An equal-percentage inherent characteristic means equal increments of travel produce approximately equal percentage changes in flow coefficient over the characterized range, at a constant valve pressure drop. It does not mean equal travel produces equal flow increments.
Inherent and installed behavior are different
A catalogue curve normally describes the valve independently under specified test conditions. In a real plant, the pressure available across the valve often changes as flow changes. Piping friction rises with flow and the pump may move along its curve. The relationship between travel and actual process flow therefore differs from the inherent curve.
A simple illustration
Consider two consecutive travel intervals in an idealized region where capacity increases by 50 percent per interval. A coefficient of 10 becomes 15, then 22.5. The absolute increments are 5 and 7.5, not equal. This is only an illustration of the term; it is not a VPORTA capacity curve or a way to calculate a specific valve opening.
What to request from a supplier
Ask for the actual Cv or Kv versus travel for the proposed size and trim. Evaluate the operating points against the installed pressure profile and expected actuator resolution. Avoid adjusting the controller to compensate for a badly oversized valve: that may reduce symptoms without fixing poor useful travel.
Practical checklist
- Check the curve’s valve size, trim and units.
- Provide the pressure drop at each flow case.
- Distinguish inherent rangeability from plant turndown.
- Use valve feedback to compare command and actual travel.
Discuss your application
Send the process conditions and required valve function for a configuration review. Request a technical quotation ↗