Valve: Ball Valves
Application: can tolerate Leakage to the level given in API 6D for metal seated valves
Service: Once in one month
Fluid: Sour Liquid/Gas mixture and Has Solid Suspensions
Size of the particulate:  Maximum size 1 cubic mm. 
Hardness of the particulate: Not known.
Erosion by the virtue of impingement not envisaged.
Balls and the seats are Tungsten carbide coated
Fear/Concern: Entrapped solid particulates may damage the seating surface during opening or closing the valve there by causing leakage during subsequent operation.
The best trick I know to reduce scratching is to install the ball with the axis of rotation horizontal. 
The ball should customarily rotate CCW to open.  Therefore the stem should be on the left when looking downstream. 
When the valve is closed, the solids drop out of suspension and lie along the bottom of the pipe.  Rotating the ball CCW tends to lift and dislodge any particles that might be at the ball/seat interface, so they don't get ground into the seat, but get flushed away as soon as flow begins. 
Also, with the horizontal rotation, the body cavity gets flushed every cycle.  Conversely with a vertical axis, the body cavity might accumulate debris and the ball will be sitting in a pile of grit. 
This trick works for butterfly valves also. 
Just because the pictures in the catalogs show the valve with a vertical stem, does not mean that they HAVE to be installed that way.

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