Equipment Design

Control Valve Noise

Predict aerodynamic and hydrodynamic noise from control valves using the IEC 60534-8-3 method. Understand noise mechanisms, pipe wall transmission loss, OSHA requirements, and noise reduction strategies.

Launch Calculator Noise reduction

OSHA action level

85 dBA

Hearing conservation program required above this threshold.

OSHA PEL (8-hr)

90 dBA

Permissible exposure limit for 8-hour time-weighted average.

Low-noise trim

10–20 dB reduction

Multi-hole cage trims shift peak frequency and reduce mixing energy.

Contents

Use this guide when you need to:

  • Predict SPL from a control valve installation.
  • Determine if OSHA hearing conservation is required.
  • Select low-noise trim or pipe schedule for noise reduction.
  • Evaluate cavitation noise in liquid service.

1. Noise Sources in Control Valves

Control valve noise originates from the conversion of fluid mechanical energy into acoustic energy as the fluid passes through the valve restriction. The two primary mechanisms are aerodynamic noise (gas/steam) and hydrodynamic noise (liquid), each with distinct characteristics and mitigation strategies.

Aerodynamic

Gas & steam service

Turbulent mixing, shock cells, and jet screech at the vena contracta. Dominant in high-pressure-drop gas applications.

Hydrodynamic

Liquid service

Cavitation bubble collapse, turbulent flow noise, and flashing. Cavitation noise can exceed aerodynamic levels.

Mechanical

Vibration-induced

Plug resonance, stem vibration, and loose trim components. Usually secondary to fluid noise.

Pipe-borne

Downstream radiation

Noise energy transmitted through the pipe wall and radiated externally. This is what operators hear.

Aerodynamic Noise Mechanism

When gas flows through a control valve, the fluid accelerates through the restriction, creating a high-velocity jet at the vena contracta. The jet velocity can approach or exceed the speed of sound in the gas. Noise is generated by three mechanisms:

  • Turbulent mixing: The high-velocity jet mixes with the slower-moving downstream fluid, creating shear layers with broadband turbulence. This is the primary noise source in subsonic flow.
  • Shock cells: When the jet velocity exceeds Mach 1 (choked flow), a series of standing shock waves form downstream. The interaction of these shock cells with turbulent structures generates intense broadband noise.
  • Screech tones: In some valve geometries, a feedback loop between the shock cell structure and the nozzle exit produces discrete high-frequency tones superimposed on the broadband noise.
Critical pressure ratio: The transition from subsonic to choked flow occurs at the critical pressure ratio xT, which is specific to each valve type and trim design. When x = (P1−P2)/P1 exceeds xT, the flow becomes choked and noise increases dramatically — often by 20–30 dB.

Hydrodynamic Noise Mechanism

In liquid service, the primary noise concern is cavitation. As liquid accelerates through the valve restriction, the local static pressure can drop below the liquid vapor pressure. Vapor bubbles form at the vena contracta and then collapse violently as the pressure recovers downstream. Each bubble implosion creates a micro-shock wave, and the cumulative effect produces intense broadband noise centered at higher frequencies (typically 2–8 kHz).

The cavitation index σ = (P1 − Pv)/(P1 − P2) characterizes the severity. Incipient cavitation begins when σ drops below σi ≈ 1/FL², where FL is the liquid pressure recovery factor. As σ decreases further, cavitation progresses from incipient to constant to severe, with noise and trim damage increasing at each stage.

2. IEC 60534-8-3 Prediction Method

The IEC 60534-8-3 standard provides a systematic engineering method for predicting the external sound pressure level produced by a control valve installation. The method calculates the total acoustic power generated by the valve, determines how much is attenuated by the pipe wall, and computes the SPL at a specified distance from the pipe exterior.

Calculation Sequence

The IEC method follows a sequential energy-based approach:

  1. Determine pressure ratio x = (P1−P2)/P1 and compare with the critical pressure ratio xT to identify the flow regime (subsonic, transition, or choked).
  2. Calculate mechanical stream power Wm from the mass flow rate, pressure drop, and fluid density: Wm = ṁ · ΔP / ρ
  3. Determine acoustic efficiency η based on the flow regime. This is the fraction of mechanical stream power converted to acoustic energy.
  4. Calculate sound power level Lw = 10 · log10(Wm · η / Wref), where Wref = 10-12 W.
  5. Determine peak frequency fpeak from the jet velocity, valve geometry factor Fd, and effective jet diameter.
  6. Calculate pipe wall transmission loss TL based on the acoustic impedance mismatch between the internal fluid and the pipe wall.
  7. Calculate external SPL = Lw − TL − 10 · log10(2πrL), where r is the measurement distance and L is the effective radiating pipe length.
  8. Apply A-weighting correction to convert from linear dB to dBA for human hearing comparison.
Measurement standard: The IEC standard specifies SPL at 1 meter (3.28 ft) from the downstream pipe wall as the reference measurement location. All noise specifications and OSHA comparisons should reference this distance unless otherwise stated.

Key Parameters

Parameter Symbol Description
Pressure ratio x (P1−P2)/P1 — dimensionless ratio of pressure drop to inlet pressure
Critical pressure ratio xT Valve-specific ratio at which flow becomes choked (sonic)
Valve style modifier Fd Ratio of hydraulic diameter of flow passage to valve seat diameter
Acoustic efficiency η Fraction of stream power converted to acoustic energy (10-7 to 10-1)
Transmission loss TL Pipe wall sound attenuation (dB) — depends on wall thickness and frequency
Liquid recovery factor FL Ratio of actual to theoretical pressure recovery; used for cavitation prediction

3. Acoustic Efficiency

Acoustic efficiency η is the most critical parameter in valve noise prediction. It represents the fraction of the mechanical stream power that is converted into sound energy. The efficiency varies over many orders of magnitude depending on the flow regime.

Efficiency by Flow Regime

Flow Regime Condition Typical η Noise Character
Subsonic (low x) x < 0.7 · xT 10-7 to 10-4 Low broadband noise, turbulent mixing only
Transition 0.7 · xT < x < xT 10-4 to 10-2 Increasing noise as shock cells begin to form
Choked x > xT 10-2 to 10-1 Intense broadband noise with shock cell interaction

In the subsonic regime, acoustic efficiency scales approximately as (x/xT)3.6, reflecting the strong dependence on the ratio of actual to critical pressure drop. In the transition zone, efficiency increases more rapidly as shock structures begin to form. Once the flow is fully choked (x > xT), efficiency levels off near 10-2 and increases only slowly with further pressure ratio increase, reaching a theoretical maximum near 10-1.

The 3.6 power law: In the subsonic regime, noise power scales as approximately the 3.6 power of the velocity ratio. This means that a modest increase in pressure drop can cause a large increase in noise. For example, doubling the pressure ratio in the subsonic regime can increase noise by more than 10 dB.

Effect of Valve Type on Efficiency

Different valve types have different critical pressure ratios (xT), which means they transition to choked flow at different operating conditions. Globe valves with their high xT (0.65–0.75) remain subsonic over a wider operating range than ball or butterfly valves with their low xT (0.15–0.35). For high-pressure-drop applications, globe valves inherently produce less noise because the flow remains subsonic at pressure ratios that would cause choked flow in a ball valve.

4. Pipe Wall Transmission Loss

The pipe wall acts as an acoustic barrier between the internal noise source and the external environment. Transmission loss (TL) is the reduction in sound level as noise energy passes through the pipe wall. It depends on the impedance mismatch between the internal fluid and the pipe wall material, the wall thickness, and the frequency of the noise.

Impedance-Based Transmission Loss

The basic transmission loss equation compares the acoustic impedance of the pipe wall to that of the internal fluid:

TL = 10 · log10(1 + ρpipe · cpipe · t / (ρfluid · cfluid · D))

where ρpipe and cpipe are the density and speed of sound in the pipe material (steel), t is the wall thickness, ρfluid and cfluid are the internal fluid properties, and D is the pipe internal diameter.

Factors Affecting Transmission Loss

Factor Effect on TL Practical Impact
Wall thickness Increases TL linearly Going from STD to XS adds 3–5 dB; Sch 80 adds 5–8 dB
Pipe diameter Decreases TL Larger pipe has less TL for same schedule; radiates more noise
Fluid density Increases denominator, decreases TL Liquid service has lower TL than gas service for same pipe
Frequency Higher frequency → more TL Mass law: +6 dB per octave above ring frequency
Ring frequency: The pipe ring frequency fring = csteel / (π · DOD) is the frequency at which the pipe wall resonates circumferentially. Below this frequency, the pipe wall is acoustically transparent; above it, the mass law applies and TL increases with frequency. For a 6-inch pipe, fring ≈ 10,700 Hz.

Typical Transmission Loss Values

Pipe Size STD Wall XS Wall Sch 80 Sch 160
4 inch 28–32 dB 31–35 dB 31–35 dB 37–41 dB
6 inch 26–30 dB 30–34 dB 30–34 dB 36–40 dB
8 inch 24–28 dB 28–32 dB 28–32 dB 34–38 dB
12 inch 22–26 dB 26–30 dB 29–33 dB 35–39 dB

Note: Ranges reflect variation with frequency and gas density. Higher values at higher frequencies; lower values for denser gases.

5. A-Weighting and Sound Pressure Level

Sound pressure level (SPL) measured in decibels (dB) reflects the physical intensity of sound at a point. However, human hearing is not equally sensitive to all frequencies. A-weighting applies a frequency-dependent correction curve that approximates the human ear's response, producing values in dBA (A-weighted decibels).

A-Weighting Curve

The A-weighting curve per IEC 61672-1 attenuates low frequencies and very high frequencies relative to the 1–4 kHz range where human hearing is most sensitive:

Frequency (Hz) A-Weighting (dB) Comment
63 −26.2 Strong attenuation of low-frequency noise
250 −8.6 Moderate attenuation
1,000 0.0 Reference frequency (no correction)
2,000 +1.2 Slight emphasis near peak hearing sensitivity
4,000 +1.0 Still near peak sensitivity
8,000 −1.1 Beginning of high-frequency rolloff
16,000 −6.6 Significant high-frequency attenuation

Distance Attenuation

For a cylindrical source (pipe), the SPL decreases with distance from the pipe surface according to:

SPL(r) = SPL(rref) − 10 · log10(r/rref)

This gives approximately 3 dB reduction per doubling of distance for a line source (pipe), compared to 6 dB per doubling for a point source. At distances much greater than the pipe length, the pipe begins to behave as a point source with 6 dB per doubling.

Practical implication: Moving the measurement point from 1 meter to 3 meters reduces the SPL by approximately 5 dB. This can make the difference between exceeding and meeting the 85 dBA OSHA action level. However, engineering controls (low-noise trim, acoustic insulation) are preferred over administrative controls (distance restrictions).

6. Liquid Service and Cavitation Noise

In liquid service, noise generation follows a different mechanism than gas service. The primary concern is cavitation, which occurs when the local static pressure at the vena contracta drops below the liquid vapor pressure. Vapor bubbles form in the low-pressure zone and then collapse violently as the pressure recovers downstream.

Cavitation Index

The cavitation index σ quantifies the margin between operating conditions and cavitation onset:

σ = (P1 − Pv) / (P1 − P2)

where Pv is the liquid vapor pressure at the operating temperature.

Cavitation Stages

Stage Condition Noise Level Trim Damage
No cavitation σ > 1.5 · σi Low (turbulent flow only) None
Incipient σi < σ < 1.5 · σi Moderate (crackling sound) Minimal
Constant 1.0 < σ < σi High (roaring sound) Moderate erosion
Severe / Choked σ < 1.0 Very high + vibration Severe erosion, potential valve destruction
Cavitation is destructive: Unlike aerodynamic noise, which is primarily an occupational hazard, cavitation causes physical damage to valve trim, body, and downstream piping. Each bubble implosion creates a micro-jet with pressures exceeding 100,000 psi at the collapse point, eroding even hardened materials. Preventing cavitation is critical for valve longevity, not just noise control.

Valve Recovery Factor (FL)

The liquid pressure recovery factor FL characterizes how much of the pressure drop is recovered downstream of the vena contracta. Low-recovery valves (high FL, like globe valves) are less prone to cavitation because less pressure is recovered:

Valve Type FL Cavitation Risk
Globe (parabolic plug) 0.85–0.90 Low (preferred for high ΔP liquid service)
Cage-guided globe 0.80–0.90 Low
Butterfly (60° open) 0.50–0.60 High (avoid for high ΔP liquid)
Ball (full bore) 0.55–0.65 High

7. OSHA Requirements (29 CFR 1910.95)

The Occupational Safety and Health Administration (OSHA) sets legal limits on workplace noise exposure. Control valve noise frequently approaches or exceeds these limits, making noise prediction essential during the design phase to avoid costly retrofits.

OSHA Exposure Limits

SPL (dBA) Max Duration Required Actions
< 85 Unlimited No specific requirements
85 8 hours Hearing conservation program, audiometric testing, hearing protection available
90 8 hours Permissible exposure limit (PEL). Hearing protection mandatory, feasible engineering controls required
95 4 hours Reduced exposure time
100 2 hours Significantly limited exposure
105 1 hour Very limited exposure
110 30 minutes Minimal exposure
115 15 minutes Maximum instantaneous continuous noise
5 dB exchange rate: OSHA uses a 5 dB exchange rate (also called doubling rate). Each 5 dB increase halves the allowable exposure time. This is more conservative than the 3 dB exchange rate used by NIOSH and international standards, and means that exposure management becomes critical even a few dB above the 90 dBA PEL.

Design Target

Most operating companies set a design target of 85 dBA at 1 meter from the pipe wall for new installations. This keeps the facility below the OSHA action level and avoids the administrative burden and cost of hearing conservation programs. For indoor installations or enclosed compressor buildings, even lower targets (80 dBA) may be appropriate.

8. Noise Reduction Techniques

When predicted noise exceeds acceptable levels, engineers have several options for noise reduction. These fall into three categories: source treatment (modifying the valve), path treatment (modifying the pipe), and receiver protection (administrative controls and PPE).

Source Treatment (Preferred)

Addressing noise at the source is the most effective approach. Options include:

Multi-hole cage trim

10–15 dB reduction

Divides flow into many small jets. Each jet has less energy and higher peak frequency (more atmospheric absorption). The most common low-noise trim solution.

Multi-stage trim

15–25 dB reduction

Stages the pressure drop across 2–4 restrictions in series. Each stage operates at a lower pressure ratio, keeping all stages subsonic. Most effective for very high pressure drops.

Diffuser plates

5–10 dB reduction

Installed downstream of the valve to break up the jet and distribute flow. Less effective than cage trims but can be retrofitted to existing valves.

Valve type change

Variable reduction

Switching from a ball/butterfly valve to a globe valve increases xT, potentially keeping the flow subsonic. Can reduce noise by 10–20 dB in high ΔP applications.

Path Treatment

  • Heavier pipe schedule: Increasing from STD to Sch 80 or Sch 160 adds 3–8 dB of transmission loss. This is a simple and reliable approach that adds minimal cost during construction.
  • Acoustic insulation: Wrapping the pipe with mass-loaded vinyl, lead sheeting, or acoustic lagging can provide 10–25 dB additional attenuation. Most effective at frequencies above 500 Hz. Requires proper installation with sealed joints to avoid flanking paths.
  • Downstream pipe length: Installing long straight pipe runs downstream allows the noise energy to distribute and attenuate before reaching elbows, tees, or other discontinuities that can re-radiate noise.
  • Inline silencers: Absorptive silencers installed downstream provide 10–30 dB reduction but add permanent pressure drop and require maintenance (fouling of absorptive elements).

Receiver Protection (Last Resort)

  • Hearing protection: Earplugs (NRR 25–33 dB) or earmuffs (NRR 20–30 dB). Required by OSHA above the PEL but considered a last resort after engineering controls.
  • Administrative controls: Limiting exposure time, rotating workers, and establishing hearing conservation zones. Required when engineering controls cannot reduce noise below the PEL.
  • Remote operation: For very high noise installations, locating the valve in an enclosed vault or away from normal work areas eliminates routine exposure.
Hierarchy of controls: OSHA requires the use of feasible engineering controls before relying on administrative controls or personal protective equipment. During the design phase, it is almost always more cost-effective to specify low-noise trim or heavier pipe than to manage an ongoing hearing conservation program.

9. Valve Selection for Low Noise

When noise is a primary concern, valve selection should prioritize the following characteristics:

Selection Criteria

  1. High xT value: Choose valve types with high critical pressure ratios to maintain subsonic flow. Globe valves (xT = 0.65–0.75) are preferred over ball (0.15) or butterfly (0.25) valves for high-pressure-drop applications.
  2. High FL for liquid service: Low-recovery valves (high FL) are less prone to cavitation. Globe valves (FL = 0.85–0.90) resist cavitation better than butterfly valves (FL = 0.50–0.60).
  3. Multi-hole cage trim availability: Cage-guided globe valves can accept multi-hole cage trims that reduce noise 10–15 dB without affecting controllability.
  4. Multi-stage options: For extreme pressure drops (x/xT > 2), multi-stage trims with 2–4 stages can keep each stage subsonic.
  5. Proper sizing: Oversized valves operating at low opening produce higher velocity jets and more noise. Size the valve so normal operation is at 40–60% open.

Valve Comparison for Noise

Valve Type xT Fd FL Noise Ranking Best Application
Globe (parabolic) 0.70 0.46 0.90 Quietest High ΔP gas/liquid, noise-critical
Cage-guided globe 0.68 0.42 0.85 Quiet High ΔP with low-noise trim
Butterfly 0.25 0.13 0.55 Moderate Low ΔP, large line size
Ball (full bore) 0.15 0.10 0.60 Loudest On/off service, low ΔP

10. Worked Example

A natural gas pressure reduction station uses a 4-inch globe valve (Cv = 200) to reduce pressure from 500 psig to 200 psig. Gas properties: MW = 18.5 lb/lbmol, k = 1.3, flow rate = 50,000 lb/hr. Temperature = 100°F. Downstream pipe is 6-inch STD. Predict the noise level at 1 meter (3.28 ft) from the pipe wall.

Step-by-Step Solution

  1. Convert pressures: P1 = 500 + 14.7 = 514.7 psia, P2 = 200 + 14.7 = 214.7 psia
  2. Pressure ratio: x = (514.7 − 214.7) / 514.7 = 0.583
  3. Compare with xT: For globe valve, xT = 0.70. Since 0.583 < 0.70, flow is subsonic (but in the transition zone since 0.583 > 0.7 × 0.70 = 0.49).
  4. Gas density: ρ = P · MW / (Z · R · T) at upstream conditions
  5. Speed of sound: c = (k · R/MW · T · gc)0.5
  6. Acoustic efficiency: In the transition zone, η is on the order of 10-3
  7. Stream power → Sound power → Transmission loss → SPL: Apply the IEC method sequentially to arrive at the external SPL in dBA at 1 meter.
Use the calculator: The manual calculation requires iterative computation of density, acoustic efficiency, and frequency-dependent transmission loss. Use the Control Valve Noise Calculator for accurate results across all operating conditions.

Design Decision Tree

  1. Calculate predicted SPL at 1 meter using the IEC method.
  2. If SPL < 85 dBA: Standard trim and pipe schedule are acceptable.
  3. If 85 ≤ SPL < 95 dBA: Consider low-noise trim (saves 10–15 dB) or heavier pipe schedule.
  4. If 95 ≤ SPL < 110 dBA: Specify low-noise cage trim plus acoustic insulation. Evaluate multi-stage trim.
  5. If SPL ≥ 110 dBA: Multi-stage pressure reduction is required. Consider separate pressure letdown stations in series.

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