When the pressure drop is too large or the downstream pressure is too low, a valve can cavitate, a phenomenon that produces noise, vibrations and valve or pipe damage, not to mention headaches for plant operators and maintenance personnel. There is no need to tolerate this nuisance when you can easily predict the intensity of cavitation and reduce or eliminate its effects.
Cavitation is a pipeline phenomenon that forms vapor cavities, which then go unstable and collapse violently in low-pressure, turbulent, flow-separation regions inside valves, elbows, pipe expansions and other fluid handling hardware. The energy release associated with the nearly instantaneous collapse manifests itself as noise, vibration and metal being ripped from the inside surfaces of cavitating devices or downstream pipe. Cavitation also restricts the maximum valve flow rate for a given upstream pressure.
Cavitations intensity and its effect on hardware can vary from insignificant to devastating. In its least violent form, cavitation produces only a light crackling sound, about the same intensity as popcorn popping. This harms neither the valve nor the system. At more advanced stages, however, cavitation noise becomes objectionable. For certain valve types, cavitation can sound like gravel rumbling through the pipeline. The noise intensity can exceed 100 db, a level that constitutes a risk of hearing damage.
|Globe valve cavitation damage (Click to enlarge)|
In its most advanced form, cavitation limits the systems maximum flow capacity, a situation referred to as choked flow or super-cavitation. At this condition, a large vapor cavity extends several pipe diameters immediately downstream from the valve. Extremely high noise levels, severe vibrations and material damage usually occur at the first significant obstruction, such as an elbow, tee or flowmeter. The exact location of the cavitation noise sometimes can create confusion. One might not suspect the valve is cavitating because the noise, vibrations and damage occur some distance downstream.
Quantify the problem
Its possible to predict cavitation intensity if you know the valves flow and pressure conditions. The analysis requires condensing the systems operating conditions into a single number -- the cavitation index -- that you compare to experimentally determined cavitation limits. For some valves, experimental data are available on four levels of cavitation, each representing a different potential impact on a valve and its system. If cavitation is excessive, specific methods can be used to reduce or eliminate it.
Analyzing valve cavitation requires a parameter to quantify the cavitation potential. Researchers have developed a variety of cavitation indices. One such index is SIGMA.
A valves potential for cavitation depends on the downstream pressure (P[-]d[-]), the barometric pressure (P[-]b[-]), the absolute vapor pressure (P[-]v[-]) and the pressure differential across the valve (DELTA P). The sigma cavitation index is defined as:
SIGMA = (P[-]d[-] + P[-]b[-] P[-]v[-])/ DELTA P
Cavitation is less likely to occur at larger values of SIGMA. For example, increasing the pressure drop across a valve or reducing the downstream pressure reduces the value of SIGMA and thus increases the likelihood or severity of cavitation.
Its usually not difficult to determine whether a valve is cavitating. One merely has to listen. However, to determine if the cavitation intensity is high enough to cause damage requires quantifying the intensity and comparing it with available experimental cavitation reference data for the valve of interest. Cavitation intensity can be quantified relative to four levels.
Incipient cavitation refers to the onset of audible, intermittent cavitation. At this lower limit, cavitation intensity is slight. The operating conditions that foster incipient cavitation are conservative and seldom used for design purposes.
Critical cavitation, the next stage, describes the condition when the cavitation noise becomes continuous. The noise intensity is often hard to detect above the background flow noise. Critical cavitation causes no adverse effects and commonly defines the no cavitation condition. This level is referred to as critical because cavitation intensity increases rapidly with any further reduction in SIGMA.
Incipient damage refers to the conditions under which cavitation begins to destroy the valve. Its usually accompanied by loud noise and heavy vibration. The potential for material loss increases exponentially as SIGMA drops below the value that initiates incipient damage. Consequently, this is the upper limit for safe operation with most valves. Unfortunately, its the limit thats most difficult to determine, and experimental data are available only for a few valves.
Choking cavitation refers to a flow condition in which the mean pressure immediately downstream from the valve is the fluids vapor pressure. This represents the maximum flow condition through a valve for a given upstream pressure and valve opening. Its a condition that damages both valve and piping. Choking cavitation is an interesting and complex operating condition. Even though the pressure at the valve outlet is at vapor pressure, the downstream system pressure remains greater. Reducing the downstream pressure increases the length of the vapor cavity, but doesnt increase the flow rate. The noise, vibration and damage occur primarily at the location where cavity collapse occurs.
Typical flow-versus-differential pressure relationships (C[-]d[-], C[-]v[-] and the like) arent valid once the valve begins to choke because increasing DELTA P doesnt increase flow. The only way to increase the flow rate through a choking valve is by increasing the upstream pressure. Choking cavitation may be an acceptable design point for pressure relief valves because their operating cycle is limited.
The values of SIGMA associated with incipient, critical, incipient damage and choking, referred to here as the reference sigma data, vary with valve opening and are determined experimentally. The reference sigma data are typically presented as a function of the valve discharge coefficient, C[-]d[-].
C[-]d[-] = V/sqrt(2 DELTA P/RHO + V^2)
where RHO is the fluid density and V is the average flow velocity at the valve inlet. C[-]d[-] is dimensionless and independent of valve size. Other discharge coefficients, such as C[-]v[-], which are commonly used in the water works industry, arent dimensionless and vary with valve size. For this reason, C[-]d[-] is used to quantify cavitation and make comparisons between valves.
Do the numbers
Evaluating the cavitation intensity for a given valve opening and flow rate proceeds as follows. Plug the valve opening and system flow conditions into Equations 1 and 2 to determine the system sigma and discharge coefficient. Then, identify the cavitation intensity by comparing the system sigma value to the SIGMA values corresponding to the four cavitation limits. The examples below are simplified because of scale effect adjustments that must be made to experimental data to account for differences in valve size and operating pressure. See Tullis (1989) and Tullis (1993) for more details on scaling cavitation data.
Figures 2 and 3 present experimental C[-]d[-] and SIGMA data for a 6-in. butterfly valve. These data will be used to demonstrate cavitation analysis. Obtain similar data for a specific valve from the valve manufacturer or the literature.
Assume that a line-size 6-in. butterfly valve is cavitating. System flow conditions include a valve opening of 63%, which corresponds to C[-]d[-] = 0.43. The flow rate is 9.36 cubic feet per second (cfs), P[-]u[-] = 60 psi upstream, P[-]b[-] = 12.65 psi, P[-]v[-] = 0.25 psi and P[-]d[-] = 33 psi.
Using Equation 1, SIGMA = (33 + 12.65 - 0.25)/(60 - 33) = 1.68.
The intersection of C[-]d[-] = 0.43 and SIGMA = 1.68 in Figure 3 falls between incipient damage (SIGMA[-]id[-] = 2.4) and choking (SIGMA[-]ch[-] = 1.2). This suggests that cavitation is damaging the valve.
Tame the beast
If a valve is cavitating excessively, you have several options to control or eliminate the problem. The first is to reduce the cavitation intensity by modifying the system operating conditions. This requires reducing the flow, reducing the pressure drop or increasing the downstream pressure. If system conditions cant be altered, consider other options. A common solution is simply to tolerate the cavitation and replace the valve when it no longer performs its intended function. Better options include installing a valve having better cavitation performance, using a valve with hardened parts or installing a downstream device to increase outlet pressure and decrease pressure drop across the valve. In some cases, injecting air into the cavitation region can suppress cavitation damage.
For example, cavitation intensity can be reduced below the damaging level by installing a second 6-in. butterfly valve downstream from the existing valve to divide the total pressure drop between two valves. For valves in series, the downstream valve can provide about 1/3 of the total pressure drop. For the preceding example, this means the downstream valve can provide a 9-psi drop and the upstream valve 18 psi.
Calculate the discharge coefficient and system sigma for each valve using the corresponding downstream pressure and associated pressure drop. Critical sigma values (SIGMA[-]cr[-]) and incipient damage sigma values (SIGMA[-]id[-]) for the upstream valve and downstream valve are scaled from Figure 3. The results are displayed in Table 1.
The system sigma values for both valves now fall between critical and incipient damage. Consequently, the valves will produce cavitation noise but no damage will occur. This would provide a long-term solution.
Another way to suppress cavitation is by using a valve with better cavitation characteristics. A number of styles of cavitation control valves are available. Three options are globe valves with cavitation trim, sleeve valves and stack valves. Each uses a combination of small jets discharging into sudden expansions or several energy dissipation stages in series. Additional information on cavitation control valves is presented in Tullis (1989), (1993) and (2005).
Cavitation intensity can be predicted and, in most cases, reduced to an acceptable level through reasonable means. Cavitation doesnt have to be tolerated.
J. Paul Tullis is President of Tullis Engineering Consultants and Blake P. Tullis is Assistant Professor of Civil and Environmental Engineering, Utah State University.