Control valve rangeability is a specification that is closely associated with the concept of flow characterization. Unfortunately, the term rangeability is often misused.
Classical – A common definition of the term rangeability (also referred to as ‘turndown’) is the ratio of the maximum to the minimum flow rate; however, the maximum and minimum flow rates are impossible for a valve manufacturer to specify because flow rates are a function of the pressure conditions of a specific application.
ISA Definition – The ISA definition of rangeability is more precise because it defines rangeability as “the ratio of the maximum control valve Cv to the minimum control valve Cv”.
By defining rangeability in terms of Cv, manufacturers can develop a rangeability specification, and users can evaluate that specification with respect to specific application requirements.
Practical Definition – A more precise and practical definition is “the ratio of the maximum control valve Cv to the minimum usable control valve Cv” .
The term usable is significant because, while some valves may have very low Cv ratings, sustained throttling at low travels may lead to accelerated erosion damage or to valve plug instability and plug slamming.