Dear boulfiza,
The only difference that in case of Gate valve you can open the valve gradually and you can use it as partly opened valve but in case of ball valve you can open it with quarter turn (90°) – it may cause Water Hummer -, Ball valve its not recommended to use as partly opened valve.
Both Gate And Ball valves are under Quick opening category.
I hope this will clarify the difference.
Regards
eyadbdc@yahoo.com
The reason we use gates valves before ball valves is expense! When you're getting above 4", ball valves start to get more pricey then gate valves, especially in a full port configuration. Good Luck! ...Mark