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Combustion Pulsation and Noise
by Dan Banks, P.E., Banks
Engineering Inc.
Posted: August 1, 2007
The natural flame instability
in any flare or furnace
produces noise. In rare
cases, the noise can be loud
enough to damage
equipment, cause nuisance
shutdowns or bother the
neighbors. Simple hardware
changes can eliminate the
problem, but sometimes
finding the right fix takes
time.
Figure 1. A gas
flame can be
distorted by
resonant
combustion noise
inside a furnace.
The frames show
one complete
cycle of the
sound wave.
Courtesy of
Cambridge
University
Engineering
Department
All flames make noise -- and larger ones
make more. Most people can’t hear the noise
from a pocket lighter, but they know when
their gas water heater or gas-fired central
heat burner starts. And any industrial-size
furnace makes enough noise that you can tell
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if it is on without looking. Usually,
combustion noise can be ignored -- it seldom
causes problems. In rare cases, though, it is
loud enough to draw complaints from the
neighbors and even vibrate equipment to
destruction. This article will consider five
cases like that.
First, A Little Theory
Any noise has a frequency and an amplitude.
You can hear noise over a frequency range of
about 20 to 20,000 Hz, or cycles per second.
Noise at a frequency less than 20 Hz might be
felt but not heard: You might notice the floor
or windows rattling even though you cannot
hear the sound. If you have good hearing, the
quietest sound you can hear is slightly louder
than 0 decibels (db). The loudest (I’m told) is
120 db, at which point your ears are quickly
damaged.
In the combustion world, there are two
categories of noise:
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Noise formed by the resonant
characteristics of ducting, stacks, ovens,
furnaces, etc. The trigger for this noise
might be a flame or some other source,
but the vessel happens to be just the
right size and shape to reinforce and
amplify. The burner flame then might add
energy to the sound, and the result can
be startling.
Noise formed by a flame only (due to
burner instability). This type of noise
usually is not very loud, but it increases
as the pressure and flow of fuel increase.
Figure 1 shows how a gas flame can be
distorted by resonant combustion noise inside
a furnace.
Case History 1: Claus Tail Gas
Incinerator
A Claus tail gas incinerator
was built at a refinery in the
Houston area. It included a
relatively long tail gas duct
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Figure 2.
Installed at a
refinery in
Houston, the
Claus tail gas
incinerator
included a
relatively long
tail gas duct
running from the
last sulfur
condenser down
to the natural
gas-fired
incinerator
burner. A
centrifugal air
blower was
installed to
overcome the
pressure drop
across a
watertube waste
heat boiler
positioned at the
furnace outlet.
Courtesy of
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running from the last sulfur
condenser down to the
incinerator burner. The
burner was natural gas fired
and included a centrifugal air
blower to overcome the
pressure drop across a
watertube waste-heat boiler
positioned at the incinerator
furnace outlet and exhausting
to a carbon steel stack. This
system was designed for
future plant expansion, so
throughput was only a
fraction of the design (figure
2).
Immediately upon startup,
the operators heard a very
loud, low frequency noise or
pulsation. The noise
disappeared when the burner
was fired at its design rate,
and, of course, it went away
when the burner was off.
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Attempts to modify the
burner fuel gas injection
geometry had little effect on the noise, even
though this step often eliminates or at least
reduces similar problems. Among the
changes tried were shifting the fuel gas gun
tip position in the burner tile and changing
the fuel gas tip drill pattern.
Callidus
Technologies
An acoustics expert was called in.
Microphones were installed at several
locations throughout the system, but the
problem source remained elusive until, with
the burner turned off, low level noise at the
problem frequency was detected. This meant
that the noise was being triggered
somewhere in the system, and the burner
was simply amplifying the signal into a vessel
accidentally sized for resonance.
Ideas to alter the resonant frequency of the
equipment were considered. For instance,
shortening or lengthening the stack would
have changed the resonant point and cut the
noise amplitude. The fix selected consisted of
installing a perforated stainless steel plate
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across the boiler outlet flange. The
perforations were sized to create as much
flue gas pressure drop as the blower and
Claus plant could handle. That fixed the noise
problem by interrupting the resonance.
Case History 2: Chemical Plant Thermal
Oxidizer
At a chemical plant also near
Houston, a thermal oxidizer
system was installed to
dispose of offgas from an
acrylonitrile plant. The
system was to have a wasteheat boiler installed later, so
the horizontal furnace was
connected via a long duct to
the exhaust stack. A forceddraft natural gas burner was
used to bring the furnace up
to temperature (figure 3).
Once the refractory was
cured out, the operator
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Figure 3. The
thermal oxidizer
system was
designed to be
used with a
waste heat
boiler, so the
horizontal
furnace was
connected via a
long duct to the
exhaust stack. A
forced-draft
natural gas
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switched the vent gas stream burner was used
to bring the
away from the atmospheric
furnace up to
vent and into the burner.
temperature.
Immediately a very loud,
high pitched noise was heard. Apparently, the
furnace system was sized just right for
resonance, and the burner provided the
amplification, while the noise was started at
the inlet valve.
Fortunately, a simple change to the inlet
valve position changed the frequency enough
to reduce the noise to acceptable levels. In
this case, the change was easy to accomplish
and worked “like magic.”
Case History 3: Elevated Flare
A refinery in California installed a large,
steam-assist, elevated flare. Steam injection
was necessary for smokeless burning of the
waste gases (figure 4). Upon startup, a loud,
low frequency (2 Hz) noise was produced. It
resulted in complaints of vibrating walls and
garage doors from neighbors well away from
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the flare site, and steps were immediately
taken to find the cause and eliminate it.
With trial and error and a few lucky guesses,
the cause was determined to be the steam jet
position in relation to the waste gas injection
passage. By raising the steam jet elevation
about 2", the problem noise was eliminated
and smokeless operation maintained.
Case History 4: North Dakota Hydrogen
Vent Incinerator
Figure 4. The
position of the
steam jet on
the steamassisted,
elevated flare
caused low
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A petrochemical plant in North
Dakota produced a waste gas
rich in hydrogen. It was to be
burned in a forced-draft
incinerator. The incinerator
floor was equipped with a
number of floor-mounted
burners, each with a central
fuel gas gun mounted inside a
refractory throat. Upon
startup, a very loud “hum” was
produced. In this case, there
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were operator complaints, and
the amplitude was so great
that it was loosening the nuts
holding the furnace platforms
and other hardware in place.
With nuts falling, the furnace
could not be operated while personnel were
below.
resonance
noise. Moving
the steam-jet
tip up 2" higher
eliminated the
problem noise.
The problem was solved by adding a central
gas passage through the center of each
burner gas gun. This reduced the waste gas
pressure drop at the gun tip and changed the
flame geometry enough to “detune” the
burners and reduce the noise to acceptable
levels. The changes delayed mixing of the
waste gas with the combustion air, but the
increased flame length was not a problem.
Case History 5: Chemical Plant Waste
Heat Recovery
A petrochemical plant near
Houston operated a large,
horizontally fired incinerator
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mated to a watertube heat
recovery boiler and
economizer. The flue gas was
ducted to atmosphere through
a 100' stack close-coupled to
the economizer. The waste gas
had very low heating value
and was injected, along with
the combustion air, through a
series of stainless pipes
installed through the furnace
refractory lining. A natural gas
burner started the combustion
and two waste liquids were
sprayed in between the burner
and the waste gas injectors.
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Figure 5. The
incinerator
floor was
equipped with
a number of
floor-mounted
burners, each
with a central
fuel gas gun
mounted inside
a refractory
throat.
Courtesy of
Zeeco
With increased plant
production, a noise began to appear. In this
case, the frequency was only one or two
cycles per second, and was detected by
rhythmic swelling of the fabric expansion
joints connecting the economizer to the
stack. Expansion joint life was reduced to
about six months, and the system had to be
taken down for fabric replacement (total
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p
(
shutdown lasting approximately three days
each time). Previously, the plant could be run
nearly two years between shutdowns.
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In this case, modifying the waste distribution
across the pipe injectors resulted in enough
amplitude reduction to solve the problem.
The low frequency noise was still present, but
flexing of the expansion joint fabric was
eliminated.
More Theory
How can a fuel gas burner act as an amplifier
for noise? It appears the pressure changes
that cause the noise are created when the
fuel and air streams are combined
improperly. If the two streams are combined
incorrectly, pockets of rich or lean mixtures
are created which are not flammable at the
operating conditions. As additional fuel or air
enters the pocket, the mixture becomes
flammable and burning continues, increasing
the volume of the pocket, which displaces the
air and fuel streams, forming a nonflammable
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,
g
pocket again. As this cycle continues, the
on/off nature of the combustion is seen as
pressure fluctuations. With many industrial
size furnaces, the pulsation frequency is in
the range of one or two per second, but with
hydrogen-rich fuel gas or fuel gas under high
pressure, the frequency can be greater.
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All burners are equipped with a special alloy
tip or a refractory tile to anchor the ignition
point. If the anchor method is insufficient,
the flame may initiate first at one point and
then jump to another point, giving rise to
pressure fluctuations and noise.
Larger burners typically are diffusion-type
designs, meaning that the fuel (gas, oil or
pulverized coal) is injected adjacent to the
combustion air stream. As the fuel diffuses
into the air stream (and vice versa), the
burner flame develops and grows until all of
the fuel is oxidized.
So, how would a fuel burner amplify an
existing noise, as apparently happened in the
field cases cited? In the first example with
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the Claus tail gas incinerator, the inert waste
gas was injected through an annular gap
surrounding the flame zone. Apparently, the
pressure fluctuations in the waste gas
distorted the fuel gas/air mixing process at
the existing frequency, and the energy
already available from the flammable mixture
pockets was redirected from standard
combustion noise to the new frequency,
which happened to be the same as the
resonant frequency of the furnace system.
Bad luck. The other cases involved a similar
effect, although the circumstances look quite
different.
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In conclusion, if your installation has
unacceptable noise, remember that the
negative effects of combustion pulsation
include:
Rhythmic flame body displacement. For
example, the flame scanner may lose
sight of the flame and trigger nuisance
shutdowns.
Brief loss of combustion airflow as oven
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or furnace pressure peaks, reducing
airflow to the burner. In these cases, the
low flow switch may trip, again causing
nuisance shutdowns.
“Breathing” of the oven or furnace shell
and expansion joints, leading to material
failure through fatigue.
Structural damage to the oven or furnace
if the amplitude is high enough.
Extremely irritated operators and
neighbors.
Eliminating combustion pulsation and
excessive noise is possible through changes
in burner operation (reduce or increase
airflow, change steam flow, etc.), but often,
some sort of hardware change is required.
There is some hope from acoustic analysis of
the resonating chamber, but this is presently
very difficult and typically too expensive to
use in the equipment design phase.
More practical approaches include changing
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the location or velocity of fuel injection.
Another involves changing the location
direction of combustion airflow, steam flow
or waste flow. And in some cases, steps must
be taken to change the resonant frequency of
the furnace/stack assembly. These changes
are typically accomplished in an experimental
manner. Sometimes, observation of the flame
body will give a clue as to the best approach.
Dan Banks, P.E., Banks Engineering Inc.
Dan Banks, P.E., is president of Banks Engineering Inc.,
Tulsa, Okla., a consulting company that provides
incinerator system consulting, flue gas scrubber design and
emergency flare help. For more information from Banks
Engineering, call (877) 747-2354 or visit
www.banksengineering.com.
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