T. McMahon, Emerson, Marshalltown, Iowa
Packing selection used to be a straightforward task that played a small part in a valve specification process that was much more involved. This changed, however, when the U.S. Environmental Protection Agency (EPA) passed the U.S. Clean Air Act amendments, which limited many valve packing emissions to 500 parts per million (ppm) and then to 100 ppm, and, in some cases, 50 ppm. Valve manufacturers addressed this challenge by introducing a wide variety of high-performance packing solutions to meet these new requirements.
The engineering rule of thumb is to apply low-emissions packing solutions to only those valves that have low-emissions requirements, and to use more typical designs for the remaining applications. Unfortunately, that logic results in many missed opportunities to reduce operating costs and improve valve performance.
This article addresses the differences in various packing solutions and suggests reasons why a low-emissions packing system may be a wise choice for many applications—even if reduced emissions are not a concern.
The basics of packing designs. To better understand the packing design selection process, it is helpful to have a good grasp of exactly what packing is, how it works and why there are so many design options.
Regardless of whether a control valve has a rising stem to open or close the plug (or a rotating valve shaft connected to a ball, disc or plug), there is a fundamental design problem. The valve stem must move relatively unimpeded, while keeping the process media inside from leaking to the environment. Even more challenging, this seal must continue to work, even as the valve cycles hundreds of thousands—and even millions—of times. These seemingly conflicting tasks are addressed by the control valve packing.
This packing consists of a series of PTFE, graphite or some other type of specialty polymer rings encircling the valve shaft (FIG. 1). In a standard packing design, the packing rings are compressed with a metal packing follower that pushes down and squeezes the rings sufficiently to seal them against the packing box walls and the valve stem.
The amount of compression, which is set by adjusting the flange bolts on top of the assembly, is critical. If the compression is set too low, the stem will move easily, but the packing will leak. If the compression is set too high, the packing will seal quite well, but the excessive force on the valve stem will impede movement and, ultimately, flow control. If the bolts are tightened too much, the valve stem cannot move at all.
Over time, the constant valve stem movement tends to wear the packing rings enough to start a leak. At that point, the packing bolts can usually be tightened to stop the leak, but constant monitoring and occasional maintenance are required to avoid packing leaks over time.
Low-emissions packing designs. The very low emissions rates mandated by the U.S. EPA Clean Air Act necessitated a completely new packing design. As mentioned previously, traditional packing designs tended to wear over time, with corresponding increases in leak rates. Current regulations require control valves to have very low leak rates, and to maintain those low values for an extended period.
Regulatory compliance necessitated significant changes in the packing design. The most important of these changes are the addition of live-loading springs to maintain a consistent and constant pressure, and the use of alternative packing ring materials. Both innovations are needed to achieve tight sealing, while still allowing smooth stem movement (FIG. 2). Most low-emissions packings use compressed Belleville springs to maintain a constant downward pressure on the packing, even as it wears over time.
This continual force allows the packing to significantly outperform standard packing designs, thus achieving and sustaining minimal emissions, even as the valve strokes through many operations. However, these springs are not useful if they compress the packing so much that the stem cannot move freely, so further design changes were necessary.
The second design modification of environmental packing involves the materials of the packing rings, as well as the valve stem itself. Highly polished and hardened stem shafts are often paired with PTFE packing rings to provide good sealing, yet still allow smooth valve movement. This works well for low-temperature applications, but not at process temperatures much above 204°C (400°F), as this is when PTFE begins to degrade.
At these elevated temperatures, graphite packing would normally be used, but it does not seal nearly as well, and it tends to bind the shaft at lower temperatures. In these applications, combinations of graphite, PTFE and carbon-reinforced PTFE are employed to achieve low leakage at elevated process temperatures (FIG. 3). These advanced combination packing designs can withstand significantly higher pressures and temperatures than PTFE packing. Note: These ratings are for higher temperatures applied to the packing itself, and process media temperatures could be higher—and often are in many applications.
Recently introduced low-emissions packing configurations incorporate a more complex series of carefully selected materials placed in very specific locations to achieve low-emissions performance at even higher pressures and temperatures. This expanded service condition range provides new opportunities to increase packing service life and reduce lifecycle costs for many valve applications.
Packing selection. Historically, packing selection has been relatively simple. If the service requires low emissions, then low-emissions packing is selected—otherwise, standard packing designs are used. In either case, the packing material combination is based on the anticipated process temperatures and pressures, along with material compatibility.
Live-loaded packing does have a significant advantage over standard packing designs since the packing compression remains constant over the life of the packing (FIG. 4). The packing in standard designs tends to wear and ultimately leak over time, so the packing compression must be adjusted on occasion. The compressed spring live-loading of environmental packing designs makes such routine maintenance unnecessary, and these valves tend to have a longer service life due to the consistent pressure loading provided by the Belleville springs.
Another major benefit of live-loaded packing occurs in high-pressure steam applications. Once steam begins to leak through packing, it establishes flow channels that can only be closed by tightening the packing to such a degree that the valve stem cannot move. Live-loaded packing maintains consistent pressure, which keeps the leak from starting in the first place, thus avoiding this problem.
When live-loaded, low-emissions packing was first introduced, it had a very limited pressure and temperature range. However, the latest environmental packing designs have greatly extended the allowable pressure and temperature limits in low-emissions service (FIG. 5A). If the application does not require < 100-ppm leak rates, then the allowable pressures and temperatures for those same packing designs are broader still (FIG. 5B), yet they leak significantly less than standard packing designs.
These much-expanded pressure and temperature limits allow live-loaded, low-emissions packing to be employed in a wide range of valve applications for both rotary and sliding-stem valves, providing lifecycle cost reductions due to minimal ongoing maintenance, very low leakage rates, consistent performance and extended service life. TABLES 1 and 2 compare various low-emissions packing styles for sliding-stem and rotary valves.
As TABLES 1 and 2 show, low-emissions packings provide a high degree of sealing. For example, the author’s company’s proprietary packing systemh has been enhanced to extend its performance with certifications at the 50-ppmv level, and with demonstrated performance as low as 3 ppm for 100,000 cycles. These packing designs also provide low packing friction and extended service life, while requiring minimal ongoing maintenance. Therefore, if a valve requires low emissions or not, most applications will see lifecycle cost improvements by switching to live-loaded, low-emissions packing.
Final considerations. When specifying a new control valve or overhauling an existing one, end users should evaluate live-loaded, low-emissions packing designs, regardless of the applicable emissions regulations. Low-emissions packing is clearly critical for environmentally sensitive service, but most any control valve will benefit from the consistently low leakage rates, minimal maintenance and extended service life provided by these packing designs. While the initial cost may be higher than a standard packing design, the long-term savings typically justify the price difference and more.
End users should consult their valve vendor to explore the options for their particular application and to determine what alternative packing designs might be available. The field of packing design offerings has expanded greatly, along with the opportunity to significantly improve valve performance and reduce operating costs with proper product selection. HP
NOTES
a Fisher™ ENVIRO-SEAL™ PTFE packing
b Fisher™ ENVIRO-SEAL™ Duplex packing
c Fisher™ ENVIRO-SEAL™ H2 Duplex packing
d DuPont KALREZ® with Vespel® CR-6100 (KVSP 500)
e Fisher™ ENVIRO-SEAL™ Graphite ULF packing
f Fisher™ V250 valves
g Fisher™ ENVIRO-SEAL™ graphite
h Fisher™ ENVIRO-SEAL™ packing system
Tim McMahon is the Global Product Marketing Director, Sustainability, for Emerson. He has more than 35 yr of experience in the valve and process control industry, and has held positions in applied research, design, testing, evaluation, marketing and sales. McMahon holds six patents and is a member of ASME B16 and ISO TC153 standards committees. He earned a BS degree in mechanical engineering from Kansas State University, and an MS degree in management of technology from Walden University.