SPECIAL TOPIC : SEALING & PACKING TECHNOLOGIES
The Current Paradigm : Focus on Braided Packing ( Wiper Rings ) and Use The PTFE
The previous paragraph highlighted that expanded mineral graphite packings need to be treated with blocking agents and lubricant to attempt compliance with LE requirements . This new element in the BOM immediately raises several questions : what to use , how much to use , and where to apply it . Starting with the last point , i . e ., where to apply the additive , current technology identifies braided graphite packing rings as the component designated to receive additives that act both as blocking agents and lubricants ( reducing the coefficient of friction ). According to this strategy , the stem graphite sealing kit consists of impregnated wiper rings , die-formed braided graphite rings according to ASTM F2191 , and dry middle rings , die-formed graphite rings according to ASTM F2168 . In this configuration , the friction-reducing and waterproofing intervention is focused solely on the wiper rings , which also transfer some of the lubrication to the stem . This is then passed on to the surfaces of the middle rings during its movements .
As for the second question , how much additive to apply to the product , this is tied to the desired weight loss limit of the packing at the designated temperatures . It is evident that anything other than graphite is less thermally stable and will degrade as temperatures rise . Finally , the most important question is what to use . According to current technology , the most widely used additive is PTFE dispersion , which is a wellknown fact since API STD 622 includes PTFE content verification among its material tests . The presence of PTFE in significant quantities to pass the LE tests brings with it a series of known issues ( corrosion , temperature limitation to 260 ° C , failure of the weight loss test ). It does not apply to the middle rings and does not meet the upcoming restrictions concerning PFAS .
A Paradigm Shift : Improving Lubrication by Banning PTFE
However , a new scenario is now possible : modifying graphite to achieve Enhanced Sealing Properties ( ESP ). New products , such as Carrara PGT4 GR8622 ESP , adopting this innovative graphite modification strategy are now available , surpassing the current production technology of graphite packings . There are four key drivers : ban PTFE ; use an additive PFAS-free and completely non-corrosive agent that is more thermostable than PTFE ; limit the modification of the graphite to comply with the weight loss test ; and both wiper and middle rings shall contribute to reducing friction across the entire sealing surface of the stem packing . To meet these requirements , it is necessary to directly modify the graphite foil to enhance its sealing properties and ensure the availability of raw material suitable for producing both braided wiper rings and die-formed rings .
Packings manufactured according to this technology tend to show significantly positive results in LE tests , also meeting the fire test , corrosion test , and weight loss requirements of TQ39-408 ° C and FSA . The reduction of friction between the middle rings and the stem , along with decreased permeability , certainly represents significant progress . However , it has also been observed that the reduction in surface friction of the graphite occurs during the molding process , affecting the relaxation behavior of the rings when installed in the valve . Therefore , the transformation of this new material requires new skills , and a complete overhaul of production technology compared to the past .
Conclusion
This article has aimed to argue that the issue of LE and graphite stem packings ultimately boils down to just two key matters . The first is chemical in nature — how to modify the graphite — and the second concerns production technology , specifically where to apply the additive , how to apply it , and in what quantity .
Thanks to the opportunities provided by new materials and the know-how of the packing
manufacturer , the technology of graphite packing opens up new prospects , enabling the production of PTFE and PFAS-free products that comply with LE technical regulations . Most importantly , it represents
ABOUT THE AUTHOR
a concrete step forward in providing products better equipped to meet modern market trends , which demand more durable and reliable performance .
Francesco Apuzzo is a Mechanical Engineer from Politecnico University of Milan . CTO of Carrara SpA Global Sealing Solution , President of VALVEcampus , the Association of Manufacturers of Industrial Valves and Components for the Oil & Gas and Power Industries , and Chairman of the Advisory Board for the Industrial Valve Summit Technical Conferences .
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