Setting the runner centerline too high above tailwater ($Z_s > Z_{s,max}$) reduces local static pressure below the vapor pressure of water ($H_{vap}$). Cavitation bubbles form along the suction side of runner blade trailing edges and collapse violently against stainless steel surfaces with micro-jet pressures exceeding 1,500 MPa. Within months, cavitation tears deep sponges out of the blades, creating severe rotor dynamic unbalance, destroyed shaft seals, and forced powerhouse shutdowns. Always ensure $sigma_{plant} ge 1.20 imes sigma_{crit}$.

At partial wicket gate openings, the water discharges from the runner with excessive tangential swirl. The swirl collapses into a precessing, corkscrew-shaped helical vapor core (the Francis vortex rope) inside the draft tube. This vortex rope precesses at 0.25 to 0.35 times runner RPM, generating massive low-frequency hydraulic pressure surges that shake the entire powerhouse foundation, cause severe megawatt power swings on the grid, and crack draft tube liner anchor bolts. Avoid continuous operation in the rough zone or install compressed air admission into the runner cone.

If full electrical load drops instantly (generator breaker trip) and governor wicket gates fail to close fast enough, water accelerates the uncoupled rotor to its runaway speed ($N_{runaway} approx 1.70$ to $2.10 imes N_{rated}$). Centrifugal stresses on the runner crown, band, and generator rotor poles quadruple ($Stress propto N^2$). If the rotor assembly and generator pole dovetails are not certified for 100% runaway overspeed, catastrophic mechanical disintegration will obliterate the powerhouse.

Closing wicket gates too rapidly following a load rejection transforms the kinetic energy of the high-velocity penstock water column into an immense Joukowsky water-hammer shockwave: $Delta H = c cdot Delta v / g$. If penstock closure time $T_{close} < 2 L / c$, overpressure can exceed 150% to 200% of nominal static head, rupturing the penstock or exploding the cast steel spiral case. Always tune governor closing laws with two-speed cushions or install pressure relief valves (PRVs).

In Himalayan or Andean glacial runoff rivers, water carries high concentrations of hard quartz sand particles (Mohs hardness 7). High-velocity water accelerated through the wicket gate clearances (up to 40-60 m/s) severely erodes top/bottom facing plates and guide vane seal lips. The enlarged gap allows water to jet through even when gates are nominally closed, preventing machine shutdown and causing continuous hydraulic energy leakage. Apply tungsten carbide HVOF coatings and install sediment desander basins.