The relationship between the pressure recovery coefficient FL and cavitation
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By consulting relevant materials, the greater the pressure recovery coefficient FL (such as in control valve cage valves, ranging from 0.8 to 0.9), the better the resistance to cavitation. The lower the pressure recovery coefficient FL (for ball valves, whose valve flow structure is simple), the worse their resistance to cavitation. However, according to the experimental data, the pressure recovery coefficient FL of the ball valve decreases as the opening angle increases, with the FL value dropping from approximately 0.9 to 0.55. In practice, however, the smaller the opening degree of the ball valve, the more likely flash cavitation will occur. However, the smaller the opening degree of the ball valve, the larger FL is. The larger the FL, isn’t it less likely to occur flash vapor erosion? With the ball valve fully open, the FL value is around 0.55, so flash vapor erosion is most likely to occur. What is the actual performance of the ball valve when it is fully open? For regulators, the higher the FL value, the greater the pressure difference before and after the valve, and the better the effect in preventing flow blockage. But I didn’t understand ball valves.
This post was last edited by HEJIYUER on 2020-4-25 at 23:39. FL is used to determine blocked flow and cannot fully reflect the degree of cavitation. Before the flow is blocked, cavitation has already occurred (initial cavitation), and it intensifies as P2 decreases. The end point of cavitation is complete flow blockage (the FL point); further reduction in P2 leads to flashing. For single-seat valves, the FL method is traditionally used to avoid choked flow, as the region of severe cavitation is very close to the choked flow region; avoiding choked flow can be \"considered\" as escaping from the region of severe cavitation. However, this approach does not work for butterfly valves/ball valves (including V-ball valves)/tilt rotators, because the cavitation development area for rotary valves is quite wide. For example, for a butterfly valve, FL=0.6; yet its initial cavitation differential pressure is 0.2 x (P1-Pv), and the region where severe cavitation occurs starts at x=0.4. The cavitation index XF under operating conditions, defined as XF = (P1-P2) / (P1-Pv), represents the depth of cavitation required for operation; when the flow rate is low, XF is high, indicating that the cavitation pits are deep. The higher the cavitation index of the valve itself, the better its resistance to cavitation; for conventional single-seat valves, this value is 0.7, while for butterfly valves it ranges from 0.25 to 0.3. Therefore, the reason why there is no need for a valve when approaching saturated liquid is that while you can avoid blocked flow, you cannot avoid cavitation, and valves are unable to handle the cavitation index under operating conditions. FL/XF/XFZ trend with flow load variation