Rangeability and Turn Down Ratio
By Torben W. Ibsen, President, M.Sc (Mech.Eng.), FlowCon International ApS, Denmark - Updated 07.2021
Rangeability is not clearly defined in regard to PICVs as PICVs hold a pre-setting element as well as a pressure independent element. The pre-setting element is often identical to the regulation element.
Controllability in control valves is related to the pre-setting and regulation element only as the control curves are created under constant differential pressure providing below mathematical reasoning:
Both the FlowCon UniQ®, FlowCon Green, FlowCon GreEQ and FlowCon SM ranges hold a 100% linear pre-setting and regulation unit and are – according to IEC 60534 standard – considered holding high control range giving a control range of 1:1000.
Another definition of rangeability, which is more suitable to PICVs as it includes the pressure regulating element, is to consider the rangeability as being the ratio between the highest to the lowest flow that the valve can control without exceeding the specified limits. These limits are defined as a total tolerance, which includes both the mechanical tolerances as well as the valve’s pressure independency and hysteresis and their tolerances.
Removing the pressure related tolerances, by reviewing the valve at a specific ΔP, significantly improves the tolerances. The tolerances based on mechanical tolerances alone are defined as 5% of the minimum adjusted flow rate for the FlowCon UniQ®, FlowCon Green and FlowCon GreEQ valves and 2% for the FlowCon SM range.
Another parameter to consider, upon discussing minimum controllable flow, is the actuator resolution as this defines how accurate the valve can be controlled by the actuator. The actuators used for the proposed valves hold a resolution of 1:100 for FN/FNP, FH and FNR actuators used for FlowCon UniQ®, FlowCon Green and FlowCon GreEQ ranges and 1:1000 for SM actuators used for FlowCon SM range when using a 0-10V signal. Based on the above, the values in table 1 are interpolated.
The rangeability in table 1, has been calculated as per below example for FlowCon SM.4.2:
Maximum flow: 51.000 l/h
Minimum flow : 340 l/h
Rangeability = (Maximum flow) / (Minimum flow)
Rangeability = (51.000 l/hr) / (340 l/hr) = 150
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