The structure and operation characteristics of thedetermine that the opening torque of the ball valve is unbalanced, and its opening torque decreases with the increase of the opening degree, which has its special torque characteristic curve.
The torque of the ball valve is the maximum at the moment when the full differential pressure is opened. At this time, it is necessary to overcome the static friction between the ball and the valve seat, that is, the disengagement torque (BTO) described in the 26th edition of API 6D draft standard. As the sphere gradually turns to the full open direction, its opening torque will decrease. When the sphere turns to 12 ° to 14 ° the opening torque will increase slightly. At this position, the valve starts to open gradually and its torque decreases with the increase of opening degree. When the opening degree reaches 45 degrees, the RTO is the minimum. When the whole circumference of the ball moves into the sealing ring again, when the ball valve reaches the full open position and is fully opened, its torque (ETO) will increase. It should be noted that there is a slight difference between the starting torque when the valve stem only bears torque and when the valve stem bears both torque and medium load.
Since the sphere is subject to the component force of the medium force which is beneficial to fixing the sphere in the closed position when it rotates, the closing torque is much smaller than the opening torque. The figure shows the torque characteristic curve of ball valve from opening to closing and then to opening. If the ball valve is started from the open or closed position, the required torque should be slightly higher than the torque that brought the ball to that position in the previous operation, namely the so-called “viscous torque”. The torque characteristic curve of most ball valves is similar.
BTO – release torque or thrust at the moment of opening
BTC – torque or thrust for closing instantaneous disengagement
RTO – operating torque or thrust during opening
RTC – operating torque or thrust during closing
ETO – torque or thrust in fully open position
ETC – torque or thrust in fully closed position