When a fluid passes through the constrictive passageways of a control valve, its average velocity increases. This is predicted by the Law of Continuity, which states that the product of fluid density (ρ), cross-sectional area of flow (A), and average velocity (v) must remain constant for any flowstream:
ρ1A1v1 = ρ2A2v2
As fluid velocity increases through the constrictive passages of a control valve, the fluid molecules’ kinetic energy increases. In accordance with the Law of Energy Conservation, potential energy in the form of fluid pressure must decrease correspondingly. Thus, fluid pressure decreases within the constriction of a control valve’s trim as it throttles the flow, then increases (recovers) after leaving the constrictive passageways of the trim and entering the wider areas of the valve body:
If the fluid being throttled by the valve is a liquid (as opposed to a gas or vapor), and its absolute pressure ever falls below the vapor pressure (Note) of that substance, the liquid will begin to boil. This phenomenon, when it happens inside a control valve, is called flashing. As the graph shows, the point of lowest pressure inside the valve (called the vena contracta pressure, or Pvc) is the location where flashing will first occur, if it occurs at all.
Note : It should be noted that vapor pressure is a strong function of temperature. The warmer a liquid is, the more vapor pressure it will exhibit and thus the more prone it will be to flashing within a control valve.
Flashing is almost universally undesirable in control valves. The effect of boiling liquid at the point of maximum constriction is that flow through the valve becomes “choked” by the rapid expansion of liquid to vapor as it boils, degrading the valve’s flow capacity (i.e. decreasing the effective Cv). Flashing is also destructive to the valve trim, as boiling action propels tiny droplets of liquid at extremely high velocities past the plug and seat faces, eroding the metal over time.
A photograph showing a severely eroded valve plug (from a cage-guided globe valve) reveals just how destructive flashing can be:
A characteristic effect of flashing in a control valve is a “hissing” sound, reminiscent of what sand might sound like if it were flowing through the valve.
An important parameter predicting flashing in a control valve is the valve’s pressure recovery factor, based on a comparison of the valve’s total pressure drop from inlet to outlet versus the pressure drop from inlet to the point of minimum pressure within the valve.
FL = Pressure recovery factor (unitless)
P1 = Absolute fluid pressure upstream of the valve
P2 = Absolute fluid pressure downstream of the valve
Pvc = Absolute fluid pressure at the vena contracta (point of minimum fluid pressure within the valve)
The following set of illustrations shows three different control valves exhibiting the same permanent pressure drop (P1 − P2), but having different values of FL:
Valve #1 exhibits the greatest pressure recovery (i.e. the amount that fluid pressure increases from the minimum pressure at the vena contracta to the downstream pressure: P2 − Pvc) and the lowest FL value. It is also the valve most prone to flashing in liquid service, because the vena contracta pressure is so much lower (all other factors being equal) than in the other two valves. If any of these valves will experience flashing in liquid service, it would be valve #1.
Valve #3, by contrast, has very little pressure recovery, and a large FL value (nearly equal to 1). From the perspective of avoiding flashing, it is the best of the three valves to use for liquid service.
The style of valve (ball, butterfly, globe, etc.) is very influential on pressure recovery factor. The more convoluted the path for fluid within a control valve, the more opportunities that fluid will have to dissipate energy in turbulent motion, resulting in the greatest permanent pressure drop for the least amount of restriction at any single point in the flow’s path.
Compare these two styles of valve to see which will have lowest pressure recovery factor and therefore be most prone to flashing:
Clearly, the globe valve does a better job of evenly distributing pressure losses throughout the path of flow. By contrast, the butterfly valve can only drop pressure at the points of constriction between the disk and the valve body, because the rest of the valve body is a straight-through path for fluid offering little restriction at all. As a consequence, the butterfly valve experiences a much lower vena contracta pressure (i.e. greater pressure recovery, and a lower FL value) than the globe valve for any given amount of permanent pressure loss, making the butterfly valve more prone to flashing than the globe valve with all other factors being equal.