Cavitation Valve
If the speed through the valve is high enough, the pressure in the liquid may drop to a level where the fluid may start bubble or flash. The pressure recovers sufficiently and the bubbles collapse upon themselves.
control valve cavitation
Cavitation may be noisy but is usually of low intensity and low frequency. This situation is extremely destructive and may wear out the trim and body parts of the valve in short time.
* The Cavitation Number An introduction to and a definition of the Cavitation Number.
* Cavitation - an Introduction
Application Ratio
A common way to characterize potential cavitation conditions is the “applications ratio” (or “the incipient cavitation index”) and can be expressed as
AR = pi - po / (pi - pv) (1)
where
AR = Application Ratio
pi = inlet pressure, absolute
po = outlet pressure, absolute
pv = vapor pressure of the fluid, absolute
For application ratios above 1 - the fluid flashes. This is not the same as cavitation, but the closer the ratio is to 1, the higher the potential for cavitation.
Note! With an increasing fluid temperature the possibility for cavitation increases.
Example - Flashing Water
If we know the boiling point and the absolute pressure of a fluid (Steam Table with saturated steam properties) the minimum outlet pressure from a valve to avoid flashing can be calculated.
For an application ratio of one equation (1) can modified to
AR = 1 = pi - po / (pi - pv)
or transformed
po = pv
Using “Steam Table” with saturated steam properties we can conclude that
* for a water temperature of 17.51 oC and absolute inlet pressure of 1 bar - the minimum outlet pressure is 0.02 bar to avoid flashing
* for a water temperature of 81.35 oC and absolute inlet pressure of 1 bar - the minimum outlet pressure is 0.5 bar to avoid flashing
* For a water temperature of 99.63 oC and absolute inlet pressure of 1 bar - the minimum outlet pressure is 1 bar to avoid flashing
Note! Flashing is not the same as cavitation. Due to local conditions in a valve cavitation may start on much higher outlet pressures.
Multi Stage Control Valves
Cavitation can be avoided by using more than one control valve or more convenient - a multistage control valve.
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September 10th, 2007 at 11:13 pm
VAPOR PRESSURE
1. A vapor pressure analyzer to determine the vapor pressure of single or multi component liquids comprising of an inlet conduit, a filter, a heating/cooling means to condition the single or multi component liquid, an upstream aeration measurement means to establish an aeration reference of the liquid, a capillary tubing pressure reduction means to decrease the pressure of the liquid smoothly to its vapor pressure oriented in a horizontal position or a pressure reducing valve which does not create pressure recovery nor a vena contracta, such vena contracta resulting in some pressure recovery and erroneous data, a downstream aeration measurement means that detects the minimum change in aeration compared to the upstream aeration measurement resulting from the pressure having reached the vapor pressure of the lightest component of the sample, a pressure measurement means to measure the actual vapor pressure at the point where the liquid starts to flash or effervesce, a temperature measurement means at the same location as the pressure measurement means, instrumentation and control means to control the liquid flow and therefore the pressure drop across the pressure reduction means at the desired condition that will result in the vapor pressure occurring at the point where the pressure measurement means is located, a pump to increase the liquid pressure back to system pressure to return the liquid back to the system at a point downstream of an extraction point, instrumentation means to process the difference in aeration between the sample upstream and downstream of the pressure reduction means, and flow tubing interconnecting the above mentioned devices.
2. A vapor pressure analyzer according to claim 1 containing instrument and control means to control the flow of the liquid admitted to the analyzer such that the pressure drop through the pressure reduction means is the minimum required to flash the fraction of the mixture having the highest vapor pressure as indicated by the minimum change in aeration that can be attributed to initial flashing or formation of bubbles in the sample as indicated by the difference between the upstream and downstream aeration measurement means.
3. A vapor pressure analyzer according to claim 1 containing downstream aeration measurement means such that the vapor pressure is obtained when the downstream aeration value reaches some pre-determined set point aeration value at the given temperature without having to compare with the upstream aeration measurement.
4. A vapor pressure analyzer according to claim 1 containing a capillary tubing pressure reduction means or a pressure reducing valve such that the turbulence and liquid shear created as the liquid passes through the pressure reduction means promote bubble formation or flashing at the actual vapor pressure of the liquid.
5. A vapor pressure analyzer to determine the vapor pressure of single or multi component liquids comprising of an inlet conduit, a filter, a heating/cooling means to condition the single or multi component liquid, an upstream density measurement means to establish density reference of the liquid, a capillary tubing pressure reduction means to decrease the pressure of the liquid smoothly for pressure reduction and oriented in a horizontal position or a pressure reducing valve which does not create pressure recovery nor a vena contracta, such vena contracta resulting in some pressure recovery and erroneous data, a downstream density measurement means that detects the minimum change in density compared to the upstream density measurement resulting from the pressure having reached the vapor pressure of the lightest component of the sample, a pressure measurement means to measure the actual vapor pressure at the point where the liquid starts to flash or effervesce, a temperature measurement means at the same location as the pressure measurement means, instrumentation and control means to control the liquid flow and therefore the pressure drop across the pressure reduction means at the desired condition that will result in the vapor pressure occurring at the point where the pressure measurement means is located, a pump to increase the sample pressure back to system pressure to return the liquid back to the system at a point downstream of an extraction point, instrumentation means to process the difference in density between the sample upstream and downstream of the pressure reduction means, and flow tubing interconnecting the above mentioned devices.
6. A vapor pressure analyzer according to claim 5 containing instrument and control means to control the flow of the sample admitted to the analyzer such that the pressure drop through the pressure reduction means is the minimum required to flash the fraction of the mixture having the highest vapor pressure as indicated by the minimum change in density that can be attributed to initial flashing or formation of bubbles in the sample and not merely to change in density caused by expansion of liquid due to drop in pressure.
7. A vapor pressure analyzer according to claim 5 containing downstream density measurement means such that the vapor pressure is obtained when the downstream density value reaches some pre-determined set point density value at the given temperature without having to compare with the upstream density measurement.
8. A vapor pressure analyzer according to claim 5 containing a capillary tubing pressure reduction means or a pressure reducing valve such that the turbulence and liquid shear created as the sample passes through the pressure reduction means promote bubble formation or flashing at the actual vapor pressure of the liquid.