TECH TALK
Carbon graphite tape . Image Credit : https :// latty . com / en / product / lattygraf-e1-carbon-graphite-tape /.
tems are some of the many reasons for an increased interest in nuclear power . Still , those working in and alongside the industry must remain vigilant as no industrial process is without risk and dangers . For example , the near miss at the Davis Besse plant in Ohio where maintenance workers discovered a football size void caused by boric acid corrosion in the reactor head in March of 2002 .
The chemical element Boron can absorb neutrons and slow the rate of the nuclear fission reaction . For this reason , Boron , in the form of boric acid is used as a control mechanism in nuclear reactors to adjust the reactor output . While boric acid plays an important role in both plant operation and safety , it also creates challenges of its own , as it is highly corrosive . In fact , it was a boric acid leak from the reactor coolant system that over time ate away at the reactor head at Davis Besse leading to the near-miss event .
One of the most critical sealing applications in nuclear facilities are the cooling system valves which can see up to 2000 ppm of boric acid within the water . In many industrial facilities , sealing of the water system is often a low priority . However , in a nuclear facility , if a valve leaks , over time the boric acid will crystalize around the stuffing box and destroy the packing . This can lead to serious leaks or failures , boric acid corrosion of critical equipment , and other related problems . Additionally , much of this system is within the containment region of the plant making replacement and repairs difficult , time consuming , and expensive due to radiation .
This is such a primary concern for nuclear power plants that most have a dedicated staff for monitoring and managing fluid leaks , and in particular boric acid-related maintenance issues . Many of these key plant personnel from plants and companies around the country come together annually at meetings of the Fluid Management Users Group ( FLMUG ) and the Boric Acid Users Group ( BAUG ) to share insights , information , and ideas . Additionally , the Electric Power Research Institute ( EPRI ) has worked with nuclear plants and key plant personnel to develop assessment guidelines and extensive technical reports to provide assistance and expertise to help drive improvement initiatives .
Valves Sealing in Reactors
Valve stem sealing is typically accomplished with mechanical packing material . The two most common packing materials in use today are polytetrafluoroethylene ( commonly referred to as PTFE or by its trade name , Teflon ) and graphite . PTFE offers advantages over graphite regarding sealability and stem friction or wear .
PTFE is well known for its lubricity ; long being a popular anti-stick coating on consumer goods like cookware . Additionally , PTFE , in its virgin form is inert to a wide variety of process media and is also an excellent dielectric insulator , helping to prevent galvanic corrosion . Most PTFE mechanical packing ‘ yarns ’ are made with an expanded form of the material which boasts exceptional tensile strength , making for a robust braided product . However , PTFE is not without its limitations . It has a limited temperature range making it unsuitable for applications above around 500F . Additionally , PTFE degrades in highly irradiated environments like those found near reactors in nuclear power plants .
Flexible graphite packing has superior temperature resistance and can also withstand high levels of radiation , making it a popular choice for nuclear plants . Like PTFE , graphite is self-lubricating and boasts very good resistance to many process media . However , flexible graphite packing generates significantly more stem friction than PTFE and has poor dielectric properties allowing for galvanic corrosion of the stem and / or bonnet metals . Additionally , flexible graphite has low tensile strength making it significantly weaker than PTFE packing ; it is highly susceptible to oxidation .
A popular form of packing for valve stems , especially in nuclear applications , is dieformed graphite . Rather than forming a braid using graphite ‘ yarns ’, this style of packing is made from thin layers of graphite ribbon that is wound around a mandrel to form a ring . The ring is then compression molded in a die to mechanically bond the graphite layers together into an essentially solid ring of a prescribed density . Similar processes can be used to form other shapes as well , like chevron-shaped ‘ cup and cone ’ ring sets . Die-formed graphite
ABOUT THE AUTHOR
ring sets often incorporate end rings of braided packing to help distribute the gland load and prevent extrusion .
It is worth noting , as it specifically applies to nuclear applications , any graphite material must be ultra-high purity , to ensure that it is free of neutron-absorbing materials that could impact the nuclear fission reaction ( particularly boron ). This high-purity graphite is commonly referred to as ‘ nuclear grade .’
Looking Forward
Time will tell if there is a renewed interest and investment in nuclear power here in the United States , though many experts believe it is inevitable as the demand for more and cleaner energy continues to rise . What is known for sure though is that , for nuclear plant operators , leak management , particularly as it relates to boric acid , is a primary focus . The stakes are high and high-profile incidents like the Davis Besse near-miss highlight the need to remain diligent .
Chris Morris is a Sales Engineer for Teadit , supporting customers and commercial interests in the Ohio Valley and Northeast Regions . Chris is a degreed engineer with over 10 years of experience in the gasketing business who specializes in field support and engineered application solutions . He resides in Parkersburg , WV with his wife and children .
Graphite braided packing . www . valve-world-americas . net • September 2023 | Valve World Americas 31