O&M Notebook: Leak Reduction Programs

Tag : Steel Die Sets

The focus of the FLMug is to bring utility and vendorrepresentatives together to share best practices and generateprogram and technical documents that will provide users with theknowledge to implement a good plant leak reduction program and gainaccess to new sealing product technologies. Although this group wasformed by nuclear, fossil would get as much—if notmore—benefit from the deliverables of this group.
A quality plant leak reduction program consists of the following: Ownership—“Go To” personnel at the site withknowledge in fluid sealing technologies and applications Training—All levels of user and personnel involved have thetraining in current fluid sealing technologies State-of-the-Art Materials—Modern sealing technologies areused and associated to critical plant equipment Database—A single database is used to identify, track andtrend leaks and another for configuration management and correctiveactions.
Using a database to identify, track, trend and correct leaks allowssites to set key performance indicators, monitor results andunderstand where their problems are. Using this programmaticapproach to fluid sealing has been proven to virtually eliminateall leaks in nuclear and fossil power plants.
Fluid Sealing Technologies
Most nuclear and fossil power plants continue to struggle withpacking and gasket leakage. Flexible graphite has been the materialof choice to replace asbestos-based sealing products since theearly 1980s. Most stations use flexible graphite-filledspiral-wound gaskets, metal pressure seals, graphite packing setsand/or bulk packing and non-asbestos synthetic sheets for generalpurpose gasket applications.
After 20 years of operational experience with these products,plants are still experiencing leaks. Because of this, the last fiveyears have seen some amazing advancements in fluid sealingtechnologies used in a power plant.
Here are some of the major issues that plants have addressed in thearea of fluid leak reduction: Graphite Packing Systems—Standard five-ring combinationgraphite yarn/die-formed flexible packing systems have highfriction that affects valve operability and will not seal manyvalves in primary pressurized water reactor (PWR) systems, leadingto boric ccid leaks in nuclear power plants. These packing systemswill not effectively seal a high-pressure valve in fossil stationsdue to the weak construction of the filament yarn. Wire Reinforced Valve Packing Material—The shift to usinginconnel jacketed braided graphite has led to serious stem scoringand difficult removal. Spiral Wound Radial Buckling FME—Graphite-filled spiral-woundgaskets used without inner containment rings will“buckle” in to the flow stream causing major FME andfuel fretting issues for nuclear. Flexible Graphite Oxidation—Flexible graphite oxidizes above600 F, the rate at which this occurs being affected by time,temperature and exposure. The condition is most prevalent inspiral-wounds in raised faced flanges. This is mainly a fossilissue with the exception of the diesel generator exhaust gaskets innuclear. Metal Pressure Seals—Silver plated metal pressure seals havebeen a major source of leaks. They require very closetolerances/finishes and become lodged in the valve
Graphite Valve Packing Systems
Most nuclear and fossil stations (as well as valve manufacturers)have standardized on a “typical” five-ring consistingof graphite filament yarn anti-extrusion rings and die-formedflexible graphite. A graphite bushing is used to take the space ofexcessive packing rings.
This packing system has provided good performance but has highfriction when used in modulating control valves, marginal MOVs andcheck valves that require “free swing” testing. Also,in PWR primary systems and fossil primary systems the gland stressrequired to seal (greater than 4,000 psi) fracture the graphitefilament yarn ring and shorten the packing life. This also leads toboric acid leakage in primary systems.
This packing system also does not address stem misalignment andside-loading, which are the most common causes of packing failures.The use of a low-density graphite bushing below the packing setonly serves as a spacer and does nothing to align the stem toreduce wear on the packing system.
Spiral-Wound Radial Buckling and Oxidization
Use of flexible graphite as a filler in spiral-wound uncontainedflange designs leads to buckling of the stainless steel windingsinto the flowstream. This foreign material intrusion has been amajor issue across the industry and is a cause for fuel fretting innuclear stations.
To address this issue, a gasket design needs to be used. Any typeof flange design that is open to the bore and not contained(including raised-face, male-female and flat-face) requires the useof an inner containment ring.
Fossil stations have experienced extensive problems with flexiblegraphite gasket sheet and graphite filled spiral-wounds inapplications above 600 F. This has been experienced in turbinecross over gaskets, check valve lids and pin gaskets, boiler manwaygaskets and raised face flanges, to name a few.
Valves that use a pressure seal as the means to seal thebody-to-bonnet joint were originally designed to utilize a platedmetal ring to provide a seal. The internal force on the bonnetdeformed the seal at the angle between the bonnet and body creatinga seal. The surface finishes of the valve components and the sealhad to be perfect to get a leak-free seal. In addition, the sealbecomes bound to the adjoining metal surfaces making disassemblyvery difficult.
Every packing and gasket leak can be solved and is avoidable withtoday’s technology and good maintenance practices. Getting toleak-free performance requires investment in training, dedicatedexpertise, high quality sealing products and web based technologybut is worth the investment in improved equipment reliability,safety, dose and corrosion prevention.
Author: Doug VanTassell is the Executive Vice President of APServices Inc. and has been involved in fluid sealing programs forpower plants for the past 22 years. He earned an MBA from QueenUniversity.