Ratio Control - Electrical Engineering (EE) PDF Download

Ratio Control
A ratio controller is a special type of feedforward controller where disturbances are measured and their ratio is held at a desired set point by controlling one of the streams. The other uncontrolled stream is called wild stream. Fig V.11 shows the schematic of a ratio controller. The ratio of flow rates of two streams are being held at a desired ratio by controlling the flow rate of one stream. The flow rates are measured through flow transmitters (FTs).

The chemical process industries have various applications for ratio controllers. Following are a few such examples:

•  Reflux ratio and reboiler feed ratio in a distillation column

•  Maintaining the stoichiometric ratio of reactants in a reactor

•  Keeping air/fuel ratio in a combustion process

 Override Control
During the operation of a process plant it is possible that a dangerous situation may arise due to unacceptable process conditions which may destruct the process or its personnel. In such case the normal operation should temporarily be stopped and preventive measures should be initiated to avert the unacceptable situation. In order to facilitate such measures, a single-purpose “switch” can be used that can take preferential instruction from one controller over the others to manipulate the final control element in such a way that the dangerous situation can be averted. This is called override control. The technique can be illustrated with the following example.

Consider a boiler shown in the Fig V.12 It has one water inlet and one steam outlet. The steam outlet is regulated by the valve in the discharge line that takes the control signal from the control mechanism in Loop1 (pressure transducer and pressure controller). In other words, the discharge of steam is regulated on the basis of its pressure desired in the supply line elsewhere. However, the water is boiled using a heating coil that needs to be always submerged below the water level so that the heating coil does not burn out. Hence, in order to ensure a certain minimum level of water inside the boiler, the control Loop 2 is set in place that contains a level transducer and a level controller. Both level controller and pressure controller give the control signal to the valve through an intermediate switch LSS (Low Selection Switch) that takes the preferential signal from the level controller. In other words, Loop 2 remains inactive during the normal operation and the Loop 1 regulates the process. Nevertheless, at critical situation when the water level drops below the minimum allowable limit, the Loop 2 takes over and takes corrective measures.