Typically when noise is present in liquid service, so is cavitation, and roughly to the same degree. Cavitation bubble implosions are the main source of disturbing noise; noise level is directly related to cavitation intensity. In the early stages of cavitation, the noise can sound like sand going through the valve. When the pressure difference across the valve rises, the intensity of the cavitation increases, as does the severity of the noise.
Unfortunately, noise not only indicates that cavitation is underway but also that premature valve failure is possible if not likely. In this situation, the horse is already out of the barn; the plant must suffer the consequences of having valves that weren't adequately sized for the service.
Fortunately, manual calculations or, better still, expert sizing software can accurately predict cavitation noise. These noise predictions are a good indicator of potential mechanical cavitation damage. If there's a substantial amount of noise, corrosive damage also is likely. Noise alone can't predict that damage, though, because it also depends upon the medium and the valve material.
Inaccurately specified flow conditions often result in the failure to size valves and line components appropriately to ameliorate cavitation. It's common practice to add safety factors to flow conditions, sometimes resulting in specified conditions that don't resemble the actual ones in any way. When this is the case, no software can help. In fact, when faced with cavitation, the very first thing to do is to compare specified to actual conditions.
Predicting cavitation damage is complicated because it depends upon many factors (e.g., pressure drop, flow medium, valve type and materials). The onset of cavitation is called incipient or partial cavitation, and shouldn't be confused with the start of damage. Incipient cavitation can be used to predict cavitation noise but not damage.
The pressure drop needed to mechanically damage valves or piping depends upon the valve type, size and material. Various studies suggest that the lower the recovery of the valve, the closer the terminal pressure drop can be approached without cavitation damage. However, if the pressure differential is very small and the time the valve is exposed to the cavitation is short, there may not be a problem.
ABATING NOISE AND CAVITATION
Because the most significant source of control valve noise in liquid applications is cavitation, it makes sense to deal with abatement of noise and cavitation together.
There are two basic approaches for dealing with control valve noise and cavitation:
1. Path treatment. This focuses on dampening the noise generated. It reduces the noise radiated by the piping system but doesn't eliminate cavitation inside the valve and adjacent piping.
2. Source treatment. This involves modifying the valve and its trim to decrease the cavitation intensity or prevent the existence of cavitation. Source treatment, whenever possible and feasible, is the preferred approach for addressing cavitation and the associated hydrodynamic noise because only it can avoid excessive generation of cavitation bubbles across a wide flow range.
Five different methods can be used (sometimes in combination) for source treatment of hydrodynamic noise and cavitation abatement:
Velocity control. Regulating the fluid velocity, which is equivalent to controlling the pressure in the valve internals (trim), is an effective means of avoiding cavitation. The aim of this approach is to get the lowest pressure in the valve trim above the vapor pressure for the liquid in question. This is accomplished by selecting valve trim with the optimal recovery coefficient (FL). For example, simply switching from a butterfly to a ball valve would alter the coefficient. If standard trim doesn't suffice, then we can select multistage trim that divides the valve pressure drop into several stages. There are two basic methods for pressure-drop staging:
• identical pressure drop across each stage; or
• identical minimum pressure in each stage.
The first approach requires a constant-flow-area trim. Due to the equal area, the flow velocity in each stage is constant and the pressure drop is equal. The second approach involves an expanding-area trim, in which the pressure drop in the first stages is very high but a higher minimum pressure is maintained in the last stages, thereby better avoiding cavitation within the trim.
Expanding-area trim is recommended when extensive reduction of cavitation is necessary. However, the velocity in the first stages of this trim will be quite high and may create erosion problems. In practice, valve designers usually combine the two methods to arrive at an optimum solution.
Acoustic noise and bubble size control. The division of flow into multiple small streams actually has two effects: it acts to create a pressure stage and also reduces the size of vapor bubbles in the fluid. Smaller bubbles cause higher-frequency noise fields in the fluid and reduce the external vibration intensity of the pipe as well as the mechanical and erosion effects of cavitation. The concept of using small jets to decrease control valve noise is based on the principle that the jet produces a characteristic frequency above the pipe ring frequency, thereby reducing the sound reradiated by the pipe.