Acoustic measurments and scenarios to reduce noise at a natural gas compression station.

Ionescu, Silviu ; Ionescu, Adina ; Codoban, Radu 等

1. INTRODUCTION

The noise pollution sources from a natural gas compression and transmission station are very important, especially when they are used in the vicinity of residential areas, where the environmental legislation for residential areas is applying. In such systems, the intake and exhaust pipe are more susceptible than the compressor nacelle to transmit noise due to the fact that the pipes walls are more thinner, focussing the propagation of vibrations and sounds. Because the internal acoustic source spread waves through the compressor and pipe lines, the source is propagated by the structure and it make it to vibrate and to produce noise. The pressure variations are the main noise and vibration source of the pipeline (*** NOVICOM, 2008).

The centrifugal compressor is considered to be the main noise and vibration source in a natural gas compression station. But there are also secondary sources generating noise and vibration, as following:

--flow tilt elements (diaphragms), which are working in a critical range;

--closing--opening elements, which are working at partial loads;

--elbows and sections variations of the distribution pipelines.

Performing acoustic measurements, accomplishing the noise map for a natural gas compression station and offering scenarios to reduce noise are the first step to solve the legislative noise issues.

2. EXPERIMENTAL MEASURMENTS

2.1 Acoustics measurements

The purpose of the measurements was to create a new database for the spectral components of the main noise sources that we identified. The acoustic measurement follows the plan presented in figure 1, which also includes 2 measurement points close to the first houses, at 1 Km from the industrial zone. Additional noise and vibration measurements have also been achieved outside the skid, at intake and exhaust pipes, in order to calculate the sound power and also for other studies regarding the correlation between noise and vibrations (Magheti & Savu, 2004). We identified that the centrifugal compressor was the main noise source, with the maximum amplitude at 2912 Hz, during those operating conditions.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

From the spectra we notice that a very high resonance with harmonics was transmitted only at the exhaust elbow pipe with a noise level of about 108 dBA.

This sound level at such frequency is disturbing for the human ear. Also the A weighting curve can show that the frequency domain between 1 kHz and 5 kHz is over the linear values. In figure 2 is presented the harmonics of the 2912 Hz.

The equipment used for measurements was a 01dB METRAVIB multichannel acquisition system with 12 microphones and a Solo sound level meter, both having a class 1 precision (Ionescu & Ionescu, 2009).

2.2 Noise map

After measurements and post-processing, we have achieved a new noise database for intake and exhaust pipes, which has very different spectral components, and we imported all octave bands in the noise mapping dedicated software (IMMI), according to Directive 2002/49/EC (Environmental Noise Directive) and the Romanian legislation the Governmental Decision 321/2005.

Our intention was to realize a strategic noise map based on a 3D model for the local zone, using also meteorological data and the noise database measured, in order to have an image of the sound propagation in free field with real data.

Following these steps, it has been accomplished a confirmation of the realized prediction with the measurement results and we evaluated also the environmental noise impact, in the vicinity (at 1 Km) of the community (Ionescu & Ionescu, 2009).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The current situation shows us that close to the first house the "Lden" is between 50 dBA and 55 dBA. From the environmental noise legislation point of view, the admissible noise level near to houses should be less than 50 dBA.

Also, it is very important to take into account the fact that cities are increasing fast and the noise limits will touch smaller values in few years in order to obtain more quiet areas.

In the figure 5 is presented the industrial noise map for the local zone, based on the 3D model presented in figure 4 and calculated with the measured noise database of the spectral components presented in figure 3.

The hypotheses taken into account for the propagation model are: calculation of 2 reflections, an average temperature of 15[degrees]C and a relative humidity of 50%.

For the influence of the local meteorological conditions, a C0/dB factor has been used with 2 dB for day and 1 dB for evening and night.

3. SCENARIOS TO REDUCE NOISE BASED ON PREDICTION

Due to the fact that the natural gas compression station is working continuously, so we couldn't work to reduce the source, we decided to offer 3 scenarios to reduce noise on propagation path close to the noise source and at the receiver place. For this purpose, we used for simulation the same 3D model with the same data, but we introduce at 1 meter from the pipelines, 3 types of sound barriers (Enescu & Magheti, 1998). The first scenario was realized with a standard cantilevered barrier selected from software database which is also used for roads (Fig.6.A). The second scenario was realized with the same transmissibility and with the same absorption loss like the first one, but with a different geometry and height (Fig.6.B.). In the last scenario we decided to use for simulation a specific sound barrier (Fig.6.C), based on a special sound-absorbent material which we designed and tested with the last generation of Kundt Tubes. We introduced in the propagation model the specific absorption coefficient "Alpha" of the custom structure and we obtained the following results presented in figure 7.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

4. CONCLUSION

These scenarios show us that we can achieve a reasonable noise reduction without expensive cost using a custom sound-absorbent structure. Similar work and case studies was presented by the authors in others articles. Further work must be done in order to reduce noise at source trough research and new concept design. In the frame of a "NOVICOM" Research Project, the next steps for the team will be to act on the centrifugal compressor, reducing noise at the source.

5. REFERENCES

Enescu, N & Magheti, I. (1998). Technical acoustics, Ed. ICPE, pp 155-159, ISBN 973-98801-2-6, Bucharest

Ionescu, S. & Ionescu, A. (2009). Case study--noise sources identification through measurements and diagnosis at a power plant, Proceedings of the 3rd International Conference on Experiments/Process/System Modeling/ Simulation & Optimization, Athens, 8-11 July

Ionescu, S. & Ionescu, A. (2009). Noise monitoring at the natural gas compression stations. Environmental impact, Proceedings of the Annual symposium of the institute of solid mechanics and Symposium of Acoustics, Bucharest, 28-29 May

Magheti, I. & Savu, M. (2004). Mechanical Vibrations, Ed. BREN, pp 135-137, ISBN 973-648-389-4, Bucharest

*** (2008). INCDT-COMOTI--Research on establishing the causes to the increase noise and vibration level in technologic units as: regulation-measurement, pipe control units and natural gas compressors--NOVICOM, National Research Project, Ctr. No. 32-163/2008, Bucharest