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pressure relief valves that unload the locked rotor, strong permanent magnets
that bias special steel assemblies to open positions (thus compromising direction
and inclination measurements and requiring the use of nonmagnetic drill
collars), and so on. It can be shown that “stable closed” tendencies are a natural
aerodynamic consequence of upstream rotor configurations - the rotors tend to
close even in clean water. Numerous unsuccessful tests addressing this problem
were performed in the 1970s: operational failures associated with jamming
valves were catastrophic.
Figure 1.4b. Streamline traces for erosion analysis.
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