Wet Column Startup

If a column is shut down by closing the steam and drawoff valves, the entire column contents accumulate in the lower section of the column. Typically the liquid level rises well above the vapor inlet from the reboiler and up over some of the lower trays. Some column designers prefer to provide enough space below the first tray that it can never be flooded. Such a shutdown may be made to accommodate production scheduling or may have been forced by process interruptions elsewhere. The column may...

Low Condensate Receiver Level Override on Reflux

As liquid level rises to the lowest position 3 psig , the output of reverse-acting override A1 with gain 4 starts to decrease from 15 psig when the level reaches 25 percent 6 psig , the output of A1 is 3 psig. This permits the reflux valve to open and the reflux controller to take over somewhere in the lower 25 percent of level. If the level drops again, this override starts to close the reflux valve. This design should be compared with that of Figure 3.26 where a PI level controller was used...

Feedforward Compensation With Overrides

Temperature Cascade Controle

A convenient method for providing feedforward compensation that does not interfere with either normal reset or antireset windup involves the use of an impulse relay and a summer as shown in Figure 9.8. In pneumatics these functions are sometimes combined into a single device. The impulse function Column base temperature control with AP override Column base temperature control with AP override

Single Loop Controls

When control of a valve is transferred from one controller to another through a selector, we would like this to happen without a bump. Since the selectors will switch on very small differential signals, they will in themselves introduce no significant jolt. The major potential source of bump is reset windup in overridden controllers. In all commercial pneumatic controllers, reset action is obtained by a positivefeedback, unity-gain circuit that feeds the controller output signal back through a...

Controllers In Override Circuits

Three types of controllers have been used in override circuits two of them widely, and the third less frequently. Proportional-Reset PI or Proportional-Reset-Derivative PID Controllers Both of these are commonly used as normal controllers, although the PI type is occasionally used as an override controller. PID controllers, if used, should be so designed that the derivative acts only on the measurement signal, not on the controller output. Both PI and PID controllers must contend with the...