Heat Recovery Steam Generator Noise Attenuation

8/19/2019 Heat Recovery Steam Generator Noise Attenuation

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HEAT RECOVERY STEAM GENERATOR NOISE ATTENUATION

John Briggs, Foster Wheeler Limited

Dr. Mal Sacks, Tacet Engineering Ltd.

Dr. Steven Kraemer

1.0) Introduction

A Heat Recovery Steam Generator (HRSG) performs a primary function as part of a

combined cycle power plant and a secondary function as an in-line silencer for

combustion turbine noise emissions. Typical combustion turbine noise emissions for

current large machines approach sound power levels of 150 dB(A). Overall plant noiserestrictions are often imposed by the community or the state, and include a limit for

sound pressure at the plant boundary for reasons of community comfort. An additional

noise limitation adjacent to the equipment is also typically necessary to comply withOccupational Health and Safety regulations. An error in the prediction of noise

attenuation by the HRSG can result in a plant that does not meet environmental limits and

safety standards. In addition, unnecessary margin in the noise specification, particularlyin the lower octave bands, can result in significant costs for noise attenuation equipment.

The task of generating the specifications to meet project requirements falls on thearchitect/engineer or project developer. The problem is divided between two major

systems: combustion turbine casing/air inlet noise paths, and the HRSG casing and stack

noise transmission paths. Over the past ten years Foster Wheeler Limited (FWL), with

the assistance of Tacet Engineering Ltd., has developed and refined methods of predicting HRSG noise emissions. The foundation of this expertise in noise prediction

includes numerous field surveys of HRSG noise emissions and four major field research

projects investigating the noise attenuation characteristics of HRSGs. These studieshave established and verified both transmission loss characteristics for HRSG casings and

insertion loss for HRSG tube bundles, and have facilitated the development of a library of

far field noise measurements of HRSGs. Of particular note is a field investigation thatutilized a unique method of cross spectrum measurement/data analysis that allowed

accurate measurement of in-duct sound pressure for an operating 250 MW CT/HRSG

train illustrated in Figure 1.

This paper will provide a review of these research projects followed by a discussion of

current FWL methodology for the prediction of HRSG noise attenuation. Appendix A provides suggested guidelines for noise specification development.


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Figure 1: Elevation of HRSG for 250 MW CC Plant

2.0) Field Investigations of HRSG Noise

Four major studies of field noise performance were completed over the past ten years.

The following section will review the first three investigations briefly and will discuss themost recent research study in greater detail.

2.1) HRSG Noise Attenuation Model Development

The initial investigation (1991) consisted of extensive measurements of both sound

pressure (near field and far field) and casing vibration for a number of HRSGs, followed by the development of a HRSG noise attenuation model. The investigation correlated

noise predicted by casing vibration levels with the noise measured as sound pressure inthe near and far field. The results were used to derive empirical formulas that permitted

the calculation of sound pressure levels at a given far field receiver location. Correction

factors were determined for combustion turbine load, wall construction, directivity, anddistance.


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All calculations of noise attenuation were based on the assumption that the in-duct noise

spectrum reported by the combustion turbine manufacturer was correct. This assumptioncould not be verified during this investigation. Overall internal tube bundle insertion loss

was determined based on stack outlet noise emissions and the aforementioned

assumption; casing transmission losses were based on sound pressure measurements and

the assumed internal noise spectrum.

This predictive technique was adequate in the early 1990’s as environmental noise

requirements were somewhat less onerous than today. Throughout the ensuing decadeour research and development projects in the field of noise prediction and control

reflected the increased importance of this aspect of the power plant environment..

2.2) Transmission Loss Versus Casing Construction

Foster Wheeler Limited initiated a research project in January, 1994 to investigatetransmission loss for a total of 18 panel constructions commonly used in steam generator

design. This project required fabricating sample panel sections for insertion in the test rignoted in Figure 2.

TEST PANEL

SAND

PERIMETER

FRAME

3“- 2 ½” lb/ft INSULATION3

PANEL SUPPORT ISOLATORS

PIT

(7’x7’x7’)

MICROPHONE

1/16“ RUBBER SHEET

4mm ACOUTHERM BARIUM

DAMPED VINYL SHEETNOISE SOURCE

Figure 2: Transmission Loss Test Rig


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The test rig consisted of a 7 ft deep by 7 ft square concrete pit with the test panelssupported on panel isolators at the perimeter of the pit to reduce noise transmission from

the concrete to the panel. The panel edges were buried using both sand and barium

damped vinyl mat to minimize the noise flanking path. A noise source was placed above

the panel and test microphones placed in the pit. Data was accumulated for each panelsection for each octave band. This work provided verification of transmission loss

coefficients versus octave band for different casing constructions.

2.3) Noise Attenuation Through Tube Bundles and Casing Walls

The third investigation undertaken was to accurately determine the sound attenuation

through the banks of tube bundles and the wall transmission loss for an HRSG prior tocommissioning. This was accomplished by placing a noise source inside the inlet duct of

a new HRSG. The inlet opening was then closed off with a heavy panel (the gas turbineunit had not yet been installed). The sound source was a continuous, steady broad-band

noise with a sound power level of approximately 118 dB(A). Measurements of sound pressure level were taken at a number of different locations along the inside and outside

of the HRSG. Vibration measurements along the exterior wall were also taken for

comparison.

Figure 3 shows the results of sound pressure level attenuation inside the HRSG between a

reference location 1.4m in front of the loudspeaker (at the inlet duct) and averaged resultsat planes normal to the HRSG axis with increasing distance from the source throughout

the tube banks, from the inlet transition to the exhaust stack base. The results have beencorrected for operational temperature and flow conditions, assuming that sound

attenuation is wavelength-dependent. All of the measurements were well above the

ambient sound levels inside the HRSG when the source was turned off. The resultsshow a small attenuation at low frequencies through the tube bundles but much greater

attenuation at high frequencies. It should be noted that, in the absence of flow, there was

no “re-generated” flow induced noise. Flow induced noise would reduce the sound

attenuation at high frequencies under operational conditions.

Figure 4 shows the averaged results of sound attenuation through the HRSG walls.

Because of high external ambient sound levels, measurements were limited to the inlettransition area. The vibration measurement results on the exterior wall were used to

predict the radiated sound for comparison. The results show very close correspondence

between the sound and vibration measurements at low frequencies, but they diverge athigh frequencies because of external sound “leakage” through the fabric inlet expansion

joint. The results based on the vibration measurements were therefore used to estimate

the sound attenuation of HRSG wall construction.


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Figure 3: Sound Pressure Vs Position in Tube Banks

Figure 4: Comparison of Internal –External SP (measured; series 1)

(calculated from wall acceleration; series 2)

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31 63 125 250 500 1000 2000 4000 8000

Octave Band Centre Frequency (Hz)

S o u n d P r e s s u r e L e v e l ( d B )

Series1 Series2

0

10

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30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11

Measurement Position

S o u n d P r e s s u r e

L e v e l ( d B )

dB(A) 31 63 125 250 5001000 2000 4000 8000


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2.4) Investigation of In-Duct On-Line Noise

Foster Wheeler Limited and Tacet Engineering recently completed an extensive

investigation of an operating HRSG to obtain near field sound pressure levels, far field

sound pressure levels, wall vibration levels and in-duct sound power levels.

Measurement of in-duct sound pressure levels provides critical information needed tocorrelate the gas turbine exhaust sound power levels (i.e. the sound source) with HRSG

acoustic performance and the resulting environmental sound pressure levels.

The ability to perform accurate measurements of in-duct sound pressure levels is

complicated by the high temperatures present inside the HRSG, the turbulent nature of

the flow at the outlet of the combustion turbine, and the fact that the gases inside theHRSG are under pressure. The methods employed during this study are the result of a

long term R&D program conducted by Tacet Engineering Ltd., Higgott-Kane Industrial

Noise Controls Ltd. and Ryerson Polytechnic University as described in references [1],[2] and [3]. This collaboration has resulted in the development of a lightweight, portable

microphone probe system that allows accurate measurement of in-duct sound pressurelevels [1] (Figure 5).

In performing these measurements, there are two exhaust sound power level spectra that

must be considered: one that is associated with sound pressure fluctuations and one that isassociated with turbulent pressure fluctuations. These two noise source spectra react

differently with the exhaust system and produce different sound pressure levels in the

environment.

COUPLING

REDUCER

LINING

DUCTWALL

ANECHOIC

TERMINATION

MICROPHONE

Figure 5: In-Duct Microphone Probe System


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In order to delineate these spectra, the research team developed a new method of

analyzing the data using a cross spectrum technique to separate the sound and turbulence

fields. This method is discussed in detail from a theoretical perspective in reference [3]

and from an experimental perspective in reference [2]. The cross spectrum techniquerequires simultaneous microphone measurements at two locations on a duct cross section.

The cross spectrum analysis attenuates the turbulence component of the signal and

extracts the embedded sound pressure level spectrum. The result is a spectrum that is theaverage of the two underlying sound spectra.

In applying these techniques to the subject HRSG, in-duct measurements were obtainedat cross sections upstream and downstream of the tube bundles and near the top of the

exhaust stack. External sound pressure levels in the plane of the exhaust outlet were also

obtained. With this data, the evolution of the dynamic pressure fields from the HRSGinlet to outlet could be observed.

Analysis of the data produced by these measurements led to the following observations:

1. The turbulence levels are strongly dominant upstream of the tube bundles and in theoutlet stack.

2. The sound levels are strongly dominant downstream of the tube bundles.3. The embedded sound pressure level in the exhaust stack, as extracted by the cross

spectrum analysis, correlates well with the sound power radiated by the exhaust

outlet.

4. The in-duct auto spectrum in the exhaust stack, which is dominated by turbulence,would predict sound power levels radiated by the exhaust stack that are too high by

20 dB. This demonstrates that the in-duct turbulent pressure fluctuations do not propagate to the external environment as sound..

5. The turbulent and sound pressure signals undergo both attenuation and regenerationalong the path from HRSG inlet to outlet. The details of these processes are not fullyunderstood and are currently under investigation.

Based on current evidence, we expect the walls of the HRSG to react in an approximately

similar manner to both turbulence and sound pressure excitation; i.e. for similar sound or turbulence levels to produce similar wall vibration levels. Consequently both spectra

would produce similar external sound levels. Therefore the in-duct auto spectrum (which

measures the total sound plus turbulence fields) would be used to estimate HRSG duct breakout and flanking noise, subject to future refinement

Figure 6 shows the sound power level upstream of the tube bundles based on themeasured in-duct auto spectra and as supplied by the turbine manufacturer. The figure

also shows the radiated sound power level at the stack outlet. Although the overall sound

power level measured in the inlet transition was very close to that supplied by the turbinemanufacturer, in the 125 Hz band the measured level was almost 9 dB lower. This

difference would have a major cost impact on noise control design where stringent noise

specifications are required if based on the manufacturer’s data for this turbine.


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Figure 6: Sound Power Levels at Various Locations

Figure 7 shows the correlation between near field sound pressure level (measured two

inches from the HRSG inlet transition duct wall) and the sound pressure level predictedfrom the measured adjacent wall vibration.

Evaluation of this data indicates that:

1. Good estimates of sound pressure level can be made from measured wall vibrationlevel.

2. These estimates are not contaminated by extraneous sound sources; i.e. we obtain thesound pressure level due to the HRSG walls only.

3. This data can be used in conjunction with the in-duct data to estimate HRSG ductwall transmission loss as well as breakout and flanking noise radiation from an on-line HRSG.

Sound Power Levels

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7580

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31.5 63 125 250 500 1000 2000 4000 8000 SumL SumA

Octave Band Centre Frequency

S o u n d P o w e r L e v e l

Inlet Outlet Plane Gas Turbine Manuf. Data


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Figure 7: Sound Pressure Vs Vibration Measurement Sound Pressure

3.0) Noise Prediction Model

There are a number of fundamental approaches to modeling noise which can be adopted

for HRSG’s, including statistical, mathematical, physical, and empirical. The relative

merits of these approaches are as follows:

• Statistical models use measurements from many installations and can predict resultsfor similar installations but not new designs.

• Mathematical models use theoretical methods (e.g. finite element or boundaryelement methods) and are potentially very accurate, but are time-consuming andcostly to set up.

• Physical scale models, built from structurally-modeled materials and measured atvery high (scaled) frequencies require expensive, specialized facilities and a separate

model for each design. These models have the drawback of high cost, but could be

useful in generic research.

Measured (VD) vs. Expected Sound Levels (based on vib. measurements), Location 4A

Expected Lp is corrected for resonant frequency at 1k, 2k, and 4kHz bands

45

50

55

60

65

70

75

80

85

90

95

100

105

31.5 63 125 250 500 1000 2000 4000 8000 SumL SumA

Octave Band Centre Frequency (Hz)

S o u

n d P r e s s u r e L e v e l ( d B )

Expected Lp (based on vibration measurements) Measured Lp (VD)


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• Empirical models use measured data and correlations in a simplified analytical model.This method can be used to predict results from existing and new designs fairly

quickly and inexpensively.

For the purposes of predicting HRSG noise attenuation characteristics, an empirical

model was set up in the form of a spreadsheet. The HRSG is modeled as an open-endedduct, divided into sections including the inlet transition, main HRSG casing, outlet

transition, stack walls, and finally the stack outlet.

Input data are the gas turbine exhaust octave band sound power levels. The sound

pressure levels inside the HRSG are calculated in octave bands at each section along the

open-ended duct, using the equation:

Lp (inside) = Lw – 10log S [dB]

Where:

Lp(inside) = average sound pressure level inside the duct sectionLw = sound power level entering each section

S = cross-sectional area

Where appropriate, values for internal gas-path sound attenuation are introduced for each

section, which reduce the sound power level accordingly. The sound power levels

radiated externally from the HRSG casing sections are calculated from:

Lw(radiated) = Lp(inside) – TL + 10logPl [dB]

Where:

Lw(radiated) = average casing breakout sound power level from each sectionTL = Transmission Loss (HRSG wall attenuation)Pl = perimeter X length of each section

The final step is to calculate near-field or far field sound pressure levels, in octave bandsand in dB(A), using the radiated sound power levels obtained for each section.

A number of parameter values and correlations obtained from experimental or theoretical

data are used in the empirical model:

• Attenuation of internal HRSG elements (tube bundles, silencers)

• Attenuation of inlet, main casing, outlet and stack walls

•

Flanking vibration of the main casing• Stack outlet directivity

This allows the model to be adjusted for different design configurations, particularly with

regard to noise control measures needed to meet specified or guaranteed sound levels.Typical noise control measures adopted include heavier casing walls, stack silencers,

inlet duct silencers, vent and safety valve silencers, and acoustic shrouds for the HRSG

casing.


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4.0) Summary

The prediction of HRSG noise attenuation characteristics is critical to the success of many combined cycle power plant installations. The preceding paper has provided an

overview of the engineering expertise Foster Wheeler Limited and Tacet Engineering

have developed through ongoing research in this important area.

The cornerstone of measuring noise attenuation of HRSG’s is the determination of the

noise emitted by the combustion turbine. The methods described herein have permittedthe accurate assessment of HRSG noise attenuation.

Further work is planned to validate the noise prediction model for HRSG’s with addednoise control elements such as stack silencers. Opportunities will be provided as more

low-noise HRSGs are completed and brought into service. More site measurements arerequired to improve the statistical basis of uncertainty determinations in the modeling

process and to determine HRSG attenuation parameters with greater accuracy.

5.0) References

1. M.P. Sacks et al – “In-duct measurement of gas turbine noise emissions using a crossspectrum method”, Paper No 2000-GT-349, ASME Turbo Expo 2000, Munich, May

2000.

2. M.P. Sacks et al – “Microphone probe for in-duct measurement of gas turbine exhaustnoise”, Paper No 2000-GT-656, ASME Turbo Expo 2000, Munich, May 2000.

3. R. Behboudi, J.G. Kawall and M.P. Sacks – “A novel technique for measuring soundwithin turbulent duct flows”, Proceedings of CANCAM 99, McMaster University,

Hamilton, Ontario, Canada, May 30 – June 3, 1999.

4. ANSI Standard B133.8-1989 “Gas Turbine Installation Sound Emissions”.

5. ISO Standard 6190:1988 “Acoustics – Measurement of sound pressure levels of gasturbine installations for evaluating environmental noise – Survey method”.

6. ISO Standard 10494:1993 “Gas turbines and gas turbine sets – Measurement of emitted airborne noise – Engineering/survey method”.

7. Robert S. Johnson – “Recommended Octave Band Insertion Losses of Heat RecoverySteam Generators used on Gas Turbine Exhaust Systems”, Noise-Con 94, Florida,May 1994.


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Appendix A : HRSG Sound Level Guarantees

And Specification Development

In most countries today, new power plants must meet municipal, state or federal noise

regulations, which specify noise limits in dB(A) or octave band sound pressure levels.The plant owner in turn, usually through the main contractor, engineer or consultant, will

prepare noise specifications based upon a model that divides the plant into a number of

discrete sources, including HRSGs. HRSG vendors are then requested to guarantee noiseemissions in accordance with the above specification.

There are several problems faced by HRSG manufacturers under these circumstances.Firstly, the sound levels radiated from the HRSG are primarily related to the sound power

level of the exhaust of the gas turbine unit, which is outside the HRSG manufacturer’s

scope of supply. Noise specifications for HRSGs are often poorly prepared and haveunnecessarily low limits, especially in the low frequency octave bands.

Noise verification methods are not always clearly defined, which make contractualguarantees difficult.

Table 1 below presents a suggested HRSG noise specification that can be used by project

developers to avoid misunderstandings and potential disputes caused by the above

problems. It can also be used as the basis for noise guarantees.

The prediction of noise from a future HRSG installation cannot be exact. A factor of

safety of 3 to 6 dB(A) should be included because of the various uncertainties in the noisemodeling and measurement procedures. Ideally, the safety factors inherent in the

specifications of gas turbine exhaust, HRSG and overall plant sound levels should becombined in a statistically valid way, but that is rarely the case. Because of the need for

guarantees at each contractual level (e.g.: developer, architect/engineer, combustion

turbine vendor, HRSG vendor), the simple addition of all of these safety factors can leadto a situation where excessive amounts of noise control are required, with a

correspondingly significant cost increase. This is especially the case when tight noise

specifications are imposed. There is no easy solution to this problem, unless all parties

concerned are prepared to disclose the safety factors inherent in their noise guaranteesand the overall plant noise uncertainty is computed by prior agreement.

1. Environmental Noise Guarantee

Sound emission from the heat recovery steam generator(s) will not exceed an average A-weighted sound pressure level of ….. dB(A) when measured as specified below at a

distance of 122m (400’) from the perimeter of the installation and 1.5m (5’) above grade.

2. Occupational Noise Guarantee


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Sound emission from the heat recovery steam generator(s) will not exceed an average A-

weighted sound pressure level of …. dB(A) when measured as specified below at adistance of 1m (3’) from the surface of the installation and 1.5m (5’) above grade (or

personnel platform).

3. Measurement Specification

Guarantee sound pressure levels are to be measured according to ANSI Standard B133.8-

1989 “Gas Turbine Installation Sound Emissions”.

4. Gas Turbine Sound Power Levels

The above sound pressure levels are only guaranteed when the gas turbine octave band

sound power levels (dB re 1 pW) are also guaranteed not to exceed the following values

at the turbine diffuser exhaust :

Band 31.5 63 125 250 500 1000 2000 4000 8000Lw … … … … … … … … …

Heat Recovery Steam Generator Noise Attenuation

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