SPECIAL TOPIC : SEALING & PACKING TECHNOLOGIES
Industrial Valves and Fugitive Emissions :
It is Time to Take Stock for Looking for a New Perspective
Industrial valves and fugitive emissions have been a consistent point of discussion for the past 30 years , drawing continuous interest and debate . The interaction between the valve , stem graphite packing , and its surrounding environment remains a key focus in scientific conferences , where it continues to garner significant attention . However , over time , the regulatory framework for Low Emission standards has been more established . Today , the requirements for both products and testing procedures are clearly defined .
By Francesco Apuzzo , R & D and Technical Manager – Carrara SpA
Additionally , the Leak Detection and Repair ( LDAR ) protocol ensures the proper monitoring of valve emissions in industrial plants . Together , these measures form what is known as Reasonably Achievable Control Technology ( RACT ), aimed at reducing and controlling volatile organic compounds ( VOC ) and hazardous air pollutants ( HAP ) emissions from valves . Therefore , it can be said that everything is in order , that the issue of fugitive emissions is under control , and that over the years the products have evolved , driven by Low Emissions test regulations , and are more efficient than those of the past . So why is it still being discussed ? This article will take a broader look and see if a paradigm shift is possible today .
Is Expanded Mineral Graphite from 2024 Different Than That of 1994 ?
Almost always the resolution of a problem and the improvement of a product ’ s performance are driven by the availability of new materials that allow for the exploration of new technical solutions . Is this the case with graphite compression packings ? Unfortunately , no . When examining the primary raw material used to produce graphite compression braided packings and die-formed graphite rings , which are the components of the sealing stem packings , it can be easily verified that the flexible graphite foil from 1994 is not significantly different from that of 2024 .
The production process of graphite foil has remained unchanged and the chemical requirements for the material are not far from the original ones ( limitations on the content of detrimental materials , mainly ash , sulphur , and halogens ). The only real innovation , introduced around the mid- 2000s , was the addition of oxidation inhibitors . This was followed by oxidation tests in accordance with the European Standard EN 14772 and the American FSA-G-604-07 ( Methods A and B ), which establish the acceptance criteria ( Weight Loss Test ).
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Therefore , from a chemical and thermal resistance standpoint , flexible graphite foil has remained unchanged , with only oxidation resistance significantly improved in modern materials ( in top-tier products ). What can be said about the mechanical behavior of graphite foil ? To fully understand its properties , it must be remembered that the constituent element of the foil is the “ flake ”, which is the expanded graphite grain . The larger the flake ( in volume ), the higher the tensile strength and recovery of the graphite foil , which can be observed in die-formed rings and braided graphite packing . The coefficient of friction and intrinsic permeability of the material have remained entirely unchanged .
The Key Factors to Ensure Good LE Performance
Based on the prior paragraph , it is clear that graphite has remained the same as it was 30 years ago . The additional contributions ( performance improvements ) that can come from the “ raw material ” ( flexible graphite foil ) to achieve the goal of minimizing Fugitive Emissions ( FE ) are minimal . The choice of “ big flake size ” graphite provides a bit of a boost in the recovery of die-formed graphite rings , but nothing more . Experience dictates that expanded graphite alone is not capable of complying with LE requirements .
The reasons why the die-formed flexible graphite rings that make up the stem sealing packing are unable to perform well within the low emission range are : 1 ) the product ’ s permeability is high ; 2 ) the coefficient of friction is very high ; and 3 ) the high friction accelerates the packing relaxation during the endurance test . Regarding permeability , it must be considered that graphite foil is a laminate of flakes without the use of binders . This makes it intuitive to understand how the foil matrix is naturally inclined to favor permeation flow .
It should also be noted that the die-formed ring is stamped from a graphite tape previously rolled into the die-forming tool . The vertical arrangement of the tape , parallel to the stem axis , creates capillary flow channels into which the fluid slowly seeps , completely permeating the sealing ring . Regarding the coefficient of friction , it is an inherent property of the material . It tends to decrease with rising temperature and with increasing load . Paradoxically , during the FE endurance tests , friction tends to reduce the radial load of the sealing rings due to vibrations and the wear of the part of the packing that rubs against the stem .
This reduction in radial load brings the system into a range where the coefficient of friction is higher . This degenerative phenomenon leads the sealing system towards emissive divergence quickly , accelerating the spontaneous trend of relaxation of the graphite sealing ring as mentioned earlier . To reduce the influence of the previously described phenomena , it is necessary to introduce other materials into the BOM of these packings . These are additives that function to reduce both permeability and the coefficient of friction of the graphite .
6 Valve World Americas | October 2024 • www . valve-world-americas . net