I have been seeking the answer of this question ‘how to decide ball valve installation direction for long time. Some of you may also have same question with me. So far, when I install a ball valve (with no hole on the ball inside the valve), I just consider for convenient operation only. There are almost no other considerations.
Basically, a ball valve can be installed where its handle at the upstream or at the downstream. At open position, the handle is inline with the piping. At closed position, the handle is perpendicular to the piping. The handle is attached to a ball inside the valve.
See image below to see a ball valve structure. This will help you easily understand how ball valve works.