What Is Ratio Control?
Holding Two Flows in Proportion

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Wild Stream and Controlled Stream

Every ratio loop has two flows. The wild stream (also called the uncontrolled or free stream) is the one that varies on its own - it is set by upstream demand or production and the ratio loop does not try to change it. The controlled stream is the one the loop manipulates to keep the proportion right.

The logic is simple: measure the wild-stream flow, multiply it by the desired ratio to compute a setpoint, and feed that setpoint to a flow controller on the controlled stream. If the wild flow doubles, the controlled setpoint doubles with it, and the ratio holds. This is a specific, disciplined form of feedforward - the wild flow is the measured disturbance driving the setpoint.

Where Ratio Control Is Used

Combustion: Air-to-fuel ratio control on heaters, treaters, and boilers keeps combustion efficient and safe as firing rate changes; too little air wastes fuel and makes soot, too much air wastes heat up the stack. Chemical injection: Dosing corrosion inhibitor, methanol, or demulsifier at a fixed rate per barrel of produced fluid is a ratio loop - injection scales with production flow. Blending: Making a product to a recipe means holding component streams in exact proportion.

A common refinement is cross-limiting on combustion loops, which leads air on an increase and lags it on a decrease so the mixture never goes fuel-rich during transients - a safety-driven variant of the basic ratio scheme.

Inside the Implementation: The Ratio Station

In most systems the ratio logic is a small block called a ratio station: it takes the measured wild-stream flow, multiplies it by the entered ratio, and writes the result as a remote setpoint to the flow controller on the controlled stream. That flow controller is an ordinary PID loop - the ratio scheme does not replace feedback, it just computes the setpoint continuously instead of leaving it to an operator. Some systems express the same thing as a ratio mode on the controller itself; the arithmetic is identical.

Two details matter more than they look. First, units: the multiplication only means something if both flows are on a consistent basis, and mixing a mass flow with a volumetric flow, or gas flows at different reference conditions, quietly builds a wrong ratio into the loop. Second, signal quality: any noise or error on the wild-stream measurement passes straight through the multiplier into the controlled stream's setpoint, so the wild flowmeter deserves the better installation, not the spare one. Where the wild measurement comes from a differential-pressure element, confirm square-root extraction is applied exactly once along the signal path.

A Worked Symbolic Example

Take chemical injection. Let the wild stream be produced fluid at flow F, and let the target dose be R units of chemical per unit of produced fluid. The ratio station computes the injection setpoint S = R times F and hands it to the injection pump's flow controller. If production doubles, S doubles with it; if the well slugs and F briefly collapses, S collapses too, and the pump turns down instead of overdosing a dribble of fluid.

Now add the practical guards. Below some minimum wild flow the measurement is no longer trustworthy - flowmeters lose accuracy at the bottom of their range - so a low-flow cutoff holds or stops injection rather than chasing a number that is mostly noise; where that threshold sits is site-specific. And because a ratio loop faithfully delivers R times whatever the meter says, a periodic cross-check closes the loop at a longer timescale: compare the chemical tank's drawdown against the totalized wild flow times R over a day or a week. Persistent disagreement means a meter or pump problem the loop itself cannot see.

Commissioning and Tuning Sequence

A ratio loop commissions cleanly if the pieces are proven in order:

1. Verify the wild-stream flowmeter: units, range, health, and agreement with an independent check.
2. Tune the controlled-stream flow loop on a local setpoint first, before connecting the ratio.
3. Confirm the ratio arithmetic at both ends of the expected wild-flow range.
4. Check that the controlled element's turndown covers the full range the ratio will demand.
5. Set and test the low-flow cutoff behavior.
6. Step the wild flow, or simulate it, and watch the controlled stream track.

The tuning philosophy: make the inner flow loop responsive and leave the ratio alone - the ratio is arithmetic, not dynamics, so there is nothing in it to tune. If the controlled stream oscillates, the flow loop is the suspect. If it tracks smoothly but at the wrong proportion, the meter scaling or the entered ratio is the suspect. Keeping those two failure classes separate is most of the diagnostic work, and it is the same reasoning used in cascade control, where an inner loop serves an outer master.

Failure Modes That Deserve an Alarm

The dangerous ratio-loop failures are the quiet ones. A drifting wild flowmeter shifts the dose everywhere without tripping any loop alarm, because the loop is doing exactly what it was told. A saturated controlled stream - the injection pump at maximum, the blend valve wide open - means the demanded ratio is silently not being met, and the flow controller cannot say so unless someone alarms on the deviation. Good practice is to compute the actual delivered ratio from the two measured flows and alarm when it departs from target for longer than a transient.

Since ratio control is a form of feedforward control, it inherits feedforward's blind spot: it corrects for the disturbance it measures and no other. Changes in chemical concentration, stream composition, or meter calibration walk straight past it. Wherever the true objective is measurable - an analyzer on a blended product, combustion readings on a burner - a slow feedback trim on the ratio value catches what the arithmetic cannot.

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