Chemical Engineering June 2010 - 45

Environmental Manager
Table 1. Sealing TypeS
Seal Type Description
Dieformed
flexible
graphite
*
Flexible graphite with a minimum carbon content of
95% is die-formed into rings with braided carbon or
graphite yarn end-rings
* The die-formed rings are flat and come in various
densities. Higher density rings are used for higher
service pressures
* Temperature capability to 850ºF in atmosphere and
1,200ºF in steam; pressure capability to 4,000+ psig
* Usually capable of 500-ppm leak performance
Braided
flexible
graphite
(Figure 2)
* 95%+ carbon purity, usually with
proprietary yarn treatment
* Wire reinforcement of the flexible
graphite yarn is common
* Temperature capability to 850ºF in atmosphere,
1,200ºF in steam; pressures to 4,500 psig
* Capable of <500 ppm and <100 ppm performance
Engineered
sets
(Figure 3)
* Combination of die-formed flexible graphite
rings of various geometries and densities
and braided yarn or braided flexible
graphite yarn packings
* Good to 850ºF in atmosphere, 1,200ºF
in steam and pressures of 10,000 psig
* Capable of <500 ppm
and <100 ppm performance
Bellows
sealed
valves
* Metal bellows seal incorporated into
the valve design. Packing type seals
are used as secondary seals
* Temperature and pressure capabilities
depend on bellows metallurgy, design and
construction; should match the pressure
class and material rating of the valve
Live
loading
(Figure 4)
* Disc spring (Belleville)
washers are compressed
on the gland
follower under the
gland stud nuts
* Temperature and
pressure capabilities
depend on the type
of seal used; live loading
does not enhance
these ratings
braided flexible graphite (Figure 2),
engineered seal sets (Figure 3), bellows
sealed valves and live-loaded packing
sets (Figure 4). Table 1 gives details for
these various seal types.
Performance standards
Two standards for valve seal performance
are API 622, " Type Testing of
Process Valve Packing for Fugitive
Emissions " and ISO 15848, " Industrial
Valves - Measurement, Test and
Qualification Procedures for Fugitive
Emissions - Part 1: Classification
* Some types date to the emission demands of California
1970s - the first clean air laws
* Superior emission performance
* Sets are made to the specific valve stem and box sizes
* Some feature simpler installation procedures
to compress the set
* Engineered sets are preferred by OEM valve manufacturers
wanting low emission performance with assembly
line speed of installation
* Virtually zero emissions
* High cost at multiple times that of a standard packed
valve
* If the seal fails, there is no possibility of adjustment.
The valve must be taken off-line and rebuilt or replaced
* In some cases elongated valve bonnets are required
to accommodate the bellow. Space can be an issue
* Can be used with any packed valve
* Effectively increases the energy in the gland stud bolts.
As packing consolidates, there is less degradation
of compressive load for better seal maintenance
* Live loading represents added expense, but provides some performance
enhancement for valves subject to numerous actuations or thermal cycles
* A good solution for valves that are difficult to monitor and access for future
adjustment
* Debate continues regarding its effectiveness since carbon/graphite packings
consolidate very little
System and Qualification Procedures
for Type Testing Valves. " Introduced
in 2006, API 622 provides test methods
for fugitive emissions, corrosion
and physical characteristics of valvestem
compression packing independent
of valve type (Figures 5 and 6).
ISO 15848 qualifies the entire valve
including its sealing components.
API 622 prescribes only test
methodology, whereas ISO 15848
prescribes both test procedure and
pass-fail criteria. Other standards
include Germany's TA-Luft for
qualifying packing and gasket performance,
as well as the standards
of individual refiners and chemical
processing companies. Table 2 provides
the basic criteria of API 622
and ISO 15848.
Proper installation
Just as important as selecting the right
seal for a particular valve application
is making sure it is installed properly.
Correct installation insures more even
compression of the packing, which results
in better emissions performance
ChemiCal engineering www.Che.Com June 2010 45
Attributes
* This method has been providing adequate emission
performance for over 30 years, but may not
attain the low leak rates demanded by the most
stringent air-quality-management districts, consent
decrees and standard performance specified
by plant end users
* Rings are made for a specific valve stem and
box sizes, and may require adjustment to obtain
and maintain low emission results. Multiple-step
installation required
* Introduced in the 1990s, it provides superior
emissions performance
* One size braid can be used to pack many different
sizes of valves, but may require adjustment
at startup to obtain low emission results
* Multiple-step installation; easy field replacement
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Chemical Engineering June 2010

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