Ratio control keeps two streams in a fixed proportion to each other, even as the total throughput changes. It is the strategy behind holding an air-to-fuel ratio steady on a burner, injecting chemical at a set dose per barrel, or blending two products to a recipe. This guide explains the wild-stream and controlled-stream setup, how a ratio loop actually works, and where it shows up in oil and gas.
Ratio Control in one line: Ratio control is a strategy that maintains a constant proportion between two flow rates. One stream (the wild or uncontrolled stream) is measured, and the controller sets the setpoint of the second stream to a chosen multiple of it, so the ratio stays fixed as the wild flow rises and falls.
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.
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.
The wild stream is the flow that varies on its own and is not manipulated by the ratio loop - it is set by upstream demand or production. The controller measures it and adjusts the second, controlled stream to keep the two in the desired proportion.
Yes. Ratio control is a specialized feedforward strategy: the measured wild-stream flow is the disturbance that drives the controlled stream's setpoint, so the proportion is maintained before any error develops. A feedback flow loop on the controlled stream trims the actual delivery.
Injecting chemical at a fixed dose per barrel of produced fluid, holding a burner's air-to-fuel ratio steady as firing rate changes, and blending two product streams to a recipe are all common ratio-control applications.
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