Q: What is the rangeability of a control valve?
A: The term rangeability does not have a well-established definition. It is usually defined in vague terms. Basically, there are two definitions of rangeability: one for inherent rangeability, and the other for installed rangeability.
Inherent rangeability is defined as the change in which the deviation from a desired inherent flow characteristic will not exceed some stated controllable limits. In practice, the inherent rangeability is the ratio between the maximum and minimum controllable Cv, where Cv is the valve flow coefficient representing valve capacity. Rules for maximum and minimum controllable Cv values vary, depending on the valve type.
Correspondingly, installed rangeability is usually defined as the range in which deviation of the installed flow characteristic will not exceed some stated tolerance limit. In this case, installed rangeability can be defined as the ratio between the maximum and minimum flow rate. This means that the gain of installed valve rangeability does not deviate from a specified gain tolerance determined by controllability. For instance, a rule of thumb of permissible limits for the installed gain is that a change in gain that is greater than 4 should be avoided. If the application is critical and the controllability requirement high, a smaller gain variation is sought – typically, 2 or 3.
One important factor, which should be taken into account when rangeability is discussed, is how well the valve actually operates, i.e. what is the smallest signal change valve response. For instance, there are a lot of high-performance butterfly valves out in the field, which provide adequate control, even though installed gain may vary. The main reason for this is that those butterfly valves have minimal friction in control and therefore also good controllability. On the other hand, in control applications, some advanced control techniques such as gain scheduling or characterization may be utilized, which make the situation for the definition of installed gain limits even more difficult. All these factors have the effect that the definition of valve rangeability may vary from one valve vendor to another. I hope that in the future international standardization organizations, such as the IEC, can reduce the confusion regarding valve rangeability.
Expert: Vesa Lempinen. Director, Control Valves Product Center.