Hydrocarbon Processing - November 2021 - 86

Valves, Pumps and Turbomachinery
The third offset consists of a seat-andseal
geometry that resembles an inclined
cone. The inclined bevel of the disc seal
and the integral body seat form a very
tight junction along the entire sealing
surface as the valve reaches the closed
end-of-travel position. This inclined cone
geometry is a critical design component
because it eliminates any rubbing between
the seat and seal.
As the valve closes, the seat and seal
contact simultaneously around the entire
circumference of the valve, and then
torque is applied to the shaft to fully seat
the disc seal. This provides a bidirectional,
zero-leakage seal, independent of line
pressure. Quality valve manufacturers can
achieve bidirectional, zero-leakage seals
with both laminated and solid disc seals.
Some TOV manufacturers claim to
have a fourth, and even a fifth, offset in
their designs. Considering that highquality
TOVs are completely non-rubbing
and have bidirectional, zero-leakage
seals, providing additional offsets is not
necessary and may even detract from the
valves' performance.
The TOV design achieves bidirectional,
zero-leakage performance utilizing
metal-to-metal seats. It is also fire safe
and rated up to Safety Integrity Level 3
(SIL-3) for use in critical areas where
safety instrumented systems are often applied.
Additionally, these valves are temperature
rated up to 815.6°C (1,500°F)
and offer alternate designs for cryogenic
service-utilizing quarter-turn actuation
to minimize fugitive emissions from the
stem packing, while providing greater reliability
and ease of maintenance.
Fugitive emissions. Fugitive emissions
have become a major concern for
operators. With consent decrees and
environmental awareness on the rise, a
valve's stem seal performance should be
taken into consideration, as it directly affects
leak detection and repair programs.
There are many facets within the scope of
fugitive emissions.
API 622 and API 624 address the
FIG. 2. The TOV design employs three offsets to
make the entire disc and body sealing surfaces
touch at the same time as the valve closes.
packing material and packing gland efficiency,
respectively. EPA Method 21, TA
Luft and ISO 15848-1 standards provide
acceptable emission levels and testing criteria
for the evaluation of the efficacy of
the valve packing gland. While the rates
of leakage and the material selection for
the packing are paramount to selecting a
valve, most manufacturers adhere to the
limits set forth by these standards.
For the purposes of this article, the focus
will be on the type of packing gland,
specifically rising stem vs. quarter-turn
operation
of the
automated isolation
valve. With respect to critical and severe
service applications, the use of risingstem
gate valves and rising-stem ball
valves have been the standard for years.
These rising-stem isolation valves can
be replaced with a quarter-turn TOV with
live-loaded packing to reduce fugitive
emissions and maintenance expenses significantly,
while increasing uptime. With
rising-stem valves, the stem is pushed/
pulled through the packing. Any foreign
particles or changes in stem diameter
due to temperature gradients will lead
to a stem seal leaking at an unacceptable
rate-requiring repair or replacement at
significant expense, along with a possible
interruption in production.
A quarter-turn valve with live-loaded
packing is a more effective solution for
fugitive emissions control. The area of
the shaft in the packing gland rotates in a
quarter-turn motion, so that the shaft in
that area is protected from external and
internal disruption. The live-loaded nature
of the packing gland automatically
adjusts to minor changes due to normal
wear and temperature changes.
This live loading is accomplished
via Bellville washers, which are concave
washers arranged in pairs, with the opposing
concave surfaces facing inward upon
themselves. These washers are completely
compressed by the gland nuts, effectively
providing a compact spring-loaded
gland packing.
While these points provide a base of
knowledge to draw upon, there are areas
that may require consultation with a valve
vendor or manufacturer for a more detailed
analysis and for recommendations
for particular applications.
FIG. 3. The diagram on the left depicts a true top-entry design, where all internal parts can be
safely, quickly and easily removed for maintenance or inspection. The diagram on the right shows
a pseudo top-entry design that makes maintenance and inspection unsafe and time consuming.
86 NOVEMBER 2021 | HydrocarbonProcessing.com
Valve selection for specific applications.
The following examples discuss
several difficult applications and provide
suggestions for addressing them.
Sulfur recovery units pose extreme
challenges for automated valves. Solidi

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Hydrocarbon Processing - November 2021

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