Valve rangeability, also known as turndown ratio, is a measure of the ability of a control valve to provide accurate and precise control over a wide range of flow rates or process conditions. It represents the ratio between the maximum and minimum controllable flow rates through the valve while maintaining acceptable control performance.

In simpler terms, valve rangeability indicates how effectively a control valve can modulate flow across its entire operating range. A higher rangeability value indicates that the valve can provide accurate control over a wider range of flow rates, while a lower rangeability value suggests that the valve's control capabilities are limited to a narrower range of flow rates.

Mathematically, valve rangeability is calculated as the ratio of the maximum controllable flow rate (Qmax) to the minimum controllable flow rate (Qmin):

Rangeability=Qmin​/Qmax​​

Where:

Qmax​ is the maximum controllable flow rate through the valve under specified operating conditions.
Qmin​ is the minimum controllable flow rate through the valve under specified operating conditions.

Valve rangeability is an important consideration in control valve selection and sizing for process control applications. A control valve with higher rangeability can provide more flexibility in controlling flow rates and can accommodate a wider range of process conditions without sacrificing control accuracy or performance. This is particularly important in industries where processes may experience significant variations in flow rates or operating conditions.

Factors that can affect valve rangeability include the valve design, trim characteristics, actuator type, and control system characteristics. It's important to consider the desired rangeability requirements when selecting a control valve for a specific application to ensure optimal control performance and efficiency.