During a power plant operating cycle, valve maintenance and reliability are key focus areas, often taking up the largest portion of an outage budget. Plant personnel responsible for maintaining the equipment must consider multiple variables present in the entire local valve “system” including: equipment such as the valve body, trim set, actuator and sub-assemblies; media parameters such as media type, temperature, pressure, piping design and environmental factors; and valve packing. Yet despite the fact packing can be a significant contributor to poor valve performance and can cause significant leakage, this part of the equipment picture is often an afterthought in the maintenance process.
The various properties of packing materials result in different performance characteristics. This article examines the features and benefits of different valve packing materials, with a specific focus on friction characteristics and their impact on control valve performance.
The Importance of Packing Friction
Some industries, such as nuclear power, frequently use sophisticated diagnostic tools that allow an end user to simulate operating conditions by cycling a valve open and closed during maintenance activities. These tools track many variables in the valve and actuator assembly during a stroke; if a control valve fails a diagnostic test or the results are abnormal, one of the first variables analyzed is the friction profile created by the resistance of stem movement generated by the stem sliding through the valve packing.
The key benefit of these diagnostic tools is the ability to generate a friction profile. Generally, the tools plot a graph showing the amount of friction generated in the valve system throughout the entire valve stroke. The diagnostic software then calculates the friction during the open and closed stroke independently, as well as the average friction for the full open and full close stroke combined. This friction profile can be used to identify several potential issues the valve may have internally before it is put back into service.
When viewing the friction profile, it is relatively easy to identify issues such as whether the valve has too much or too little friction or whether the valve has a non-linear friction, which potentially indicates stem wear or necking. Though there are situations where too little friction can be an issue, this article focuses on reducing packing friction to increase valve margins and operability. Increasing friction can be achieved relatively simply by increasing sealing rings or packing stress; decreasing friction while maintaining leak-free service is much more complicated.
Challenges to Decreasing Friction
Valves with too much packing friction can suffer many negative effects, including hunting or actuator overcorrection; stem wear; actuator degradation; and reduced equipment reliability and operating life. All these factors may lead to downtime or equipment failure. Reducing packing friction while maintaining leak-free operation requires a delicate balance that is greatly dependent on valve packing materials.
The friction attributed to valve packing is generally represented as the product of two variables, f and y, in the packing friction equation:
Packing Friction (L) = Gs*π*f*y*Di*H*N, where Gs is gland stress, π is 3.14159, fy is the coefficient of packing friction, Di is the inside diameter of valve packing (stem diameter), H is the packing ring height and N is the number of packing rings.
The dimensionless fy value is a product of two individual values, the coefficient of friction of the packing material, represented by “f,” and the transfer ratio of axial stress to average radial stress, represented by “y.” These two values are multiplied together to form a single variable, fy, which represents the overall coefficient of friction of the packing on the stem.
Valve packing manufacturers have focused on reducing the fy values of their packing products for years, seeking to identify the ideal combination of materials to provide end users with packing that offers consistent sealing capabilities while maintaining low friction. Many packing types excel at one of these two aspects while lacking in the other. In other words, where one material might be effective at sealing but produce a high friction load, another material might produce low friction load but struggle to maintain sealing properties for multiple cycles.
This imbalance often forces the end user to select a packing type based on its strength in one capability, while compromising on its performance in the other.
Valve Packing Materials
For many years, most valve packing was made from asbestos and installed with various forms of artificial lubricants. Eventually, the negative side effects of asbestos inhalation were recognized, and the industry searched for and found a replacement that was relatively inexpensive and abundant: graphite. Graphite is readily available and was used as a lubricant for many decades.
This widely accepted replacement material for most valves was a combination of braided graphite yarn and die-formed flexible graphite. The braided yarn rings and die-formed flexible graphite packing rings were used together to achieve stem sealing. The configuration would typically include three, die-formed flexible graphite rings in the middle and two braided yarn rings on the ends to keep the inner die-formed rings from extruding (Figure 1, above).
While this solution is inexpensive and provides effective sealing, diagnostic testing has shown that this packing set induces relatively high friction and has a relatively limited life in control valves. One of the characteristics of using the die-formed flexible graphite rings is that when these rings are manufactured, they are generally pre-formed to 90 pounds per cubic foot density. To achieve this density, the graphite has to be compressed in a die to roughly 2,000-2,200 psi of axial stress. This means a die-formed ring needs to overcome the forming pressure to seal. Therefore, typical recommended gland stresses for die-formed packing sets range from 3,000–5,000 psi. When considering the aforementioned packing friction equation, it is evident that as greater gland stresses are applied, greater packing friction is applied to the stem.
Properties and benefits of PTFE Packing
The introduction of packing containing polytetrafluoroethylene (PTFE) was revolutionary in the world of control valve packing. Many of the mechanical properties of PTFE are attractive for use as a stem sealant in control valves; it is non-porous, chemically inert and stable up to relatively high temperatures (generally having upper temperature limits ranging from 350°F or 177°C to 615°F or 324°C, depending on the properties of the PTFE used to manufacture the packing). During the last few decades, valve packing containing PTFE has been used to reduce the friction between the stem and packing, thus allowing the valve to operate more smoothly while decreasing wear and increasing equipment reliability.
PTFE-based packing is not without its limits and not all operating conditions are suitable for this material; temperature and radiation limits must be taken into consideration.
In an attempt to verify and quantify the benefits of PTFE-based packing using the information available, the author of this article and of the study it references compared the friction coefficients in control valves using various forms of valve packing. The study was not intended to be considered scientific—a proper statistical analysis has yet to be performed. But this article compares information to examine differences in packing types. The study uses data obtained from a web-based valve packing database software used in the domestic power industry.
The packing database used in the study allows end users to input certain valve parameters to predict the friction induced by a packing set. Users input various pieces of data, such as stem and stuffing box diameter, packing gland stud diameter, the number of gland studs, and the desired packing (gland) stress; the software then uses a set of stored default fy values to predict the total friction induced by the valve packing. These predicted friction values become especially important for valves that are required to operate within a certain time domain. If the predicted friction is too high, the end user may choose to use alternative packing materials or reduce the number of rings to bring the predicted friction back within an acceptable range.
The default fy values associated with each packing type are based on packing manufacturer input, oftentimes coming from internal testing. Generally, this testing is performed in lab conditions and may represent an “ideal” situation, while the real-world coefficient of friction may vary widely based on several factors in the field. To more accurately predict the fy values, a feature was added to the software application to allow end users to input the actual measured friction obtained from diagnostic testing for each control valve. Based on this real-world data from the field, the program reverse-calculates the actual measured fy value of the valve packing for each individual valve.
The goal was to gather enough data points so that the default values could be revised based on field data to more accurately predict real-world friction. This data presented the ability to compare packing data in aggregate. When charted to a spreadsheet, the data shows how certain packing types compare to one another with respect to their real-world fy values. One of the most significant comparisons was real-world fy values of standard yarn and graphite packing sets versus PTFE-based packing sets.
Comparing Real-World Values
Roughly 1,000 records were compared, and the average real-world fy value was calculated for each packing type. As shown in Table 1, the average real world fy value for packing sets containing braided yarn rings and die-formed flexible graphite and no PTFE was 0.065; the average real-world fy value for PTFE braided rings with die-formed flexible sealing rings was 0.053; and the real-world fy value of those packing sets that contain PTFE braided rings and no die-formed flexible graphite rings was 0.043.
While the data set is relatively small because of the number of valves diagnostically tested since data tracking began, the results support a consistent ability of PTFE-based valve packing to reduce friction. The data demonstrates that using PTFE-based backing can reduce the coefficient of friction by roughly 34%, which equates to a significant increase in actuator margin and valve operability.
Based on this available data, PTFE valve packing offers improved performance over graphite and yarn packing in control valve applications.