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When it comes to the material used in the packing material , Asbestos was considered to be optimal . Asbestos is flexible enough to be woven into rings , suitable for very high process temperatures , and is not conductive of electricity and so not liable to galvanic corrosion . Its classification as a hazardous substance now precludes its use as packing material for modern valves .
The two most common packing materials today are Teflon ( Polytetrafluoroethylene , or PTFE ) and graphite . Teflon is exceptionally good at creating a tight seal , is highly chemical resistant , and offers an extremely low coefficient of friction . It can be trusted to create a reliable and durable seal . However , it has a limited temperature range , foreclosing some common valve usages , such as at nuclear power plants . The partner to Teflon in packing material dominance is graphite . It has a far greater temperature range than Teflon , and is able to endure harsh nuclear radiation . However , graphite creates more friction against the stem , and permits galvanic corrosion due
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to its electrical conductivity . As Control Automation adds , ‘ sacrificial zinc washers are sometimes added to graphite packing assemblies to help mitigate this corrosion , but this only postpones rather than prevents corrosive damage to the stem .’
Other hybrid packing materials have been developed . Carbon fiber packing combines high strength and high thermal resistance , making it ideal for high-pressure applications . Aramid fiber packing is high-strength and has good abrasion resistance , making it suitable for high-speed rotating equipment .
Key Elements to Consider - Sealing and Packing Solutions
Many factors must be taken into consideration when identifying appropriate sealing
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mechanisms for valves .
• Identify the pressure and temperature of the process substance ;
• Know the valve type , manufacturer , and model ;
• Determine the legislated fugitive emissions standard for the process substance .
With these variables in mind , one can seek out the right vendor providing the right packing solution . Manufacturers will provide instruction for installation and maintenance that must be followed in order to achieve optimal performance .
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In terms of maintenance , the tightness of the packing nuts on the valve flange is a common point of human error . Care is needed to achieve sufficient packing flange force to create a seal , but not to tighten too far as to over-compress the packing material |
, leading to excessive friction against the stem and premature packing failure . ‘ Valve Packing , the latter scenario [ over-tightening ] is what one usually finds in industrial settings , where well-intentioned but uninformed personnel over-tighten valve packing in an effort to prevent leaks . The proper remedy for a packing assembly that leaks despite having been properly torqued is the replacement , not further tightening . 1
REFERENCES
1 . https :// control . com / textbook / control-valves / valvepacking
/
2 . https :// www . processingmagazine . com / valves-actuators /
article / 21248111 / simplifying-control-valve-packing-
selection
3 . https :// www . arthomson . com / blog / info-1 / understandingvalve-packing-13
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