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Natural Science Forum / Physics / Acoustics / August 2007noise from flare
Hello, I've got to find a noise reducing solution for a flare at a water treatment plant. The flare basically burns the methan from the wasted waters. The noise isn't high, but an old lady leaving 500m away says the noise causes her severe problems. I've been there to measure but couldn't even hear the noise. My calculations show a noise level at - 5 dBA (yes, - 5!) at her place... The oldlady migth therefore be crazy but she makes such a mess that the mayor is willing to do something with the flare even if the noise level is way below any relevant regulation. Anyway, The flare is about 5m high and 1m in diameter. The methan is coming at low speed, cirka 4 m/s. My measurements show that the mean frequency is in the 31,5Hz 1/3-octavband. The noise is as created by the flames in the flare. Imagine flaming "bubbles" with low ascending speed. I "balance" between two solutions: lower the frequency to 20 Hz or so in order to make the noise less audible. Other solution is to increase the flow speed such as to raise the frequency around 125 Hz, a frequency more "convenient" for a silencer. In any case, it's a long shot and the cost for modifications is pretty high, so I'd reallly appreciate if anyone who's encountered something of that sort could give me some tips. Alain Bradette abr kirjoitti: > Hello, > [quoted text clipped - 22 lines] > > Alain Bradette Have you been to the old lady's house? What i'm after is that is it really the noise from the flare or rather e.g. low frequency pulsations from the flare which is generating noise in the house structures (first guess windows) or is it noise from the flare that "feels". BR, ari > abr kirjoitti: >> Hello, [quoted text clipped - 32 lines] > > ari I fully agree with Ari, and moreover, dBA is not suitable at all for describing the nuisance from low frequency sound. You should get the frequency spectrum, preferably, down to 10 Hz or lower, both outside and inside the house, and preferably both with the source on and off if possible. Be careful to avoid wind noise on the outside microphone. But if the 31.5 Hz is dominating the 5 dBA-level, the sound pressure level at 31.5 Hz is about 45 dB, which is below the normal treshold of hearing at that frequency, and should not be a problem. More likely the real problem is tinnitus, but the old lady will not like that. Does anybody else there get annoyed? Asbjørn Thanks everyone for your answers, I've been at the old lady's place, both outside and inside. I've measured the noise level inside, 18dBA, where the clock on the wall was the main noise source. I meausred also outside but it was useless as I could hear just noise from traffic and found obviously no correlation with measurements close to the flare. All measurements made in 1/3-octavband down to 8 Hz. Nobodyelse complains, it's just the old lady. In plain english, the old lady is crazy, there's no "technical problem". Crazy or not, the owner of the water treatment plant, the city, wants to do something. That's the "something" I'm after. After some further thoughs I'm thinking of buliding an enclosure around the flare with baffels as "roof". Any better idea? Alain > Have you been to the old lady's house? What i'm after is that is it > really the noise from the flare or rather e.g. low frequency pulsations [quoted text clipped - 6 lines] > > - Show quoted text - abr kirjoitti: > Thanks everyone for your answers, > [quoted text clipped - 24 lines] >> >> - Show quoted text - OK. Quick advise: "out sight out of mind". (Hopefully it was english;) Another thing 18 dB(A) is really low. At least i don't have a mic (in LD 3000+ Virte) which goes that down. It is questinable even with their best mic. (With a resonable to resolution to background noise). But if you didn't hear/feel the noise (with "sensitive mind"), i trust there is no real acoustic problem. And third. When my grandfather still lived at home, he was annoyed by he's own clock. In my opinion, not his... BR, ari Alain, My gut tells me an enclosure is not going to fly with the water treatment folks. I'm no expert, but assuming that the flaring is, for all intents and purposes, a rare occurrence, burning the excess methane to generate electricity, while noble, may not be economical. If it's a frequent enough occurrence, then capture, storage, resale, etc., should all be considered. As I understand it, fuel gas for a combustion turbine (to ultimately generate electricity, if that's what's being suggested above) needs to be quite pure. So, equipment would have to be installed to purify the methane. This equipment would require electricity to operate, thus reducing the system's efficiency. I would imagine there's a "break even" point where the volume of methane gets to be high enough that capturing its energy for something more useful than flaring becomes a viable option. If this has not been considered by the water treatment plant folks, it should be. If the flaring is frequent enough and there is enough fuel, they may be able to generate themselves enough electricity to run some of their facility, such as office A/C & lights, etc., and turn their waste into a windfall. But, like I said, I'm no expert on this. IMO, it would seem that they should be considering *something* as an alternative to flaring. State laws limiting carbon emissions will probably be widespread in the near future. If the plant is on the east or west coast, they may already be looking at having to limit their emissions. At a minimum, they should be considering their alternatives. Being a good neighbor involves more than just getting rid of nuisances like noise. :) All that aside, I did fine some papers on the 'net that might help you on flare stack noise: http://hfpacoustical.com/HFP_site/Downloads/Publications/pdfs/dinn_effect_of_flo w_parameters.pdf Explains the noise generating processes in flaring quite well. http://www.johnzink.com/products/flares/pdfs/tp_important.pdf Provides some "food for thought" on mitigation from flaring. Good luck, ---Savant--- You might reconsider the combustion process. Catalytic combustion (might be able to take a converter off a car and put it on there for some effect) or planarized combustion (passing the methane through a bed of charcoal or zeolite before burning it) could be ways of addressing the problem. On 8/16/07 2:02 AM, in article 1187254924.139883.280190@b79g2000hse.googlegroups.com, "abr" > After some further thoughs I'm thinking of buliding an enclosure > around the flare with baffels as "roof". Any better idea? Maybe all you need is a baffle to keep the lady from seeing the flare. You might even be able to test her to find out if she can tel when the flare is going when she cannot see the flame. Bill -- Fermez le Bush--less than 18 months to go. On 8/15/07 6:23 AM, in article 1187184223.090082.5440@a39g2000hsc.googlegroups.com, "abr" > I've got to find a noise reducing solution for a flare at a water > treatment plant. The flare basically burns the methan from the wasted [quoted text clipped - 5 lines] > with the flare even if the noise level is way below any relevant > regulation. Anyway, <snip> Sell the methane. Use a catalyst (platinized asbestos or something like that) to get flame free oxidation. There must be a use for that energy. Get a natural gas internal combustion engine driving an alternator to push the energy onto the grid. Bill  SignatureThe PC conservative does not believe in evolution but likes to see natural selection proceed. The PC liberal believes in evolution but will do almost anything to prevent natural selection from working. >On 8/15/07 6:23 AM, in article >1187184223.090082.5440@a39g2000hsc.googlegroups.com, "abr" [quoted text clipped - 15 lines] >a natural gas internal combustion engine driving an alternator to push the >energy onto the grid. I am familiar with a plant near my house. Their flame is silent. Its also small. Conversion to power their own equipment was planned, but it seems its too complicated. Each state or country will likely have different rules and methods. greg >>On 8/15/07 6:23 AM, in article >>1187184223.090082.5440@a39g2000hsc.googlegroups.com, "abr" [quoted text clipped - 22 lines] > > greg I once knew a dust control engineer who designed a baghouse to collect the dust, The client thought it too expensive ($30K), so the engineer offered to build it himself, provided he could keep the dust. The client was suspicious until it was explained that the dust was valuable and would pay for the baghouse within 6 months - followed by years of profit. Methane to electricity is old technology. Collection, storage, and electricity generation is an "off the shelf" technology. Local power supply persons would know what companies supply these conversion units. Burning methane reduces its "greenhouse gas impact" twenty fold, plus producing usable marketable energy. This "flare noise" could be a problem of an annoying noise - plus a very visible flare. The sight of the flare may be the visual cue that she relates to a noise that has nothing to do with the flare. Or it could be that the flare may be producing perceptible air borne pulsations. Or it could be an old lady with inner ear problems. I once spent several night searching for a "pump noise" for an old lady (85+) with tinnitis, high blood pressure, and tremors. Nothing was detectable even down to 10Hz with a background less than 20dB(A). The last I heard, she is still employing acoustical consultants to find the "pump"(?) producing her late night "buzzing" noises and "tremors". We only parted company (thankfully) when I suggest a physiological cause rather than a physical cause. >> In article <C2E888D4.9308D%salmonegg@sbcglobal.net>, Salmon Egg > <salmonegg@sbcglobal.net> wrote: [quoted text clipped - 55 lines] >only parted company (thankfully) when I suggest a physiological cause >rather than a physical cause. At my new house, I live up on a hill, over a river with some industrial plants down over the hiil near the river. I get this fluttering once in a while, but it only lasts for 10-15 seconds. Maybe 10 Hz or less. Dosen't quite shake the house, but I am interested in finding the source. Others I have asked don't detect it. I have not metered it. greg clip, clip > At my new house, I live up on a hill, over a river with some industrial plants > down over the hiil near the river. I get this fluttering once in a while, but [quoted text clipped - 3 lines] > > greg An airplane up in the sky braking 1 Ma limit? Cavitation of really big pump? An (small) explosion somewhere? Or something else as fancy?... Have you got a wooden house (and others a brick house)? Resonating structures? Hard rock or mud in the ground... Possibilities are many. The key might be a systematic observation (a 1700 centuary scientist's way, updated into this millenium). BR, ari >clip, clip >> At my new house, I live up on a hill, over a river with some industrial [quoted text clipped - 16 lines] >Possibilities are many. The key might be a systematic observation (a >1700 centuary scientist's way, updated into this millenium). When I first heard it I assumed it was a hellocopter. Its louder in my bathroom, a brick house. I was walking over to my neighbors one day and I heard or sensed it. They were sitting outside, but they did not hear it after i asked. I am probably more familiar with sounds or pressure changes. At work, ther is one area on one floor, where a pump or air handler puts out very strong pulses in the same vicinity of what i am hearing. If I had some type of directional microphone setup, perhaps I could tell which direction the house noise is comming from. greg GregS kirjoitti: clip, clip > If I had some type of directional microphone setup, perhaps I could tell > which direction the house noise is comming from. I would not expect too much inside, otside possibly (if the frequency is around 10 Hz). 10 Hz requires quite is big "microphone grid". Hundreds of meters, either in 2D to 3D (depending on wheather the sound is coming from groud or air)? See for army studies on localizing (enemy) airplane take offs and similar. > > greg BR, ari < SNIP > > At my new house, I live up on a hill, over a river with some industrial plants > down over the hiil near the river. I get this fluttering once in a while, but [quoted text clipped - 3 lines] > > greg Once dealt with a charcoal retort (16 storeys high). A few times a day it would deviate from its stability point and start "hunting" for a few minutes until it achieved stable operation again. Each time, it would generate a 10Hz tone. With the wind in the right direction, it would rattle windows and doors of houses 3.5 kilometres to the west. The only other 'distant' indication was a low level rising-and-falling hum. All analysis done by sound level meter - no need for special exotic equipment. Once it was identified and located, I offered a solution, but the client and their neighbours leave it alone - as a local curiosity. >> In article <krMwi.18221$tp3.98847@nasal.pacific.net.au>, Brian Marston > <marstonbb@pacific.net.au> wrote: [quoted text clipped - 21 lines] >equipment. Once it was identified and located, I offered a solution, but >the client and their neighbours leave it alone - as a local curiosity. I have some chemical plants plus a power plant burning coal. every once in a while it seems they shut down the collector to clean the system, makes a steamy sound. I also have trains, and tug boats. Seems much better now, but some of the engineers would get out of control with the horns. When the air conditions are right, they get pretty loud. Normally they are at a reduced level, and I have become acustomed to ignore them. greg > I've got to find a noise reducing solution for a flare at a water > treatment plant. The flare basically burns the methan from the wasted [quoted text clipped - 10 lines] > is in the 31,5Hz 1/3-octavband. The noise is as created by the flames > in the flare. Imagine flaming "bubbles" with low ascending speed. 5 dBA... If it dominated by 31 Hz (you said so above) one is obliged to back calculate what the linear level it was that you measured. My conversion scale shows -39 dB weighting for 31 Hz, making the sound 44 dB linear. My one encounter with such "burner noise" was in a fellow's house 500 feet from the offending plant. He heated with a wood burning fireplace (no HVAC fans!). I measured 48 dB in the 31 Hz band in the center of his living room (60 dB in the corners of that room where I fully expected to find the standing wave pressure maximum). I discovered that the noise source was a strong 40 Hz combustion resonance in a gas furnace used to heat plant process solutions, painfully audible in that process room by that furnace. I designed a 40 Hz branch resonator to place in the stack from that furnace to the roof. Cured the problem. First you must confirm that the flame column is indeed the source of this 31 Hz band sound. If it is not, the city should be aware of this. It it is, then you have to produce convincing data that it is (or is not). Then, if it is, fix it; but not until the proof is made. Share with us your data and results.... Angelo Campanella I just returned from vacation. Have you googled "Taos hum"? Do this first. In cases where some persons hear LF-noise, but most (the person studying the case, too) not, it may be helpful to apply the procedure described in http://www.defra.gov.uk/environment/noise/research/lowfrequency/pdf/nanr45-proce dure.pdf At very low frequencies loudness may double when sound pressure level increases by 2...4 dB near the hearing threshold. At 1 kHz we need ca. 10 dB increase to double loudness. Personal hearing threshold varies. If we assume that the hearing threshold of persons having normal hearing is normally distributed, we may meet persons that hear sounds inaudible to, say, 99 % of people. We know that some persons, for example, can not fall asleep, if the LF-sound they complain about, is audible. This is some kind of ON/OFF-response. Perhaps adequate behavioural therapy might help them to "learn-off" this response. In cases where the complainer suffers significant health effects, for example, can not sleep in certain rooms/houses, but does not have difficulties to sleep in environments where he/she cannot hear the sound/noise in question: one should make careful measurements and observations in the case when the complainer hear the sound and does not hear. Unfortunately we very often can not find such signal components that could explain what is heard and when. Another method is trying to simulate the sound. Unfortunately in many countries they do not have any LF-chambers with necessary signal generators to produce LF-sound with different adjustable frequency and time domain characteristics. - - - By heart: Flare noise papers were published, for example, in 1970's in hydro-carbon and oil industry journals. Should consult my archives. I remember that I have several papers copied. BR Kari Pesonen > Hello, > [quoted text clipped - 22 lines] > > Alain Bradette Interesting information Kari. I'll have a closer look at the article. But, in the case of my project, I don't have to prove wether there's really a problem or not at the lady's place. I simply have to find a solution. I'm thinking of an enclosure on 4 sides with baffles in the opening as "roof". I must calculate the dimensions and see whether it is a feasable solution. I'll keep the news group informed if the solution sees daylight one day and what are the results. Alain > I just returned from vacation. > [quoted text clipped - 67 lines] > > - Show quoted text - > The flare is about 5m high and 1m in diameter. The methan is coming at > low speed, cirka 4 m/s. My measurements show that the mean frequency > is in the 31,5Hz 1/3-octavband. The noise is as created by the flames > in the flare. Imagine flaming "bubbles" with low ascending speed. I am taking a fresh ook at this since it appears to be a unique problem. There is a phenomenon demonstrted in classrooms called a "Singing flame", where a horizontal sound wave is sent along a long horizontal tube with closely spaced hole perforations. Illumination gas (e.g. methane) is sent into the tube to flow from those perforations. The sound wave modulates the gas flow rate from ech perforation to establish the length of the flame sent forth, to be short or tall. But that is not quite the phenomenon here. Here, we have a gas flow rate from a single pipe opening that may already be modulated by conditons inside that feed pipe. In particular, that feed pipe may have velocity fluctations or standing waves within it. The wavelength of this standing wave depends on the speed of sound in methane, or in the gas mixture in tat stream whci can cpntaon CO, CO2, etc. but it wiil generally be different that for air. > I "balance" between two solutions: lower the frequency to 20 Hz or so > in order to make the noise less audible. Yet another possibility is to quell any oscillation of flow speed in the pipe or at its outlet. That is, implement a damper at the outlet point in the form of a different "jet" aperture; a hole of a different size, or some sort of a throttle (aka damper) plate.. > Other solution is to increase > the flow speed such as to raise the frequency around 125 Hz, a > frequency more "convenient" for a silencer. That would be a modified "throttle" that is frequency selective... > In any case, it's a long > shot and the cost for modifications is pretty high, so I'd reallly > appreciate if anyone who's encountered something of that sort could > give me some tips. If the length of the feed pipe is changed, and if the frequency of the sound also becomes changed, then we know it's a standing wave resonance in the feed pipe that is inspiring that tone. My solution here for the 40 Hz tone here included the notion that a branch (Helmholtz) resonantor placed anywhere anywhere along the side of the 22" diameter stack (only two 7" interface holes needed to be cut in the stack wall) would absorb the 40 Hz sound power. That it did indeed do!; so much so that the 40 Hz tone completely disappeared! Therefore, the badsic phenomenon had been that of a feedback osclilator, where the feedback was diminished by the branch, and the 40 Hz oscillation simply could no longer exist. Angelo Campanella > > The flare is about 5m high and 1m in diameter. The methan is coming at > > low speed, cirka 4 m/s. My measurements show that the mean frequency [quoted text clipped - 53 lines] > > Angelo Campanella interesting "angelo Campanella" <a.campanella@att.net> ???????:aMCzi.51751$ax1.25793@bgtnsc05-news.ops.worldnet.att.net... >> The flare is about 5m high and 1m in diameter. The methan is coming at >> low speed, cirka 4 m/s. My measurements show that the mean frequency [quoted text clipped - 9 lines] > the gas flow rate from ech perforation to establish the length of the > flame sent forth, to be short or tall. Here is a video clip on Youtube showing this experiment. Really interesting. http://www.youtube.com/watch?v=RI6qGJ1SL8c CSL |
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