Reducing noise from an oil refinery catalytic distillation columnR. D. Rawlinson,a� J. Alberola,b� P. F. Joseph,b� and M. G. Smithb�
�Received 2006 February 14; revised 2006 July 03; accepted 2006 July 04�
This paper concerns the reduction of the exhaust noise from a CatalyticDistillation Column, commonly called a Cat Cracker. A Cat Cracker is used toconvert heavy oil into gasoline products. Following the upgrading of the CatCracker, there were persistent community complaints of an irregularly varyingnoise that sounded like an “overflying jet aircraft”. The paper describes adetailed study of the Cat Cracker noise involving: field tests on-plant and in thecommunity; scale model tests in the laboratory; theoretical predictions of thein-stack sound power level; and a study of atmospheric propagation effects usingthe Parabolic Equation method. The objectives of the study were to i… identifymethods of reducing the noise levels, and ii… establish the cause of theirregularity of the noise levels in the community. The laboratory tests used a 1
This paper concerns the noise from the exhaust stackof an oil refinery Cat Cracker. A Cat Cracker is aprocess plant that converts crude oil into itsby-products. Following an upgrade of the Cat Crackerat a major oil refinery in the UK, complaints had beenreceived from the local community about a noise,which occurred during certain meteorological condi-tions, that was likened to the sound of an overflyingaircraft. The noise was observed to rise and fall irregu-larly when the wind was blowing towards the commu-nity. The community extends along the southernboundary of the site and the prevailing winds are fromthe southwest.
a� ISVR Consulting, Institute of Sound and Vibration Re-search, University of Southampton, Highfield, Southamp-ton, SO17 1BJ, ENGLAND; email: [email protected]
b� ISVR Consulting, Institute of Sound and Vibration Re-search, University of Southampton, Highfield, Southamp-ton, SO17 1BJ, ENGLAND.
360 Noise Control Eng. J. 54 �6�, 2006 Nov-Dec
The noise levels measured in the community werefound to be more than 20 dB greater than thosepredicted using classical theories of valve noise andsound propagation, suggesting that an additional noisemechanism was present.
This paper addresses the procedures that werefollowed to identify:
• the source of the anomalously high noise levels• the reason for the fluctuations in the noise levels
in the community.A number of approaches were adopted to resolve
these issues, which included:• field measurements in the community—during
normal operation and when the process was de-liberately adjusted from the noisy condition toone that provoked less community complaints
• field measurements in the plant—includingmeasurements at the top of the exhaust stack
• scale model tests in the laboratory—using a 13
scale model to investigate possible interactionsbetween components of the process
• theoretical predictions of the in-stack soundpower level
• a study of the variability in atmospheric propa-gation effects using the Parabolic Equationmodel
2 OUTLINE OF THE PROCESS
The role of the Cat Cracker is to convert heavy oilinto gasoline products within the Reactor by the distil-lation process. The exhaust gases discharge toatmosphere through a tall chimney. The process runs at735 °C. The reactor catalyst is fluidised at thistemperature and spent catalyst is fed back to a Regen-erator. There are 55 tonnes per minute of catalystmovement. The flue gases then pass through waste heatrecovery units �WHB� to the Tertiary Cyclone Vessel�TCV�, which contains 25 sets of cyclones to removeprocess catalyst. The flue gas then passes through ValveC, as shown in Fig. 1. The volume of flue gas passingthrough the stack can be varied between 4800 and6000 Sm3/min, where the units Sm3/min indicate theflow rate normalised to standard conditions of tempera-ture and pressure �i.e. 15 °C and 1 Bar�.
The flow rate through the system is normallycontrolled by Valve C. Downstream of this valve is theMulti-Holed Orifice �MHO�, which was installed toassist in controlling the velocity through the system tominimise erosion of the waste heat recovery units.Control can be maintained using Valve A but thisadversely affects the process.
Two silencers are positioned downstream of theMHO plate. The stack discharges to atmosphere at anelevation of 89 m. The process runs 24 hrs per day,7 days per week. It is a steady process and not subjectto sudden changes. Prior to December 2001 the catcracker ran without complaints. At that time there wasa single stack silencer to reduce an earlier tonal
Fig. 1—Schematic diagram of the process.
Noise Control Eng. J. 54 �6�, 2006 Nov-Dec
problem. A new blower was installed during theupgrade to increase the air rate from approximately3500 Sm3/min to 6000 Sm3/min. Following commu-nity complaints the stack was extensively altered andan additional silencer was fitted. Although this reducedthe stack tip noise by 14 dB�A�, complaints persisted.
It had been observed that the noise problem mainlyoccurred when Valve C was in control and the windwas blowing from a northerly direction. The commu-nity complaints could be reduced by switching controlof the process to Valve A. However, the process couldnot be run in this configuration for extended periods.
3 FIELD SURVEYS
3.1 Noise Levels and Frequency Content
An initial environmental noise survey was carriedout in the community to determine typical backgroundnoise levels during the day and night. This surveyincluded periods when community complaints werevigorous and when they were reduced by switchingcontrol to Valve A. The methodology adopted wasbased on that in current standards.
A second survey was carried out when the processwas changed deliberately from Valve A being in controlto Valve C being in control, then returning to Valve A.This changeover took about 8 hours to complete andthroughout this time simultaneous recordings of thenoise were taken on-site and in the community. Fiverecording systems were used: three were in the commu-nity, one recorded the noise at the stack tip and a fifthwas at ground level in the refinery. Norsonic NOR 121sound analysers were used to make direct measure-ments and audio recordings for later analysis. The stacktip measurement technique did not conform to anynational standards because of the difficulty of gainingaccess. However, repeated measurements, taken undersimilar conditions, demonstrated that good repeatabil-ity was obtained.
During the second survey some dynamic pressuremeasurements were taken in the stack, downstream ofthe MHO, to try and determine the in-duct soundpressure levels. Unfortunately there was only one smallport into which the probe could be inserted and timeconstraints did not allow special techniques to beemployed. It was recognised at the time that thesemeasurements would need to be treated with cautionbecause the probe could not differentiate betweenacoustic signals and turbulence.
From the audio recordings individual noise fluctua-tions, or “roars,” were isolated more easily by low passfiltering the recordings then analysing them to deter-mine their temporal and frequency characteristics.Since initial results indicated that the frequency of the
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noise of concern lay predominantly between about125 Hz and 630 Hz, a low-pass cut-off frequency of800 Hz was used to isolate the noise from the other,higher frequency sounds that were also recorded.
Figure 2 shows a comparison of the 1/3 octave bandanalyses for three typical “roars” occurring during a1-hour period when Valve C was in control. Similarrepeatability was obtained for the recordings immedi-ately before and after the roars, and when Valve A wasin control.
3.2 Noise Variability
Analysis of the data measured at the stack tip did notreveal any significant time-varying fluctuations in noisethat corresponded to the variations heard, andmeasured, in the community at distances between650 m and 1250 m from the stack. This stronglysuggested that the fluctuations in the community noisewere a propagation effect and not caused by changes inthe characteristics of the source. Meteorological datawas routinely collected by the refinery and the impor-tant tests, that were fundamental to the diagnosis of theproblem, were only carried out when the weatherpattern was stable, with light winds blowing from theNortherly sector. The weather data was only collected
Fig. 2—Comparison of the 1/3 octave band spec-tra for three “roars” when Valve C was incontrol.
Fig. 3—Noise levels in the community for loud and
362 Noise Control Eng. J. 54 �6�, 2006 Nov-Dec
at ground level because of the difficulties of supportinginstrumentation at elevated positions in the refinery.
Comparison between the community noise record-ings suggested that the largest fluctuations in noiseoccurred downwind of the exhaust stack. When thetime histories of the noise at each location werecompared, after adjustments to allow for the propaga-tion times from the stack, there was no correspondencebetween the fluctuations at each location. This providesfurther support for the conjecture that the communitynoise fluctuations were due to meteorological effects.
The roar was characterised by averaging threerandomly selected samples which were taken from a1-hour recording of the noise with the process heldsteady. The three samples of the noise were taken at thebeginning, middle and end of the 1-hour period. Figure3 shows a comparison between the: averaged noiselevels for the “roar,” the averaged noise levels immedi-ately before or after the “roar,” and the difference inthese levels. These spectra correspond to the conditionof maximum community complaints when Valve C wasin control.
The noise survey results confirmed that the noiseemitted at the stack tip was very stable, did not fluctu-ate over short periods and was broadband and lowfrequency in character. The difference in overall,low-pass filtered noise level at the stack tip increasedby 4 dB from Valve A being in control to Valve C beingin control although the 1
3 octave band sound pressurelevels varied by up to 10 dB.
A simple analysis of the total sound power level inthe stack was carried out using established valve noiseprediction theory.1 The approach was to combine thesound power levels generated by the individual sourceswithin the system leading to and including the stack. Itwas assumed that the only form of attenuation in thesystem was due to the stack silencers. Since there wasno valve type in the published data that compared to the
t parts of the “roar” and the difference in levels.
quie
Valve C the nearest matching type was chosen. Predic-tions were made for the two situations when Valve Cwas in control, and Valve A was fully open, and thenwhen the operating conditions of the valves werereversed.
A sensitivity check was made for the range of valvetypes in the prediction method to establish the possiblerange of noise levels that might be generated. No inter-action effect, between the valve and the MHO, wasincluded in the calculation.
When the results were compared to the levels thatwere measured at the stack tip, and in the stack, it wasapparent that there was another stronger source of noisethat had not been accounted for.
The in-stack measurements were made using asingle high pressure transducer. Some contamination ofthe acoustic measurements by turbulence was antici-pated so the results were treated with caution. Never-theless, they gave an upper limit to the in-stack noise.Figure 4 shows the three estimates of the in-stacksound pressure levels, two upstream of the silencersand one downstream of the silencers. The levels are thelinear, 800 Hz low pass filtered sound pressure levels.There is a 20 dB difference between the two estimatesof the sound pressure levels upstream of the silencer.When the predicted upstream noise level is comparedto the downstream level the 16 m of stack silencingappears to give only 5 dB of dynamic insertion loss.
The exact value of the upstream sound pressure levellay somewhere between the two estimates. It wasconjectured that the noise source that was notaccounted for in the predictions was possibly an inter-action effect between the turbulence shed from Valve Cand the downstream MHO.
5 LABORATORY MODEL TESTS
The interaction hypothesis was explored further inthe laboratory by the use of a 1
3 scale model of theduct—valve—MHO system and exploring the changesin sound pressure level when changes were made to thearrangement. Figure 5 shows a schematic representa-
Fig. 4—Overall low-pass filtered sound pressurelevels in exhaust.
Noise Control Eng. J. 54 �6�, 2006 Nov-Dec
tion of the rig and Fig. 6 shows a view down the modelduct. The test rig was made of plywood with a squarecross-section, for ease of construction. A high volume,high pressure air supply was attached to the duct torepresent the flow in the stack. The in-coming airentered through a 70 mm diameter hole, which repre-sented the outlet of the valve when Valve C was incontrol.
The multi-holed orifice was represented by a panelwith scaled holes to the pattern of the MHO. A numberof other patterns of MHO were tested, as well as acombination of MHO’s in series.
The laboratory tests were carried out in the ISVR’sreverberation chamber. Not all of the controllingparameters could be scaled, however, such as thetemperature of the fluid in the stack, which can reachup to 500 °C.
Noise levels in the reverberation chamber weremeasured for:
• varying separation distance between the valveand the MHO
• varying jet velocity for a fixed arrangement ofvalve and MHO
• different designs of MHO plate
Fig. 5—Schematic diagram of laboratory arrange-ment.
Fig. 6—View downstream of the model duct.
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• using multiple MHO’s downstream of the valveThe results of the laboratory tests clearly demon-
strated that an “interaction effect” occurred when theMHO was close to the valve outlet. The interactioneffect increased the low frequency content of the noise.
Figure 7 shows the mid-frequency noise levels whenthe separation between the MHO and the valve wasincreased for a constant exhaust velocity.
Figure 8 shows the effect of varying the exhaustvelocity for fixed arrangements of the valve and MHO.
The relationship between the increase in noise andthe change in velocity of the air was approximately
�Lp = 10*Log10� v2
v1�5.5
where v2 and v1 are the two values of velocity.The 5.5 exponent is commonly associated with a
dipole sound source, which gives an increase of16.5 dB in the noise level for a doubling of velocity.
The laboratory tests concluded that:• the close proximity of the MHO to the valve
caused an increase in the low frequency noise,which was consistent with that found at thestack tip
Fig. 7—Effect of separation between valve andMHO �outlet velocity=164 m/s�.
Fig. 8—Effect of increasing jet velocity for MHO A
364 Noise Control Eng. J. 54 �6�, 2006 Nov-Dec
• noise level increased for reduced separation be-tween the valve and the MHO
• noise level increased for increased outlet veloc-ity for a fixed arrangement of valve and MHO
• the interaction effect behaved like a dipolesound source.
6 THEORETICAL PREDICTIONSOF THE SOUND PROPAGATION USINGA PARABOLIC EQUATION MODEL
The most annoying aspect of the noise in thecommunity was its high level of variability. One of theconclusions of the surveys described in Sec. 3 abovewas that the noise measured at the stack tip was steadyand the fluctuations were due to atmospheric effects.The Parabolic Equation model was used to try andunderstand this behaviour more clearly.
The Parabolic Equation model considered both thesteady state observed wind effect and the effect ofatmospheric turbulence along the sound propagationpath from the chimney stack to the observer. The turbu-lence model used here only accounts for typical turbu-lence fields produced naturally in the atmosphere, asthe characteristics of the induced turbulence fields bythe heat transfer activities of the refinery wereunknown.
It is assumed that the turbulence field µ, does notchange as the sound waves propagate through it. Thisapproach is known as the frozen medium approach andis based on the fact that sound waves take less time totravel from source to receiver than the sound speedprofile takes to fluctuate. This means that each realiza-tion will be like a “snapshot” of the turbulentatmosphere.
Atmospheric turbulence is included in the PE modelas small fluctuations of the sound speed, where n is thesound speed fluctuation, described mathematically as:
00 mm from valve.
at 8
n = n̄ + µ �1�
where n̄ is the average value of the sound speed and µdenotes the random perturbation representing theturbulence �with µ� n̄ and µ̄=0�.
The mathematical function describing the turbu-lence has been derived assuming that the fluctuatingpart of the sound speed µ�r ,z� has an autocorrelationfunction defined by:
C�s� � �µ�R + s� · µ�R�� �2�
where � � denotes an ensemble average over manyrealizations of µ, R= �x ,y ,z� is a position vector and srepresents some spatial separation distance in the r-z
Fig. 9—Theoretical predictions of variability in soulence.
Noise Control Eng. J. 54 �6�, 2006 Nov-Dec
plane. It is assumed that for small-scale atmosphericturbulence, C�s� can be approximated by a Gaussiandistribution,
C�s� = µ02 · e−s2/l2 �3�
where µ0 is the root-mean-square fluctuation of µ�r ,z�and l is the correlation length.
There are other possible distributions, like theKolmogorov and von Karman distributions, howeverthe majority of authors use the Gaussian distribution,and its universality makes it more suitable for thisstudy. Daigle, Gilbert and Salomons2–5 recommendorders of magnitude for µ and l of about 10−3 and 1 m,
ropagation due to wind and atmospheric turbu-
nd p
0
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respectively. Following these recommendations, thevalues that were used were 1.42�10−3 for µ0 and1.41 m for l.
To obtain approximations of µ�r ,z�, the square-rootof the wave number spectrum is calculated from theautocorrelation function C�s�, then multiplied by arandom phase function and finally computed using theinverse Fourier transform.
The PE model was run using 100 approximations ofthe fluctuating sound speed for each frequency of inter-est. Figure 9 shows the predicted transmission loss forfrequencies from 63 Hz up to 500 Hz, where the trans-mission loss is defined here as the difference in soundpressure levels between the stack tip and the commu-nity.
The results indicate increased variability withincreased frequency, up to 500 Hz. Some of the sharpdips in the propagation characteristics are due to inter-ference effects but the enhancements are solely windand turbulence effects.
Subject to the restriction that the predictions arepresented in single frequencies and cannot be directlycompared to the 1
3 octave band measurements, thepredictions provide a useful indication of the effects of
Fig. 11—Sound pressure levels at the stack tipbefore and after the modifications.
Fig. 10—Process changes to the stack.
366 Noise Control Eng. J. 54 �6�, 2006 Nov-Dec
atmospheric turbulence for the particular site condi-tions. However, the magnitude of the variation experi-enced at the site is greater than predicted. The predic-tion at 250 Hz, and at a distance of 1200 m from thestack, is ±5 dB about a mean attenuation of 60 dB fromthe stack. Comparing these predictions with themeasured results from Figs. 11 and 12, the measureddifferences in levels between the stack and the commu-nity values at 250 Hz are 35 to 38 dB. This suggests anunder prediction of the fluctuations by about 20 dB.Such discrepancies due to atmospheric effects areunusual but have been noted elsewhere.6
7 THE NOISE CONTROL SOLUTION
No one test of the many described above provedconclusively the causes of the excessive noise levelsand their fluctuations but the accumulated evidencestrongly supported the theory that the source of noisewas due to turbulent interaction between Valve C andthe MHO. This arose because of the close proximity ofthe MHO to the valve. The effects of the wind andturbulence caused the irregular fluctuations in themeasured noise in the community.
Following the investigation the client considered arange of solutions. The first solution that was consid-ered was to increase the separation between the twocomponents. Unfortunately there was not sufficientspace to do this. The final solution was a compromisebetween the requirements to meet stringent chemicalengineering objectives and the noise control objectives.The chosen solution was to replace the existing MHOand Valve C with three, widely separated MHO’s inseries. This arrangement enabled Valve C to beremoved and the required total pressure dropmaintained. The arrangement is shown in Fig. 10.
Following these modifications another noise surveywas carried out in the community. The results are givenin Fig. 11 and show a reduction in the stack tip noise of
Fig. 12—Sound pressure levels in the communityat “loud” part of fluctuation.
between 10 and 14 dB. A similar level of reduction inthe community noise is illustrated in Fig. 12.
8 CONCLUSIONS
This study has investigated the cause of the fluctu-ating noise heard in the community near to an oil refin-ery. The range of tests carried out in the field, and in thelaboratory, strongly suggested that the noise was due tothe turbulence shed by the Valve C striking thedownstream Multi-Holed Orifice Plate �MHO�. Thenoise emitted by the stack tip was steady but the effectsof atmospheric turbulence caused the noise received inthe community to fluctuate by more than 10 dB abovethe mean level at some frequencies. Removing thevalve and MHO and replacing them with three new,widely separated MHO’s reduced the noise at the stacktip and in the community by up to 14 dB at some 1
3
octave band frequencies.
Noise Control Eng. J. 54 �6�, 2006 Nov-Dec
9 ACKNOWLEDGMENTS
The authors would like to thank Stuart Dyne, ProfFrank Fahy, Matt Parker and Christos Karatsovis, all ofISVR Consulting, for their assistance in the project.
10 REFERENCES
1. D. A. Bies and C. H. Hansen, “Engineering Noise Control,”Third Edition, E & FN Spon.
2. Daigle, G. A. “Effects of Atmospheric Turbulence on the Inter-ference of Sound Waves above a Hard Boundary,” J. Acoust.Soc. Am. 64�2�, pp. 622–630 �1978�
3. Gilbert, K. E., White, M. J., “Application of the Parabolic Equa-tion to Sound Propagation in a Refracting Atmosphere,” J.Acoust. Soc. Am. 85�2�, pp. 630–637 �1989�
4. Gilbert, K. E., Raspet, R., Di X. “Calculation of TurbulenceEffects in an Upward-Refracting Atmosphere,” J. Acoust. Soc.Am. 87�6�, pp. 2428–2437 �1990�
5. Salomons, E. M., “Computational Atmospheric Acoustics,”Kluwer Academic Publishers, Dordrecht �2001�.
6. “Guide to the Use of Noise Procedure Specification,” The Engi-neering Equipment and Materials Users Association �EEMUA�,
