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 mid2000s, 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).
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.
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 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.