Column
Control valve Q & A
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 .
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 tradeoff 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 :
• Reduce dead band
• Improve control loop stability
• 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 .
12 www . valve-world . net Valve World September 2024