Himani, It might be best to have two valves, one at the top of the penstock for isolation and one at the bottom for the control that you mention. It is difficult and extremely unwise to try and control from the valve at the top of the penstock, like trying to push a string. The likely scenario would be to partially drain the penstock and then open the valve at the top. When the water columns collide you will develop a water hammer such as you have never seen and don't want to see. If you don't have enough money for two valves go with a single valve at the bottom of the penstock. S l o w l y is operating rule number 1, avoid shutting the valve down too rapidly. Some of my big penstocks require 1/2 hour to make a major rate change. Your small Hydro is a bit more forgiving but the same physics apply. Good luck. Bob Robert K. Weir P.E. Hydroscreen CO. _www.hydroscreen.com_ (http://www.hydroscreen.com)
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