Tuesday, May 26, 2009
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Pressure reducing device such as control valve, pressure relief valve, restriction orifice, etc, there will be pressure drop and mass passing through these device, internal acoustic energy is generated and transmitted to downstream piping and potentially lead to severe piping excitation, vibration and stresses on downstream piping and potentially lead to fatigue failure. Internal acoustic energy transmitted along the pipe may also transmitted to through the pipe and emitted as Noise.
One of the common safety requirement is limit the noise level to 85 dBA (Noise level with A-weighted) in continuous exposure and 115 dBA during intermittent exposure. In earlier conceptual or Front End Engineering Design (FEED) stage, the noise level across pressure reducing device may be estimated to determine overall noise control philosophy. The following will present a simple method to estimate noise level generated at 1 meter from a pressure reducing device.
Noise Level Estimation
Noise level at 1 meter from a pressure reducing device can be estimated from Sound Power Level (PWL) as discussed in "Sound Power Level (PWL) Prediction from AIV Aspect". The Sound Power Level will be transmitted across the pipe wall and emitted to atmosphere. There will be noise correction when Sound power is transmitted across pipe wall (metal). The noise correction is subject to wall thickness. Thicker wall will result higher noise correction. Following are typical noise correction for pipe size and wall thickness.
Noise level at 1 meter from a pressure reducing device,
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Pressure reducing device such as control valve, pressure relief valve, restriction orifice, etc, there will be pressure drop and mass passing through these device, internal acoustic energy is generated and transmitted to downstream piping and potentially lead to severe piping excitation, vibration and stresses on downstream piping and potentially lead to fatigue failure. Internal acoustic energy transmitted along the pipe may also transmitted to through the pipe and emitted as Noise.One of the common safety requirement is limit the noise level to 85 dBA (Noise level with A-weighted) in continuous exposure and 115 dBA during intermittent exposure. In earlier conceptual or Front End Engineering Design (FEED) stage, the noise level across pressure reducing device may be estimated to determine overall noise control philosophy. The following will present a simple method to estimate noise level generated at 1 meter from a pressure reducing device.
Noise Level Estimation
Noise level at 1 meter from a pressure reducing device can be estimated from Sound Power Level (PWL) as discussed in "Sound Power Level (PWL) Prediction from AIV Aspect". The Sound Power Level will be transmitted across the pipe wall and emitted to atmosphere. There will be noise correction when Sound power is transmitted across pipe wall (metal). The noise correction is subject to wall thickness. Thicker wall will result higher noise correction. Following are typical noise correction for pipe size and wall thickness.
| Noise Correction | ||
| Nominal Dia. (Inch) | Wall thickness (mm) | Noise Correction (dB) |
| 25 | 3.38 | 54 |
| 50 | 3.91 | 50 |
| 100 | 6.02 | 50 |
| 150 | 7.12 | 49 |
| 200 | 8.18 | 48 |
| 250 | 9.3 | 47 |
| 300 | 9.53 | 47 |
| 350 | 9.53 | 46 |
| 450 | 9.53 | 46 |
| 600 | 9.53 | 45 |
| 750 | 9.53 | 43 |
| 900 | 9.53 | 43 |
| 1050 | 9.53 | 42 |
Noise level at 1 meter from a pressure reducing device,
L1m = PWL - LA
where
PWL = Sound Power Level (from Sound Power Level (PWL) Prediction)
LA = Noise correction from above table
Example
PWL = 10 x Log [((87-7) / (87+1.01325))^3.6
x (100,000 / 3600)^2
x ((50+273.15)/22)^1.2]
+ 126.1
PWL = 167.5 dB
Noise level at 1m,
L1m = PWL - LA
L1m = 167.5 - 46
L1m = 121.5 dBA
As the noise level at 1m (normal trim) is 121.5 dBA, this is far too big from normal requirement. A low noise trim control valve may be considered and/or acoustic insulation to be provided.
Related Topic
PWL = Sound Power Level (from Sound Power Level (PWL) Prediction)
LA = Noise correction from above table
Example
A pressure control valve (PV) passing 100,000 kg/h of gas with molecular weight (MW) of 22. The inlet condition is 87 barg and 50 degC and downstream pressure is about 7 barg. The pipe diameter is 18 inch with wall thickness of 9.53mm, estimate noise level at 1 m from PV.
PWL = 10 x Log [((87-7) / (87+1.01325))^3.6
x (100,000 / 3600)^2
x ((50+273.15)/22)^1.2]
+ 126.1
PWL = 167.5 dB
Noise level at 1m,
L1m = PWL - LA
L1m = 167.5 - 46
L1m = 121.5 dBA
As the noise level at 1m (normal trim) is 121.5 dBA, this is far too big from normal requirement. A low noise trim control valve may be considered and/or acoustic insulation to be provided.
Related Topic
- Assess AIV with "D/t-method"
- Extra Attention to Common Point and Similarity on AIV Failure
- Piping Excitation When Expose to Acoustic Energy
- Acoustic Induced Vibration (AIV) Fatigue
- Sound Power Level (PWL) Prediction from AIV Aspect
- Several Criteria and Constraints for Flare Network - Piping
- Flow Element (FE) Upstream or Downstream of Control Valve (CV) ?
Labels: Air Pollution Control, AIV, Safety
posted by Webworm, 1:26 PM
3 Comments:
Hi,
In the above link article to determine PWL, you showed an example of PWL calculation, which gives an answer in units of dB.
In the above table, the noise correction factor "LA" is in units of dB as well.
Therefore, should the above L1M anwer be in units of "dB" too, instead of "dBA"?
If this is so, how would you convert dB to dBA?
Thank you.
The noise correction factor contain Sound Power Level (PWL) attenuation and correction from dB to dBA. Therefore the PWL generated at source minus the noise correction factor (attenuation plus correction) lead to noise level in dBA.
hi thanks for the info. I find it really useful. Could you please provide me the reference for the attached noise correction table for the different pipe and wall thickness and the reference for the equation to determine noise correction.
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