Lowdown on Turndown

Feb. 3, 2020
There are a lot of misconceptions on what is the turndown capability of measurements and final control elements (e.g., control valves and variable frequency drives). Here is a very frank concise discussion of what really determines turndown and things to watch for in terms of limiting factors. For flow measurements and final control elements, the term rangeability is often used.

There are a lot of misconceptions on what is the turndown capability of measurements and final control elements (e.g., control valves and variable frequency drives). Here is a very frank concise discussion of what really determines turndown and things to watch for in terms of limiting factors. For flow measurements and final control elements, the term rangeability is often used.

The turndown of vortex meters and magmeters is determined by a minimum velocity. The actual turndown experienced is typically a lot less than stated in publications because the maximum velocity for the meter size is usually greater than the maximum velocity for the process. Much larger than needed meters are often chosen because of conservative factors built into the stated requirements by process and piping engineers and the desire to minimize pressure drops across the meters. Less than optimum straight runs of upstream and downstream piping can also reduce rangeability for vortex meters and particularly differential herd meters (flow being the square root of pressure drop) because of the introduction of flow sensor noise that becomes large relative to size of sensor signal at low flows. Also, physical properties of the fluid most notably fluid kinematic viscosity for vortex meters and fluid conductivity for magmeters can significantly reduce turndown capability.

The turndown for valves more commonly stated as rangeability is severely limited by backlash and stiction that is often a factor of two or more greater near the seat than at the mid stroke range where response testing is normally done.  Also, valve actuator sizes should provide at least 150% of the maximum torque or thrust requirement to deal with less than ideal conditions and tightening of stem packing. Valve rangeability is also greatly limited by the installed flow characteristic particularly if the valve to system pressure drop ratio at max flow is less than 0.25 in a misguided attempt to reduce pressure drop and provide more flow capacity than what is actually needed.

The literature does not alert users to the fact that variable frequency drives can have a very nonlinear installed flow characteristic and poor turndown. To maximize rangeability of variable frequency drives, use a pulse width modulated inverter with slip control, speed to torque cascade control in the field (not control room), a pump head that is at least 4 times the maximum static head, totally enclosed fan cooled inverter rated motor, high resolution signal card, and minimal dead band setting in drive setup.

Transmitters and some sensors have an error that is expressed as a percent of span that reduces turndown. Transmitters selected that have a range narrowed to be closer to the actual maximum consequently improve turndown.  The use of thermocouples (TCs) and resistance temperature detectors (RTDs) input cards instead of transmitters introduce a huge error and resolution limit and reduction in real rangeability due to the large spans. The use of RTDs instead of TCs greatly increases rangeability due to much smaller sensitivity errors and drift of RTDs provided the temperature is in the recommended range for RTDs.

You can achieve greater rangeability by putting small and large flow meters and control valves in parallel. The process control loop manipulates the smaller valve using the smaller flow meter for cascade control for more precise control. A valve position controller (VPC) manipulates the large valve to keep the small valve in a good throttle range. External-reset feedback is used to reduce interactions and provide a fast correction if the small valve is moving toward the lower or upper best part of its installed flow characteristic. Feedforward or flow ratio control can be used to provide quicker correction. See the Control article “Don’t Over Look PID in APC” for much more on the many uses of VPC. Note that the use of split range control is not as good because you are normally manipulating the large valve and using the large flow meter with error and resolution limitations that are large due to being a percent of span.

Going for more flow capacity, lower pressure drop or a cheaper installation generally hurts turndown. Remember bigger is not better and cheaper is not really cheaper in the long run.

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About the Author

Greg McMillan | Columnist

Greg K. McMillan captures the wisdom of talented leaders in process control and adds his perspective based on more than 50 years of experience, cartoons by Ted Williams and Top 10 lists.

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