THE MODELING AND CALCULATION OF SOUND RADIATION FROM FACILITIES WITH GAS FLOWED PIPES INTRODUCTION

Transcription

1 THE MODELING AND CALCULATION OF SOUND ADIATION FOM FACILITIES WITH GAS FLOWED PIPES INTODUCTION Analysis o the emission caused by industrial acilities like chemical plants, reineries or other production areas is the irst and most important step while calculating the noise pollution o the environment. Pipework with gas or luid lows are oten contributing relevant to the sound radiation o a complete acility. This radiation can be determined applying the methods described in technical papers like VDI 3733 and ISO FIGUE 1: Gas pipes inside industrial acility In all cases where measurements are not possible or during a planning and optimization process, a practical technique is to derive the sound power level rom signiicant parameters which do have inluence on the noise emission. Ater the deinition and modeling o an emitting system, all contributing system parameters inluencing the emission can be altered according to dierent operating scenarios. As a result the spectrum o the noise emission o a gas pipe can be calculated or the real conditions. ACOUSTICAL ASPECTS OF GAS FILLED PIPES Pipes are used to transport solid, liquid and gaseous media. The ollowing only deals with round pipes or gaseous media without a solid content, e.g. granulates etc., up to a diameter o 800 mm. For pipes with rectangular cross-sections and pipes or liquid media, dierent considerations must be taken into account. From the acoustical point o view, pipes with large diameters are to be viewed as even plates. Signiicant or the emission o a pipe is the contribution o sound power via the outer walls o the pipe introduced into the pipe rom external sources such as ans, compressors etc. As a rule, this external sound power is signiicantly higher than the natural noise generated by the low. Noise absorption in Gas illed ound Pipes Compared to lat plates, pipe noise absorption has signiicantly higher values below the ring expansion requency (see /1/]). educed sound absorption occurs in the range o the critical requency that is dependent on each wave and vibration orm (mode) (see /1/, /5/). Above the ring expansion requency, the acoustic characteristics

2 o pipes are the same as or lat plates with a characteristic drop in sound absorption in the range o coincidental requency (see also /3/). From /1/, the ollowing equations are used. In addition, the inluence o coincidence according to /3/ and /4/ is taken into account. Formula or attenuation is: D 1 10Log 4 3 E = ( 10 d ) sinh 10Log B ( B) + L (1) Where: c ρ s + Log /5 w w r = Log + cf F d ρ r + K (2) c F = p 10 (3) ρ F r cw = π d 10 3 (4) K = 35 10Log 1 + g g + 1 (5) B = K =0 or g < r 4 10 g =.4 (6) 1 3 ( d 10 ) 6 W ( c ( s 10 3 ) ( d 10 ) + α (7) t α = ( + M ) ( ) A d p 1 11 (8) M A = v/c F L = α (9) 3 ( d 10 )

3 Considered parameters are: Lm [m] section length d [mm] pipe diameter v [m/s] low speed in the pipe p [bar(abs)] absolute gas pressure in the pipe ρ F [kg/m3] gas density in the pipe t [grd C] gas temperature in the pipe s [mm] pipe wall thickness ρ W [kg/m3] pipe wall density c W [m/s] expanding wave speed o the pipe material Flow Noise Generation Flow noises are caused in lows through pipes by turbulent boundary layer low. In accordance with /1/, a proportion o 10-3 o the low energy is converted into sound energy. In industrial acilities, low noise is usually o subordinate signiicance, but can be a dominating sound source in piping ittings with high pressure loss (e.g. Valves / Armatures) and the outlow rom silencers. To determine low noise, various equations are speciied in the literature. The A-evaluated sound power level increases with around the power o 6 o the low speed. Flow noise is independent o the pipe length, but is also dependent on the pressure loss. L WA ( w) + 10Log( S) + 10Log( p) 25log( N T ) 15Log( κ )db = s 0.16 w + 60Log (10) Where T = t The considered parameters in this case are: w [m/s] low speed S [m2 ] cross-section area p [bar] pressure in the pipe or channel N [kj/kg/k] speciic gas constant T [ C] temperature Κ [---] adiabatic exponent CALCULATING THE SOUND POWE LEVELS OF GAS PIPING SYSTEMS FOM TECHNICAL PAAMETES USING SIMULATION SOFTWAE Sound power levels o technical sound sources depend on source-typical parameters. Those can depending on the kind o source or example be the nominal value, the length, the diameter, etc. The sotware program CadnaA /6/ eatures a module based on the idea (SET), that the resulting sound power levels can be calculated by the program depending on the values o theses parameters. This eature can be used to automatically calculate sound power levels o sources as well as transmissions to urther elements, depending on the appropriate requency dependent attenuation; single sources can thus be combined to assemble complex noise emitting systems. Updates in these systems or example due to the insertion o new elements or the position change o an existing one lead to technical correct changes o the complete system; this way, the eects o a silencer can or example easily be tested, or dierent system s operation conditions can be evaluated with minimum time and eort. Technical source elements are deined in the sotware according to the ollowing igure, and can later be used in the model. Up to 10 parameters can have inluence on the calculation o sound power levels or the attenuations;

4 Linkages or consideration o transmission can be deined with up to 9 inputs and outputs; urthermore there is an output or the transmission. adiation AD IN1 OUT1 IN2 OUT2 IN3 OUT3 Input IN4 IN5 SC Source OUT4 OUT5 Output IN6 OUT6 IN7 OUT7 IN8 OUT8 IN9 OUT9 a b c d e g h i j Parameter FIGUE 2: Theoretical model o a SET-S element In the ollowing example a an puts pressure in a piping system consisting o 4 elements, which inishes in a steel chimney. Apart rom the radial an these are the pipes, the steel chimney and the element or the mouth relection. Transmission between the dierent elements o the piping system is also considered. These are all assembled linearly ater each other, which allows the structure to still be relatively simple (as seen in the ollowing igure). Per element, not more than one in- and output is used. AD AD AD AD AD adial Ventilator OUT IN OUT OUT OUT Pipe 1 Pipe n Steel Chimney Chimney Mouth IN IN IN a..j a..j a..j a..j a..j Flow ate (m^3/s) Pressure Di. (Pa) Length Cross-Section Flow ate Gas Density... Length Cross-Section Flow ate Gas Density... Length Diameter Wall Thickness Exhaust Temp.... Cross-Section ad. Angle FIGUE 3: Logic structure o a conduit s elements (rom let to right). Deinition o the system Single blocks can be easily created line by line in the so- called SET-T table. All the necessary lines are shown in the ollowing igure. FIGUE 4: Conduit with a an, 4 pipes and a steel chimney with opening

5 Single modules need to get the necessary values or the parameters. The ollowing igure shows values on the example o a pipe section. Furthermore, linkages between system elements are relevant. In this example, the start point is the radial an, where pressure side (output 2) is linked with the pipe 1. For all urther elements, the deault setting next can be kept or output 1, as the linked elements are directly connected to them in the SET-T table (Figure 4) Ventilator Volumenstrom 56 m³/s Gesamt-Druckdierenz 7000 Pa ohrleitung Teilstücklänge 10 m³/s ohrdurchmesser 2000 mm Strömungsgeschw m/s Gasdruck 1 bar Gastemperatur 20 C ohrwanddicke 2 mm ohrmaterialdichte 7850 kg/m³ Dehnwellengeschw m/s Kamin Teilstücklänge 20 m Kamindurchmesser 3000 mm Kaminwanddicke 2 mm Abgastemperatur 20 C Verschmutzungsgrad 2 Kaminmündung Querschnitt 7.2 m² FIGUE 5(A): Exemplary values or the required elements in SET-T FIGUE 5(B): Parameter coniguration o a pipe section. Modeling and calculation The modeling is conducted with regular industrial sources or all the elements. The an can be modeled with a point source. The ollowing pipes can be modeled with line sources and the chimney is created with a line source entered vertically. At last the chimney opening is modeled; this is done with a point source with a speciic directivity. The described model is shown in the ollowing igure. FIGUE 6: Conduit with an, 4 pipes and a steel chimney. The values shown at the industrial sources are sound power levels calculated with exemplary parameter values.

6 Once the model is set, a calculation can be done in order to get the sound pressure levels at the receiver points. GAS PIPEWOK MODEL S MODIFICATION AND UPDATING Once a pipework model is created, it can easily be updated or modiied. For example, elements can be moved via drag & drop; urthermore new elements can easily be inserted. In both cases, all emission spectra and the transmitted sound can be updated with just one click. Modeling non - physical aspects In some cases, urther elements need to be inserted even i no physical elements are added to the system. This should be considered when working with SET and is shown with the change o a cross-section here. The cross-section o a pipe has inluence on noise emission and transmission, which is considered correctly in SET. But the sudden jump between 2 diameters also has an inluence; this is not considered with the single parts so ar. So beside the adjustment o pipe parameters (diameter, luid speed, etc.) a urther element must be entered to consider the changing cross-section. To achieve this, the block Cross Section Jump can be utilized. In SET-T it can be inserted at the desired position between two pipes; depending on the cross-sections entered in the element, a urther attenuation between the respective pipe elements is considered. FIGUE 7: SET-S input parameters dialog or a Cross Section Jump The cross-section jump is only mentioned as an example among others. Especially or pipe systems, many elements are documented and can be used. Entering and changing position o urther elements in the system For already existing pipework systems, new elements can be created easily in order to evaluate dierent scenarios. In this example, a silencer should be included between the last pipe and the chimney. The silencer shall be a carriage with attenuation according to a predeined, requency dependent curve. Once the new element is entered the noise transmissions and emissions are completely re-calculated. Figures 8(a) and 8(b) show the system s table beore and ater the entry o a silencer with exemplary values. As can be seen, the silencer itsel doesn t emit any noise. To change this, an output or AD could have been added in the appropriate table.

7 FIGUE 8(A): Pipework system without any noise reduction device FIGUE 8(B): Inserting a silencer between Pipe 4 and steel chimney This easy tool is also drag & drop sensitive, allowing quick assessment o all possible actions. In the ollowing igure, the silencer, which was between Pipe 4 and the Steel chimney, will be moved to the position between pipe 1 and pipe 2. Ater a recalculation the complete system s table is automatically renewed, which means that the changed emission- and transmission values and (as ar as receivers are present) the changed sound pressure levels are immediately visible. FIGUE 9: Inserting a silencer between Pipe 1 and Pipe 2 This possibility to place a sound-inluencing element simply by clicking on this table line and dragging enables the study o any solution with minimum time and eort. The silencer or the data structure behind this line can be copied and allocated in other positions. Cut and paste becomes the technical data pendant to removal and installation. Whereas in the case o conventional programming, all radiation suraces and sources downstream o the silencer have to be "touched" to reduce their sound power accordingly, this is now dealt with by inserting the silencer with one operation. The same procedure continues until immission calculation. I a calculation is run ater changing the plate thickness o a pipe in an industrial system, the consequences have been correctly taken into account or all calculated levels and also or entire noise maps.

8 FIGUE 10: esulting Sound Powers and Sound Pressure level at receiver when placing the silencer between pipe 1 and 2 EFEENCES 1. VDI 3733 Noise in pipes, July DIN 2458, Welded steel pipes, February Schmidt, H.: Handbook o acoustics, VDI publications, Düsseldor, Measurements and calculations within the ramework o project work at Bayer AG 5. W. Straßen: Noise creation through inserts in pipes and solids in the luid, lecture 'House o Technology', CadnaA: Computer Aided Noise Abatement, sotware developed by DataKustik GmbH,