Control Valve Noise Causes and Reduction Methods
What Causes Control Valve Noise?
The three main causes of noise generated by valves are:
- Vibration. The mechanical vibration of the valve, its components or connected piping or steelwork.
- Turbulent gas flow through the valve. This is known as aerodynamic noise and is generated by the valve trim, and at the valve exit. Aerodynamic noise results from the shear forces created as the gas hits obstructions in the flow stream e.g. the valve trim, where it decelerates, expands, or changes flow direction.
- Cavitating liquid flow through the valve. This is known as hydrodynamic noise, and similiar to aerodynamic noise it is generated by the valve trim and, at the valve exit. Cavitation is the formation and collapse of vapour cavities in the flowing liquid.
Why is Valve Generated Noise an Issue?
Legislation, and company standards often limit the noise allowed in a working environment in an effort to eliminate potential hearing damage to plant operators and mainteneance crews e.g. OSHA 1910.95 in the United States, EU Directive 2006/42/EC in Europe, and The Control of Noise at Work Regulations in the United Kingdom. The noise generated by valves can contribute significantly to the overall plant noise.
Legislation and standards differ, however typical limits are:
- For continually operational control valves noise shall not exceed 85 dBA one meter downstream of the valve and one meter out from the valve,
- For intermittently operated atmospheric discharge vent valves noise shall not exceed 90 dBA at a point four meters downstream from the vent exhaust.
Control valve noise can introduce process control issues, and if left untreated lead to costly repairs to valves, pipes, instrumentation, and surrounding equipment.
How To Estimate Valve Noise
It is better to avoid installing a noisy valve rather than trying to retrofit noise abatement solutions to an existing valve. Therefore estimating valve noise at the design stage is good practice. The ISA and IEC organizations have developed a method of calculating valve noise in the international standard IEC 60534-8-3, "Industrial process control valves – Part 8-3: Noise considerations – Control valve aerodynamic noise prediction method".
All reputable valve suppliers will provide noise estimates for the given service conditions when they quote for supply of a valve.
Controlling Valve Noise
The two basic approaches for controlling valve noise are source treatment and path treatment. Source treatment attempts to curb excessive noise that would have been generated within the valve, while path treatment attempts to reduce noise after it has been generated.
How to Reduce Control Valve Noise at Source
The common source treatments include:
- Proper sizing of valve trim. Use of valve trims that are too small, or too large, can lead to the unnecessary generation of noise,
- Selection of low noise vlave trims. These tend to be more expensive than conventional trims, and if the flowing medium is dirty this can result in the trims blocking.
- Use of an inline diffuser. A diffuser is a pressure reducing device that is installed downstream from the control valve. When installed, the total pressure drop of the system is divided across the valve and diffuser. This enables the valve to operate at a lower pressure drop ratio, thereby lowering the noise level generated.
- Use of a vent diffuser. Vent diffusers reduce the noise generated by the venting of compressible fluids to atmosphere.
Path Treatment Options
The principle of path treatment is to reduce the propagation and transmission of sound to the external sound field. The common path treatments include:
- Increasing pipe thickness. The use of a heavier walled pipe downstream of a valve can reduse noise. A method not over popular with piping engineers due to the increased complexity of the piping design.
- Use of accoustic (or thermal) insulation. When correctly applied this can be a very effective method of reducing noise. However this method can lead to Corrosion Under Insulation (CUI). CUI is a form of localised, external corrosion of pipe resulting from water trapped between the pipe and insulation. Undetected CUI on pipes can result in leaks.
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