POWER March 2020 - 44

GAS-FIRED GENERATION
quickly. A faster valve will allow more
cleaning gas to scour the inside of the
pipe at maximum velocity. A quarter-turn
ball valve or knife gate valve that opens
in 0.5 to 1 second (or better) is a good
choice. An automatic operator is preferred
for safety reasons.
The closer the valve is to the tail pipe;
the more volume will be available upstream
for pressurization. More volume
gives greater stored energy and longer
blows. Alternatively, locating the valve
upstream of the piping being cleaned
will provide an additional cleaning benefit
when the valve pops open and the pressurized
gas impacts the dirty pipe. However,
most importantly, the valve should
be installed away from flow disruptions,
and if a manual valve is used, it should be
in an accessible and safe location.
Because the blow valve will be subloads,
jected
to
high
stresses,
and
debris,
it can be easily damaged. It's
words, the ratio of cleaning to operating
CF, known as the cleaning force ratio
(CFR), must be greater than 1.0, as calculated
in the following equation:
CFR = CFB
where CFB
/ CFM
blow and CFM
≥ 1
is the cleaning force of the
is the maximum cleaning
force during operation. Assuring CFRs
are greater than 1.0 is the fundamental
criteria upon which the gas blow starting
pressures are based. Experience
has shown that drag forces from a CFR
greater than or equal to 1.2 will provide
acceptable results. Higher CFRs do not
show significantly faster cleaning and
are not recommended because more
gas is exhausted. However, a higher CFR
margin should be used if required by the
equipment manufacturer.
CFR increases as you move downstream
in a constant diameter pipe,
The principle behind gas blow cleaning is to
create high gas velocities and drag forces,
which will shear debris off the pipe interior
walls.
therefore considered good insurance to
have a spare.
Rupture disks are sometimes used
instead of a blow valve. They are disposable
and open very quickly. However, because
they must be replaced after each
blow and are manufactured to a single
predefined burst pressure, they may not
be suitable when many gas blows or
starting pressures are needed.
Cleaning Calculations
The principle behind gas blow cleaning
is to create high gas velocities and drag
forces, which will shear debris off the
pipe interior walls. This cleaning force
(CF) is defined as follows:
CF = (mB
2 x vB
) / (2 x gc
x A)
where mB is the mass flow rate of the
blow gas during cleaning, vB is the specific
volume of the blow gas during cleaning,
gc
is the gravitational constant, and A is
the area at the tail pipe exit. (All variables
in equations referenced in this article use
customary U.S. units, such as ft, sec., lb,
psia, and F, unless otherwise stated.) CF
during the gas blow must be greater than
the maximum CF during operation for
effective cleaning to take place. In other
44
therefore, the majority of the pipe will
see CFR's many times greater than 1.0.
Also, the calculations do not take credit
for the dynamic shock that occurs when
the blow valve is suddenly opened. Because
of this conservativism, a minimum
CFR of 1.0 is considered reasonable and
sufficient for service blows.
To calculate CFR, the mass flow and
specific volume is needed along the entire
blow path. Sonic and subsonic velocities,
choked flow, and density changes
occur, which make the calculations difficult.
Fortunately, compressible fluid flow
software greatly simplifies the effort.
First, construct a software model of
the system in its normal operating configuration.
Runs of each operating mode
are then used to find the largest values
corresponding to the terms in the denominator
of the CFR equation.
Next, modify the model to include
any configuration changes that will be
made for cleaning. All temporary pipes,
bypasses, the tail pipe, blow valve, and
blow path block valves should be added.
The objective is to use the model to
find the blow gas reservoir starting and
ending pressures that will result in acceptable
CFR values throughout the length of
pipe being cleaned. This is done by a trial
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and error process in which different pressures
are tested, and the mass flow and
specific volume results are imported into
a spreadsheet that calculates the CFRs.
The following is some advice for determining
starting pressures:
■ As the pipe depressurizes (is blown
down), gas velocity will decrease.
When the source pressure drops to
a point where the CFRs are below
1.0, the gas blow can be terminated.
By not completely depressurizing,
the recycle time and quantity of gas
used can be minimized. However,
use some judgement. If the blow
down times are very short (less than
10 seconds), complete depressurization
will allow more time to eject the
debris.
■ Increasing the gas mass flow rate or
specific volume will increase the CFR.
Mass flow rate can be increased by
increasing reservoir pressure. Specific
volume can be increased by decreasing
pipe pressure or increasing the
temperature.
■ When gas flows through a pipe, the
pressure drops along the path due
to friction. The pressure drop causes
the gas specific volume and velocity
to increase. If the sonic velocity
is reached, a shockwave forms. The
velocity will no longer increase beyond
the shockwave and the flow is
considered " choked. " Sonic choking
can occur at locations where the pipe
diameter suddenly increases, at the
pipe exit, and at constrictions such
as a reduced bore valve. Ideally, sonic
choking should only occur at the pipe
exit and not inside the pipe, where it
could cause severe pipe vibrations.
■ Increasing the tail pipe diameter will increase
the CFR. However, the tail pipe
diameter should be no larger than that
of the piping to which it connects to
avoid creating an internal choke point.
Pressure and temperature instrumentation
with data recording capability,
installed near the gas inlet and outlet,
should be used to confirm that acceptable
CFRs are being achieved. Use a
table or chart of gas properties to get
the specific volume based on measured
pressure and temperature. Studies have
shown that the most reliable method of
determining the mass flow rate (mB
) is to
base it on a calculation of sonic velocity
(c) at the exit.
mB = A x c / vB
POWER | March 2020
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POWER March 2020

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