The noise generated by control valves is a result of the turbulent flow of fluid through the valve and the pressure drop across the valve. As the fluid passes through the valve, it experiences sudden changes in direction and velocity, which can create turbulence and vortices that generate noise.
When the pressure of the liquid flowing through the valve drops below its vapor pressure, bubbles can form and collapse rapidly, creating shock waves that generate noise. This formation and collapse of vapor bubbles in a flowing liquid is called cavitation.
The severity of cavitation noise depends on several factors, including the pressure drop across the valve, the fluid properties (such as density and viscosity), and the valve design. Some common methods to reduce control valve cavitation noise include:
Increasing the pressure drop across the valve: This can be done by installing a pressure-reducing valve upstream of the control valve. By reducing the pressure upstream of the valve, the pressure drop across the valve is reduced, which can reduce the likelihood of cavitation.
- Increasing the valve size: A larger valve size can reduce the velocity of the fluid flowing through the valve, which can reduce the likelihood of cavitation.
- Using a different valve trim: The valve trim is the internal components of the valve that come into contact with the fluid.
- Using a different trim design, such as a multistage trim or a cage-guided trim, can reduce the likelihood of cavitation.
- Using a different valve material: Some valve materials, such as hardened steel, can be more resistant to cavitation than others.
- Adding a noise-reducing insert: A noise-reducing insert, such as a diffuser or an orifice plate, can be installed downstream of the valve to reduce the noise generated by cavitation.
It is important to note that while these methods can reduce the severity of cavitation noise, they may not eliminate it entirely. In some cases, additional noise mitigation measures may be necessary, such as installing a sound barrier or using ear protection.
The noise generated by control valves can be problematic for a number of reasons. For example, excessive noise can be a nuisance for workers or occupants in a facility, and it can even be a safety hazard if it interferes with communication or causes distraction. Noise can also be damaging to equipment and structures if it causes vibrations or resonance.
The noise level of control valves can be calculated using several methods, including empirical equations, computational fluid dynamics (CFD) simulations, and experimental measurements. Here are some common methods:
- Empirical Equations: Empirical equations are mathematical formulas that relate the noise level of a control valve to its flow rate and pressure drop. One such equation is the Masoneilan-Kates equation, which is commonly used in the industry. This equation is:
Lp = K1 + K2 * log10(Q) + K3 * log10(P1-P2) + K4 * log10(Q) * log10(P1-P2)
where Lp is the sound pressure level in decibels (dB), Q is the volumetric flow rate in cubic meters per hour (m3/h), P1 is the upstream pressure in kilopascals (kPa), and P2 is the downstream pressure in kPa. K1, K2, K3, and K4 are constants that depend on the valve size, type, and characteristics.
- Computational Fluid Dynamics (CFD) Simulations: CFD simulations use computer software to model the flow of fluids through a control valve and predict the resulting noise level. These simulations can provide detailed information about the flow patterns and turbulence that cause noise. However, CFD simulations require significant computational resources and expertise to perform.
- Experimental Measurements: Experimental measurements involve installing a control valve in a test rig and measuring the noise level using a sound level meter. This method provides direct, accurate measurements of the noise level but may be time-consuming and expensive.
Overall, the choice of method depends on the accuracy required, the resources available, and the expertise of the user.