Hi, I need to open and close 2 valves of 1", I am thinking about servos because their high power, accesibility and because I only need to turn 90 degrees. The valve I want to manipulate is this one:
Do you think a 15Kg/cm servo is right for the job? Any other idea?
Thanks !!
Start by measuring how much torque it takes to rotate the handle. Measure the distance from the axis of rotation and the maximum force needed to complete the rotation. Multiply the two numbers to get torque. Multiply by two for safety margin. The result is the torque of servo you need.
If your just opening and closing the valve, why not just a solenoid valve? 
15 kg cm is more than enough. Where did you find one? Is it a MGBB 1/4 scale?
That appears to be a ball valve, which often require a good amount of torque to operate.
What kind of servo I need to move this valve?
A BIG one. These suckers need a lot of torque to move, heck sometimes it's hard for a human to do it.
You may be better off using an actuator at the end of an extended handle.
Rob
Here is a hint. Search out industrial electrically operated 1" ball valves and look at the massive size of the motor they use and then see if you still think you can adapt a hobby servo to do the task. Not saying it's impossible as there are very high torque servos available but it will not be simple. The proper non-flexing bracket assembly you will have to come up with will be a challenge in itself.
As someone already posted, see if you can get by with a solenoid operated (on/off) valve, it may to cheaper and easier in the long run, especially if you don't have a need to regulate the flow rate.
Lefty
see if you can get by with a solenoid operated (on/off) valve
Yep, these are a dime a dozen. As long as you have pressure (to crack them open) and don't need to adjust the flow they are good.
Rob