POWER September 2011 - 64

PLANT COMMISSIONING
Developing the Plan
As an engineering, procurement, construction,
and commissioning (EPCC) contractor,
our planning for commissioning of a plant
begins at the start of the project, with assignment
of a commissioning manager who will
lead the startup. The engineering part of the
EPCC team remains fully involved in facility
commissioning to provide technical support.
One key site startup activity is the gas supply
line cleaning process. Fluor's practice is to
develop a project- and site-specific solution
that ensures appropriate line cleaning while
minimizing gas consumption and release. If
natural gas cleaning is not an option in your
locale, several alternatives are discussed later
in this article.
Fluor follows the cleaning requirements
provided by the original equipment manufacturer.
As a general guideline, Fluor's recommendation
for gas blows is for a minimum
cleaning force ratio (CFR) of 1.0 (based on
normal maximum flow) throughout the entire
length of the pipe, unless the manufacturer has
different or more stringent recommendations.
The CFR represents the momentum of the gas
used to clean the piping with respect to the
normal natural gas flow design conditions.
Also, the CFR will change throughout the
piping system due to changes in design flow
rates, gas temperature and pressure changes,
and pipe diameter variations. Pipe segments of
the blow path must be analyzed to ensure adequate
CFR at the beginning of each segment.
For the two combined cycle projects scheduled
to be completed by Fluor in 2011, the
combustion turbine vendor is General Electric
(GE). GE's GEK 110483b, Cleanliness Requirements
for Power Plant Installation, Commissioning,
and Maintenance, states that the
minimum CFR along the entire length of pipe
that is to be cleaned should be 1.03.
Selecting the Point of Release
In the case of the Kleen Energy explosion, the
CSB final report found that " natural gas was
blown outside the power generation building
into a congested outdoor area in an almost
horizontal orientation limiting gas dispersion. "
Methane has an NFPA flammability
rating of 4, the highest degree of hazard; has
a lower explosive limit (LEL) of 4.4 volume
percent; and an upper explosive limit (UEL)
of 16.5 volume percent. This means that any
mixture of air and methane (the principal
component of natural gas) that falls within
this range forms an explosive mixture that is
easily ignited.
If natural gas is to be used to purge gas
piping, the point of release of the gas must be
carefully determined " to minimize the extent
of the flammable atmosphere, " according to
the CSB report. The report also notes that
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63
to make this analysis, " a complex technical
evaluation of various factors is necessary,
including height, location, orientation of the
vent pipe, velocity and density of the natural
gas being discharged, potential sources of ignition,
personnel location, wind speed, and a
dispersion analysis to verify that the natural
gas will rapidly dissipate. "
During the plant design, the gas vent
point of discharge is jointly determined by
the engineering and commissioning teams
in the 3-D plant model. We then develop
isometric piping drawings defining the
physical routing and materials of construction
of all temporary and permanent systems
involved in the gas blow. Pipe supports are
designed for both the permanent and temporary
piping for the conditions experienced
during the blow process. This approach also
ensures that the plan for gas release location
is adequately investigated and not just
a field judgment.
In past years, the vent point of discharge
was located away from existing structures
and at a height sufficient to provide good
dispersion. Sufficient dispersion was determined
by good engineering judgment and
prior field experience. Today, Fluor requires
a site-specific gas dispersion analysis to predict
the extent of the flammable atmosphere
when the gas is vented. Fluor uses DNV Software's
Phast Version 6.6, a comprehensive
hazard analysis software and modeling tool.
Several scenarios are modeled in Phast
using user-defined parameters such as discharge
velocity, discharge rate, temperature,
LEL, and UEL, based on the natural gas
used. For choked flows (as is normally the
case for gas vents), Phast requires the isentropic
temperature and isentropic velocity
as inputs to the dispersion model as well as
the assumed wind speed and meteorological
conditions (Pasquill stability class). Phast
caps the gas exit velocity at 500 m/s as a
conservative estimate; higher velocities promote
elevated dispersions.
Phast produces a prediction of the maximum
dispersion contour that usually remains
steady until the pressure inside the gas vent
system drops. Interestingly, the maximum
horizontal dispersion, near the exit point,
occurs only at the end of the gas release, at
low exit velocities. The plume dispersion is
calculated until the released gas is dispersed
to nonhazardous concentrations.
During the design of one plant, designers
considered five separate blow paths with
different vent discharge locations to determine
that the maximum horizontal extent
of hazardous concentrations of natural gas
was 4 to 8 meters and the maximum vertical
extent was 5 to 38 meters. In one option, the
predicted flammability cloud overlapped an
existing structure within the 3-D model, so
the discharge vent location was relocated to
ensure that no overlap occurred (Figure 2).
In addition to indicating potential " impingement "
areas that could create a spark
on impact, the dispersion modeling also provides
good guidance on where it might be
best to locate the combustible gas detection
monitors. This additional protection can be
utilized to ensure that the modeling appropriately
represented the discharge conditions.
Avoid Outside Influences
Even with proper placement of the vent to
ensure that no explosive mixtures of air and
methane are possible, past experience has
shown that there may be unexpected ignition
sources that should be considered during
plant design and commissioning.
2. Dispersion models guide vent location. Multiple cloud maps were conservatively
modeled as simplified cylinders at the exhaust vent locations in a plant 3-D design model
so that objects within the flammability cloud can be identified. In one of the five scenarios for
the specific site layout analyzed, the cloud cylinder overlapped an existing structure within the
model for the planned vent locations (left). The discharge vent location for this gas blow scenario
was relocated to ensure no overlap (right). Source: Karl Welsch, Fluor Corp.

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POWER September 2011

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