Rangeability and Turn Down Ratio

By Gitte Pedersen, R&D Manager, B.Eng (Integrated Design), FlowCon International ApS, Denmark - Updated 05.2024


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 Essentia, 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 Essentia, 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 Essentia, 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/hr and Minimum flow1: 340 l/hr.

Note 1: The minimum flow rate is defined as being the smallest flow the valve can control without exceeding the specified limits. For FlowCon SM valves and FlowCon Essentia.0 valves, this value is defined by the mechanical tolerances and not the minimum flow the actuator can modulate to. For FlowCon Essentia.1/.2, FlowCon Green and FlowCon GreEQ, the lowest value is defined by the actuator and not the machanical tolerance.

Model Highest Settable
Max. Flow
Lowest Settable
Max. Flow
Minimum Controlled
Mechanical Tolerance
at Lowest Setting
(note 2)
  [l/hr] [l/hr] [l/hr] [l/hr] [lx:1]
FlowCon Essentia.0 465 35 30 2 15
FlowCon Essentia.1 1.000 105 10 5 100
FlowCon Essentia.2 1.700 150 17 7 100
FlowCon Green.0 575 37 6 2 100
FlowCon Green.1 1.110 64 11 3 100
FlowCon Green.1HF 2.650 620 27 31 100
FlowCon Green.2 4.630 865 46 43 100
FlowCon Green.3 13.647 1.900 136 95 100
FlowCon GreEQ.0 454 17 5 1 100
FlowCon GreEQ.1 805 27 8 1 100
FlowCon GreEQ.2 2.160 850 22 43 100
FlowCon SM.1.1 2.470 633 3 13 190
FlowCon SM.2.1 8.420 1.850 9 37 228
FlowCon SM.3.0 15.000 5.310 15 106 141
FlowCon SM.3.1 27.500 9.240 28 185 149
FlowCon SM.3.2 35.600 12.800 36 256 139
FlowCon SM.4.1 33.800 12.600 34 252 134
FlowCon SM.4.2 51.000 17.000 51 340 150
FlowCon SM.4.3 72.700 13.300 73 266 273
FlowCon SM.5.1 83.800 23.300 84 466 180
FlowCon SM.5.2 106.000 25.600 106 512 207
FlowCon SM.6.0 277.000 33.100 277 662 418

Note 2: The flow rates used to calculate the turn down ratio may contain additional decimals compared to the rounded numbers listed in the table.

Turn Down Ratio

Turn down ratio is used to describe the ratio between the maximum and minimum controllable flow in a given system. A product holding a spindle height limitation will result in limitation to its available resources and therefore have a lower turn down ratio than rangeability. E.g. a valve holding a 100:1 rangeability and sized for a specific system to utilize only 40% of its capacity, results in a 40:1 turn down ratio.

The FlowCon Essentia, FlowCon Green and FlowCon GreEQ valves – unlike most other PICVs – do not have any stroke limitation regardless of setpoint as these valves always utilize 100% of its capacity. The turn down ratio is therefore 100:1.

The FlowCon SM range is controlled by rotary actuator motion and holds a very high rangeability. When calculating the FlowCon SM’s turn down ratio, actual degree rotation is divided by the maximum degree rotation. As an example, FlowCon SM.3.1 has a rotation limitation to 1.080° when selected for a max. flow of 11.600 l/hr compared to 2.160° in highest maximum flow settings. This effectively means that the valve’s turn down ratio is 50%3 of its specified rangeability or full capacity or in other words 50:1 for this selected lower flow setting. If lower flow rates are required, it is recommended to select a SM.3.0 instead in order to keep turn down ratio up.

Note 3: 1.080°/2.160° = 50%


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