POWER March 2020 - 45

GAS-FIRED GENERATION
where c is √(k x gC
x 144 x PE
x vB
), k is
the ratio of specific heat (about 1.4 for air
and nitrogen), and PE
the gas exit.
is the pressure at
Noise Control
Gas blows are loud. In fact, sonic shock
waves may be visible in the gas plume exiting
the tail pipe. Hearing protection will
be required for anyone working nearby.
Sound pressure level (SPL) indicates
the intensity of the sound at a given
point with respect to a reference level.
Measurements are usually expressed in
dB(A) units where the " A " indicates an
adjustment for the frequency sensitivity
of human hearing. Based on an Occupational
Safety and Health Administration
(OSHA) noise limit of 90 dB(A), a reduction
to less than 85 dB(A) at a distance
of 3 feet is recommended. SPL can be
estimated with the following expression
derived from American Petroleum Institute
(API) Standard 521:
SPL ≅ 20 log (dE
log [(4 x vB
π x r2) - 22.232
where dE
/ 1000) - 10 log (vB
x mB) / (π x dE
) + 80
2)] - 10 log (2 x
is the tail pipe gas exit inner
diameter and r is the distance from pipe
exit. If the plant is near a residential
area, a silencer will be necessary. When
specifying a silencer, pressure loss at the
maximum flow rate should be restricted
to no more than about 1 psid. A high flow
resistance will result in higher blow pressures
and more blow gas being used.
Time and Mass Calculations
When depressurization begins, the pressure
differential between the reservoir
and the atmosphere is high, and the
exit flow is at sonic velocity. As the differential
decreases, the exit velocity remains
constant until a critical pressure is
reached and it becomes subsonic. As the
reservoir pressure is reduced further, the
volumetric flow and mass flow drop until
the differential pressure reaches zero.
Blowdown time (tBD
) can be estimated
with the following empirical expression
recommended by the American Gas Association
(AGA):
tBD ≅ (0.1225 x P1
where P1
pressure, SG
0.333 x SG
0.5 x V x Fc
) / dE
2
is the gas reservoir starting
is the specific gravity relative
to air (1.0 for air and approximately
1.0 for nitrogen), V is the storage volume
under pressure (including tank, reservoir,
and piping), and FC
is the choke factor (1
March 2020 | POWER
for an ideal exit to 1.8 and higher for a
typical gate valve). Ensure the blowdown
time is sufficiently greater than the blow
valve opening time and the reservoir is
not pressurized above the rated pressure
capability of the components.
A positive displacement compressor
will fill the reservoir at a relatively constant
flow rate. The time to recharge the
reservoir (tCHARGE
) can be estimated with
the following expression:
tCHARGE
≅ [V x (P2
where P2
pressure, Pa
- P1
) x 60] / (Pa
x Q)
is the gas reservoir ending
is atmospheric pressure
(14.7 psia), and Q is compressor output
capacity for filling the reservoir. To
estimate the calendar days needed for
cleaning, the number of blows required
for each blow path is needed.
Unfortunately, an accurate prediction
is nearly impossible. The number
can vary from 1 to 100. If there was an
emphasis to " build it clean, " fewer blow
cycles should be needed. Whatever
number is used, it's going to be a rough
estimate, so add margin. Additional time
should be added to account for gathering
and checking instrumentation data, calculating
CFRs, removal and insertion of
target plates, and inspecting the target.
Experiencing a shortage of nitrogen
during the cleaning process will delay
completion. To estimate the quantity of
nitrogen required for a gas blow (qGAS
),
use the following expressions based on
the perfect gas laws:
qGAS ≅ (2.7 x SG
x P1
x V) / T
where T is the gas temperature in Rankine
(F + 459.67), or:
qGAS ≅ V / vB
Pressure Testing
ASME B31.1 covers the design and
testing of power plant piping systems.
ASME B31.3 is used for industrial piping
such as at a refinery or chemical plant.
ASME B31.8 applies to gas transportation
and distribution piping. The plant
owner should identify which codes to
use and where they apply.
Prior to performing gas blows, the
piping must be pressure (leak) tested.
Normally, water is used. However, if
permitted by the piping code, pneumatic
testing could instead be performed
using the cleaning gas. This may be
desired in situations where the hydro
test water cannot be easily removed,
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cannot be tolerated in the pipe, or is
not sufficiently available.
The applicable piping code describes
the process for conducting a pneumatic
pressure test. ASME PCC-2 gives a
method for determining the distance enveloping
the pipe that should be marked
off with caution tape and cleared of
people. The decision to perform a pneumatic
test should not be taken lightly.
Pneumatic pipe ruptures occur suddenly,
usually without warning; therefore,
pneumatic testing is discouraged.
Discharge Force
Much like thrust from a rocket engine,
the open discharge will create a static
reaction force at the tail pipe exit. Usually
an anchor is required to restrain the
tail pipe from movement. API RP 520-2
provides the following expression which
can be used to estimate the thrust force
at the tail pipe exit (F):
F = 9.836 x mM x √{(k x T) / [(k+1) x Mwt
+ [144 x (PE
- Pa
) x A]
where mM is the mass flow rate of gas
during maximum operation and MWT
is
the gas molecular weight (about 28.5 for
air and nitrogen). When the blow valve
suddenly opens, the pipe will be subjected
to dynamic loads as the gas starts
flowing. The dynamic forces are accounted
for by a dynamic load factor (DLF).
This factor represents the ratio of the
peak stress from a rapidly applied load
to the stress that would occur, if applied
slowly. DLF ranges from 1.1 to 2.0. Typically,
a DLF of 2 is used. If a lower value
is desired, see ASME B31.1 for details.
Do the Job Right
In a 2010 CSB-conducted survey on gas
blow cleaning, about half of the respondents
stated they did not have a technical
basis for determining the gas flows
and pressures they should use. Without
the guidance of supporting calculations,
it's likely that more time and gas will be
required for cleaning. An analytical approach
to gas blow cleaning will help to
ensure that the process proceeds safely
and efficiently.
In sports, a blowout is when one team
outperforms another by a large margin.
When it comes to blowout pipe cleaning,
preparing a technical basis for the work will
make sure you are on the winning team. ■
-Michael F. Czyszczewski, PE
(mczyszczewski@asme.org) is a
mechanical engineer with 40 years
of experience in the power industry.
45
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POWER March 2020

Table of Contents for the Digital Edition of POWER March 2020

Contents
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