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.