Valve Noise Reduction: Aerodynamic and Hydrodynamic Noise Sources and Solutions

Noise generated by control valves is a significant concern in process plants for several reasons. Excessive noise levels create occupational health hazards requiring hearing protection for plant personnel working near loud valves. High-velocity flow through valve trim generates acoustic energy that propagates both through the fluid downstream of the valve and through the pipe wall as structure-borne noise, creating risk of downstream pipe and fitting fatigue failure in severe cases. Understanding the mechanisms of valve noise generation and the available noise reduction technologies allows engineers to specify quiet valve designs for applications where noise is a constraint.

Aerodynamic Noise in Gas Service

When a gas flows through a control valve at high velocity, turbulence in the jet of gas emerging from the valve trim generates broadband acoustic noise. The noise level increases approximately as the eighth power of the jet velocity, so small increases in velocity produce large increases in noise level. Noise becomes particularly severe when the pressure drop ratio across the valve is large enough to cause the gas to reach sonic velocity at the vena contracta within the trim, a condition called choked flow. Under choked flow conditions, aerodynamic noise levels can easily exceed 100 to 110 dBA at one meter from the downstream pipe, sufficient to cause hearing damage with prolonged exposure.

• Noise level increases with approximately the eighth power of jet velocity

• Choked flow (sonic conditions at trim): most severe noise condition for gas valves

• Downstream pipe diameter: larger pipes attenuate noise more effectively than smaller pipes

• Pipe wall thickness: heavier schedule pipe provides more noise attenuation than standard wall

• IEC 60534-8-3: standard calculation method for aerodynamic noise prediction in control valves

Hydrodynamic Noise and Cavitation

In liquid service, cavitation is the primary source of high-intensity noise. When the local static pressure within the valve trim falls below the vapor pressure of the liquid, vapor bubbles form. When these bubbles collapse as pressure recovers downstream of the vena contracta, they generate intense local pressure pulses that create broadband noise, vibration, and material damage. Cavitation noise has a characteristic crackling or gravel-in-a-pipe sound and is accompanied by severe vibration. The onset of cavitation can be predicted using the valve's published cavitation index (Km or sigma) and the process pressure conditions.

Noise Reduction Technologies

Several technologies are available to reduce valve noise below acceptable limits. Multi-stage pressure reduction trim reduces noise by dividing the total pressure drop into multiple smaller steps, each producing lower velocity jets that generate less noise. Noise-attenuating cage trim uses a large number of small holes arranged so that the many small jets cancel each other acoustically (destructive interference) before combining into a single downstream flow. Downstream diffusers and silencers installed in the pipeline after the valve provide additional noise attenuation by absorbing acoustic energy before it reaches occupied areas. For severely noisy applications, a combination of multi-stage trim and downstream silencer may be required.