Proceedings of the 8th International Pipeline Conference
IPC2010
September 27-October 1, 2010, Calgary, Alberta, Canada
FINAL
IPC2010-
IPC2010-31346
INNOVATIVE METHOD TO DOUBLE BLOCK AND BLEED
Frank Dum
T.D. Williamson Inc
Tulsa, Oklahoma, United States
Colin Drysdale
T.D. Williamson Inc
Tulsa, Oklahoma, United States
ABSTRACT
Double Block and Bleed (DBB) is the preferred approach
used to maximize safety while performing piping maintenance.
The aim of the paper is to outline an innovative method of
achieving DBB on pressurized piping or pipelines.
shutting off a branch of the pipeline supplying natural gas to a
large shopping mall. Part of the pipeline company’s safety
requirements included a double block and bleed method of
pipeline isolation. There were existing valves and a bleed port
to provide DBB downstream of the installation but there was
no method of DBB upstream. To add to the complexity of the
job, the pipeline was next to a very busy street. Because of
rush hour traffic, the pipeline could only be repaired between
the hours of 9am to 4pm. In addition, other requirements of
the customer included minimized intrusion into piping, less
number of fittings, less welding time and the material and
labor costs associated with purchasing and installing half the
fittings. In response to the requirements and for a double block
and bleed capability, the STOPPLE Train plugging system was
deployed to meet these customer needs.
DBB is traditionally achieved by several methods, for
example, through using Two Valves, a Valve & a STOPPLE®
plugging head, a STOPPLE plugging head & an Isolation Plug
or Tandem STOPPLE plugging heads. There are also several
DBB methods which utilize inflatable bags or bladders.
The STOPPLE Train is a patent pending tool that uses 2
STOPPLE® plugging heads. Only one hot tap is required in
the piping for which the STOPPLE Train is inserted. The first
STOPPLE plugging head is the trailing plugging head and
forms a primary seal. The second STOPPLE plugging head is
the leading plugging head and forms a secondary seal. Once
sealed, the bleed is accomplished through a relief port allowing
downstream maintenance to commence.
INTRODUCTION
On 20 November 2008, an isolation valve needed to
be added to a natural gas distribution pipeline. The pipeline
needed to be cut and the isolation valve installed without
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Supply to Shopping Mall
Isolation valve
to be installed
STOPPLE
Train
&
Bleed Port
Figure 2 A traditional method of DBB
Existing downstream
block valves and
bleed port.
HOW THE STOPPLE® TRAIN WORKS
The patent pending STOPPLE Train plugging system is
unique in that it links two plugging heads into a “train”
capable of providing a double block and bleed function.
Figure 1 Job Site Schematic
The STOPPLE Train could achieve DBB through a single
STOPPLE fitting resulting in less excavation, and only half of
the tapping and STOPPLE equipment. This resulted in less
congestion and a significant decrease in the time needed for
welding, tapping, and setting/pulling STOPPLE plugging
heads.
Bleed
TRADITIONAL METHODS OF DOUBLE BLOCK AND
BLEED
Historically, pipeline operators have used a number of
different methods to achieve double block and bleed. For
example, two separate valves with a bleed port between them
can achieve this. Or, the combination of a valve and a plugging
head with a port between them can work, as can the tandem of
a plugging head and an isolation plug with a port between
them. Many operators have simply used two plugging heads
and a bleed port. What all of these methods have in common is
the dual barrier concept. Whether the barriers are valves,
plugging head seals or combinations thereof, the basic premise
is that a second barrier serves as a failsafe for the first.
Primary
Block
Secondary
Block
Figure 3 STOPPLE® Train in Cutaway Fitting
This new innovative system requires only one hot tap
and a single standard STOPPLE® fitting to achieve DBB
compared to traditional methods. Its linked plugging heads can
be inserted into a line through just one opening. A single hot
tap and fitting thus yields the double block benefit of two inline plugs. A pressure bleed port positioned between the two
plugging heads allows the void between the heads to be bled
down, creating a zone of zero energy. This port also allows for
seal verification and monitoring. If the operator so chooses,
this system can also be used without the bleed port to simply
get the safety advantage of redundant seals.
EQUIPMENT TESTING
The STOPPLE® Train system is also robust enough to
handle both low pressure and high pressure environments.
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SUBSTANTIAL SAVINGS
As part of the customer request to reduce costs, the
STOPPLE Train plugging system was selected and provided
substantial cost savings over other double block and bleed
methods. This was achieved by reducing the material cost of
fittings placed on the line. Further savings resulted from less
excavation, fewer pieces of tapping and STOPPLE equipment
required on site, less welding and a shorter overall job time,
resulting in lower labor costs and less exposure time to
potential hazards for personnel and the general public at a
busy intersection.
Tests have shown that at low pressures, compression between
the sealing elements and the pipe ID creates an excellent
barrier. In fact, the sealing element does not even need to be
energized to seal. At 0.5 psi to 5 psi, the compression fit alone
is sufficient. Once energized, the seal will remain intact until
broken by an external force, such as retraction of the plugging
head.
The most cost-effective aspect of this new plugging
system stems from the fact that it is based on nearly six
decades of proven STOPPLE technology. The STOPPLE
system was first designed and patented in 1952 with minimal
changes to the initial design. Pressure ratings for the
STOPPLE Train system follow traditional STOPPLE
parameters, and all standard STOPPLE tools apply to the
maintenance of the STOPPLE Train heads. Overall, use of the
STOPPLE Train plugging system requires nearly identical
training, operation and maintenance as for all STOPPLE
systems, meaning many experienced users will see very little
change (other than a little extra plugging head travel length)
from their typical plugging operations.
Figure 4 Low Pressure sealing element tests
Since there are a variety of methods of achieving
double block and bleed, the STOPPLE Train provides
substantial cost savings especially compared to using two
conventional STOPPLE heads. As illustrated in the following
figure, the cost savings vary over various pipe sizes and the
number of fittings that are required in order to successfully
perform the job. But the point to note is that a conventional
double and double tandem could potentially cost a significant
amount more to achieve the same level of safety that the
STOPPLE Train can provide.
On the other end of the pressure spectrum, both
plugging heads in a STOPPLE Train system are rated to 1,000
psi at 180°F, or to 600 psi at 350°F. They have been tested
successfully at 1.5 times this rating, and the pressurized
plugging heads have been successfully tested together and
independently.
Because the STOPPLE Train plugging system
requires only one hot tap and a standard fitting, fewer potential
leak paths are left on the piping once work is complete.
Compared to other methods for achieving double block and
bleed, which typically require twice as many line penetrations,
this represents a major improvement. Fewer fittings also
results in less expense.
Savings when using a STOPPLE Train compared to various
other methods of DBB
$35,000
$30,000
$25,000
$20,000
MAXIMUM SAFETY
There was no initial means of isolation for this section of
pipeline. Since the project was driven by safety concerns, the
added safety of the STOPPLE Train system comes from its two
seals, thus providing double block and bleed. With a tap
between the two seals, that area can be monitored for leakage
before and during work on the pipeline downstream. In the
unlikely event that the first seal leaks, the second seal is fully
capable of holding the line pressure by itself.
$15,000
$10,000
$5,000
$0
4”
6”
8”
10”
12”
Range of Pipeline Inches up to 12'
CONVENTIONAL DOUBLE & DOUBLE TANDEM VS. STOPPLE TRAIN DOUBLE & DOUBLE TANDEM
CONVENTIONAL STOPPLE VS. STOPPLE TRAIN
Figure 5 Projected savings comparison when using a
STOPPLE Train vs. other traditional forms of DBB.
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Figure 6 Project was on a pipeline located next to a very
busy street
ACTUAL FIELD JOB
On the actual day of the job, standard pressure tests on the
STOPPLE Fittings were carried out, the hot taps were made
and the STOPPLE Train Housing was installed along with
blow down and equalization hoses. The STOPPLE Train was
inserted and sweeping conducted. The pipe was bled and the
DBB seal was established.
With the STOPPLE Train set in place, cold cutting
welding of both sections of the pipe were executed
completed. Completion plug was set in place and
STOPPLE Train was retracted. The housing was unbolted
completion plug inserted and set.
and
and
the
and
CONCLUSION
The STOPPLE Train performed according to expectations.
The sealing elements sealed properly and the bleed port
allowed the gas to be evacuated to a safe location. The project
was successfully completed with all the objectives and
requirements satisfied.
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