The requirement for noise control is a function of legislation to protect our wellbeing and to prevent physical damage to control valves and piping. Noise prediction is a well defined science. Actual results will be within 5 dBA of predicted levels. Prediction is based upon contributions for:
1. Pressure drop 2.Flow rate 3.P/P1 and trim style 4.Piping and insulation 5.Downstream pressure
Noise reduction is accomplished in two general ways: 1. Source treatment, which acts upon the amount of noise generated.
Noise reduction is accomplished in two general ways: 1. Source treatment, which acts upon the amount of noise generated. 2. Path treatment, which blocks transmission on noise to the environment.
There are two common source treatments:
1. Valve noise trim is based on principles of dividing the flow to create many small noise sources which combine to a lower level than a single large flow noise. Diffusers used with control valves share pressure drop creating two lower noise sources which again combine to an overall lower level. 2. Path treatment involves use of insulation or absorptive devices to lower the sound level which reaches observers. Hydrodynamic noise from liquid flow streams can mainly be traced to cavitation. In this case, damage from the cavitation is of more concern than the noise. Appropriate treatment of the cavitation source should be initiated through staging the pressure drop. Two-phase, or pure flashing, applications do not create noise problems, and there is no technically appropriate two-phase noise prediction method
Noise has always been present in control valves.It is a natural side effect of the turbulence and energy absorption inherent in control valves. This chapter will address how noise is created, why it can be a problem, and methods to attenuate noise created in control valves.
The major problem with industrial noise is its affect on humans. Companies usually build town border stations on sites remote from residential developments. Isolation, however, is not always possible, and noise prevention is a must.
The U.S. Occupational Safety and Health Act (OSHA) establishes maximum permissible noise levels for all industries whose business affects interstate commerce. These standards relate allowable noise levels to the permissible exposure time. Notice in table 6-1 that the maximum permissible levels depend upon the duration of exposure. For example, the maximum sound level a person should be exposed to for an eight hour day is 90 dBA. These maximum sound levels have become the accepted noise exposure standard for most regulatory agencies. Thus, they have become the standard by which much noise generating equipment has been specified and measured.
Frequent maintenance needs; for example, a user will say that ‘we have to rebuild this valve every shutdown.’ The maintenance cycle has been done so many times it has become the norm.
Valves that work perfectly after rebuild maintenance but in a relatively short period become a headache again, performing poorly and unreliably.
Cause concerns during startup and shutdown as to whether they will get through the transient. However once the system is at full load the problems go away.
Valves for which the control system has to be put in manual operation because trips occur when the system is in automatic.
Valve bypass loops are put into service during startup and shutdown transients to allow stable operation of the main valve.
Extensive perturbations to the system when transferring from a parallel startup valve. The upset can cause trips or lifting of relief and safety valves. The corrective procedures become the standard to work around the problem.
These problem valves exist because of:
1 Misapplication of the valve design selected. 2 Over or under sizing the valve for the process need. 3 Poorly specified requirements to the supplier
Analyzing noise, in the context of piping and control valves, requires consideration of its origin. This indicates how the noise will propagate.Generally speaking, noise originates from either a line source or a point source.A sound level meter is used to determine sound pressure levels. Readings for line source noise levels are normally measured one meter from the pipe’s surface and at a point one meter downstream from the valve outlet. Measurements should be made in an unobstructed free field area with no sound reflecting surfaces nearby. Line source noise levels are radiated from the piping in the form of an imaginary cylinder, the pipe centerline as the axis. As you move away from the pipeline, the sound pressure level decreases inversely to the changes in surface area of the imaginary cylinder. The following equation defines the sound pressure level (LpA) at distances other than one meter from the pipeline surface:
What this equation tells us is that the sound pressure level decreases dramatically as the distance from the pipeline increases. Keep in mind that this equation determines the noise level radiated only by the pipeline. Other noise sources could combine with the pipeline noise source to produce greater overall sound pressure level .The other type of noise source needed to be discussed is point source. Point source noise measurements are taken at a three meter distance in the horizontal plane through the source. Vent applications are typical examples of point source noise. Point source noise levels are radiated in the form of an imaginary sphere with the source at the center of the sphere. As you move away from a point source, the sound pressure level decreases inversely in proportion to the changes in the surface area of the imaginary sphere. The equation that defines the sound pressure level at distances other than three meters from the point source and below a horizontal plane through the point source is:
This procedure determines the noise level radiated only by the point source. Other noise sources could combine with the point source noise to produce a greater overall sound pressure level .
Noise has always been present in control valves.It is a natural side effect of the turbulence and energy absorption inherent in control valves. This chapter will address how noise is created, why it can be a problem, and methods to attenuate noise created in control valves. The major problem with industrial noise is its affect on humans. Companies usually build town border stations on sites remote from residential developments. Isolation, however, is not always possible, and noise prevention is a must. The U.S. Occupational Safety and Health Act (OSHA) establishes maximum permissible noise levels for all industries whose business affects interstate commerce. These standards relate allowable noise levels to the permissible exposure time. Notice in table 6-1 that the maximum permissible levels depend upon the duration of exposure. For example, the maximum sound level a person should be exposed to for an eight hour day is 90 dBA. These maximum sound levels have become the accepted noise exposure standard for most regulatory agencies. Thus, they have become the standard by which much noise generating equipment has been specified and measured .
Decibels (dB) are a measure to give an indication of loudness. The “A” added to the term indicates the correction accounting for the response of the human ear. The sensitivity of our ears to sound varies at different frequencies. Applying this “A” correction is called weighting, and the corrected noise level is given in dBA. The A-weighting factor at any frequency is determined by how loud noise sounds to the human ear at that particular frequency compared to the apparent loudness of sound at 1000 hertz.At 1000 hertz the A-weighting factor is zero, so if the sound pressure level is 105 dB, we say it sounds like 105 dB. On the other hand, if we listen to a sound at 200 hertz with a sound pressure level of 115 dB, it sounds more like 105 dB. Therefore, we say that the A-weighted loudness of the noise with a sound pressure level of 115 dB is 105 dBA. Essentially, if two or more sounds with different sound pressure levels and frequencies sound like the same loudness, they have the same dBA, regardless of what their individual, unweighted sound pressure levels may be. The effect of A-weighting on control valve noise depends upon the flowing medium since each develops its own characteristic spectrum. Noise levels for hydrodynamic noise, or liquid flow noise, have appreciable energy at frequencies below 600 hertz. When the levels are A-weighted, it makes the low frequency terms more meaningful and the government standards somewhat more difficult to meet. On the other hand, aerodynamic noise levels produced by steam or gas flow are the same in either dB or dBA. This is because aerodynamic noise occurs primarily in the 1000 to 8000 hertz frequency range. The human ear has a fairly flat response in the frequency range of 600 to 10,000 hertz, and the A-weighting factor is essentially zero in this range. Thus, there is negligible difference between the dB and dBA ratings .
Velocity distribution is affected by several factors, including pipe surface roughness, turbulence level, flow area changes. The shear stress of the fluid creates the velocity distribution in the flow. The flow velocity very close to the pipe wall is zero. This results in a significant change in velocity across the pipe cross section. The laminar and turbulent velocity distributions are significantly different as a result of different shear stresses in the flow. In turbulent flow, the velocity gradient is greater near to the pipe wall than it is in laminar flow. In laminar flow, the center line velocity is higher than in turbulent flow of equal mean velocity .
With a smoothly shaped outlet from tank to pipe, the velocity profile is a function of distance. The fully developed profile is constant, if it is not disturbed. In a pipe elbow, the fully developed velocity profile is disturbed by inertia forces in the fluid .
A more drastic change in velocity profile can be caused by an orifice or a valve that create a high velocity jet into a flow stream. On the downstream side of an orifice there are strong vortices which cause flow pressure losses. High velocity jets are also accompanied by the noise, especially in the case of compressible flow. The velocity profile after the valve is dependent on the valve type and design, and on valve travel. A fully open ball valve behaves as an extension to the pipe and thus does not disturb the velocity profile in the way that, for instance, butterfly and globe valves do
The flow in conventional control valve installations is almost always turbulent. Laminar flow can occur with very viscous fluids such as lube oil and with very small flow velocities. Laminar flow can be explained as a microscopic viscous interaction between several layers of fluid. In laminar flow the fluid particles move in parallel paths or streamlines. A particle has only axial velocity along a streamline. Fluid layers slide relative to each other, and the streamline of an individual particle can be predicted . If the velocity is increased above a certain limit the laminar flow field is disturbed and fluid particles start to move in erratic paths. When this happens, the flow becomes turbulent. Transition from laminar to turbulent flow can be predicted by a single parameter,the Reynolds number (Re), as defined in equation : Re = flow velocity * nominal diameter / kinematic viscosity Above a critical value of the Reynolds number the flow goes through a transition to become turbulent. The critical value of Reynolds number is around 3000 for flow in a straight pipe. When the flow is turbulent, part of the flow energy is used to create eddies, which cause increasing pressure losses. Turbulent flow can be described as irregular secondary motion of fluid particles. Secondary motion does not correspond to the principal direction of the flow. The flow path of a single flow particle is irregular.
To achieve good rangeability and linearity of the valve flow to stem position (installed linearity), the valve trim must be characterized. Thus, at the low load conditions, a valve capacity versus position curve (characterization) will look like that shown in Figure 3. This characterization not only provides better rangeability and control, but also assures that the valve closure member does not operate near the seat and thus minimizes damage to the critical seating surfaces due to excessive fluid velocities. Depending upon the low load pressure drop, other measures may be necessary to limit high fluid velocity erosion such as the use of multi-stage trim designs. These designs become a consideration when pressure ratios, p1/p2 across the valve exceed three or pressure drop across the valve could result in cavitation or excessive noise. Cavitation and excessive noise can occur at pressure drops as low as 30 psi (0.2 MPa) for some fluid conditions. The characterization and trim erosion considerations are very important because they contribute significantly to the most vital function of this valve and that is accuracy of feed control.