When there is more than one manipulated flow, split range control is commonly used, the subject of Part 1. In succeeding parts to this series we will see how regulatory control connoisseurs use valve position control and advanced control enthusiasts have gotten increasingly innovative in the use of model predictive control. The manipulated flows are set points of secondary flow controllers or the signals to final elements (control valves and variable speed drives). The primary conceptual objectives in the manipulation of multiple flows to control a single process variable are as follows:
(1) Extend rangeability
(2) Improve resolution
(3) Enable preferential use of flows based on cost
(4) Send flows to multiple destinations possibly based on priorities
(5) Provide counteracting effects
Some applications require more rangeability than can be achieved by a single valve or flow meter. The standard definition of valve rangeability that uses a low flow limit based on the uniformity of the inherent valve characteristic at low positions is too large and misleading. The lowest flow that can be reasonably throttled is the minimum flow of a repeatable installed characteristic multiplied by the stick-slip or backlash, whichever is greater. Per the standard definition, linear valve maybe cited as having the best rangeability even though the installed characteristic is quick opening and it is missing a positioner. Alternatively, a rotary valve may be stated as best because of its high flow capacity even though it may have high sealing friction and breakaway torque and excessive backlash in its linkage. The best rangeability is achieved by a sliding stem valve with an equal percentage characteristic, low friction packing, a generously sized actuator, and a tuned digital valve controller, which also provides the best resolution as discussed in the article “Improve Control Loop Performance”
The differential head flow meter rangeability of 4:1 is the lowest although it can be extended to 16:1 in cases where the differential pressure is large enough and the noise is low enough by the use of extended range d/p transmitters. The vortex meter, magmeter, and coriolis flow meter have an approximate rangeability of 16:1, 100:1, and 250:1, respectively if the maximum velocity of the meter size matches the maximum velocity of the process.
When the valve or flow meter rangeability (turndown) is not good enough, split range control is used where the controller output is split between a small and large flow meter or valve. The split range point between small and large flow control is conventionally chosen as 50%. However, to keep the control loop more linear, the split range point should be based on the capacity of the meter and control valve. For example if the big meter or big valve has a capacity 9 times larger than the small meter or small valve, the split range point should be 10%. In terms of control valves, this means the small control valve should be stroked 0 to 100% as the controller output goes from 0 to 10% and the big control valve should be stroked 0 to 100% as the controller output goes from 10% to 100%. In the old days, the split ranging of valves was done in the positioner. Today it is widely recognized that split ranging should be done in the control system for better maintainability, flexibility, and visibility.
The most common loop where you see small and large valves is the pH loop because of the exceptional rangeability and sensitivity of the pH measurement. The 0-14 pH scale covers fourteen orders of magnitude of hydrogen ion concentration and control at neutrality (e.6. 7 pH) involves controlling concentration to 7 or more significant figures. Often the small and large reagent valves are split ranged. However, for split ranged control, the resolution needed to meet the sensitivity requirement only occurs at low reagent demands before the transition from the small to large valve.
Split range creates a severe discontinuity not only from the change in gain but also because one valve is often opening and/or another control valve is closing. The stick-slip and the uniformity of the valve characteristic are worst as the valve goes into or trys to come off the seat. Many loops oscillate across the split range point. A hysteresis or deadband setting at the split range point can prevent both valves from being open but this deadband adds deadtime and the controller output will never settle out within the deadband but will always be traversing back and forth across the deadband for manipulated flows needed that are close to the split range point. The deadband provides some useful purpose in preventing noise or a short term disturbance from causing a transition from one valve to another. A deadband is commonly used in pH control of bioreactors in an attempt to decrease the number of excursions from carbon dioxide (acid) and sodium bicarbonate (base) to reduce the rise in internal pressure of the cells from an increase sodium ion concentration.
In some cases, cost dictates the preferential use of a flow. For example a waste reagent stream would be maximized compared to a purchased reagent stream for pH control, a waste fuel would be maximized compared to a purchased fuel for combustion control, air flow would maximized compared to a oxygen flow for bioreactor dissolved oxygen control, suction flow would be minimized via speed control compared to a flow vented for air compressor discharge pressure control, and a letdown valve position would be maximized compared to a flow vented, for header pressure control. Sometimes the allocation is made based on the priority of the user. For example, the more profitable reactors would loaded up with feed first for reactant compressor pressure control and the more efficient heat exchangers would be loaded up with coolant flow first for chiller pressure control.
In Parts 2 and 3 we will see if there are better solutions than split range to achieve objectives (1) through (4). Split range control is admittedly the best solution for loops that manipulate flows with competing effects, objective (5). Primary examples are acid and base reagent valve manipulation for pH control and heating and cooling valve manipulation for temperature control. The use of split range readily prevents the acid and base valve from both being open and the heating and cooling valves from both being open wasting reagent and energy, respectively.
The Emerson 2005 Exchange Presentation titled ControlUsingTwoManipulatedVariables.pdf offers implementation details and examples of split range control, valve position control, and model predictive control for the manipulation of multiple flows to control a single process variable.