By Doug VanTassel, A.P. Services

Many sites, nuclear and fossil, lack a program and subject matter expert for fluid sealing. External leakage is accepted as “normal” and an inevitable part of plant operations. In review of industry operating experiences from nuclear, there have been more than 70 operating experiences for packing, gasket and seals in the last 10 years. Many have led to plant downpowers and lost generation. External leakage is still a major issue in many pressurized water nuclear reactors due to boric acid corrosion.

There is a lack of information regarding plant leak reduction programs. Most sites do not attempt to implement one until a major failure occurs or an Institute of Nuclear Power Operations “area for improvement” is brought against the site. To address this, the nuclear industry organized a users group in 2007 called the Fluid Leak Management Users Group (FLMug).

The focus of the FLMug is to bring utility and vendor representatives together to share best practices and generate program and technical documents that will provide users with the knowledge to implement a good plant leak reduction program and gain access to new sealing product technologies. Although this group was formed by nuclear, fossil would get as much—if not more—benefit from the deliverables of this group.

A quality plant leak reduction program consists of the following:

• Ownership—“Go To” personnel at the site with knowledge in fluid sealing technologies and applications
• Training—All levels of user and personnel involved have the training in current fluid sealing technologies
• State-of-the-Art Materials—Modern sealing technologies are used and associated to critical plant equipment
• Database—A single database is used to identify, track and trend leaks and another for configuration management and corrective actions.

Using a database to identify, track, trend and correct leaks allows sites to set key performance indicators, monitor results and understand where their problems are. Using this programmatic approach to fluid sealing has been proven to virtually eliminate all leaks in nuclear and fossil power plants.

Fluid Sealing Technologies

Most nuclear and fossil power plants continue to struggle with packing and gasket leakage. Flexible graphite has been the material of choice to replace asbestos-based sealing products since the early 1980s. Most stations use flexible graphite-filled spiral-wound gaskets, metal pressure seals, graphite packing sets and/or bulk packing and non-asbestos synthetic sheets for general purpose gasket applications.

After 20 years of operational experience with these products, plants are still experiencing leaks. Because of this, the last five years have seen some amazing advancements in fluid sealing technologies used in a power plant.

Here are some of the major issues that plants have addressed in the area of fluid leak reduction:

• Graphite Packing Systems—Standard five-ring combination graphite yarn/die-formed flexible packing systems have high friction that affects valve operability and will not seal many valves in primary pressurized water reactor (PWR) systems, leading to boric ccid leaks in nuclear power plants. These packing systems will not effectively seal a high-pressure valve in fossil stations due to the weak construction of the filament yarn.
• Wire Reinforced Valve Packing Material—The shift to using inconnel jacketed braided graphite has led to serious stem scoring and difficult removal.
• Spiral Wound Radial Buckling FME—Graphite-filled spiral-wound gaskets used without inner containment rings will “buckle” in to the flow stream causing major FME and fuel fretting issues for nuclear.
• Flexible Graphite Oxidation—Flexible graphite oxidizes above 600 F, the rate at which this occurs being affected by time, temperature and exposure. The condition is most prevalent in spiral-wounds in raised faced flanges. This is mainly a fossil issue with the exception of the diesel generator exhaust gaskets in nuclear.
• Metal Pressure Seals—Silver plated metal pressure seals have been a major source of leaks. They require very close tolerances/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 consisting of graphite filament yarn anti-extrusion rings and die-formed flexible graphite. A graphite bushing is used to take the space of excessive packing rings.

This packing system has provided good performance but has high friction when used in modulating control valves, marginal MOVs and check valves that require “free swing” testing. Also, in PWR primary systems and fossil primary systems the gland stress required to seal (greater than 4,000 psi) fracture the graphite filament yarn ring and shorten the packing life. This also leads to boric acid leakage in primary systems.

This packing system also does not address stem misalignment and side-loading, which are the most common causes of packing failures. The use of a low-density graphite bushing below the packing set only serves as a spacer and does nothing to align the stem to reduce wear on the packing system.

Spiral-Wound Radial Buckling and Oxidization

Use of flexible graphite as a filler in spiral-wound uncontained flange designs leads to buckling of the stainless steel windings into the flowstream. This foreign material intrusion has been a major issue across the industry and is a cause for fuel fretting in nuclear stations.

To address this issue, a gasket design needs to be used. Any type of flange design that is open to the bore and not contained (including raised-face, male-female and flat-face) requires the use of an inner containment ring.

Fossil stations have experienced extensive problems with flexible graphite gasket sheet and graphite filled spiral-wounds in applications above 600 F. This has been experienced in turbine cross over gaskets, check valve lids and pin gaskets, boiler manway gaskets and raised face flanges, to name a few.

Valves that use a pressure seal as the means to seal the body-to-bonnet joint were originally designed to utilize a plated metal ring to provide a seal. The internal force on the bonnet deformed the seal at the angle between the bonnet and body creating a seal. The surface finishes of the valve components and the seal had to be perfect to get a leak-free seal. In addition, the seal becomes bound to the adjoining metal surfaces making disassembly very difficult.

Every packing and gasket leak can be solved and is avoidable with today’s technology and good maintenance practices. Getting to leak-free performance requires investment in training, dedicated expertise, high quality sealing products and web based technology but is worth the investment in improved equipment reliability, safety, dose and corrosion prevention.

Author: Doug VanTassell is the Executive Vice President of AP Services Inc. and has been involved in fluid sealing programs for power plants for the past 22 years. He earned an MBA from Queen University.