Leak
quantification
Leak is quantified as parts per
million (ppm).
Cost About 50¢ per component
Frequency
Data analysis Straightforward interpretation
of survey data
Measurement
challenges
Wind and technician adherence to
M21 procedure affect the data.
Training
Repair
AWP (optical gas imaging)
AWP is a qualitative, not quantitative tool, which can generally detect leaks of
5,000 ppm and greater. Latest advances have improved sensitivities.
Some estimate less than 50¢ per component (as low as 25¢), but because AWP
requires more frequent surveys, the two methods can cost about the same.
Quarterly and yearly monitoring Every 30, 45 or 60 days depending on sensitivity level chosen, plus one annual
Method 21 survey3
Video records needed to demonstrate regulated components are monitored. Video
data are open to interpretation.
AWP sensitivity is affected by temperature gradient. Ambient temperature and
media being monitored also affect video records. Components need to be viewed
from various angles and lens adjustments are necessary for sharp images.
Training required. Generally viewed
as basic technician skill level.
Higher skill level requiring training on camera use and leak recognition.
Monitor and repair only regulated
components.
Safety
Technician is in potentially
dangerous areas.
Entire areas are scanned; regulated and unregulated equipment are monitored.
Unregulated components are subject to repair. Repair of unregulated components is
voluntary, but if leaks are found, they are typically repaired consistent with the spirit
of the regulations and general housekeeping.
Technician is at a safe distance, could be 20 to 50 ft., from the leak point, whereas
M21 sniffer wands, typically 6” to 12” long, require extremely close proximity to
leaks of potentially harmful substances.
ing and properly installing the right valve
stem packing can keep the valve in compliance without such costly measures,
while also reducing process fluid loss.
SUCCESSFUL PACKING
TECHNOLOGIES
In many plants, a conflict exists between
the LDAR/environmental and maintenance departments. This is because the
LDAR/environmental department is
charged with meeting regulations, but
maintenance needs to keep the plant running. Good packing technologies and
practices, combined with proactive maintenance and LDAR programs, can meet
the objectives of both those departments.
The success of such programs
requires information on a variety of factors, including valve and service conditions, packing box dimensions and leak
histories. Success also requires a common database linking the needs of maintenance and LDAR groups. Based on this
data, intelligent choices can be made
regarding the type of valve stem packing
that can provide the longest-lasting compliant seals.
Valve stem sealing for VOC/HAP
service involving different types of
graphite materials are summarized in
Table 4 (below).
Packing sets can be live-loaded to
reduce degradation of compressive load
for better seal performance. Live loading
uses spring-disc or Belleville washers
Table 4. Packing types for low fugitive emissions
Seal Type
Die-formed flexible graphite
with braided carbon yarn
packing end-rings.
Braided flexible graphite
Description
Most basic of today’s valve stem sealing solutions.
Flat die-formed rings come in various densities.
Temperature capabilities of 850º F (454º C) in
atmosphere, 1,200º F (649º C) in steam, and
pressures of 4,000+ psig
Wire-reinforced flexible graphite yarn.
Same temperature and pressure capabilities as
die-formed graphite.
Engineered sets
(See Figure 3)
Advanced technology spool
packings
Combination of die-formed flexible graphite rings of
various geometries and densities and braided yarn or
braided flexible graphite yarn packings.
Same temperature ratings but good to pressures of
10,000 psig.
Allow on-site creation of ultra-low-emission packing
sets using different types of braided flexible graphite
in combination.
Effectiveness
Usually capable of 500 ppm leak performance. This
method has been providing adequate emission
performance for over 30 years, but may not attain
the low leak rates demanded by some air quality
management districts and end users.
Capable of <500 ppm and <100 ppm performance.
Introduced in the 1990s, it provides superior
emissions performance.
One size braid can be used to pack many different
sized valves.
Capable of <500 ppm and <100 ppm performance.
Some types date to the emission demands of the first
clean air laws in California in the 1970s.
Qualification testing indicates average leakage of as
low as 50 ppm.
