Chemical Engineering June 2023 - 23

Facts At Your Fingertips
Control-Valve Emissions and Packing
Department Editor: Scott Jenkins
F
ugitive emissions - defined
as
unintended
releases
volatile organic compounds
(VOCs) from process equipment
to the atmosphere - have been a
long-standing issue in the chemical
process industries (CPI). Valves and
pumps are two of the leading sources.
With valves, the primary cause of
fugitive emissions is failure of packing
seals and gaskets, which are
designed to maintain a seal between
the valve bonnet and stem. This column
describes potential solutions to
address valve-packing failures.
Two of the most common ways to
prevent fugitive emissions from valves
are using live-loaded packing and bellows
seals. Live-loaded packing, a
method where a compressive force is
applied to the packing material so that
it tightly seals against the interior of the
valve bonnet, can be paired with nearly
any valve configuration to achieve very
low leakage rates. Bellows seals are
only available with sliding stem valves,
but provide even lower leakage rates
- near zero in most cases.
Seal selection
Seal-system selection is based on
the specific requirements of the application,
but some some general
guidelines apply in most cases. Solutions
emitting no more than either
500 parts per million by volume
(ppmv) or 100 ppmv, depending on
the location of the facility, are usually
required. Live-loaded packing is
sufficient in most cases to achieve
compliance, but bellows seals are
often used to further reduce (or even
eliminate) emissions.
End users typically work with a valve
vendor to determine the best solution
for an application by balancing price
and performance. Live-loaded packing
is less expensive than, but not as
effective as, bellows seals. When process
media is lethal, volatile or radioactive,
bellows seals are the industry
standard because they prevent the
stem and packing from coming into
contact with the process media. The
result is leakage rates down to 1 ×
10-6 cm3/s of helium.
Materials
of
When live-loaded packing is used,
the material of choice is typically either
polytetrafluoroethylene (PTFE) or
graphite. PTFE offers near-universal
chemical compatibility with many
types of process media. It also provides
very low friction, which allows
for smaller sizing of actuators, but it
has process-media temperature limitations.
Typically, vendors use 450°F
as the upper limit for PTFE packing.
Similar to PTFE, graphite is compatible
with most process media,
but graphite-based packing systems
can be used in applications for VOCs
where the process media temperature
is up to about 750°F. Graphite
also has the additional upside of being
inherently fire-safe.
Some packing systems provide capabilities
of both PTFE and graphite,
making them the best fit in certain applications.
This is achieved by using a
layer of graphite packing rings closer
to the process media, and a layer
of PTFE rings farther away from the
process media. These systems provide
low friction and emissions, and
in some cases, an API 589 fire-tested
solution, for operation at process media
temperatures up to 450°F.
Leak testing
For all types of valves, testing is required
to ensure effectiveness in operation
(Figures 1 and 2).
International Organization for Standardization
(ISO) 15848 applies to
both control and isolation valves, and
has two components. A vendor test
(ISO 15848-1) proves the capabilities
of a technology, and a production test
(ISO 15848-2) proves the production
equipment meets the requirements
while in operation. The testing requirements
for ISO 15848-1 include
compliance with several combinations
of leakage classes, thermal cycles
and mechanical cycles to qualify
for certification.
ISO 15848-1 specifies use of either
the vacuum or flushing " total
leakage " measurement methods described
in Annex A of the standard.
Leakage is recorded as a leakage
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JUNE 2023
FIGURE 1. Testing for requirements for ISO 15848-1
include compliance with several combinations of
leakage classes and thermal and mechanical cycles
FIGURE 2. Leakage is recorded as a leakage rate
per measured valve stem size
rate per measured stem size. ISO
15848-1 states there is no correlation
intended between the leakage
tightness classes, whether the test
fluid is helium or methane. The other
popular method to detect leakage is
Method 21 from the U.S. EPA, which
is also specified in ANSI/FCI 91-1.
This method stipulates leakages between
500 and 100 ppmv.
Performing these leakage measurements
in the field is both costly
and time consuming, as it requires
sophisticated listening devices operated
by a skilled technician to detect
leaks. But if a company has demonstrated
success with detecting and
correcting issues, monitoring-period
extensions of up to one year, instead
of every three to six months, can be
implemented. For frequent offenders,
monthly measurements need to be
taken. Specifically, facility personnel
must prove that no more than 0.5%
of its total valve population has issues
with stem or shaft leakages if they are
to extend monitoring periods.
Editor's notes: 1) The content for this edition of " Facts at your
Fingertips " was developed by Andrew Prusha, director - Chemical
Process Industries, Emerson; 2) For more on valve emissions, see:
www.emerson.com/documents/automation/control-valve-handbooken-3661206.pdf.
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Chemical Engineering June 2023

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