Ball valve which suffered failure is connected to manifold end and severe erosion found on connecting/ball valve flanges.  No erosion noted on opposite flange end.  Suspect there may be dispersion of sand in manifold piping from other wells.  What should be normal condition of header valve open position, whether full or partial?
Throttling with sand suspended in the fluid causes high velocities in the venaContracta-the narrowest portion of the flow just after it goes thru a restriction.  In this case the restriction is the sliver between the partially open ball and the seat.  Damage in this case would be a rounding of one side of the ball opening, scouring of half of the inside of the body, Sharp-edged wear of the downstream edge of the the ball port, assymetical wear of the valve seats, and one-sided wear of the valve tailpiece. 
The answer to reducing the damage is to use harder materials (316 instead of Carbon steel, or stellite instead of 316, ot Tungsten carbide instead of stellite) , reduce the valve size so it is run farther open or(even better) fully open.  Install the valve with the shaft hotizontal, and the bonnet on the left side when looking downstream.  When the ball is closed you get a layer of sand on the bottom of the pipe.  Opening a properly installed valve pushes the sand up and away from the seat until flow is established, than it flushes thru.  A valve with a vertical spindle just gets a bucket of sand in the bottom that grinds away at it whenever the ball is rotated.

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