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Noise Control Manual

Bulletin OZ3000 01/02

Experience, Knowledge & Technology...In Control

Table of Contents

1. Control Valve Noise ............................................................................................

1.1 Introduction ................................................................................................................

1.2 Acoustic Terminology ................................................................................................

1.3 Human Response to Noise........................................................................................

1.4 Major Sources of Noise ............................................................................................

2. Aerodynamic Noise Prediction ..........................................................................

2.1 An Introduction to the Prediction Method ..................................................................

2.2 Further Explanation of the Prediction Method ............................................................

2.3 Additional Comments ................................................................................................

3. Aerodynamic Control Valve Noise Reduction ..................................................

3.1 Methods ....................................................................................................................

3.2 Equipment ..................................................................................................................

3.3 LO-DB Static Restrictor Selection ............................................................................

4. Atmospheric Vent Systems ................................................................................

4.1 Introduction ................................................................................................................

4.2 Noise Calculation Procedure ....................................................................................

5 Hydrodynamic Noise ..........................................................................................

5.1 Prediction ..................................................................................................................

5.2 Application Guidelines and Equipment Selection......................................................

6. References ..........................................................................................................

Appendix: Installation Considerations ....................................................................

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1. Control Valve Noise

1.1 Introduction

Noise pollution will soon become the third greatest menaceto the human environment after air and water pollution.Since noise is a by-product of energy conversion, there will be increasing noise as the demand for energy for transportation, power, food, and chemicals increases.

In the field of control equipment, noise produced by valveshas become a focal point of attention triggered in part byenforcement of the Occupational Safety and Health Act,which in most cases limits the duration of exposure to noisein industrial locations to the levels shown in Table 1.

1.2 Acoustic Terminology

NoiseNoise is unwanted sound.

SoundSound is a form of vibration which propagates through elastic media such as air by alternately compressing andrarefying the media. Sound can be characterized by its frequency, spectral distribution, amplitude, and duration.

Sound FrequencySound frequency is the number of times that a particularsound is reproduced in one second, i.e., the number of

times that the sound pressure varies through a completecycle in one second. The human response analogous tofrequency is pitch.

Spectral DistributionThe spectral distribution refers to the arrangement of energy in the frequency domain. Subjectively, the spectral distribution determines the quality of the sound.

Sound AmplitudeSound amplitude is the displacement of a sound wave relative to its "at rest" position. This factor increases withloudness.

Sound PowerThe sound power of a source is the total acoustic energy radiated by the source per unit of time.

Sound Power LevelThe sound power level of a sound source, in decibels, is10 times the logarithm to the base 10 of the ratio of thesound power radiated by the source to a reference power.The reference power is usually taken as 10-12 watt.

Sound Pressure Level: SPLThe sound pressure level, in decibels, of a sound is 20 times the logarithm to the base of 10 of the ratio of the pressure of the sound to the reference pressure. The reference pressure is usually taken as 2 x 10-5 N/M2.

Decibel: dBThe decibel is a unit which denotes the ratio between twonumerical quantities on a logarithmic scale. In acousticterms, the decibel is generally used to express either asound power level or a sound pressure level relative to achosen reference level.

Sound LevelA sound level, in decibels A-scale (dBA) is a sound pressurelevel which has been adjusted according to the frequencyresponse of the A-weighting filter network. When referring tovalve noise, the sound level can imply standard conditionssuch as a position 1 m downstream of the valve and 1 m fromthe pipe surface.

Duration of Exposure Sound Level(Hours) (dBA)

8 904 952 1001 105

1/2 1101/4 or less 115

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Foreword

This noise manual contains informative material regarding noise in general and control valve noise in particular.

Noise prediction methods used by Masoneilan for aerodynamic noise and hydrodynamic noise are based on the latest publications of the Instrument Society of America (ISA) and the International Electrotechnical Commission (IEC), see refer-ence Section 6.0. The calculations required by these methods are quite complex, and the solution of the equations is best accomplished by computer. For this purpose, the Masoneilan valve sizing and selection computer program provides a convenient and efficient working tool to perform these calculations.

Table 1.

Table 2Comparison of Energy, Sound Pressure Level,

and Common Sounds

Table 3Changes in Sound Level

1.3 Human Response to Noise

FrequencyGiven a sound pressure, the response of the human earwill depend on the frequency of the sound. Numerous testsindicate that the human ear is most sensitive to sound in the frequency region between 500 and 6000 Hz and particularly between 3000 and 4000 Hz.

Sound Weighting NetworksA weighting network biases the measured sound to conform to a desired frequency response. The most widely used network for environmental noise studies, theA-weighting network, is designed to bias the frequencyspectrum to correspond with the frequency response of thehuman ear, see Figure 1.

RelativeEnergy Decibels Example

1x1014 140 Proximity to jet aircraft1x1013 130 Threshold of pain1x1012 120 Large chipping hammer1x1011 110 Near elevated train1x1010 100 Outside auto on highway1x109 90 Voice - shouting1x108 80 Inside auto at high speed1x107 70 Voice - conversational1x106 60 Voice - face-to-face1x105 50 Inside general office1x104 40 Inside private office1x103 30 Inside bedroom1x102 20 Inside empty theater1x101 10 Anechoic chamber

1 0 Threshold of hearing

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Increase in SoundLevel

3 dBA5 dBA

10 dBA20 dBA

Human SubjectiveResponse

Just perceptibleClearly noticeableTwice as loudMuch louder

1.4 Major Sources of Noise

Mechanical VibrationMechanical noise is caused by the response of internalcomponents within a valve to turbulent flow through the valve. Vortex shedding and turbulent flow impinging on components of the valve can induce vibration againstneighboring surfaces. Noise generated by this type of vibration has a tonal characteristic.

If this turbulence induced vibration of trim parts approachesa natural frequency of the plug-stem combination, a case ofresonance will exist. A resonant condition is very harmful,since it can result in fatigue failure of trim parts. Noise frommechanical vibration does not occur often in control valves,especially since the introduction of top and cage guidedvalves. Should it occur, steps must be taken to eliminate thatresonant condition, to reduce the noise but more importantlyto preclude fatigue failure.

Possible cures for this type of noise include change in trimdesign or capacity, reduction of guide clearances, largerstem sizes, change in plug mass, and sometimes reversalof flow direction. These steps are intended to shift the natural frequency of parts and the excitation frequencyaway from each other. There is presently no reliable methodfor predicting noise generated by mechanical vibration incontrol valves.

Aerodynamic NoiseAerodynamic noise is a direct result of the conversion of the mechanical energy of the flow into acoustic energy asthe fluid passes through the valve restriction. The propor-tionality of conversion is called acoustical efficiency and isrelated to valve pressure ratio and design. See Sections 2,3 and 4.

Hydrodynamic NoiseLiquid flow noise, cavitation noise, and flashing noise canbe generated by the flow of a liquid through a valve and piping system. Of the three noise sources, cavitation is themost serious because noise produced in this manner canbe a sign that damage is occurring at some point in thevalve or piping. See Section 5.

1. Control Valve Noise (cont.)

20

10

0

-10

-20

-30

-40

-50 2 5 1 2

Figure 1IEC Standard A-Weighting Curve for

Sound Level Meters

20000

10000

5000

2000

1000

500

200

100

5020

Frequency (Hz)

Rel

ativ

e S

ound

Pre

ssur

e Le

vel (

dB)

2. Aerodynamic Noise Prediction2.1 An Introduction to the Prediction Method

Aerodynamic noise prediction described in this section is based on the equations and nomenclature of the international standard for control valve noise prediction,IEC-534-8-3. Because of the extent and complexity of these calculations, only a general description of the calculation methods are included here.

The IEC control valve aerodynamic noise method consistsof four basic processes. (1) The method determines theprocess conditions to calculate the trim outlet velocity andsolves for the valve noise source strength at the valve. (2) This method estimates the portion of the sound generated at the valve that propagates into the downstreampiping. (3) The third step of the method models how thepipe walls attenuate the noise as it passes from the insideto the outside of the pipe. (4) This method describes theradiation of the sound from the pipe wall to estimate the A-weighted sound-pressure level (SPL) at a distance of onemeter from the piping wall. In addition, the method takesinto account noise generated by flow expansion upon exit-ing the valve body and adds this expander noise to thevalve noise, yielding the aerodynamic noise produced bythe valve system one meter downstream of the valve exitand one meter from the piping wall.

2.2 Further Explanation of the Prediction Method

The problem of predicting control valve noise is two-fold.First, the sound power generated in the fluid inside thevalve and piping due to the throttling process must be estimated. Secondly, the transmission loss due to the piping must be subtracted to determine the sound level ata predetermined location outside the piping.

Noise prediction for a freely expanding jet is based on multiplying the mechanical energy conversion in the jet byan efficiency factor. This theory is modified to take intoaccount the confined jet expansion in a control valve, andthe inherent pressure recovery.

In order to accommodate the complex nature of valve noisegeneration, the prediction method addresses the calcula-tion of significant variables in five different flow regimes.Among the significant variables are an acoustic efficiency,sound power, and peak frequency. From these and othervariables, the internal sound power is calculated.

The transmission loss model is a practical simplification ofcomplex structural transmission loss behavior. The simpli-fication is rationalized on the basis of allowable tolerancesin wall thickness.

The downstream piping is considered to be the principalradiator of the generated noise. The transmission lossmodel defines three sound damping regions for a given pipehaving their lowest transmission loss at the first coincidencefrequency. The transmission loss is calculated at the first

coincidence frequency and then modified in accordancewith the relationship of the calculated peak frequency to thecoincidence frequency.

A correction is then made for velocity in the downstreampiping.

The predicted sound level is then based on the calculatedinternal sound pressure level, the transmission loss, velocitycorrection, and a factor to convert to dBA.

2.2.1 The flow regime for a particular valve is determined from inlet pressure, downstreampressure, fluid physical data, and valve pressurerecovery factor.

Five flow regimes are defined as:

Regime I - SubsonicRegime II - Sonic with turbulent flow mixing

(recompression)Regime III - No recompression but with

flow shear mechanismRegime IV - Shock cell turbulent flow interactionRegime V - Constant acoustical efficiency

(maximum noise)

The following explanation is based on Regime Iequations, but will serve to illustrate the method-ology employed.

2.2.2 The stream power of the mass flow is determined(for Regime I) as:

Noise Source Magnitude• Magnitude: Proportional to Stream

Power, Wm, at Vena Contracta

2.2.3 For the confined jet model, the acousticalefficiency is calculated as:

• Mixed Dipole – Quadruple Source Model

2.2.4 In Regime I, the peak frequency of the generatednoise is determined as:

Noise Frequency• Peak frequency of noise generation, fp• Varies with flow regime• Always scales with jet diameter and velocity at

the throttling vena contracta

Jet vena contracta diameter is a function of jet pressure recovery and valve style modifier, Fd(throttling flow geometry).

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2.2.5 Only a portion of the sound power propagatesdownstream. That portion is designated as a fac-tor rw. This factor varies with valve style.

Valve Noise Propagation• A portion of valve noise propagates

downstream• This ratio, rw, varies with valve style• Reflects “line-of-sight” through valve

2.2.6 The sound pressure level in the downstream piping is determined as:

Downstream Piping Internal Noise• Average valve sound pressure level over

cross-section of downstream piping

2.2.7 An increase in noise occurs with increased Machnumber on the downstream piping.

Downstream Noise Propagation• Higher Mach number in the downstream piping,

M2, increases noise by Lg• Alters wave propagation (Quasi-Doppler)

Note: Moderate M2 Controls Noise

2.2.8 The sound transmission loss due to the down-stream piping is determined as:

Basic Sound Transmission Through Piping Wall

Note: Increasing Wall Thickness Increases Loss

2.2.9 The transmission loss is dependent upon frequency.

Frequency-Dependent Sound TransmissionThrough Piping (I)• Pipe Ring Frequency, fr

• Pipe Coincidence Frequency (Minimum Transmission Losses), fo

2.2.10 The transmission loss regimes can be illustratedgraphically:

Transmission Loss Regimes• fp < fo: Larger TL (non-resonant wall fluid

coupling)• Smallest TL, fp = fo (~ circumferential

bending & acoustic modes coincident)• fp > fr: TL increases markedly

(~ flat-plate radiation)

The slope to the transmission loss in the threeregimes can be determined by the following relationships:

Transmission Loss Regimes

Based on the above, Frequency Factors Gx andGy are applied per the IEC standard.

Note: Higher fp (smaller dvc) can increase pipingdamping and reduce control valve throttling noise

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TL

fo

fr

Peak Frequency

Valve Style rw

Globe (21000, 41000) 0.25

Rotary Globe (Varimax) 0.25

Eccentric Rotary Plug (Camflex) 0.25

Ball 0.5

Butterfly 0.5

Expander 1

2. Aerodynamic Noise Prediction (cont.)

2.2.13 In the case of an expander downstream of avalve, the noise generated in the expander is calculated in a manner similar to the Regime Imethod, and added logarithmically to the valvenoise to determine an overall sound level (Le).

High expander noise occurs when high Machnumber exit flow jets into the larger downstreampiping (> 0.3 Mach). This is very important as thisnoise source can readily overwhelm trim noiseand result in damaging low frequency noise whichcan excite piping structures.

2.2.14 A flow chart illustrating the aerodynamic noiseprediction method is shown below.

2.2.11 The net sound level at the pipe wall converted todBA is:

• Sum Noise, Add +5dB for A-Weighted SPL

2.2.12 At one meter from the pipe wall, the valve noise is:

• Cylindrical Spreading Model Yields Noise at 1m from Pipe Wall

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Control Valve Aerodynamic Noise Prediction Flow Chart

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2.3 Additional Comments

Examples of the use of the prediction methods are shownin detail in the respective standards. These examples maybe used to verify a computer program.

The IEC standard also provides for prediction for propri-etary low noise trim designs and other valve configurationsnot specifically covered by the standard. The manufactur-er is required to incorporate additional changes in soundpressure level as a function of travel and/or pressure ratio,in addition to the sound pressure level obtained by usingthe appropriate clauses applying to valves with standardtrim. Masoneilan has accomplished this requirement in ourvalve sizing and selection computer program.

A = flow area

Cv = valve capacity

c = sound speed of gas

cp = sound speed of piping

dp = outlet pipe inner diameter

dv = outlet valve inner diameter

dvc = trim jet vena contracta diameter

Fd = valve style modifier

FL = pressure recovery coefficient

f = sound frequency

fo = acoustic-structural Coincident Frequency

fp = flow peak frequency

fr = pipe ring frequency

Le = expander and pipe flow noise sound-pressure level, A-weighted and 1 meter from the pipe wall

Lg = pipe Mach number correction factor

LpAe = A-weighted sound-pressure level

•m = mass flow rate

LpAe.1m = A-weighted sound-pressure level,1 meter from pipe wall

Lpi = pipe internal sound-pressure level

M = Mach number

Mw = molecular weight of gas

p = pressure

pa = actual pressure outside pipe

ps = standard pressure outside pipe (1 atmosphere)

R = universal gas constant

T = gas temperature

TL = pipe transmission loss

TLfr = pipe transmission loss at ring frequency

tp = pipe wall thickness

U = velocity

Wa = sound power

Wm = stream power of mass flow

Greekγ = ratio of specific heats

η = acoustical efficiency factor

ρ = gas density

Subscripts1 = upstream of valve or vena contracta

2 = downstream of valve or vena contracta

e = expander

v = valve outlet

vc = vena contracta

I = Regime I

Nomenclature


3. Aerodynamic Control ValveNoise Reduction

3.1 Methods

Reduction of control valve aerodynamic noise can beachieved by either source treatments (preventing the noisegeneration) or path treatments (pipe insulation, silencers,or increasing pipe schedule). Source treatment oftenbecomes the preferable method. Sound, once generated,propagates virtually unattenuated in downstream pipe. In addition, as discussed in the Appendix, very high soundlevels inside piping systems can damage the pipe and mechanical components located downstream by inducingexcessive vibration.

3.1.1. Source TreatmentThe generation of noise can be controlled by using trim components specially designed for low noise production. There are basically twomethods employed in reducing noise generated inthe valve trim:

1. Use of Small, Properly Spaced Fluid JetsThe size of the fluid jets affects noise generationin three ways. First, by reducing the size of thefluid jets (and consequently the size of theeddies), the efficiency of conversion betweenmechanical and acoustical power is reduced.Second, the smaller eddies shift the acousticenergy generated by the flow to the higher frequency regions where transmission through the pipe walls is sharply reduced. Third, the higher frequency sound, if raised above 10000 Hz,is de-emphasized by both the A-weighting filternetwork and the human ear.

The spacing of the fluid jets affects the location of the point downstream at which the fluid jets mutually interfere. The mutual interaction of the fluid jets at the proper location downstreamthereby reduces the shock-eddy interaction that islargely responsible for valve noise under criticalflow conditions. This factor further reducesacoustical efficiency.

2. Adiabatic Flow with FrictionThe principle of “Adiabatic Flow with Friction” is toreduce pressure much like the pressure losswhich occurs in a long pipeline. This effect is pro-duced by letting the fluid pass through a numberof restrictions, providing a tortuous flow patterndissipating energy through high headloss ratherthan through shock waves.

The flow area of the valve trim is graduallyincreased toward the downstream section. This compensates for expansion of the gas withpressure loss and ensures a nearly constant fluidvelocity throughout the complete throttling process.

As shown on Figure 2, in conventional orifice typevalves, internal energy is converted into velocity(kinetic energy). This results in a sharp decreasein enthalpy. Downstream turbulence accompaniedby shock waves, reconverts this velocity into thermal energy with a permanent increase inentropy level (corresponding to the pressurechange P1-P2).These same shock waves are the major source of undesirable throttling noise. Ina LO-DB valve, however, the velocity change isminimized and the enthalpy level remains nearlyconstant.

Most Masoneilan LO-DB valves use both of thepreviously mentioned methods to limit noise generation to the minimum levels possible. Whencontrolling noise using source treatments, such as LO-DB valves, it is imperative that the fluidvelocity at the valve outlet is limited to avoid regenerating noise at this potential source. Low noise valves are inherently quieter (less effi-cient noise generators), due to their special trim designs. The noise generated by the outlet, if not properly limited, can easily dominate over the noise generated by the trim, rendering the low noise trim virtually ineffective. There are twomethods used to control outlet velocity. First, thedownstream pressure can be increased by usingMasoneilan LO-DB cartridges and expansionplates. This method, from Bernoulli’s principle,decreases the velocity at the valve outlet byincreasing the pressure immediately downstreamof the valve. The second method is simply tochoose a valve size that is adequate to ensure theproper outlet velocity.

S0

S0

2

En

tha

lpy

h

ConventionalSingle Orifice

Valve

MultistepInternal

Friction Device

Ideal Adiabatic Flow with Friction

Entropy S

Figure 2

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10 OZ3000 01/02

3.1.2. Path TreatmentThere are three basic methods of incorporatingpath treatment into control valve systems:

1. SilencersSilencers can be effective in reducing controlvalve noise provided they are installed directlydownstream of the valve. However, there are several technical problems often encountered in their use. First, to be effective, they require low flow velocities which often make them imprac-tical, especially for use in high capacity systems.Second, the acoustic elements are not alwayscompatible with the flowing medium, and third,the operating conditions may be too severe.

2. Increase in Pipe ScheduleAn increase in the wall thickness of downstreampiping can be an effective means to reduce control valve noise. However, since noise, oncegenerated, does not dissipate rapidly with down-stream pipe length, this method must normally beused throughout the downstream system.

3. Pipe InsulationThis method, like that of increasing pipe thick-ness, can be an effective means to reduce radiated noise. However, three restraints must benoted. First, as with the pipe schedule method,insulation must be used throughout the down-stream system. Second, the material must becarefully installed to prevent any “voids” in thematerial which could seriously reduce its effec-tiveness. Third, thermal insulation normally usedon piping systems is limited in its effectiveness inreducing noise. Unfortunately, more suitablematerials often are not acceptable at high temperature, since their binders may burn out,radically changing their acoustical and thermalqualities. In application, noise reduction ofacoustical insulation reaches a practical limit of11-12 dBA due to acoustical “leaks” from the valvebonnet and top works, see Figure 3.

3.2 Equipment

3.2.1 Historical PerspectiveMasoneilan's innovative research and develop-ment has pioneered solutions to control valveapplication problems for years. Before OSHA wasestablished Masoneilan developed the first highperformance valves for reducing control valvenoise and minimizing the effects of cavitation.Among these were the 77000 and 78000 Seriesvalves, followed by the introduction of our firstglobe valves with special LO-DB trim.

Since 1975, Masoneilan laboratory studies haveled to a steady stream of innovative designs.Examples include new LO-DB trim for popularcage guided and top guided globe valves.Masoneilan has led in the application of the stat-ic restrictor as an effective means to reduce noisecontrol costs.

In addition to the development of new designs,Masoneilan has continued to conduct both pureand applied research at Masoneilan's corporatelaboratories. The result has been numerous inter-nationally published technical articles, and thefirst universal noise prediction method.

Masoneilan has contributed to the work of ISAand IEC standards organizations, whose effortshave resulted in the noise prediction methodsnow employed by Masoneilan.

3.2.2 Products and General Selection CriteriaMasoneilan offers a wide variety of low noisevalves and valve systems. Some LO-DB valvesprovide low cost solutions to relatively general purpose applications. Others, such as the 77000Series valve and numerous special variations ofthe standard products can be custom-made forparticular applications. This wide selection provides a cost effective solution to virtually anycontrol valve problem. A brief description of eachunit, its typical uses, and noise reduction perform-ance is given below in order of increasing cost.

LO-DB Static Restrictors - Cartridges and PlatesThese units used downstream of either conven-tional or LO-DB valves reduce noise generated bydividing the total pressure drop between the valveand the restrictor yet retaining control by thevalve. Because the restrictor is a multiple-stage

3. Aerodynamic Control ValveNoise Reduction (cont.) 25

20

15

10

5

1.0

Noi

se R

educ

tion

(dB

A)

Insulation Thickness (in.)

Maximum practical limit of a pipe insulation system due toinstallation restrictions andacoustic "short circuits" suchas the valve bonnet and top works.

Mineral wool or fiberglass(published data)

Calcium silicate

2.0 3.0 4.00

Figure 3Additional Noise Reduction from Typical Pipe Insulation Systems

OZ3000 01/02 11

device, the result is a much quieter system oper-ation. Also, because the intermediate pressurebetween the valve and the restrictor is increased,sonic valve outlet velocity and its associatednoise generation are avoided. This often facili-tates the use of smaller valves, typically resultingin total system cost savings of 50%. When usedwith conventional valves, up to 20 dBA noisereduction can be achieved at modest cost.Thesavings over a valve alone installation are attrib-uted to use of smaller (because of reduced outletvelocity), relatively low cost valves such as theCamflex® and Varimax™. With LO-DB valves, over30 dBA noise reduction can be achieved with thesame sizable cost advantages. These devices areparticularly effective at pressure ratios of 3 or morewhere the maximum advantage of multiple-stagingeffect can be realized.

21000 Series with LO-DB TrimThis valve model fills the moderate cost, moderatenoise reduction category of the product line.Operated flow-to-open (FTO), it produces noiselevels approximately 16-19 dBA lower than con-ventional valves. The LO-DB trim is based onMasoneilan's multiple-orifice cage concept. It iscompletely interchangeable with other 21000Series parts. A two-stage noise reducing trim isnow available for greater noise reduction.

The 21000 LO-DB is the optimum choice for abroad range of process applications due to itssimple construction, tight shutoff, and effectivenoise reduction.

2600 Series with LO-DB TrimThis valve is ideally suited to chemical and otherindustries. Its key features include a modularapproach to valve construction that results in angle, globe or other configurations, availabili-ty of numerous body materials, quick change trimand separable flanges. The LO-DB trim based on Masoneilan's multiple-orifice cage concept,generates up to 12 dBA less noise than conven-tional valves.

41000 Series with LO-DB TrimThe 41000 Series control valve can be equippedwith four different efficient noise reduction pack-ages which comply with process conditions.These trim packages are directly interchangeablewith conventional construction. These packagesinclude:

1. Standard Capacity LO-DB2. High Capacity LO-DB3. Reduced Capacity LO-DB4. Multi-Stage LO-DB

The cage, which is the LO-DB element, has beendesigned using the latest in hole sizing and spac-ing technology from both Masoneilan researchand NASA funded programs. Proper hole sizingand spacing prevents jet reconvergence andshock-induced effects, which reduce acousticenergy formation.

30000 Series Varimax™ with LO-DB TrimThe Varimax LO-DB valve provides control of highpressure compressible fluids without the erosion,vibration and high noise levels associated withconventionally designed rotary valves. Becausethe Varimax has relatively large flow passages itis particularly well suited for applications involvinggases. For high pressure ratios, LO-DB cartridgesin the globe adaptor are recommended.

The high rangeability 100:1 of this Varimax LO-DB valve allows wide variations in controlledflow. Operation is stable because the plug isequilibrated. This uniquely balanced plug has nosecondary balancing seal and mounts with astandard seat.

72001 Series LO-DBUsing the proven 41000 LO-DB trim design in anangle body configuration, the 72001 Series fabri-cated, low noise, angle valve provides high flowcapacity with high noise attenuation. Typicalapplications involve gas collection systems, com-pressor surge control, and gas-to-flare lines.Noise prediction and attenuation are identical withthe 41000 LO-DB Series. Furthermore, an opti-mal second stage cage provides added noisereduction on high pressure drop service whenrequired. The 72001 Series valves are availablewith outlet sizes up to 36" and capacities up to5000 Cv, and with expanded outlet to reducevalve outlet velocity.

72003 Series V-LOGUsed on very high pressure ratio applications(usually > 10 to 1), where the 2-stage drilled cagedesign cannot provide acceptable noise levelsand/or some trim velocity limitations are required.The trim design is a brazed stack of overlappingdiscs which form individual tortuous flow paths. High path flow resistance is achieved byright angle turns with some contractions andexpansions. Because each stack uses individuallaser cut discs, customized staging and flow

3. Aerodynamic Control ValveNoise Reduction (cont.)

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21000 Series Valvewith LO-DB Trim



41000 Series Double Stagewith LO-DB Trim


and Optional Diffuser 77000 Series LO-DB Valve 72000 Series LO-DB Valve

LO-DB Cartridge LO-DB Expansion Plate

30000 Series Varimax with LO-DB Trim

characteristics can be achieved. The small flowpaths shift sound frequency to increase transmission losses while area expansion andpath resistance reduce trim velocities anddecrease sound source strength. The V-LOG trimis available in the very large and versatile 72000Series style angle bodies with expanded outletsto reduce valve outlet velocities. Like the 72001Series this product is typically used in large gasline applications, Vent-to-Flare, Soot Blower, andCompressor Recycle.

77000 Series LO-DBThis is a specialized valve, of extremely toughconstruction fitted with an effective multiple-

staged LO-DB trim. The multi-step labyrinth typeplug and seat ring incorporate Stellite-faced seat-ing surfaces which, when coupled with leveragedactuator force, provide tight shutoff. The labyrinthflow pattern, with a large number of steps, resultsin gradual pressure reduction and quiet operation- approximately 20 dBA quieter than conventionalvalves. Perhaps most importantly, the shape ofthe flow passages are designed to preventdeposits and entrapment of solids that may beentrained in the fluid stream. Combined with lowfluid velocity, longer wear is ensured. These plusmany other features make the 77000 Series valveideal for high pressure drop applications, espe-cially those involving solid-entrained fluids typicalof drilling rig platforms, where it has achievednotable success.


3.0

2.5

2.0

1.5

1.0

0.5

0dBA

to b

e A

dded

to L

arge

st L

evel

1 2 3 4 5 6 7 8 9 10 12 14 16

P3P1

Flow Direction

Decibel Difference Between Two Sound Levels dBA

P2

Figure 4

3.3 LO-DB Static Restrictor Selection

Used with either conventional or low noise valves, LO-DBcartridges and expansion plates can be an extremely costeffective low noise system.

The static restrictor should be sized using valve sizingequations. Normally, the pressure ratio across the restric-tor should be taken as 2 to 1 for initial sizing purposes. Theaddition of a restrictor holds a higher downstream pressureon the control valve, reducing the noise generation of thevalve. A pressure drop of at least 20% of the total pressuredrop should be taken across the valve to assure good control. If a conventional valve requires a pressure drop of less than 20% to meet the acceptable noise level, a LO-DB valve must be considered.

For high system pressure ratios, two or more restrictorsmay be used. For sizing purposes, a pressure ratio of 2 to1 should be taken across each restrictor.

3.3.1 Estimation of Sound LevelAerodynamic noise generated by a low noise stat-ic restrictor (LO-DB cartridge or expansion plate)can be calculated by using the same procedureemployed to estimate low noise control valvenoise level.

When a valve and a restrictor are in series, themethod for calculating the overall noise level willvary somewhat depending upon how the valveand restrictor are connected (i.e., reducer orlength of pipe). The following methods are usedto calculate system noise.

Case IValve and downstream restrictor(s) are close coupled by reducer(s).1. Calculate the aerodynamic valve noise for a

conventional valve or a low noise valve,using the restrictor inlet pressure as valvedownstream pressure, and pipe wall thick-ness and pipe diameter downstream of the restrictor(s).

2. Calculate the sound level of the restrictor(s)by using the methods for low noise valves.

3. Find the total sound level for the valve andrestrictor(s) combination.

a. From the sound levels calculated inSteps 1 and 2, subtract 6 dBA for eachrestrictor downstream of a noise source.The limit of 12 dBA applies with 2 ormore downstream restrictors.

b. Determine the final sound level by loga-rithmic addition. Logarithmically add theresults above according to Figure 4 toobtain the estimated sound level down-stream of the final restrictor.

Note that the close coupling of the valve and LO-DBcartridges and expansion plates results in a lowerpredicted noise level than when separated by pipe.

Case IIValve and downstream restrictor(s) are separatedby a length of pipe (not close coupled).1. Calculate the sound level downstream of the

final restrictor as in Case I.

2. Calculate the sound level for the controlvalve using restrictor upstream pressure asvalve downstream pressure, and pipe wallthickness and pipe diameter of the connect-ing pipe. This is the sound level radiated bythe connecting pipe.

3. Compare sound levels of the connecting pipeand downstream of the final restrictor. Theconnecting pipe is an effective noise sourcethat should be examined to determine over-all system performance.


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5. Hydrodynamic Noise5.1 Prediction

The basic sources of hydrodynamic noise include:

• Turbulent flow

• Flashing

• Cavitation

• Mechanical vibration resulting from turbulent flow,cavitation, and flashing

Problems resulting from high hydrodynamic noise levelsare flashing erosion, cavitation erosion, and combined ero-sion/corrosion. Unlike aerodynamic noise, hydrodynamicnoise can be destructive even at low levels, thus requiringadditional limitations for good valve application practice.

The international standard for control valve hydrodynamicnoise prediction is IEC-534-8-4. The method in this standard is based on physics principles, and can beapplied to any valve style. Like the aerodynamic standardthe intended accuracy is plus or minus five decibels.

4. Atmospheric Vent Systems4.1 Introduction

Noise emitted from atmospheric vents, using either conventional, low noise valves, or valve-restrictor systemscan be calculated using the procedure below. Sphericalradiation is assumed which reduces noise by 6 dBA foreach doubling of distance. However, at long distances,much lower noise levels would be expected due to atmos-pheric absorption and attenuating effects of topography,wind, temperature gradients, and ground effects.

LO-DB static resistors (cartridges and plates) used witheither LO-DB or conventional valves, can often provide the most cost effective solution to vent applications. If these systems are used, only the final system soundlevel is considered.

4.2 Noise Calculation Procedure

Step 1 Calculate the base sound level for a conventionalvalve, low noise valve, or static restrictor by themethods given in the previous sections. However,in each case, use transmission loss, TL, equal tozero. Correct for distance, r, by subtracting 20 logr/3 for distance in feet (or 20 log r for distance inmeters) to obtain the corrected sound level.

Step 2 Correct for DirectivityThe directivity index is important in vent applica-tions because of the directional nature of high frequency noise typical of control valve signature.Figure 5 is based on typical average peak fre-quencies of 1000 to 4000 Hz. If a silencer is used,the directivity index will change appreciably.Silencers, by design, absorb the high frequency(directional) components from the valve spectralsignature, leaving predominantly low frequencynoise. Consequently, for silencer applications,use one half the directivity index at each angleshown. Add the directivity index to the sound leveldetermined in Step 1.

Step 3 For large distances, make appropriate correctionsfor wind and temperature gradients, topography,and atmospheric absorption, for a specific application.

Figure 5Directivity Index

135˚

90˚

0˚

-5dBA

0dBA

45˚

-10dBA

-15dBA

VerticalVent Stack

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The factors used in the calculations are:

• FL Pressure Recovery, Choked Flow Factor

• XF Differential Pressure Ratio

• XFZ Pressure Ratio at Which Cavitation Inception isAcoustically Detected

• ηF Acoustical Efficiency Factor (Ratio of Sound

Power to Stream Power)

1x10-8 for Std. Globe Valve

• ∆LF Valve Specific Correction Factor for Cavitating

Flow

Lwi Measured - Lwi Calculated

• Lwi Internal Sound Power

• FB Factor to Account for Cavitation of Multi-Component Fluids Having a Range of VaporPressures

5.1.1 The graph below depicts a typical liquid flowcurve for a control valve over a wide range of pressure drops, with constant inlet pressure.Flow rate is plotted on the vertical axis versus thesquare root of pressure drop. At relatively low to moderate pressure drop, in the range of fullyturbulent and non-vaporizing flow, flow is propor-tional to the square root of pressure drop. At high-pressure drop, flow is choked; that is, furtherdecrease of downstream pressure does not resultin an increase in flow rate. Note that the pressurerecovery factor, FL, is determined by test at theintersection of the straight line representing non-vaporizing flow and the straight-line repre-senting choked flow. The factor Kc is determinedas the point of deviation of the straight-line flowcurve. The newer factor XFZ is determinedacoustically as the point where an increasingnoise level is detected. Although not required foruse in this standard, the cavitation factor Sigmadenotes the inception of vaporization determinedby high frequency detection through the use of anaccelerometer. Note that the XFZ and Sigma maybe very close, and for all practical purposes canbe considered the same point, unless Sigma

5.1.2 Non-Cavitating FlowThe basic equations for non-cavitating flow are shown below:

• Stream Power

• Sound Power

Key Factors: Mass Flow and Pressure Drop

5.1.3 Cavitating FlowThe equation for cavitating flow has additional quantities:

∆P is Limited by Critical Pressure Drop

Key Factors: ∆LF, Ratio XFZ / XF

NOTE: FB factor is included in the cavitation noise adderfor multi-component fluids.

Q

XFZ

Kc

σi(High Freq.)

FL ∆P

5. Hydrodynamic Noise (cont.) incipient is found by use of very high frequencydetection.

Flow Curve

• XFZ is approximately σi (not high frequencydetection)

5.1.6 A generalized hydrodynamic noise curve isdepicted in the graph shown below. Sound levelin dBA is plotted on the vertical axis versus thepressure drop ratio (pressure drop divided by inletpressure minus vapor pressure). The most inter-esting feature of this curve is that a rounded curveis superimposed on the intermediate straight line(shown in the turbulent region) and the dotted lineprojection. This illustrates the result of the use ofthe sound power equation that adds an addition-al quantity in the cavitating region.

The method predicts that all globe valves havingequal pressure recovery will produce the samenoise level in the non-cavitating region. However,by selecting a valve with higher XFZ (or lowerSigma), the inception of increased noise due tovaporization and cavitation will be forestalled tohigher-pressure drop. With the selection of anti-cavitation valve trim, the resulting noise levels willbe dramatically reduced.

Non-Cavitating Cavitating

SounddBA

Laminar Turbulent

Hydrodynamic Noise Prediction

XF = ∆P/(Pl-Pv)

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Acoustical Version of the Sigma (σ) Curve

Hydrodynamic Noise

0

5.1.4 Pipewall Transmission LossThe internal frequency spectrum is first determined:

• Standardized Spectrum Based on Std. SingleSeated Globe Valve Water Testing

• Noise Spectrum in the Single Octave BandRange of 500 Hz through 8000 Hz

The pipewall transmission loss is then calculated for eachfrequency band:

• Pipe Wall Transmission Loss

Key Factors: Pipe Diameter, Wall Thickness, Ratio ofCenter Frequency to Ring Frequency

5.1.5 External Sound Pressure Level

The external unweighted sound power level isnext calculated in each frequency band:

lp = 3 Meters Min.

Key Factors: Diameter and Length of Pipe and TL

Then the A-weighted external sound power levelis determined:

• A-Weighting Sound Levels (LWA)

fm, Hz 500 1000 2000 4000 8000Correction Values, dB -3.2 0.00 +1.2 +1.0 -1.1

• A-Weighted Sound Power Level

LwAn is the External A-Weighted Sound Power Level of thenth Octave Band

Finally, the external sound pressure level is calculated:

• Based on open field conditions and cylindrical radiation, the sound pressure level 1 meter downstream of the valve outlet flange and 1meter lateral of the pipe is:

5. Hydrodynamic Noise (cont.)

16

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5.1.7 A flow chart illustrating the hydrodynamic noise prediction method is shown below.


Hydrodynamic Noise Prediction Flow Chart

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cF = speed of sound in the fluid

cp = speed of sound of the longitudinal waves in the pipe wall

Cv = flow coefficient

di = inside diameter of the downstream pipe

do = outside diameter of the downstream pipe

f = frequency

fm = octave center frequency

fr = ring frequency

FB = Factor to account for cavitation of multi-component fluids having a range of vaporpressures.

FL = liquid pressure recovery factor

lo = reference length of pipe = 1

lp = length of pipe

LpAe = A-weighted sound-pressure level external of pipe

LWAn = A-weighted sound power level of the nth

octave band

LWe = external sound power level (unweighted)

LWAe = A-weighted sound power level external of pipe

LWi = internal sound power level

∆LF = valve specific correction value

•m = mass flow rate

Po = reference sound pressure = 2 x 10-5

pv = absolute vapor pressure of fluid at inlet temperature

p1 = valve inlet absolute pressure

p2 = valve outlet absolute pressure

∆P = differential pressure between upstream and downstream (p1-p2)

T1 = inlet absolute temperature

TL = transmission loss (unweighted)

t = thickness of wall pipe

U2 = fluid velocity at outlet of valve

Wm = fluid power loss in the valve

Wo = reference sound power = 10-12

x = ratio of differential pressure to inlet absolute pressure (∆P/p1)

xF = differential pressure ratio (∆P/p1-pv)

xFz = characteristic pressure ratio for cavitation

ηF = acoustical efficiency factor for liquid (at φ = 0.75)

ρF = density (specific mass) at p1 and T1

ρp = density (specific mass) of pipe material

Nomenclature

5.2 Application Guidelines and Equipment Selection

5.2.1 Cavitating FluidCavitating fluid, usually water, can be one of themost devastating forces found in control valveapplications. Caused by high localized stressesincurred by vapor implosion, it can quickly destroycritical valve parts if not properly controlled oreliminated. Fortunately, because the imposedstresses are highly localized, the vapor implosionmust occur at or very close to valve metal sur-faces to cause damage. This attribute providesmany methods of controlling these destructiveforces, some of which are described below.

The damage potential of any cavitating fluid isdirectly proportional to:

1. Inlet Pressure P1: The inlet pressure isdirectly related to the amount of energy avail-able to cause damage. The greater the inlet

pressure, the greater the potential energyapplied to the cavitating fluid and the greaterthe damage potential.

2. Degree of Cavitation: This factor, related tothe percentage of the fluid which cavitates,is proportional to the required vs. actualvalve FL and to the degree that the fluidvapor pressure is well-defined. For example,using a valve with a FL of 0.9, a system witha required FL of 0.98 will have a muchgreater percentage of fluid cavitating than asystem with a required FL of 0.92, both at thesame P1, and will, therefore, experiencegreater damage. Secondly, a fluid that doesnot have a well-defined vapor pressure, thatwill boil over a wide temperature range, willlikely be self-buffering in a cavitating appli-cation. Consult Masoneilan Engineering.


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3. Fluid Surface Tension: Since fluid surfacetension affects the amount of pressure recov-ery experienced before vapor implosion, itdirectly affects the amount of energy soreleased. Consequently, fluids with low sur-face tension will tend to cause less damage.

5.2.2 Equipment ApplicationPreventative Measures: There are several pre-ventative measures that can eliminate cavitationdamage. First, however, it cannot be over-stressed that cavitation must be eliminated orcontrolled. Further use of hard materials is not asolution and will only delay ultimate valve failure.On all but the lowest pressure systems, this delaywill be insignificant. Several steps which can betaken are as follows:

1. Use a valve with low pressure recovery (highFL): Often on a moderately cavitating system,cavitation can be eliminated by using a low pressure recovery valve such as a cageguided globe. The goal is to increase the critical pressure drop, FL

2 (P1-Pv), above thevalve ∆P.

2. Reduce ∆P: If the ∆P can be reduced so that the vena contracta pressure does notdrop below the vapor pressure, cavitation willbe eliminated. Often this can be done bychanging the physical location of the valve(elevation, etc.).

3. Use of back pressure plates: If system range-ability permits, use of back pressure plates toincrease P2, reducing ∆P below the critical∆P1 can be the most cost effective solution.

Cavitation Control: At low to moderately highpressures, cavitation can be controlled by use ofspecially designed trims. These trims function intwo ways:

1. High FL: Recall, cavitation damage is directlyproportional to the percentage of fluid cavitating. Consequently, valves with lowpressure recovery (high FL) will experienceless cavitation damage.

2. Containment: Because cavitation is a highlylocalized phenomenon which requires directimpingement on metal surfaces to causedamage, use of a design which diverts thebubble implosion away from metal surfacescan be effective.

Most cage guided single and two-stage anti-cavitation valves including Masoneilan's LO-DB41000, 21000 and 2600 Series are examples of

cavitation control. Although they can be cost ef-fective solutions, there are limitations to theamount of energy that can be absorbed in thismanner. Consult Masoneilan Engineering.

Cavitation Prevention: Where high potential en-ergy exists (high P1) on cavitating fluid, cavitationmust be eliminated through use of good multiple-stage trim, designed specifically for anti-cavitationservice. Ideally, the pressure staging should be such that the smallest pressure drop occurs atthe last stage to minimize overall valve pressurerecovery. To minimize plug damage, the flowshould be axial, parallel to the plug; for good control, there should be no dead spots in the trim,providing a good smooth flow characteristic.Finally, since most valves of this type will be seated much of the time, extra-tight shutoff shouldbe provided. See Masoneilan's VRT® product catalogs, 78000, 78200 LINCOLNLOG™, and77000 product catalogs.

Flashing Fluid: When flashing exists in a controlvalve, potential physical damage to the valve mustbe considered. Flashing fluid vapor carries liquiddroplets at high velocity, quickly eroding carbonsteel. Use of higher alloys such as chrome-molywill result in acceptable performance.


Variable Resistance Trim Type SSectioned to Show Flow Passages

Variable Resistance Trim Type CSectioned to Show Flow Passages

20 OZ3000 01/02

6. References

6.1 CEI/IEC 60534-8-3, 2nd Edition, 2000“Control Valve Aerodynamic Noise Prediction Method”

6.2 CEI/IEC 534-8-4, 1st Edition, 1994“Prediction of Noise Generated by Hydrodynamic Flow”

6.3 CEI/IEC 534-8-1, 1st Edition, 1986“Laboratory Measurement of Noise Generated by Aerodynamic Flow Through Control Valves”

6.4 CEI/IEC 534-8-2, 1st Edition, 1991“Laboratory Measurement of Noise Generated by Hydrodynamic Flow Through Control Valves”

6.5 ISA Standard ISA S75.17, 1989“Control Valve Aerodynamic Noise Prediction”

6.6 ISA Standard ISA S75.07, 1987“Laboratory Measurement of Aerodynamic Noise Generated by Control Valves”

6.7 ISA Recommended Practice ISA RP75.23, 1995“Consideration for Evaluating Control Valve Cavitation”

21OZ3000 01/02

Appendix:Installation Considerations In closed systems, control valve noise generated by thethrottling process is radiated to the atmosphere throughdownstream piping. Noise calculations are based on laboratory conditions, including an acoustic “free field” (anenvironment without acoustic reflections) and with pipingsystems designed so that they will not contribute to generated noise. Consequently, like any other equipmentin a facility, these factors should be considered whendeveloping expected installed control valve noise levels.

Acoustical EnvironmentThe acoustical environment refers to the type of “field” inwhich the valve is installed. It is a measure of the soundbuild-up expected due to acoustic reflections from bound-aries, other equipment, as well as the total size (volume)of the installed environment. These factors are explainedin any basic acoustics text but cannot be anticipated by thecontrol valve manufacturer.

Piping Design GuidelinesThe following guidelines should be considered for optimumresults.

1. Straight run before and after valveStraight pipe for at least 10 diametersupstream and 20 diameters downstream ofthe valve is recommended.

2. Isolating ValvesIsolating block valves must be selected to ensure minimum resistance to fluid flow. Fullbore type is preferred.

3. Fluid VelocityDepending on velocity, fluid flow may createnoise levels higher than that produced by thecontrol valve. Masoneilan provides a meansfor calculating the Mach number (M) at serv-ice pressure and temperature conditions.

Average Velocity of Flowing MediumM =

Sound Velocity in the Flowing Medium

With LO-DB trim and fluid velocities above 1/3

Mach, fluid velocity noise must be calculatedand total system sound level reevaluated.

4. Expanders and ReducersLike any other source of turbulence in a fluidstream, expanders and reducers may be thecause of additional system noise. Concentricexpanders and reducers with includedangles smaller than 30˚ upstream and 15˚downstream of the valve are recommended.

As an exception to the above, short reducers(large included angles) are recommendedwith LO-DB restrictors because of their inher-ent stiffness and the fact that velocity is lowupstream of the restrictors.

5. Bends,T's and other Piping ConnectionsDrastic disruptions in the fluid stream, espe-cially if high fluid velocity exists, are potentialnoise sources. Possible improvements toconventional design for piping connectionsare shown in Figure 6.

Piping SupportsA vibration free piping system is not always possible toobtain, especially when thin wall piping such as Schedules5S and 10S are used. Supports in strategic locations, how-ever, will alleviate a lot of the potential structural problems.At the same time, they reduce the possibility of structureborne noise. In some cases, piping may be buried toreduce noise and vibration problems.

Flow

Flow

Shortest PossibleIntermediateExpanders

Restrictors

D1 D2

10 D1 20 D1

22 OZ3000 01/02

Appendix:Installation Considerations (cont.)

Extreme Sound LevelsFluid borne valve generated noise induces mechanicalvibration in the piping system which is radiated to the envi-ronment as valve noise. The valve sound level is indicativeof this surface motion. Excessive vibration can cause fail-ure or damage to valve and pipe mounted instruments, andaccessories. Piping cracks, loose flange bolts, and otherproblems can develop. For this reason, valve noise shouldbe limited to 115-120 dBA. If higher levels are expected,LO-DB valves, LO-DB static restrictors or other alternativesshould be used to reduce noise below the recommendedlevels. Note that pipe insulation and certain other “add on”noise control treatments, which do not change the pipe wallsurface motion, are ineffective. In most cases, suchextreme sound levels are precluded by occupational andenvironmental noise requirements anyway.

Reference Articles:

1. “Escape Piping Vibrations While Designing,” J. C. Wachel and C. L. Bates, Hydrocarbon Processing, October 1976.

2. “How to Get the Best Process Plant Layouts for Pumps and Compressors,” R. Kern, Chemical Engineering, December 1977.

3. “Predicting Control Valve Noise from Pipe Vibrations,” C. L. Reed, Instrumentation Technology, February 1976.

4. "Improving Prediction of Control Valve Noise," H. Boger, InTech, August 1998.

5. “Avoid Control Valve Application Problems with Physics-Based Models," J. A. Stares and K. W. Roth, Hydrocarbon Processing,August 2001.

Preferred Design

Bend

A. Pipe Turns

B. Inlets

C. Elevation Changes

D. Junctions

E. Connections

Elbow

Angular Lateral

One-Plane Turn Double Offset

Streamlined Opposing

Streamlined Branching Conventional Branch

Usual Design

Figure 6

23OZ3000 01/02

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