This chapter discusses the practical acoustic noise aspects of control valves. Valve noise-generation mechanisms, prediction, and reduction information are covered. Control valve noise generation and the prediction of valves' noise levels are highly complex subjects and our understanding of them is still incomplete. Although our general knowledge may be somewhat incomplete, it is a testimony to the experts in the field that we have gained a reasonably accurate ability to predict control valve noise and find workable practical solutions for reducing it.

Introduction

Noise is considered by many to be the world's most prevalent form of pollution. Consider the following items:

• A German study by the Max Planck Institute for Industrial Physiology suggests that noise contributes to cardiovascular problems such as heart disease and high blood pressure. Workers in high and low noise exposure areas were compared. The high noise group had more circulatory problems, cardiac disturbances, neurosensory motor impairment, and even more social conflicts at work and at home.
  
  
• The Fells Research Institute in Yellow Springs, Ohio, found that noise can influence the unborn fetus. The fetus's heart rate can quicken, the muscles can be made to contract, and prolonged exposure can produce malformations in the nervous system that may affect behavior later in life. Experiments with rats show that an unborn fetus subjected to auditory stress demonstrated disorganized behavior when grown.
  
  
• The contribution of noise to hypertension can result in gastrointestinal problems such as peptic ulcers. Russian studies in this area have indicated that workers in high noise areas had twice the incidence of peptic ulcers. There was ten times the incidence of impaired hearing and twelve times the subjective complaints as compared to workers in a low noise environment.
  
  
• Noise exposure reduces the depth and quality of sleep and thus may affect overall mental and physical health. A two-year British study found that persons living near London's Heathrow Airport had a significantly higher rate of admission to mental hospitals than persons living in socially similar but quieter areas.

All of these items show that noise is extremely detrimental to humans. Some have called it "a slow agent of death." The one end result that most often comes to mind is hearing loss and deafness. This type of noise damage is so subtle and insidious that by the time it becomes apparent, it is too late. The World Health Organization estimated a number of years ago that noise costs the United States more than $4 billion annually in accidents, absenteeism, inefficiency, and compensation claims. Of course, not all of this is due to industrial noise, but it certainly bears the lion's share. What is noise pollution? It can be defined as unwanted sound, or sound without value. Noise fighters fondly point out that the word apparently derives from the Latin nausea. Figure 8-1 shows a table of relative noise levels for common sounds and activities and provides a perspective for judging aspects of noise in our lives.

There are many noise sources in industrial plants, but major contributors are control valves operating under conditions of high mass flow and/or pressure drop, with gases and vapors. This aerodynamic noise is one of the few sources, and sometimes the only source, of noise levels exceeding 100 dBA, and in some cases up to 140 dBA About the only thing louder in normal operation is a rocket or aircraft jet engine at takeoff power. Being able to predict such valve noise problems in advance of valve purchase and then being able to generate corrective action are therefore vitally important. Control valve noise can vary from a low frequency rumble to a high frequency scream or screech, depending upon the basis of the noise being generated. For those not completely familiar with noise terminology, the decibel, or dB, is the most commonly used unit of measurement. The decibel measures a sound's intensity or, to the ear, its loudness.