offering even better solutions to the leakage-friction trade-off.
About the author
Dr. Hans D. Baumann is an internationally renowned consultant with extensive experience in the valve industry. Throughout his career, he held managerial positions in Germany and France, and his innovative spirit led to the creation of 10 novel valve types, including the well-known Camflex valve. Dr. Baumann has authored 8 books, including the acclaimed “Valve Primer,” and has been granted 115 US patents. He also founded his own valve company, which he later sold to Emerson, and served as Vice President at Masoneilan and Fisher Controls Companies.
About this Technical Story
This Technical Story is an article from our Valve World Magazine, June 2024 issue. To read other featured stories and many more articles, subscribe to our print magazine. Available in both print and digital formats. DIGITAL MAGAZINE SUBSCRIPTIONS ARE NOW FREE.
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