Mud Siren Torque and Erosion Analysis - Measurement While Drilling Signal Analysis, Optimization and Design - page 181

hydraulically developed by the valve as the mud flows therethrough will be a
desired function of the flowrate and/or density of said mud.”
7.1.3 Stable-opened designs.
The above solution methods are “brute force” in nature because they address the
symptoms and not the cause of the root problem. The primary reason for stable-
closed behavior is an aerodynamic one: the rotor will naturally close by itself
(when turning torque is not supplied through the shaft) even when the fluid is
clean and free of debris. Of course, the presence of debris worsens the problem.
In other words, the torque acting on the rotor is such that it will always move to
the most stable position, which happens to be closed. In airplane design,
commercial jetliners are engineered so that, despite wind gusts and turbulence,
aircraft always return to a stable horizontal cruise configuration. By analogy, a
safe stable-opened position is the objective of good mud siren re-design. But
how is this achieved?
A completely aerodynamic solution to the stable-closed problem was
developed and reported by Chin and Trevino (1988). In this re-design, the stator
is located upstream while the rotor is placed downstream. As illustrated in
Figure 7.2, the rotor also contains several important physical features.
Importantly, (1) its sides must be “slightly” tapered, (2) the azimuthal width of
its top should be “a bit” less than that of the stator bottom, and (3) rotor-stator
gap distance should not be “too small.” While stable-open behavior is desirable,
even a stable-closed characteristic is tolerable provided opening hydraulic
torques are small. Torques needed to open closed valves for sirens of the type in
Figure 7.2 are much smaller than those for Figure 7.1 for the same flow rate.
Figure 7.2. New 1990s “stable-opened” improvement.