Control Valves Turndown and Rangeability

Turndown and rangeability is usually associated with control valves. The definition of these are (from Perry):

Rangeability, R= maximum controllable flow / minimum controllable flow

Turndown ratio, T= normal maximum flow / minimum controllable flow

Generally valves are designed so that T=0.7*R where R is between 20 and 50.

This definition complies with the ISA standard.

Turndown is often used not only in connection with control valves but also as a more “global” parameter e.g. for whole plants.

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