27.14. CONTROL VALVE PROBLEMS |
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interference between the moving element and the stationary seat, providing information valuable for predicting the remaining service life of the valve before the next rebuild.
27.14.2Flashing
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:
Control valve
P1
P2
Pvc
Upstream Trim Downstream
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 pressure51 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.
Flashing is almost universally undesirable in control valves. The e ect 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 e ective 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.
51It 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.
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CHAPTER 27. CONTROL VALVES |
A photograph showing a severely eroded valve plug (from a cage-guided globe valve) reveals just how destructive flashing can be:
A characteristic e ect 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.
r
FL =
P1 − P2
P1 − Pvc
Where,
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)
27.14. CONTROL VALVE PROBLEMS |
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The following set of illustrations shows three di erent control valves exhibiting the same
permanent pressure drop (P1 − P2), but having di erent values of FL: |
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Valve #2 |
Valve #3 |
P1 |
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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.
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CHAPTER 27. CONTROL VALVES |
Compare these two styles of valve to see which will have lowest pressure recovery factor and therefore be most prone to flashing:
Butterfly valve
Sharp constriction
Globe valve |
Sharp constriction
(drops all the pressure at
a single point in the flow path) Multiple points of constriction (distributes the total pressure drop)
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 o ering 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.