Noise produced in pipelines may be pump-generated (changes in power and pressure. or varying amplitudes of pressure pulsations) or fluid-generated (flow instability, turbulence or simple fluid friction).
Noise caused by the operation of valves, regulators and control elements is transient and related to the degree of turbulence or cavitation produced although. in specific designs and certain circumstances. individual elements may be subject to vibration and generate a continuous noise.
The noise level of such devices is dependent on the design and the localised flow velocities produced, and also on the response time, where applicable.
When the turbulent liquid flow is stable, it does not usually cause any significant noise. Cavitation is the most common cause of noise in the liquid flow.
Hydrodynamic noise can be reduced by affecting the intensity of cavitation. The best way to prevent cavitation is to intensify flow losses, which reduces the intensity of pressure recovery and increases the acoustically determined the differential pressure ratio of incipient cavitation.
Valves can be designed so as not to direct any cavitation jets at the valve trim; this helps to lower the effect of cavitation corrosion. For hydrodynamic noise reduction, cage type valve trims are used. It’s immediate aim is to eliminate or minimise cavitation.
Each orifice with special shape directs a jet of cavitating fluid that impacts with the jet that enters from the opposite hole in the centre or the cage. Therefore, a continuous cushion is formed which prevents the cavitation liquid from coming into contact with the metal surfaces and ensures that the collapse of the vapour bubbles takes place at the centre of the flow stream.
The above-shown cage consists of one or more concentric cylindrical sections referred to as stages. The number of stages required depends on the inlet pressure and the pressure drop. In operation, the liquid undergoes a portion of the total pressure drop in each stage of the cage. This prevents the liquid in any one stage or the cage from falling to or below its vapour pressure. Therefore, the formation of vapour bubbles and their subsequent collapse is eliminated.
Cage trim for butterfly valve:
With conventional butterfly valves, increasing differential pressure causes a high dynamic torque. thus jeopardising controllability at high opening angles and causing noise with gases and cavitation with liquids.
The non-symmetric pressure distribution pattern on both sides of the fin has been made symmetric with a partial flow obstacle downstream within the valve body. This design helps eliminate dynamic torque and, due to the more turbulent flow pattern, reduces recovery behavior.
The noise generated in the gas or vapor is called aerodynamic noise. Most of it occurs during the deceleration stage in the bottleneck process. The area where the noise is generated can extend from the hole to the downstream pipe. The pressure waves inside the pipe make the wall vibrate.
The noise attenuates very slowly in pipes filled with gas or steam. The sound pressure level of a gas control valve generally has a broadband frequency distribution. The maximum sound pressure levels are between 1000 and 4000Hz. There are a number of methods to decrease aerodynamic noise. Two, in particular, are effective:
- Reduction of pressure and velocity gradients generated during the throttling process:
- using, for example, multi-stage throttling and splitting the flow into several jets.
The mean flow velocity and its profile downstream of the valve have a particularly marked effect on the valve noise level. Splitting the flow into smaller parallel jets reduces noise.