Chemical Engineering June 2010 - 43

Flat
washer
Gland
stud
Solids Processing
Environmental Manager
Containing
Fugitive
Emissions
Practical ways to seal valve stems
and prevent unwanted emissions
FIGURE 2.
Jim Drago
Garlock Sealing Technologies
C
ontaining fugitive emissions
of volatile organic compounds
(VOCs) and hazardous air
pollutants (HAPs) is a challenge
to the chemical process industries
(CPI). It has been estimated that
these industries account for half of all
fugitive emissions, and 60% of these
emissions are the result of valve stem
leaks. In addition to environmental
benefits, effectively containing these
emissions can yield significant operational
and economic benefits and avoid
punitive penalties for non-compliance
with regulatory standards. If cap-andtrade
or carbon-tax programs become
law, then reducing emissions below
mandated levels could yield carbon
credits or reduced tax liabilities. Overall,
emission reduction improves operating
efficiency and creates a safer,
more productive workplace.
This article describes how to effectively
seal valve stems, including leak detection
and repair (LDAR); various types of
sealing solutions, their advantages and
limitations; performance standards and
testing; and proper installation.
Environmental impact
VOCs and HAPs are major contributors
to ground-level ozone, a significant
component in smog, which can cause
respiratory illnesses. Some VOCs and
HAPs are known or suspected carcinogens.
The U.S. Environmental Protection
Agency (EPA) prescribes proactive
LDAR programs, including identifying
leaking components, comparing leakage
levels to compliance standards,
making the necessary repairs, ongoing
monitoring and measurement, recording
and maintaining data, taking
corrective actions, training and audits.
These programs are costly and time
consuming, and involve thousands
of plant components, such as valve
stems,
Braided, flexible graphite
packings deliver low leak
performance for field repacks
flanged-joints, pump seals,
pressure relief devices, end connections
and others. Plant personnel devote
much time and effort to gathering
information, maintaining databases
and generating the requisite reports,
all for the ultimate objective of stopping
leaks.
To avoid leaks in the first place, the
EPA encourages the use of low-leak
valve and packing technologies. Current
consent decrees are giving attention
to the most prevalent method of
controlling valve stem emissions -
compression packing (Figure 1).
Valve stems
Studies have indicated that leaking
valve stems are by far the single largest
source of fugitive emissions in processing
plants. These emissions can be
controlled by following simple guidelines
that take into account the valve
and its service conditions, the seal
supplier's recommendations,
proper
seal installation and ongoing performance
monitoring.
Obtaining clear input on the type of
valve to be sealed and its mechanical
condition is the logical starting point.
The most-effective sealing solution
depends on whether the valve is occasionally
actuated, such as a manually
operated gate valve, or a continuously
actuated control valve. Poorly maintained
equipment can cause stem
packings to fail, so it is important to
inspect the physical condition of the
valve for damage to the gland studs
or stem, which if bent or gouged can
push into or tear the packing.
Next considerations include the
temperature, media and pressure to
which the valve seal will be subjected,
as well as the level of sealing performance
required to comply with federal,
state and local regulations, consent
decrees and company standards.
It should be noted that while federal
regulations may require seal performance
with a maximum leakage of
10,000 ppm, most states and consent
decrees mandate 500 ppm and lower.
Some local air-quality-management
districts may require levels as low as
250 ppm.
Sealing types
Different types of seals have different
performance attributes in terms
of valve actuation force; interaction of
axial compression to radial expansion
of the packing; friction; emission level;
and the ability to retain and adjust a
seal for compliance. There are a number
of viable choices for valve stem seals,
notably die-formed flexible graphite,
ChemiCal engineering www.Che.Com June 2010 43
Yoke
Nut
Gland
follower
Stem
FIGURE 1. This cross-sectional view
shows a typical valve-stem
sealing assembly
Packing
http://www.Che.Com
Chemical Engineering June 2010

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