Control valve with controlling actuator are used in vast field nowerdays. We know that control valve are selected and sized to provide a range of flow for a specified pressure drop accoss the valve, they are never moved to fully open or fully closed in normal work condition.
When people required a control valve, they usually offer the datasheet with elaborate specifications for seat leakages. And the suppliers offer valves as per the seat leakage standards are setted up by control valve bodies. Considering that a control valve is not used to shut-off flow, the leakage characteristic of control valve in the closed postion is selected as opposed to on/off valve.
If the work condition required the flow to be fully shut down, we are choose another on/off valve (such as gate valve) positioned near the control valve. Because the high cost and less value to the control valve's real life performance is unnecessary.
That's way, for control valve it is very important to offer enough correct data to avoid any default.
