accident. The minimum required spac-
ing of these valves is prescribed in
ASME B31.4, “Gas Transmission &
Distribution Piping Systems” and
ASME B31.8, “Pipeline Transportation
Systems for Liquid Hydrocarbons &
Other Liquids.”
Several factors influence valve spac-
ing, including: 1) the amount of poten-
tial fluid leakage, 2) the impact of a
release, 3) future development in the
pipeline area, and 4) the time required
to blow down (empty) an isolated sec-
tion. Other criteria include how close the
line is to occupied buildings and houses.
According to B31.4, the distance
between block valves could be as little
as four miles apart for a gas pipeline.
Liquid pipelines have their own criteria for valve placement. They are
placed: 1) at the suction end and discharge ends of a pump station, 2) on
each line entering or leaving a storage
tank area, 3) on each mainline at locations along the pipeline that will limit
damage or pollution from accidental
hazardous liquid discharge, 4) on each
lateral take-off from the trunk line, 5)
on each side of a water crossing that is
more than a 100 feet wide, and 6) on
each side of a reservoir holding water
for human consumption.
Additionally, check valves may be
installed on grades and the downstream
side of rivers and streams for more protection from backflow conditions in case
of a line breach.
Many block valve installations are
outfitted with automatic shutdown controls. These controls are set to close the
valve if pressure or flow rates change,
indicating a possible breach in the line.
By having these valves spaced throughout the line, the amount of potential
fluid leakage that might occur during a
line break is limited. Additionally, many
pipeline valves are designated as emergency shutdown valves (ESD), which are
remotely operated from the pipeline
control center.
These block valve location requirements account for the numerous small,
fenced-in valve installations visible
when driving around areas with many
pipelines—numerous pipeline block
valves are located above the ground for
easy maintenance. However, some are
Slab gate valves
are used along
the pipeline
systems.
buried, with only the operating mechanism and auxiliary lubrication and bleed
lines showing. These installation areas
used to be the exclusive domain of gate
valves. However, today welded body
trunnion-mounted ball valves are very
popular, especially for clean natural gas
transmission lines. The unique welded
body construction eliminates the potential body-bonnet leak path, while the
only remaining leak path is up through
the packing area.
Though fugitive emissions (FE) leak-
age has been a focal point in the refining
industry for over 20 years, the upstream
and midstream markets have been fairly
immune from FE scrutiny. However,
that situation is changing. According to
MPL’s Daigle, “LDAR [leak detection
and repair] for pipelines is becoming
popular and required, especially since
packing leaks are the most common
leaks we deal with.”
One place where emissions of any
type are unacceptable to almost every-
one is in undersea pipelines. Because
they are surrounded by water and
vibrant marine life, undersea pipelines
certainly have their own set of chal-
lenges. However, there are other key dif-
ferences from on-land pipelines that
affect design, including the design of the
valves attached to the pipelines.
For example, undersea pipelines that
connect wellheads to gathering points
often operate at much higher pressures
than their onshore counterparts. It is
not uncommon for these lines to see
10,000 psi. Valves designed for this
submerged service are critical, purpose-built flow control devices that absolutely
must work properly when called upon to
operate. Because of the unique undersea
environment, standard API 6D requirements are not deemed tough enough, so
a special underwater valve specification
was written to cover these products:
API 6DSS “Specification for Subsea
Pipeline Valves.”
TESTING
Although interior pressures are also
quite high in subsea pipelines, it is
sometimes the outside pressure from the
extreme depths that introduces the most
stress on valves and piping. As a result,
pipeline valves designed for installation
at great depths are often tested in a
hyperbaric chamber, where extreme
pressure is exerted on the outside of the
valve, while the inside is sealed against
the external pressure.
All pipeline valves receive seat and
shell tests per API 6D or 6DSS, not
unlike their downstream counterparts,
which are usually tested in accordance
with API 598, “Valve Inspection &
Testing.” One difference between the
two testing documents is that, with API
6D pipeline valves, the holding times for
the tests are much longer. For example,
a 24-inch valve shell tested per API 598
requires a five-minute duration, while
the same size valve tested per API 6D
requires a 30-minute duration. These
longer holding times for pipeline valve
tests are often extended into hours by
the supplementary test requirements of
many pipeline owners.
While pipelines and pipeline valves
lie mostly invisible beneath six feet of
earth or under 600 feet of ocean, they
are nonetheless highly “visible” when
an accident occurs. As a result, pipeline
valves are closely scrutinized members
of the valve family. They are built to
tougher standards and must work every
time because they must protect lives and
property that lie near their installations.
Pipeline valves could borrow the Latin
motto of the United States Coast Guard,
which is “Semper Paratus,” which
means: always ready.VM
GREG JOHNSON is a contributing editor to Valve
Magazine and president of United Valve
( www.unitedvalve.com), Houston. Reach him at
greg1950@unitedvalve.com.
