Abstract: A butterfly valve having a pair of disk tails provided at the back of a valve body in the rotation direction in an opening operation, a water flow hole formed in each disk tails, and a steady region found in relationship between loss factor and opening degree of the valve body where the loss factor becomes substantially constant with a change in opening degree of the valve body when water is filled from the water flow hole. Water filling velocity in an upstream pipe of the butterfly valve from the water flow hole, and pressure head upstream of the butterfly valve are set. Optimal value of the loss factor is determined based on the set water filling velocity and pressure head, and a total opening area of the water flow holes is set so that the loss factor equals the optimal value in the steady region.

Abstract: A butterfly valve having a pair of disk tails provided at the back of a valve body in the rotation direction in an opening operation, a water flow hole formed in each disk tails, and a steady region found in relationship between loss factor and opening degree of the valve body where the loss factor becomes substantially constant with a change in opening degree of the valve body when water is filled from the water flow hole. Water filling velocity in an upstream pipe of the butterfly valve from the water flow hole, and pressure head upstream of the butterfly valve are set. Optimal value of the loss factor is determined based on the set water filling velocity and pressure head, and a total opening area of the water flow holes is set so that the loss factor equals the optimal value in the steady region.