Q: What is a control valve authority?
A: When the term authority is used in connection with control valves, you might well think that it is some kind of organization or body which classifies or confirms a particular characteristic of a control valve. But this is not the case. A control valve authority is a factor which is defined as the differential pressure across a valve at a certain flow rate divided by the differential pressure across the valve when the valve is closed. The flow rate commonly corresponds to valve travel under maximum design process conditions.
If the valve authority is high (close to one), the control valve takes most of the total system pressure drop, i.e., the flow resistance of the pipe and other equipment is low compared to the control valve’s flow resistance. By contrast, if the valve authority is low, this means that the pressure loss in the pipeline system is significant compared to the control valve’s pressure loss. The term valve authority is quite commonly used, but it is not standardized internationally. Hence other terms may also be used to describe the same factor. In Valmet’s control valve sizing program, Nelprof, the parameter called DPm factor corresponds to control valve authority. The DPm factor (or valve authority) can be utilized to study a valve’s installed flow characteristic or to provide rules of thumb for required inherent flow characteristics.