PPantha,
Please note that this is not a direct calculation, but a result of an evaluation of your total situation. Both layout, normal process situation, normal filling and emptying and eventual breakdown situations must be considered.
Filling: Must be controlled and in a rate where air can escape 'naturally' or if insufficient by air outlet valves. Normal waterpipe mains calculation/air outlet valve types based on this should be OK
Emptying: For a hydroelectric plant worst case is equipment breakdown. Air inlet, top placed, must in any cases be large enough to avoid collapse from vacuum. Too fast emptying can be caused by false or wrong operation, bottom valves breakdown, other bottom equipment failure, bursting main pipeline or other causes.
The rate/amount of water leaking can vary widely, for instance will a buried pipe probably leak a bit less than an open pipe if it bursts.
Necessary inlet valve size in your case may come out to be in a larger range. I have supplied air inlet valves up to 1000mm in diameter to hydroelectric plants.
A rough estimate can be done as simple as taking the water volume escaping as necessary volume air to be let in by an opening Xmm if the air velocity is a fair 'gas speed normal through pipelines' say 30-90 m/s. Remember air noise level!
(PS: 400-500mm????)
