Control Valve Noise and Cavitation

Introduction

Sound is generated from the movement of fluid through a valve. It is only when the sound in undesirable that it is termed ‘noise’. If the noise exceeds certain levels then it can become dangerous to personnel. Noise is also a good diagnostic tool. As sound or noise is generated by friction, excessive noise indicates the possible damage occurring within a valve. The damage can be caused by the friction itself or vibration.

There are three main sources of noise:

Mechanical vibration
– Hydrodynamic noise
– Aerodynamic noise

Mechanical Vibration

Mechanical vibration is a good indication of the deterioration of valve components. Because the noise generated is usually low in intensity and frequency, it is generally not a safety problem for personnel. Vibration is more of a problem with stem valves compared with cage valves. Cage valves have a larger supporting area and are therefore less likely to cause vibration problems.

Hydrodynamic Noise

Hydrodynamic noise is produced in liquid flows. When the fluid passes through a restriction and a pressure change occurs it is possible that the fluid forms vapour bubbles. This is called flashing. Cavitation is also a problem, where the bubbles form but then collapse. The noise generated is generally not dangerous to personnel, but is a good indication
of potential damage to trim components.

Aerodynamic Noise

Aerodynamic noise is generated by the turbulence of gases and is a main source of noise. The noise levels generated can be dangerous to personnel, and are dependent on the amount of flow and the pressure drop.

Cavitation and Flashing

Flashing

Flashing is the first stage of cavitation. However, it is possible for flashing to occur by itself without cavitation occurring.
Flashing occurs in liquid flows when some of the liquid changes permanently into vapour. This is brought on by a reduction in pressure forcing the liquid to change to the gaseous state. The reduction in pressure is caused by the restriction in the flowstream generating a higher flow rate through the restriction and therefore a reduction in pressure.
The two main problems cause with flashing are:

– Erosion
– Reduced capacity

Erosion

When flashing occurs, the flow from the outlet of the valve is composed of liquid and vapour. With increased flashing, the vapour carries the liquid. As the velocity of the flowstream is increased, the liquid acts like solid particles as it strikes the internal parts of the valve. The velocity of the outlet flow can be reduced by increasing the size of the valve outlet which would reduce the damage. Options of using hardened materials are another solution. Angle valves are suitable for this application as the flashing occurs further downstream away from the trim and valve assembly.

Reduced Capacity

When the flowstream partly changes to a vapour, as in the case of flashing, the space that it occupies is increased. Because of the reduced available area, the capacity for the valve to handle larger flows is limited. Choked flow is the term used when the flow capacity is limited in this way

Cavitation

Cavitation is the same as flashing except that the pressure is recovered in the outlet flowstream such that the vapour is returned to a liquid. The critical pressure is the vapour pressure of the fluid. Flashing occurs just downstream of the valve trim when the pressure drops below the vapour pressure, and then the bubbles collapse when the pressure recovers above the vapour pressure. When the bubbles collapse, they send severe shock waves into the flow stream. The main concern with cavitation, is the damage to the trim and body of the valve. This is primarily caused by the collapsing of the bubbles. Depending on the extent of the cavitation developed, its effects can range from a
mild hissing sound with little or no equipment damage to a highly noisy installation causing severe physical damage to the valve and downstream piping Severe cavitation is noisy and can sound as if gravel were flowing through the valve.
The noise produced is not a major concern from a personal safety point of view, as it is usually low in frequency and intensity and as such does not pose a problem to personnel.

Also read 

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How to choose a control valve? Conditions affecting control valve selection

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