Figure 3. Engineered packing sets are capable
of average leakage rates of less than 100 ppm.
compressed on the gland follower under
the gland stud nuts to effectively increase
the energy in the gland stud bolts. Tests
have shown that it can enhance the performance of all types of packing sets,
especially those subject to thermal
cycling. However, debate continues as to
its effectiveness with carbon/graphite
packings, which tend to consolidate very
little.
To determine which sealing solution is
best for which valve, start with a basic
check list of application parameters,
Static Leakage Chart
Maximum Reading
4.5E-05
including temperature, valve type, valve
function, media passing through the
valve, pressure, stuffing box dimensions,
condition of the valve, actuator sizing
limitations and valve friction requirements.” Armed with this information, a
packing supplier can provide reliable
recommendations.
The importance that valve condition
plays in sealing performance and
longevity cannot be overstated. For
instance, a valve with a bent stem will
maintain low emissions only until it is
actuated. After that, it will become a
chronic leaker, forever out of compliance
until the stem is repaired. Valve function
is also a critical factor in packing selection. Choosing packing for a continuously actuating control valve with
thousands of actuations a year requires
more careful consideration than a block
valve actuated two or three times a year.
Packings can be tested to qualify
them for various types of service, which
450
500
also helps to assure fugitive emissions
requirements are met. Two popular independent standards for valve seal performance are API Standard 622, “Type
Testing of Process Valve Packing for
Fugitive Emissions” and International
Organization for Standardization (ISO)
15848, “Industrial Valves – Measurement, Test and Qualification Procedures
for Fugitive Emissions – Part 1: Classification System and Qualification Procedures for Type Testing Valves.”
Additional standards are published by
TA Luft, VDI and others.
Both API 622 and ISO 15848 qualify valve stem packings by subjecting
them to a number of actuations and temperature excursions for several days or
weeks. ISO 15848-1 requires prototype
testing at full pressure and temperature,
with global (100%) capture of the leakage out of the packing box. ISO 15848-
1 allows the use of helium or methane
for a test media. For production testing,
Static Leakage Chart
Maximum Reading
4.0E-05
400
400
3.5E-05
Leakage (atm cc/sec)
3.0E-05
2.5E-05
2.0E-05
Leakage
Temperature
1.5E-05
350
300
250
200
Nominal Test Temperature (C)
150
>500 PPMv
Leakage (PPMv)
200
300
1.0E-05
100
5.0E-06
50
0.0E+00
0
125
250 375
Cycle Number
0
500
Figure 4. Shown here are ISO 15848 helium emissions and temperature
data. Leaks are detected in a pressurized valve as temperature cycles from
ambient to 752° F [400° C].
100
0
0
250
500 750 1000
Cycle Number
1250 1500
Figure 5. These sample results of an API 622 methane emissions test
indicate adjustment of the packing after emissions spiked above 500
ppm. It should be noted the test has no inherent pass/fail criteria, but
rather are established by the party requesting the test.
