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By J. Paul Tullis and Blake P. Tullis
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.
Cavitationís 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 systemís 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
Itís possible to predict cavitation intensity if you know the valveís flow and pressure conditions. The analysis requires condensing the systemís 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 valveís 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[-] Ė 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.
Itís 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 ďno cavitationĒ 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. Itís 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, itís the limit thatís 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 fluidís vapor pressure. This represents the maximum flow condition through a valve for a given upstream pressure and valve opening. Itís 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 doesnít increase the flow rate. The noise, vibration and damage occur primarily at the location where cavity collapse occurs.
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