Hushing Up A Serious Problem: Gaseous Noise

No joke, it’s getting a lot quieter out there. You, your equipment and your processes have a number of cutting-edge noise attenuation solutions to thank for it.

Uncontrolled noise in process operations is a serious problem. Unresolved issues with noise can lead to health problems, vibration and, in the most extreme cases, equipment damage. All noise attenuation solutions are not created equal, and no one product will be effective in every situation. It is, therefore, important to understand what is creating noise before attempting to fix the problem.

Noise 101
The most significant factor impacting noise is fluid velocity—a significant rise in velocity can produce noise beyond safe limits.

When fluid travels through a conventional singleseated, globe-style valve, a “vena contracta” (point of narrowest flow restriction) develops directly downstream of the narrowest throttling point. At this point the fluid reaches a minimum pressure and maximum velocity that rapidly recovers to a lower pressure than the inlet pressure. When fluid pressure in the valve drops, the fluid velocity rises—this is called the “Bernoulli Principle.” As the velocity of the fluid increases, the noise generated by turbulence in the fluid also increases (see Figs. 1 and 2).

The driving force behind velocity and, accordingly, noise, is the difference between the inlet pressure and outlet pressure, which represent the energy available to generate noise. When this difference is low, the energy contained in the fluid stream will be low and the noise that is generated typically will be low as well. Each noise solution will have a range of pressure drops where the design is most effective.

Noise attenuation solutions
Most globe valve attenuation solutions use cages with a variety of designs available on the market. A typical solution with drilled holes is shown in Fig. 3. (Pressure through a multi-stage valve is shown in Fig. 4.) Different solutions using one of the noise reduction mechanisms listed in the accompanying sidebar—or a combination of such mechanisms— also are available.

Reducing pressure while controlling velocity is a common method for reducing noise, accomplished by dividing a large pressure drop into smaller pressure steps, which will produce far lower velocities at each step. For example, a sudden contraction followed by a sudden expansion can decrease pressure by creating turbulent zones in the fluid flow. The turbulence takes energy out of the fluid in the form of pressure. This is the effect primarily used by orifice plates. Using several orifice plates will create a high overall pressure drop while generating lower velocities than would a single plate designed to create the same pressure drop.

Design solutions, such as small flow passages, also can help reduce noise. Small passages accentuate the friction formed by the passage walls. As the passage grows smaller more pressure is required to force the fluid to flow.

A mutual impingement design also will reduce pressure without adding velocity to the flow. Mutual impingement is created when two flows impact at 180°, forming a highly turbulent zone that dissipates energy.

Sudden turns in the fluid path are another way to cause the pressure in the fluid to drop. The angle of the turn can have a dramatic effect on the energy loss—angles sharper than 90° are difficult to manufacture but are more effective in reducing pressure.

An acoustical attenuation solution can provide a barrier that blocks noise. This can be accomplished in a number of ways, including insulating the pipe and increasing the distance to the noise source.

Careful engineering of the noise solution includes evaluating any existing attenuation. Often thermal pipe insulation can be used in the evaluation of the noise to reduce the predicted noise level without adding cost to the system.

Understanding the Peak Frequency Effect can offer alternative options for decreasing noise levels. Most noise in a control valve produces a range of frequencies that have a bell curve type distribution and a peak frequency—changes in the geometry of the valve design will shift this peak. It is possible to shift this frequency out of the range of human hearing, which lowers the perceived noise and damage to human organs. Shifting the peak higher also reduces the level of noise that will pass through the pipe, which has a naturally low frequency. A common way to raise the peak frequency is to make smaller outlet holes in the noise control device—cutting a hole diameter in half can lower the overall noise level by up to five decibels. This type of solution is available from all major control valve manufacturers as a cage with small holes.

WaveCracker® technology is a patented technology that reduces noise as flow passes through irregularly shaped cross sections. Tests have shown this technology can effectively reduce noise by more than 10 decibels. WaveCracker works by forming an irregular cross section shape. Sound waves reflecting off the walls of the passage have irregular patterns that cause the sound pressure wave to lose intensity as it moves down the passage (see Fig. 5).

One cause of noise could be harmonic vibration—something that occurs when the valve and pipe approach a common frequency. This problem is characterized by a tell-tale “screech,” where a single frequency is pronounced. Because screech occurs when the frequency of the valve and pipe match, it is not easily predicted.

Noise also can worsen due to reflective surfaces that amplify noise coming from a pipe. A single, flat surface near a valve, like a concrete floor, can add three decibels. Two hard, flat surfaces (like a floor and flat ceiling) that are parallel to one another will add more than six decibels. Adding walls, a ceiling and a floor can add 30 to 40 decibels.

Noise prediction
Careful noise predictions will prevent most noisy applications. A number of noise prediction techniques exist with varying degrees of accuracy for different applications. Unfortunately, no standard exists that is the most accurate for all possible conditions.

Most manufacturers have proprietary techniques that will produce acceptable prediction under a range of conditions and with equipment the manufacturer is familiar with. When used outside the acceptable range or with other equipment, however, proprietary techniques can be significantly different than actual noise produced.

The IEC committee has developed the IEC standard 60543-8-2 in an effort to provide an accurate standard that can be used to compare products from different manufacturers. Although it’s not perfect, this method does create a clear baseline that can be used to compare equipment from a variety of suppliers. If noise control is critical around your operations, it is important to study all factors, such as flow conditions, valve design, system installation and available noise prediction methods.

Before you buy…
Before purchasing expensive noise suppression equipment, you should ask yourself a few important questions:

• How much noise attenuation is actually required?
• What are the low-cost alternatives to noise attenuation?
• And, if noise attenuation devices are necessary, what lower-cost equipment can be specified?

If the predicted sound pressure level (SPL) exceeds 85 or 90 dBA, noise suppression devices should be considered. If the noise is not associated with equipment damage and is located in a remote location away from people, however, higher noise levels may be acceptable. Other possible low-cost alternatives to noise suppression are piping insulation, discharging the valve into a vessel, relocating the noise source outside an enclosed area, reversing the flow direction through the valve and reducing the pressure drop across the valve.

When noise levels are critical, it always will be important to consult a control valve noise expert. In these cases, the expert will need to gather information and data on your specific application. The more information you are able to provide, the higher the expert’s success rate will be. MT